Language：Others   Publishing type：Research paper (scientific journal)   Publisher：International Journal of Fatigue  

Hierarchical anisotropic nanostructured Mg-RE alloys, containing LPSO lamellae and dense nano-precipitates, exhibit superior mechanical properties. However, their reliability as structural materials for long-term service, especially in terms of fatigue, is still a matter of concern. Here, scattered oxide nodules are detected as a special kind of fatigue damage that assists crack initiation, rather than the typical slip-band structure. The LPSO lamellae and dense β′ nano-precipitates lead to the high localization of fatigue damage. The fatigue-induced oxide nodules form and grow within the soft α-Mg layer, but their growth is limited by the adjacent strong LPSO lamellae, resulting in microcrack nucleation. Finally, clusters of microcracks along the LPSO/oxide interface converge to form the trans-granular crack initiation.

DOI： 10.1016/j.ijfatigue.2023.107820

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Friction  

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation, thus potentially causing unanticipated catastrophic fractures. The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous, which obstructs the understanding of fretting fatigue issues, and is unable to guarantee the reliability of structures for long-term operation. Here, fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of an α/β titanium alloy up to 108 cycles. Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone. The grain size in the topmost nanostructured layer could be refined to ∼40 nm. The grain refinement process involves the initial grain rotation, the formation of low angle grain boundary (LAGB; 2°–5°), the in-situ increments of the misorientation angle, and the final subdivision, which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays. The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary (HAGB) and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer, thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress. Eventually, it facilitates surface degradation and the formation of catastrophic fractures.

DOI： 10.1007/s40544-022-0729-z

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：International Journal of Fatigue  

Heterogeneities of microstructure, tensile and fatigue behavior of FSWed titanium alloy joint were in-situ studied by utilizing replica, DIC and electron back-scattered diffraction methods. Banded TMAZ consisting of needle-like α precipitates and elongated αp within DRXed β grains bridges the HAZ and SZ. Fine β grains in SZ contribute to the highest tensile strength in the 3rd layer. Fatigue cracks prefer to nucleate from HAGBs and {1 1 2} 〈1 1 1〉 slip deformation of β structure in the 2nd layer of SZ below 210 MPa and 3rd layer of HAZ otherwise. Transgranular crack propagation along the {1 1 2} planes exhibits a higher crack growth rate in HAZ than SZ.

DOI： 10.1016/j.ijfatigue.2022.107426

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Scripta Materialia  

Bismuth titanate(BIT) ceramics have been widely studied for high temperature applications thanks to their high Curie temperature and large spontaneous polarization. The underlying domain structures play significant roles in regulating the macroscopic properties of ferroelectric materials, while there is only limited knowledge of the domain structures for BIT ceramics until now, substantially bad for the development of BIT-based new materials. In this study, the intriguing domain walls(DWs) in BIT ceramics were investigated in detail. The atomic-scale observation reveals that there primarily exist energetically favorable {110}-type 90° DWs in BIT ceramics. Combined with the theoretical analysis, these DWs are not only elaborated under multiple piezoelectric response modes, but also further corroborated by the quantitative reconstruction of spatial polarization distributions of twin-related ferroelectric variants. This work sheds insight into domain structure and piezoelectric response mechanism in BIT ceramics, and contributes to promoting their more applications in advanced electron devices.

DOI： 10.1016/j.scriptamat.2022.114793

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

High cycle and very high cycle fatigue (VHCF) crack initiation mechanism of TC17 alloy joint with heterogeneous microstructures in FZ has been studied at the stress ratio of 0.1. Intrinsic deficiencies of slip deformation in β grains with few martensites and cracking coarse grain boundary between prior β grains with martensite colonies account for the high cycle fatigue (HCF) of joints. The average diameter of intergranular welding pores regarding VHCF crack nucleation is measured to be ∼36 μm. The driving force governing the crack nucleation from intrinsic deficiencies in HCF to porosity defects in VHCF is ∼1.66 MPa·m1/2.

DOI： 10.1016/j.ijfatigue.2021.106525

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

In most cases, welding processes promote the application of structural materials at the expense of strength and ductility. Post-weld treatment is proposed to regain mechanical properties of weldment, which necessitates that microstructural and the associated mechanical behavior evolutions being clearly understood. In this study, post-weld heat treatment was conducted to enhance the mechanical properties of the electron beam welded Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy joint. Microstructural evolutions and monotonic mechanical properties were characterized to optimize the post-weld heat treatment parameters. It was found that direct aging at 630 °C for 2 h can upraise the strength and ductility of the joint by ∼31% and ∼511% respectively. Metastable needle-shaped martensite α′ within rapidly solidified fusion zone had been completely decomposed, and thin recrystallized α lamella precipitated in the prior β phase without forming consecutive coarse grain boundary α after above direct aging treatment. Such microstructural characteristics enable the direct-aged joint to exhibit a higher fatigue strength from high to very high cycle region than the as-welded joint. Moreover, the fatigue crack nucleation resistance was enhanced significantly through direct aged treatment.

DOI： 10.1016/j.msea.2021.142168

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1111/ffe.13520

Language：Others   Publishing type：Research paper (scientific journal)  

The underlying low-stress cyclic deformation and associated dislocation activities within the slip bands (SBs) in a rare earth-containing magnesium (RE-Mg) alloy were characterized. The results show that basal slip is the predominant deformation at grain scale, but the gliding of 〈c + a〉 dislocation is also visible near grain boundaries to accommodate localized inhomogeneous deformation. The interaction between basal 〈a〉 dislocations and non-basal 〈c + a〉 dislocations results in the nucleation of dislocation intensity zones (DIZs), which subsequently hinders the development of the basal slip bands. Therefore, it is suggested that the DIZs contributed to the enhanced high cycle fatigue resistance of RE-Mg alloys.

DOI： 10.1016/j.ijfatigue.2021.106302

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.msea.2021.141850

Language：Others   Publishing type：Research paper (scientific journal)  

This paper investigated the effects of Gd and Y solutes on aging behaviour and corresponding mechanical properties of the extruded Mg–Gd(-Y)-Zn-Mn alloys at room and elevated temperatures. The results show that aging treatment provided significant improvement of ~100 MPa in strength by forming ellipsoidal β′ nanophases in the as-extruded alloys. Partially substituting Y for Gd in the as-extruded Mg-Gd-Zn-Mn alloys can delay age-hardening response, but improve the strength increment after aging treatment. As the Y/Gd atomic ratio changed from 0 to 1, the Mg-1.75Gd-0.75Y-0.5Zn–Mn(at.%) alloy with a Y/Gd atomic ratio of 0.4 obtained the higher peak-hardness and mechanical properties. Enhanced age-hardening response and better mechanical properties were detected after separate additions of Y and Gd. The extruded-T5 Mg-2.5Gd-0.75Y-0.5Zn-0.3Mn alloy exhibited superior ultimate tensile strengths of 520 MPa at room temperature, 344 MPa at 250 °C, and 225 MPa at 300 °C. Fracture behaviours reveal that a change in predominant deformation mechanism from one based on dislocations to one mediated by grain boundary (GB) processes was found as the tensile temperatures arise from 250 °C to 300 °C. The activation of GB sliding of the fine grains partially resulted in the decrease of tensile strength at 300 °C.

DOI： 10.1016/j.msea.2021.141019

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In this paper, the dissolution, refinement and re-precipitation of long period stacking order structure and solute-segregated stacking faults (LPSO structure/SFs) in an Mg-Gd-Zn-Mn alloy fabricated by hot extruding were studied. The results demonstrate that the refinement of LPSO structure/SFs was dominated by the misalignment mechanism and re-dissolution mechanism. Dislocation slips on the LPSO structure/SFs, as well as the rapid diffusion of solute atoms was responsible for the re-dissolution of LPSO structure/SFs. As the LPSO structure/SFs dissolved, a region with supersaturated solute atoms was formed and resulting in re-precipitation of SFs in continuous dynamically recrystallized grains. Finally, the evolution model of LPSO structure/SFs during continuous dynamic recrystallization was proposed.

DOI： 10.1016/j.matchar.2020.110756

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Ultrasonic fatigue tests were performed on notched specimens to investigate high and very high cycle fatigue behaviors in the laser shock peened region of a compressor blade titanium alloy (Ti–8Al–1Mo–1V). The plastic deformation and compressive residual stress, induced by the laser shock peening (LSP), present gradient distributions along the depth direction. High-angle grain boundaries (HAGB, >15°) are increased by deformation twins, which are introduced into primary α phases during the plastic deformation. The enhancement in fatigue resistance is ascribed to the comprehensive retardation effects of the compressive residual stress, high-density dislocations, and HAGBs.

DOI： 10.1016/j.ijfatigue.2020.105868

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Very high cycle tensile and torsional fatigue of TC17 titanium alloy with bimodal microstructure have been firstly and comparatively studied at the stress ratio of 0.1. Like the tensile cracks, torsional cracks were substantiated to nucleate from specimen surface or interior sites and presented in a fatigue life-dependent mode. Internal tensile and torsional fatigue cracks with faceted morphology were analyzed using focus ion beam and unraveled to initiate in modes II + III that driven by maximum shear stress. Particularly, short crack branching and deflection beneath the crack initiation rough area can be observed for torsional loading condition.

DOI： 10.1016/j.ijfatigue.2020.105720

Language：English   Publishing type：Research paper (scientific journal)  

Microstructure evolution and mechanical properties of Mg96.5Gd2.5Zn1 alloys in as-cast and solution-treated states were systematically studied during hot extrusion. The as-cast alloy mainly contains eutectic (Mg,Zn)3Gd compound, block-shaped 18R-LPSO structure and Mg matrix with a few lamellar long-period stacking order (LPSO) structures. The solution-treated alloy has a block-shaped 14H-LPSO structure at grain boundaries (GBs) and high-density lamellar LPSO structures within the matrix. After hot extrusion, the (Mg,Zn)3Gd phase and 18R-LPSO structure were broken an kinked, respectively. Bimodal microstructure with fine recrystallized (DRXed) grains and coarse-deformed grain were formed in both studied alloys. Sandwich-like distribution of the fine DRXed grain was observed in the as-cast-extruded alloy, which shows obviously different DRX behavior compared with the solution-treated-extruded alloy that recrystallizes at GBs. The result reveals that lamellar LPSO structure promotes DRX formation in the LPSO-free region of the matrix by accumulating dislocations at the interface frontier of lamellar LPSO/matrix. Moreover, the LPSO structure, fine DRXed grains and nanoparticles at GBs enhanced the yield strength of the wrought alloy.

DOI： 10.1016/j.jmatprotec.2020.116666

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Selective variant growth of precipitates was found in an as-extruded Mg-13.7Gd-1.7Zn-0.5Mn (wt.&#37;) alloy. In the coarse-deformed grains, β' phases and hexagonal clusters mainly precipitate along the [101-0]Mg orientation. While in the fine dynamic recrystallized (DRXed) grains, the β' phases and a few of the hexagonal clusters are relatively uniform distributed along the three < 101-0>Mg directions. This phenomenon is attributed to the coupling of dislocation strain field and residual stress component in the as-extruded alloy. Moreover, dislocations in the deformed grains result in a significant increase in the number density of hexagonal clusters comparing to those in the DRXed grains. This selective variant growth of precipitates is expected to change the deformation behavior and modify the mechanical properties in Mg-RE (rare earth) alloys.

DOI： 10.1016/j.matlet.2020.127853

Language：English   Publishing type：Research paper (scientific journal)  

In this work, a high-strength Mg-12Gd-2Y-1Zn-Mn alloy with an ultimate tensile strength of 509 MPa and a fracture elongation of 5&#37; at room temperature tensile was developed by using hot extrusion and subsequent ageing. Progressive observations of crack initiation and early propagation of the as-extruded and peak-aged alloys were conducted during the tensile process. The results show that the activation of fine-grain boundary sliding and multiple non-basal slip systems in coarse unDRXed grains is the dominant deformation mechanisms of the as-extruded alloy, which eventually lead to crack initiation and propagation. After peak-ageing treatment, dense β′ precipitates are formed within the bimodal grains, resulting in changes in deformation mechanisms and crack initiation and propagation behaviours during the tensile process. The grain boundary sliding is suppressed and the grain boundary strength is enhanced due to the precipitation of β′ phases on the hindering effect of dislocation mobility. Therefore, no-basal slip and cross-slip in coarse unDRXed grains become the major roles in crack propagation. The intra-grain LPSO (long-period stacking ordered) phase can suppress crack initiation and propagation in both as-extruded and peak-aged alloys. For the thick LPSO phase at the grain boundaries of the as-extruded alloy, it inhibits crack propagation to some extent. While the thick-deformed LPSO phase at the grain boundaries of the extruded-peak-aged alloy more easily causes crack initiation with increasing tensile strains.

DOI： 10.1016/j.matchar.2020.110396

Language：English   Publishing type：Research paper (scientific journal)  

Ultrasonic fatigue tests were performed to investigate high and very high cycle fatigue behaviors of a laser additively manufactured Inconel 718 (IN718) alloy in the as-deposited condition. The results indicate that the competition failure behavior between the surface and interior crack initiation results in the separate S-N curve. Both manufacturing defects (e.g., gas pore, lack of fusion) and columnar grains (matrix) observed in the microstructure could act as the original fatigue micro-cracks due to the effective restriction on manufacturing defects. The fatigue sensitivity levels increase successively in terms of the type, size, and location of the maximal micro-crack.

DOI： 10.1016/j.ijfatigue.2020.105580

Language：English   Publishing type：Research paper (scientific journal)  

Effects of high-frequency cyclic loading on the banded ferrite–pearlite steel were analyzed through crack initiation and propagation. Interfaces of ferrite and pearlite colony with a small angle deviation from the loading axis were verified to be the most potential sites to fabricate the microcracks caused by the high strain gradient. The initial crack extension inside ferrite grain was driven by shear stress in model II along the direction with a 45° angle to the loading axis. Banded pearlite colony and the high-angle grain boundaries were considered as the dominant factors that promote the fatigue resistance of the material through arousing crack deflection in short crack propagation range and crack branching in long crack propagation range to reduce the crack propagation driving force in the crack tip. P–S–N curves were used to quantify the dispersion of fatigue lifetimes and evaluate the effect of elevated volume content of pearlite colony on the fatigue performance of the material.

DOI： 10.1007/s42243-019-00347-2

Language：English   Publishing type：Research paper (scientific journal)  

We explored the very high cycle fatigue (VHCF) behavior of solution treated Mg–10Gd–3Y–1Zn–0.5Zr (wt. &#37;, GWZ1031K-T4) alloy containing long-period stacking ordered (LPSO) structures. The fatigue strength of GWZ1031K-T4 alloy at 109 cycles is about 115.0 MPa, and the corresponding fatigue ratio is approximately 0.44. Crack initiation that causes fatal failure was found to occur primarily from the specimen surface in the high cycle fatigue regime and the subsurface in the very high cycle fatigue regime, accompanied by crystallographic facet formation irrespective of a lifetime. Our results reveal that the basal slip activated in the matrix grain substantially alleviated strain localization during VHCF testing. The unique microstructure, featuring block-shaped LPSO structures scattered at the grain boundary, a large amount of thin LPSO phase and stacking faults enriched with solute atoms (SFs) homogenously distributing in the alloy matrix, and supersaturated Gd and Y atoms in the nano-scale Mg layers in the grain, is proposed to account for the enhanced fatigue failure resistance of GWZ1031K-T4 alloy.

DOI： 10.1016/j.mtla.2020.100672

Language：English   Publishing type：Research paper (scientific journal)  

The precipitates inside deformation twins may block the dislocation motion and consequently affect the mechanical property of materials. Herein, at the atomic level, we directly visualize that the basal dislocation slips shear the twinned stacking faults (TSFs) within the deformation twins in an Mg–Zn–Y alloy containing long-period stacking ordered (LPSO) structures. The TSFs, enriched with solute atoms, could be considered as precipitates inside deformation twins. They are sheared by a single step or multiple shearing steps on the basal plane. The microstructural fingerprints, i.e., the width of basal shearing steps, enable a quantitative assessment of the local and total plastic shear strain due to the basal dislocation within the deformation twins. The TSFs can block dislocation slip, while the dislocation shearing induces large lattice distortion and even solute atoms redistribution at local intersection. The TSFs-dislocation interaction is expected to lower the basal dislocation motion and resultantly modulate the mechanical properties of magnesium alloys. These results may offer a novel strategy for strengthening and toughening magnesium alloys via tailoring the shearable precipitates.

DOI： 10.1016/j.msea.2020.139109

Language：English   Publishing type：Research paper (scientific journal)  

Supersaturated coarsening β phase and submicron acicular martensitic α′ phase originated by the high cooling rate during the welding process resulted in the electron beam welded TC17 titanium weldment to be poor in strength and ductility. Post-weld heat treatment was performed at 630 °C for 2 h to improve the mechanical properties of electron beam welded TC17 titanium alloy joint through modifying the microstructures of the weldment. Results showed that undesired α′ martensites in the fusion zone and heat affected zone, which inhibited the mechanical response of the weldment, were completely eliminated. The supersaturated β phase with martensitic α′ structure in the as-welded joint was recrystallized into α lamella with a width in nanoscale embedded in the β matrix after the post-weld heat treatment, which conspicuously enhanced the mechanical performances of welded joints. Micro-hardness of the FZ presented to be more stable around HV410. Tensile strength was regained from 868 MPa to 1138 MPa, which was very closed to the raw material. Intrinsic metallic deficiency inducing fatigue failure of the as-welded joint was eliminated after the post-weld heat treatment accompanied with the fatigue strength enhancing from about 140 MPa of the as-welded joint to 240 MPa with an equal fatigue life of 107 cycles under the stress ratio of 0.1.

DOI： 10.1016/j.jallcom.2019.151937

Language：English   Publishing type：Research paper (scientific journal)  

The tensile fracture was originated from the strain concentration in equiaxed grain region adjacent to the fusion center, where a lower micro-hardness occurred when compared to the columnar grain region in the fusion zone. A higher stress level was more potential to fabricate a fatal fatigue crack initiating from the intrinsic metallic deficiencies. At a lower stress of 180 MPa, fatigue crack appeared to initiate from the intergranular welding pores other than the intrinsic metallic deficiencies. Furthermore, intergranular welding pores induced fatal crack initiation became the major fatigue crack initiation mechanisms when the fatigue stress amplitude down to 140 MPa.

DOI： 10.1016/j.ijfatigue.2019.105210

Language：English   Publishing type：Research paper (scientific journal)  

The interfacial segregation plays an important role in affecting the mechanical performance of various materials. Here we study the segregation behaviors along deformation-induced interfaces in the matrix of an Mg97Zn1Y2 (at&#37;) alloy compressed at 473 K using atomic-scale high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging techniques. The kink boundary, twinning-like boundary, and tilt boundary within the matrix grain are detected to be segregated with both solute Zn and Y atoms. The segregation along kink boundary and twinning-like boundary is closely linked to the associated partial dislocations, and these boundaries also could be regarded as the nucleation sites for the formation of long period stacking ordered (LPSO) structures of nanometer scale and stacking faults (SFs) during deformation. For the various tilt boundary, the segregation could be explained based on either the O-lattice theory or partial dislocations. The experimental results may shed some new light on tailoring microstructures for improving mechanical properties and thermal stability of magnesium alloys.

DOI： 10.1016/j.mtla.2019.100287

Language：Others   Publishing type：Research paper (scientific journal)  

Microstructural cyclic deformation mechanisms and their relation to small fatigue crack initiation and early propagation behavior were experimentally investigated in a rare earth-containing magnesium alloy (Mg-Gd-Y-Zr). The results indicate that basal slip is the dominant deformation mechanism, especially in coarse grains, and eventually leads to fatigue crack initiation. Early crack propagation behavior was strongly influenced by local microstructural heterogeneities in grain size and orientation. Three kinds of microstructures—favorably-oriented coarse grains, fine grain clusters and unfavorably-oriented coarse grains—are discussed in terms of their deformation mechanisms and resistance to crack propagation. These microstructural effects caused highly variable crack propagation rates within the first ∼200 μm of cracks.

DOI： 10.1016/j.ijfatigue.2018.10.002

Language：English   Publishing type：Research paper (scientific journal)  

Understanding the interactions between deformation twins and plate-like phases in magnesium alloys is one of the key issues to tailor the microstructure of magnesium alloys for better mechanical properties. The {1012} twin boundary with the local ring-like atomic pattern in magnesium alloy, accompanied by the interaction between deformation twin and solute atoms, has been investigated using aberration-corrected scanning transmission electron microscopy. We found that these boundaries featured local overlapping morphology near the intersection between deformation twins and stacking faults (SFs) enriched with solute atoms. The overlapping morphology is proposed to be induced by the asynchronous shuffling of the SFs and matrix during the twinning. The local ring-like atomic patterns shown here imply that the shearable specific SFs in magnesium alloys will increase twinning energy and resultantly hinder twinning propagation.

DOI： 10.1080/14786435.2018.1539262

Language：English   Publishing type：Research paper (scientific journal)  

High cycle fatigue testing was performed on an as-extruded Mg-10Gd-3Y-0.5Zr alloy to investigate its fatigue crack initiation and early propagation behaviors. Experimental results showed that only basal slip was activated at a low cyclic stress amplitude. Fatigue crack initiated from the slip bands along the basal plane, leading to the formation of cleavage-like facets on the fracture surface. The formation of facets around the crack initiation sites consumed a vast majority of the cyclic loadings in high cycle fatigue regime, and small fatigue crack propagation was significantly retarded by local microstructure heterogeneity of neighboring grains. This can be ascribed to the incompatible deformation at low cyclic stresses, resulting in the localization of slip bands within isolated grains.

DOI： 10.1016/j.msea.2018.10.015

Language：English   Publishing type：Research paper (scientific journal)  

The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.

DOI： 10.1080/02670836.2017.1407556

Language：Japanese  

A unique fracture in fatigue of age-hardened Al alloys and its macro and microscopic aspects
<p>There are many studies reported that a fatigue crack propagated in a tensile mode macroscopically and the fracture surface was mainly occupied by striations in many metals. Whereas, a unique fracture in which a crack propagated in a macroscopic shear direction accompanied by ductile facets was observed in some aluminum alloys under specific conditions. In the present study, fatigue tests of age-hardened Al alloys of extruded 2017-T4 and 7075-T6 were conducted in relative humidity environments of 25% and 85% under rotating bending and ultrasonic loading conditions to clarify the crystallographic feature of a shear mode crack and propose its growth mechanism. Many facets showing a feature of shear mode crack propagation with an equivalent size to the grain size were observed at the fracture surface under both conditions of rotating bending in high humidity and ultrasonic loading irrespective of humidity. In addition, it was confirmed that the angle between the loading axis and the growth direction of the shear mode crack composed a constant value, ~35°, relating to the marked texture in the propagation process of the macroscopic shear mode crack. However, a crack growth rate was lower in the ultrasonic loading than in the rotating bending in high humidity. Based on their differences in occurrence conditions of the shear mode crack, two mechanisms for this unique propagation were proposed as follows; that is, one was a shear mode crack occurred by the promotion of the slip deformation to one direction due to hydrogen generated by reaction of Al alloy with water vapor in high humidity, and the other was a crack by the suppression of the deformation to one direction due to re-welding of crack faces under ultrasonic loading.</p>

DOI： 10.1299/transjsme.17-00273

Language：English   Publishing type：Research paper (scientific journal)  

Very high cycle fatigue behavior of nugget zone in AA 7075 friction stir welded joint was experimentally investigated using ultrasonic fatigue testing system (20 kHz) to clarify the crack initiation mechanism. It was found that the fatigue strength of nugget zone decreased continuously even beyond 10⁷ cycles with no traditional fatigue limits. Fatigue cracks initiated from the welding defects located at the bottom side of the friction stir weld. Moreover, a special semicircular zone could be characterized around the crack initiation site, of which the stress intensity factor approximately equaled the threshold of fatigue crack propagation rate. Finally, a simplified model was proposed to estimate the fatigue life by correlating the welding defect size and applied stress. The predicted results are in good agreement with the experimental results.

DOI： 10.1155/2017/7189369

Language：Japanese   Publishing type：Research paper (scientific journal)  

Effect of shot peening on fatigue properties of high strength Al alloy in high humidity

Language：Japanese   Publishing type：Research paper (scientific journal)  

Growth mechanism of a fatigue crack of extruded high strength Al alloy 7075-T6 in high humidity

DOI： 10.1299/transjsme.14-00694

Language：Japanese   Publishing type：Research paper (scientific journal)  

Aging condition for improvement of fatigue properties of maraging steel
The effect of aging condition on fatigue properties and the mechanism of decrease in fatigue strength in high humidity in 18&#37; Ni maraging steel of grade 350 were investigated under rotating bending in relative humidity of 25&#37; and 85&#37;. Aging conditions investigated were under-, peak- and over-aging ones at the conventional aging temperature of 753K. In addition, double-aging treatments which were under- peak- and over-aging treatments at 673K and under-aging one at 473K after the peak-aging at 753K were also examined. Both of static and fatigue strengths were increased by the double-aging without any decrease in ductility and fatigue fracture toughness. Fatigue strength was markedly decreased by high humidity in all of the steels, and the decrease in fatigue strength was mainly caused by the accelerations of crack initiation and its growth at the early stage of fatigue process. The decrease in fatigue strength in high humidity was also suppressed by the double aging. A few facets comparable to a grain size of a prior austenite were observed at the fracture origins in high humidity but transgranular cracks by slip deformation were in low humidity. However most of the fracture surfaces were covered with lath boundary cracking regardless of the humidity and aging conditions. River pattern was observed in the facets, suggesting that the acceleration of crack growth in high humidity was a behavior related to hydrogen generated in cathode reaction. Based on the results, new aging treatment for improvement of fatigue properties of maraging steel was proposed. © 2013 The Society of Materials Science, Japan.

DOI： 10.2472/jsms.62.756

Language：Japanese  

2B2 Improvement of Fatigue Strength of Age-hardened A1 Alloy in High Humidity by Shot Peening

Language：English   Publishing type：Research paper (other academic)  

Effect of plant oil on the fatigue strength of a squeeze cast Al-Si-Mg alloy was investigated. Crack growth rate was higher in plant oil than in air, inducing a decrease in fatigue strength. The reason included is explained from the viewpoint of high water absorbency of plant oils, on basis of hydrogen enhanced localized plasticity. © (2014) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/KEM.577-578.89

Language：Others   Publishing type：Research paper (other academic)  

A novel aging treatment was proposed to improve the fatigue strength of maraging steel by taking the effects of aging condition and humidity into account. Rotating bending fatigue tests were carried out for a 350 grade of 18% Ni maraging steel in the long life region up to 108 cycles in the relative humidity of 25% and 85%. Aging conditions under investigation included a conventional aging or the so called single aging at 753K, and a two-step aging or double aging by ageing at lower temperature of 473K succeeded to the single aging. Through the double aging, the susceptibility of fatigue strength to humidity was significantly improved without any deleterious effects. The main reasons for the decrease in fatigue strength in high humidity are due to the promotion of crack initiation and the acceleration of small crack propagation. The improvement of fatigue property in high humidity by the double aging was explained from the roles of the additional precipitation of supersaturated Mo atoms during the second lower temperature aging. Copyright © (2013) by International Conference on Fracture.

Language：Others   Publishing type：Research paper (other academic)  

The effect of humidity on the fatigue properties of 18% Ni maraging steels with different hardness and aging structure was investigated under rotating bending fatigue by varying relatively humidity from 25% to 95%. It was found that the initiation and the early propagation of a fatigue crack were accelerated in high humidity, which caused to a large decrease in fatigue strength of the alloys. In fact, the fatigue strength at 107 cycles obtained in RH 85% was less than a half of that in RH25%. Meanwhile, successive observation on surface fatigue progress and the fractographic analysis of fractured specimens elucidated that anodic dissolution was the main reason for promoting crack initiation whilst hydrogen embrittlement due to cathode reaction was responsible for the acceleration of crack propagation. However, the acceleration of crack propagation was suppressed by the formation of reverted austenite.

Language：English   Publishing type：Research paper (scientific journal)  

The fatigue micro crack initiation and propagation tests of a TiAl alloy with 8&#37; Nb content were carried out by using scanning electron microscopy in situ technology at room temperature and at 750°C. These results indicated that the fatigue micro crack initiation was mainly caused by the stress concentration at room temperature, but at an elevated temperature (750°C) the multi-cracks were caused by the coupled factors of both lamellar microstructure and stress concentration. Therefore, fatigue micro crack initiation behavior is much more dependent on the lamellar structure at an elevated temperature. One of the reasons is that the elevated temperature degrades the interface strength between the lamellar of the TiAl alloy with 8&#37; Nb content. Therefore, the small fatigue crack propagation behavior of the alloy exhibited a mixture damage model of interlamellar and translamellar at a micro scale. The crack growth path and fracture characteristics provided a proof of crack deflection, branching and/or bridging induced either by interlamellar or by translamellar failure mode. © 2012 by the authors.

DOI： 10.3390/ma5112280

Language：Japanese   Publishing type：Research paper (scientific journal)  

Effect of humidity on growth behavior of a fatigue crack of 7075-T6 extruded Al alloy
In order to investigate the effects of humidity change and loading frequency on growth behavior of a fatigue crack of an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out in relative humidity of 25&#37; and 85&#37; at frequencies of 50Hz and 6Hz. At 50Hz, macroscopic appearances of fractures were a shear mode at high stress levels in high humidity, though those were a tensile mode in other conditions of humidity and stress levels. The crack growth rate was accelerated by high humidity. By changing humidity, both of the growth rate and the growth mode of a crack were immediately changed to those corresponding to the changed humidity. That is, the growth behavior and fatigue life under humidity change may be estimated by those in constant humidity. Fatigue life in high humidity at 6Hz was longer than that at 50Hz in spite of accelerations of both of the crack initiation and its growth rate in the early growth process in high humidity at both frequency, though there was no or little influence of frequency on fatigue life in low humidity. Moreover, the crack propagated in the tensile mode even at high stress levels where the shear mode crack propagated at 50Hz. These results were explained by the effect of hydrogen on the crack growth behavior. © 2012 The Society of Materials Science, Japan.

DOI： 10.2472/jsms.61.712

Language：Japanese   Publishing type：Research paper (scientific journal)  

Effect of microstructure on fatigue properties of Al alloy 2017 in high humidity
In order to investigate effects of a texture and a slip characteristic on fatigue properties of Al alloys in high humidity, rotating bending fatigue tests were carried out using plain specimens of an extruded bar and a drawn one of age-hardened Al alloy 2017-T4 and an Al alloy annealed the extruded one in relative humidity of 25&#37; and 85&#37;. The extruded Al alloy and the annealed one had a marked texture of a (111) plane at the cross section of each bar, but a specified orientation was not observed in the drawn one. In addition, the aged-hardened Al alloys show a planar slip property and the annealed alloy shows a wavy one. In the all Al alloys, the obvious retardation of a small crack growth due to blocking effect of a grain boundary was observed in low humidity, though the retardation disappeared in high humidity. Moreover the acceleration of a crack growth was yielded due to the change in growth mechanism of a crack from a tensile mode in low humidity to a shear mode at high stress levels in high humidity in the extruded alloy, though the effect of humidity on the growth mechanism was very small in the drawn Al alloy and the annealed one. In the extruded Al alloy, the fracture surface yielded by the shear mode crack in high humidity was occupied by many slip planes and voids and was a (100) plane, though the fracture surface formed by the growth of the tensile mode crack in low humidity was covered with striations. In the drawn and annealed Al alloys, fracture surfaces were mainly occupied by striations irrespective of humidity and stress levels. © 2012 The Society of Materials Science, Japan.

DOI： 10.2472/jsms.61.556

Language：English   Publishing type：Research paper (scientific journal)  

Fatigue properties of age-hardened Al alloy 2017-T4 under ultrasonic loading frequency (20 kHz) were investigated and compared with the results under conventional loading of rotating bending (50 Hz). The growth of a crack retarded at about 500 µm in surface length under ultrasonic loading, while at about 20 µm under rotating bending. Although striations being a typical fracture mechanism were observed under conventional loading, most of fracture surface was covered with many facets under ultrasonic loading. These facets were also observed under rotating bending in nitrogen gas. The difference in growth mechanism depending on the loading frequency and the retardation of a crack growth under ultrasonic loading may be caused by the environment at the crack tip due to high crack growth rate under ultrasonic loading.

DOI： 10.1063/2.1203108

Language：English   Publishing type：Research paper (scientific journal)  

As possible substitutes for high-strength Cu-Be alloys, Cu-6Ni-2Mn-2Sn-2Al alloys have been developed. To clarify the physical background of the effect of trace Zr on the fatigue strength of such alloys, the initiation and propagation behavior of a major crack that led to the fracture of the tested specimens was monitored. When the stress amplitude was less than sigma(a) = 350 MPa, the fatigue life of the alloys with Zr was about 2-2.5 times larger than that of the alloy without Zr. When sigma(a) > 350 MPa, the effect of Zr addition on the fatigue life dramatically decreases as the stress amplitude increases. The increased fatigue life due to Zr addition resulted from an enhancement of the crack initiation life and microcrack growth life. The enhanced crack initiation life was mainly attributed to the strengthening of grain boundaries due to the precipitation of SnZr compounds. A statistical analysis of the behavior of multiple cracks was made to quantitatively evaluate the scatter in fatigue behavior. The statistical analysis supported the conclusions obtained from the behavior of a major crack.

DOI： 10.1007/s10853-011-5935-4

Language：Japanese   Publishing type：Research paper (scientific journal)  

Growth mechanism of shear mode fatigue crack of age-hardened al alloy
In order to investigate the growth mechanism of a shear mode fatigue crack in an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens of the alloy in environments of controlled relative humidity of 25&#37;, 50&#37;, 75&#37; and 85&#37;, distilled water and nitrogen gas. Fatigue strength was decreased by high humidity. The growth mechanism of a fatigue crack was different depending on environment and stress level. Although most of fractures occurred by the growth of a tensile mode crack, a crack propagated in a shear mode accompanying with slip planes and voids at high stress levels in high humidity and at all stress levels tested in nitrogen gas. The shear mode crack was related to the marked texture of the alloy. Growth rates of the shear mode crack were higher in high humidity than in nitrogen gas. Growth mechanisms of the shear mode crack were different between environments in nitrogen gas and in high humidity. That is, the reason for the growth of a shear mode crack in nitrogen gas was the suppression to growth of a tensile mode crack by reversible slip due to absence of oxide film. On the other hand, in high humidity, the growth of a shear mode crack was promoted by the formation and coalescence of voids, suggesting that the acceleration of the growth rate of a crack may be assisted by hydrogen accumulated around precipitated particles on glide planes. © 2011 The Society of Materials Science, Japan.

DOI： 10.2472/jsms.60.890

Language：English   Publishing type：Research paper (other academic)  

Effect of high humidity on growth mechanism of a fatigue crack was investigated for an extruded bar of an age-hardened Al alloy 7075-T6 which had the marked texture of plane (111). Fracture in high humidity occurred by the growth of a shear mode crack under high stress levels, though a crack propagated in a tensile mode under low stress ones, macroscopically. Many voids and slip planes were observed on the fracture surface yielded by the shear mode crack. Especially most of the fracture surface was occupied by voids where the crack was small. The void percentage decreased with increasing in the crack depth. Fracture surface yielded by the shear mode crack was a plane (100). The growth direction of the shear mode crack to the cross section of specimen was about 55° corresponding to the angle composed by this plane and the texture of plane (111). These results suggest that the shear mode crack was related to microstructure, stress and environment. The growth mechanism of the shear mode crack assisted by hydrogen was proposed based on the results of the acceleration of crack growth and the formation of voids in high humidity. © (2012) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/KEM.488-489.45

Language：Japanese  

In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075T 6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%~70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.

DOI： 10.1299/kikaia.76.1651

Language：Japanese   Publishing type：Research paper (scientific journal)  

Fatigue strength of age-hardened & extruded al alloy under high humidity (rotating bending and ultrasonic loading)
In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075T 6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%~70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.

DOI： 10.1299/kikaia.76.1651

Language：English   Publishing type：Research paper (scientific journal)  

We present an optical technique to measure five-degree-of-freedom error motions of a high-speed microspindle. The measurement system consists of a rod lens, a ball lens, four divided laser beams, and multiple divided photodiodes. When the spindle rotates with its concomitant rotation errors, the rod and ball lenses, which are mounted to the chuck of the spindle, are displaced, and this displacement is measured using an optical technique. For this study, we decide the design parameters of the optical system using ray tracing, fabricate a prototype of the measurement system, and evaluate it experimentally. The results show that the measurement system has a resolution of 5 nm and can be used to evaluate high-speed microspindle rotation errors.

DOI： 10.1007/s12541-010-0102-4

Language：English   Publishing type：Research paper (scientific journal)  

In order to study the fatigue damage formation process of ultrafine grained copper, fatigue tests were carried out for two samples: oxygen-free copper and deoxidized low-phosphorous copper. After the processing by equal channel angular pressing (ECAP) using eight passes, equiaxed grains with an average size of 300 nm were formed for both the samples. The formation behavior of surface damage was monitored. The effects of trace impurities on surface damage formation behavior were discussed from the viewpoints of change in surface morphological feature during stressing.

DOI： 10.1142/S0217979210065179

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out at room temperature and 500 degrees C for alloy 718 with nearly the same static strength but different precipitated particles, i.e. a peak aged condition (720 degrees C-10h) and a double aged one (720 degrees C-8h, 620 degrees C-8h), in order to investigate the effect of precipitated particles on crack initiation and propagation behavior. Fatigue strength was higher in the double aged material than in the peak one at both temperatures. The main reason for high fatigue strength of the double aged material was that the propagation of a small crack with a few grain sizes was suppressed by the carbide particles precipitated in a grain.

DOI： 10.1142/S0217979210065751

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075T 6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%~70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.

DOI： 10.1299/kikaia.76.1651

Language：English   Publishing type：Research paper (scientific journal)  

Two-layered cutting tool tips with thin sintered carbide plates on the rake faces are introduced as a means to significantly reduce tungsten consumption in cutting tools as a response to the sharp and manifold rise of global tungsten raw material prices. The objective of this study is investigation of factors leading to premature failure of the two-layered cutting tool tips as compared to homogeneous sintered carbide tips. Cutting tests were performed and the effects of coolant, thin sintered carbide plate material type and its thickness on stress state and fracture are discussed. It was established that the absence of coolant, as well as, higher Young's modulus and less thickness of the thin sintered carbide plate results in higher stresses on the tip's rake face, leading to its premature failure. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijmachtools.2009.06.003

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/jmmp.2.1399

Language：Others   Publishing type：Research paper (other academic)  

To investigate the effects of loading frequency on fatigue crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy 7075-T6 in ambient air and in N2 gas. The results obtained were compared with those in rotating bending fatigue. Fatigue strength was higher in ultrasonic fatigue than in rotating bending fatigue. This may be caused by the retardation of crack initiation and early crack propagation. In ultrasonic fatigue, the growth direction of cracks changed macroscopically from tensile mode to shear mode with the direction oblique about 35 degrees to the stress axis. Fracture mechanism involved changed from striations featured to transgranular facets and microvoids predominated. Crack growth rate at growth mode transition was ~3x10-9 m/cycle. The relation between applied stress, a, and crack depth, bT, at the transition sites can be approximated as anbT = constant. The results were discussed from viewpoints of crack tip plasticity, time dependent environment effect and the texture microstructure of the alloy.

Language：English   Publishing type：Research paper (scientific journal)  

Ultrasonic fatigue tests of plain specimens with different grain sizes were carried out for Ni-base super alloy, Inconel 718, in ambient air in order to investigate the effect of grain size on fatigue properties. Fatigue strength was increased with decrease in grain size. The increase in fatigue strength by refining grain size was mainly caused by the suppression of crack initiation. That is, the effect of grain size on crack growth rate was hardly recognized, though crack morphology was rougher in the specimen of larger grain, meaning that the crack growth in the large-grained specimens was suppressed by roughness induced crack closure. On the other hand, however, more flat facets caused by twin boundary cracking were observed in the large-grained specimens, which inversely led to crack growth acceleration.

Language：Japanese  

Effect of Grain Size on Ultrasonic Fatigue Properties of Ni-Base Super Alloy Inconel 718
Ultrasonic fatigue tests of plain specimens with different grain sizes were carried out for Ni-base super alloy, Inconel 718, in ambient air in order to investigate the effect of grain size on fatigue properties. Fatigue strength was increased with decrease in grain size. The increase in fatigue strength by refining grain size was mainly caused by the suppression of crack initiation. That is, the effect of grain size on crack growth rate was hardly recognized, though crack morphology was rougher in the specimen of larger grain, meaning that the crack growth in the large-grained specimens was suppressed by roughness induced crack closure. On the other hand, however, more flat facets caused by twin boundary cracking were observed in the large-grained specimens, which inversely led to crack growth acceleration.

DOI： 10.1299/kikaia.74.1000

Language：Japanese   Publishing type：Research paper (scientific journal)  

Influence of shot peening on fatigue strength of maraging steels with different hardness
In order to investigate the hardness dependence on the fatigue strengths for surface fracture and internal fracture, rotating bending fatigue tests were carried out for maraging steels using specimens with different hardness obtained by heat-treating at different under-aging conditions. Fatigue limit for surface fracture was evaluated in electro-polished specimens and the one for internal fracture was in shot-peened ones, where fatigue limits were defined as fatigue strengths at 107 cycles for surface fracture and 108 cycles for internal fracture, respectively. Fatigue limit for internal fracture increased with increase in hardness similar to the one for surface fracture. However, the proportional relationship between the fatigue limit and the hardness existed till higher hardness in the fatigue limit for internal fracture than in the one for surface fracture. This difference in the hardness dependence on fatigue limit was discussed from the viewpoint of the influence of humidity on the fatigue strength. © 2007 The Society of Materials Science.

DOI： 10.2472/jsms.56.1126

Language：Others   Publishing type：Research paper (other academic)  

Rotating bending fatigue tests were carried out to investigate the influence of grain size on the resistance to crack growth of Ni-base super alloy, Inconel 718, using the materials with grain sizes of about 20, 50,100 and 200 たm, at room temperature. The larger grain size, the smaller crack growth rate, though the static strength was decreased with increase in grain size. The growth rate of a small crack was uniquely determined by the term ja na (Small crack growth law), where ja and a are the stress amplitude and the crack length, respectively, and n is a constant. The resistance to crack growth among materials was evaluated based on the law and showed a good correlation with tensile strength jB, meaning that fatigue life can be predicted by using only j B BB.

DOI： 10.4028/www.scientific.net/kem.348-349.541

Language：English  

OS4-4-1 Influence of grain size on fatigue properties in carbon steel
In order to investigate the influence of grain size on the resistance to crack growth, fatigue properties of commercial fine-grained carbon steels with grain sizes of 6.5 and 20 μm were investigated based on the crack initiation and propagation behavior through the successive observation of surface state of smooth specimen under rotating bending. A crack initiated at the early stage of stress repetitions and most of fatigue life was occupied by the growth life of a crack smaller than 1～2 mm. The crack growth rate determined by the term σ_a^nl, uniquely in both steels, where σ_a and l are the stress amplitude and the crack length and n is constant. The fine grained- steel has an excellent resistance to crack growth in comparison with many annealed carbon steels.

Language：Others   Publishing type：Research paper (other academic)  

In order to investigate the influence of loading frequency on the fatigue behaviors of the high strength steel, ultrasonic fatigue tests were carried out for a high-carbon-chromium steel and the results were compared with those of fatigue tests using conventional rotary bending fatigue test machine with a frequency of 52.5Hz. The different of fatigue strength at ultrasonic frequency level and conventional frequency level is very small and the S-N curve obtained from 20 kHz or 52.5 Hz shows the step-wise shape. The fatigue crack occurred from inclusions on the subsurface site in the long life regime and the typical surface fracture occurred in the short life one though the loading frequency level is different. It is indicated that ultrasonic fatigue method is an effective method to investigate the fatigue properties in super-long life region.

DOI： 10.4028/www.scientific.net/kem.353-358.227

Language：Others   Publishing type：Research paper (other academic)  

Maraging steel is one of typical high strength steels used for light-weight mechanical structures due to its significantly high strength over 2000 MPa. It was melted in vacuum furnace in order to reduce the contents of unexpected inclusions so that higher mechanical properties can be achieved. After solution treatment, appropriate aging treatments were applied to some specimens to provide the desired strength levels. Ultra-long life fatigue properties were investigated by using both solution treated and aging hardened specimens under rotating bending fatigue. The effects of aging conditions were examined from the viewpoints of fatigue life, strength distribution, fatigue mechanism, and fracture morphology.

Language：English   Publishing type：Research paper (scientific journal)  

Very long life fatigue and near-threshold fatigue crack growth behaviour of 7075 and 6061 Al-alloys in T6 condition were investigated using accelerated fatigue testing at 19.5 kHz on a piezoelectric machine. Significant interfacial voids were observed in the two Al-alloys during crack initiation and early fatigue crack growth process. Fatigue failure in Al-alloys could be ascribed to: the formation of a number of fatigue voids, their growth and coalescence, and the subsequent formation and propagation of macroscopic cracks. Moreover, fatigue crack growth rates of small cracks in these alloys were found to be greater than those of large cracks for the same stress intensity factor range.

DOI： 10.1016/j.ijfatigue.2005.09.017

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the growth mechanism of a fatigue crack of Ni-based super alloy at ultrasonic frequency, ultrasonic fatigue and rotating bending fatigue tests were carried out for Inconel 718 in ambient air and in N ₂ gas using plain and notched specimens and the results in both tests were compared by focusing on the effects of strain rate and environment. In ambient air, fatigue strength was higher in ultrasonic fatigue than in rotating bending fatigue in plain specimen, while the results in notched specimen were the reverse. Many transgranular cracks and cleavage like cracks were observed on the fracture surface in ultrasonic fatigue in addition to striation and transgranular facets observed in rotating bending fatigue and there was no difference in the fracture mechanism between plain specimen and notched one. Moreover, these effects of frequency on the growth mechanism of a crack in ambient air were nearly the same in N₂ gas, though fatigue strength in N₂ gas was higher than that in air. The increase in fatigue strength in N₂ gas was mainly caused by the retardation of a crack initiation.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Effect of Loading Frequency on Fatigue Strength of Ni-Based Super Alloy

DOI： 10.1299/kikaia.72.1542

Language：English   Publishing type：Research paper (scientific journal)  

Fatigue induced from various aeromechanical sources can reach gigacycle regime due to vibratory stress cycles at high frequencies. Composite repairs offer various advantages over mechanical fastening or riveting, including improved long-term durability behavior and reduced corrosion. In this study, by using ultrasonic frequency cyclic loading, fatigue crack growth tests in gigacycle regime were conducted on notched 7075 and 6061 aluminum alloy substrates (in T6 condition) with and without bonded Boron/epoxy patches. A considerable increase in the fatigue life was observed with more number of patch plies. The effect of patch configuration and thickness on enhancing the fatigue strength by orders of magnitude has been demonstrated.

DOI： 10.1016/j.ijfatigue.2006.02.028

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T 6, in ambient air and in N₂ gas, and the results were compared with those in rotating bending fatigue. Fatigue strength was higher in ultrasonic fatigue than in rotating bending one. This was caused by the retardation of a crack initiation and its early propagation. In ultrasonic fatigue, the growth direction of a crack changed macroscopically from a tensile mode to a shear one which the direction was about 35 degrees against to the specimen axis, and fracture mechanism also changed from striations to transgranular facets with voids microscopically. Crack growth rate at the transition of crack growth direction was about 3×10^-9 m/cycle. The relation between an applied stress σ_a and a crack depth at the transition of crack growth direction b_T was expressed by σ_a ⁿb_T=constant. These results were explained from the time dependent environmental effect and texture of microstructure.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Crack Growth Behavior of Al Alloy 7075-T6 under Ultrasonic Fatigue

DOI： 10.1299/kikaia.72.1356

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T6, and the results were compared with those in rotating bending fatigue. Fatigue strength in ultrasonic was higher than that in rotating bending. This was mainly caused by the retardation of crack initiation. Growth direction of a crack changed from a tensile mode to a shear one in ultrasonic fatigue, though fracture occurred by the growth of a tensile mode in rotating bending. The growth direction of a shear mode crack was inclined about 55 degrees to the tensile axis. The relation between an applied stress σ_a and a crack depth at transition of growth direction b _T was expressed by σ_a ⁿ b_T=C, where C and n are constants. These results were discussed from the points of view of the time dependent environmental effect and the texture of material.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Propagation behavior of an internal crack of a radical nitrided bearing steel

Language：Others   Publishing type：Research paper (scientific journal)  

In order to investigate the propagation behavior of an internal crack in surface treated steel, rotaing bending fatigue tests were carried out for a radical-nitrided bearing steel. The fracture occurred from a specimen surface under high stress levels, whereas a fish-eye fracture occurred under low stress levels. Consequently, S-N curve was duplex S-N one. A circular trace was observed inside the fish-eye when the fracture occurred under two-step variable stress amplitude test. It was suggested that the trace was formed by the change in stress amplitude. Moreover, from the change in the trace size depending on the stress level and the number of stress cycles, it was concluded that there is a fatigue limit for the internal fracture meaning the limit for crack propagation and most of fatigue life the internal fracture was occupied by the growth life of a small internal crack.

DOI： 10.1299/kikaia.71.1362

Language：English   Publishing type：Research paper (scientific journal)  

Fracture and small crack behavior in the very high cycle domain of 10 ⁹ cycles were investigated with a nickel-based superalloy under ultrasonic fatigue in ambient air at room temperature. The influence of ultrasonic frequency is examined by comparing the results with those in conventional low frequency fatigue. It is found that fatigue strength increases as frequency is raised up to 19.5 kHz and the most of fatigue life is consumed in nucleating and propagating small cracks up to 100 μm. Transition of fracture mode from transgranular ductile fracture to cleavage-dominated fracture occurs beyond a critical stress intensity factor range of approximately 21MPam, leading to the catastrophic failure under ultrasonic fatigue.

DOI： 10.1016/j.ijfatigue.2005.07.022

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the fracture mechanism in fatigue of Ni-base Superalloy at elevated temperatures, rotating bending fatigue tests were carried out for Inconel 718 at 500°C and 600°C up to 10⁸ cycles. At both temperatures, fracture occurred from a subsurface of the specimen in long life region, though the origin of fracture was the specimen surface in short life region. Consequently, S-N curves showed a two-step shape in elevated temperature tests. Although a surface crack was observed similar to the result at room temperature even in long life region at elevated temperatures, the surface crack stopped propagating. In this case, interganular cracks were observed at an origin of subsurface fracture. The subsurface damage was initiated at the early stage of fatigue life.

Language：Others   Publishing type：Research paper (scientific journal)  

In order to investigate the fracture mechanism in fatigue of Ni-base Superalloy at elevated temperatures, rotating bending fatigue tests were carried out for Inconel 718 at 500°C and 600°C up to 108 cycles. At both temperatures, fracture occurred from a subsurface of the specimen in long life region, though the origin of fracture was the specimen surface in short life region. Consequently, S-N curves showed a two-step shape in elevated temperature tests. Although a surface crack was observed similar to the result at room temperature even in long life region at elevated temperatures, the surface crack stopped propagating. In this case, interganular cracks were observed at an origin of subsurface fracture. The subsurface damage was initiated at the early stage of fatigue life.

DOI： 10.1299/kikaia.71.1226

Language：Japanese   Publishing type：Research paper (scientific journal)  

Fracture Mechanism in Fatigue of Ni-Base Superalloy Inconel 718 at Elevated Temperatures

DOI： 10.1299/kikaia.71.1226

Language：Japanese   Publishing type：Research paper (scientific journal)  

Propagation Behavior of an Internal Crack of a Radical-nitrided Bearing Steel

DOI： 10.1299/kikaia.71.1362

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the influence of loading frequency on the fatigue properties of Al alloy, ultrasonic fatigue tests were carried out for an age-hardened Al alloy, 6061-T6. The results were compared with those of rotating bending fatigue in view points of initiation and propagation behavior of a crack and fracture mechanism. Fatigue life was longer in ultrasonic fatigue than in rotating bending fatigue, which was mainly caused by the suppression of crack initiation. In ultrasonic fatigue, many fatigue voids and cleavage cracks were observed on the fracture surface in addition to the shear mode cracks, striations and dimples, which were typical fracture modes in rotating bending.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Ultrasonic Fatigue Properties of an Age-Hardened Al Alloy 6061-T6

DOI： 10.1299/kikaia.70.1139

Language：English   Publishing type：Research paper (scientific journal)  

Defective metallic components and structures are being repaired with bonded composite patches to improve overall mechanical and fatigue properties. In this study, fatigue crack growth tests were conducted on pre-cracked 7075/T6 Aluminum substrates with and without bonded Boron/epoxy patches. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of patch plies increased. The experimental results demonstrate that the patch configurations and patch thickness can enhance fatigue life by order of magnitude. Quantitative comparisons between analytical and experimental data were made, and the analytical model based on a modified Rose's analytical solution appears to best estimate the fatigue life.

DOI： 10.12989/sem.2004.18.3.277

Language：English   Publishing type：Research paper (scientific journal)  

The fatigue behavior of 7075/T6 and 6061/T6 aluminum alloys in the very long life regime was investigated by using piezoelectric accelerated fatigue tests at 19.5 KHz. The mechanism of fatigue crack initiation and propagation by void formation and growth was identified using scanning electron microscopy (SEM).

DOI： 10.1023/B:JMSC.0000008091.55395.ee

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the effect of radical nitriding on fatigue behavior of high-strength steels, very long life fatigue tests were carried out for two kinds of steels, 18Ni Maraging steel and SUJ2 bearing steel, by using conventional and ultrasonic machines, respectively. It was shown that radical nitriding played the principal role in the subsurface crack initiation. Fatigue strength of Maraging steel appears to be significantly enhanced by radical nitriding, however, radical nitriding has a destructive effect on the fatigue behavior of SUJ2 steel.

Language：English   Publishing type：Research paper (scientific journal)  

The objective of this study is to determine very long life fatigue and near threshold fatigue crack growth behaviors of 7075/T6 and 6061/T6 Al-alloys using piezoelectric accelerated fatigue at 19.5KHz. The experimental results show the fatigue failure can occur beyond 107, even 109 cycles, and endurance limits could not be obtained in the Al-alloys until 109 cycles. Fatigue voids are noticed on fatigue fracture in both alloys. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviors have been examined. Fatigue crack growth rates of small cr acks in the Al-alloys are found to be greater than those of large cracks at the same stress intensity factor range.

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the influence of loading frequency on the fatigue strength and fracture mechanism of a high strength Al alloy, ultrasonic fatigue tests were carried out for an extruded Al alloy 7075-T 6 and the results were compared with those of rotating bending fatigue tests. Fatigue strength of ultrasonic was higher than that of rotating bending. The crack initiation and the crack growth rate in ultrasonic fatigue were delayed compared with those of rotating bending fatigue. In case of ultrasonic fatigue, the direction of crack growth changed from tensile mode to shear mode and many voids were observed on the fracture surface in shear mode growth, whereas the crack propagated in tensile mode and striations were observed in rotating bending fatigue.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Ultrasonic Fatigue Properties of a High Strength Extruded Al Alloy

DOI： 10.1299/kikaia.69.1672

Language：Others   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The effects of pitting corrosion on very high cycle fatigue behavior of aluminum alloys were discussed. The results indicated that the presence of pre-existing corrosion pits, produced by 1-day, 4-day and 7-day immersion in salt water significantly reduces the fatigue life of the alloy by a factor of 10-100. It was also found that the pittings accelerated crack initiation and promoted multiple-site damage.

DOI： 10.1016/S1359-6462(03)00365-8

Language：English   Publishing type：Research paper (scientific journal)  

Corrosion and fatigue properties of aircraft materials are known to have a considerable scatter due to the random nature of materials, loading, and environmental conditions. A probabilistic approach for predicting the pitting corrosion fatigue life has been investigated which captures the effect of the interaction of the cyclic load and corrosive environment and all stages of the corrosion fatigue process (i.e. the pit nucleation and growth, pit-crack transition, short- and long-crack propagation). The probabilistic model investigated considers the uncertainties in the initial pit size, corrosion pitting current, and material properties due to the scatter found in the experimental data. Monte Carlo simulations were performed to define the failure probability distribution. Predicted cumulative distribution functions of fatigue life agreed reasonably well with the existing experimental data.

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the effect of load frequency on the fatigue strength and fracture mechanism of a nickel-base superalloy, Inconel 718, fatigue tests were carried out under an ultrasonic frequency (19.5 kHz) in ambient air environment. The results were compared with those obtained under conventional rotatary bending fatigue at a frequency of 50 Hz. Fatigue strength increased at ultrasonic frequency, which is mainly caused by the suppression of crack initiation and its growth at the early stage. Under both ultrasonic and rotary bending fatigue tests, most of fatigue life consumed in the growth of a crack smaller than 1 mm. In the ultrasonic fatigue, intergranular and cleavage crack propagations were observed in addition to striation, which was a dominant fracture mechanism in the conventional fatigue.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Ultrasonic Fatigue Properties of Ni-Base Superalloy

DOI： 10.1299/kikaia.69.626

Language：English   Publishing type：Research paper (scientific journal)  

The fatigue properties of Inconel 718 in long life region up to 10⁸ cycles were investigated under rotating bending at room temperature and 500°C. The early propagation of a crack was suppressed by oxide induced crack closure. Consequently, fatigue strength was higher at 500°C than at room temperature. Surface fracture occurred at high stress levels and internal fracture occurred in long life region beyond 10⁷ cycles at 500°C. Correspondingly, S-N curve at 500°C presented in a step-wise shape, compared to the conventional one-step shape at room temperature. Intergranular cracking was observed at the origins of internal fracture at 500°C.

Language：English   Publishing type：Research paper (scientific journal)  

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was > 10⁷ cycles. A significant change in the slope of the S-N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10⁷ and 10⁹ cycles are > 99%.

DOI： 10.1016/S0142-1123(02)00037-3

Language：English   Publishing type：Research paper (scientific journal)  

The fatigue properties of a nickel-base superalloy, Inconel 718, in long-life region up to 10⁸ cycles were investigated under rotating bending at room temperature and elevated temperature of 300°C, 500°C and 600°C. S-N curves at 500°C and 600°C showed the stepwise shapes, though those in cases of room temperature and 300°C were the conventional one-step ones. All the fractures originated from the specimen surface at room temperature and 300°C, while the internal fractures occurred in long-life region beyond 10⁷ cycles and the surface fractures at the stress levels over the horizontal line of S-N curves corresponding to the fatigue limit for the surface fracture at 500°C and 600°C. Intergranular cracking were observed at the origin of internal fracture.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Fatigue Properties of Ni-Base Superalloy in Long Life Region at Elevated Temperature

DOI： 10.1299/kikaia.68.1192

Language：Others   Publishing type：Research paper (other academic)  

DOI： 10.1117/12.468825

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out to investigate the influence of radical nitriding on the surface integrity and the fatigue strength for a maraging steel. Fatigue strength increased by nitriding. Although the initiation site of fracture was the surface in the aged steel, it was the specimen suface at high stress levels and the interior at low stress levels in the nitrided steel. The main reason for the surface fracture was the surface embrittlement and the one for the interior fracture was the surfce hardening due to nitriding. As the results, S-N curves of the nitrided maraging steel showed a double staged curve.

DOI： 10.1299/kikaia.67.314

Language：English   Publishing type：Research paper (other academic)  

Undeformed chip thickness may be smaller than sharpness radius of a cutting edge at small feed rate when workpieces are cut by a finishing tool with a large round corner. Therefore, in finish cut, sharpness of a cutting edge has to be taken into consideration in cutting process. The purpose of this study is to find mechanism of cutting force occurrence at smaller undeformed chip thickness than sharpness radius of a cutting edge. In this study, effect of sharpness of a cutting edge on cutting force and its effect on burnishing force were investigated in orthogonal cutting process. Furthermore, contact length between a tool flank face and a workpiece during cutting was measured in order to consider the relation between the burnishing force and the contact length. As a result, principal conclusions reached were as follows. The following parameters were in inverse proportion to the nth power of undeformed chip thickness normalized by sharpness radius of a cutting edge: (i) cutting forces per unit area of chip section, ( ii) specific burnishing force defined as burning force divided by undeformed chip thickness and width of cut, and ( iii) specific burnishing length defined as burnishing length divided by undeformed chip thickness.

Language：English   Publishing type：Research paper (scientific journal)  

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter, σanl (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanical parameters, σanl and ΔK. The applicable conditions of the two parameters were discussed and manifested.

DOI： 10.1007/s11661-000-0228-6

Language：English   Publishing type：Research paper (scientific journal)  

The strength of Inconel 718 under rotary bending fatigue is investigated at room temperature, 300, 500 and 600 °C in air. It is found that in the long-life region, the fatigue strength of a plain specimen is much higher at elevated temperatures than at room temperature, though the static strength decreases with the increase in temperature. The effect of temperature on the fatigue strength is examined in terms of the initiation and early growth behaviour of a small crack. The results are discussed in relation to the competition between the softening of the nickel matrix (γ phase) and the surface oxidation at elevated temperatures.

DOI： 10.1046/j.1460-2695.2000.00263.x

Language：English   Publishing type：Research paper (scientific journal)  

In-process measurement of tool flank wear is very important for judgment of tool life with the object of running fully automated machine tools economically. The purpose of this study is to propose a new method of detecting tool flank wear by using characteristic values of dynamic cutting process during steady-state cutting in turning operation. In this report, a simple calculation model of dynamic thrust force was proposed to derive equations of characteristic values from it, and effectiveness of the equations was experimentally verified under many cutting conditions and excitation conditions of a workpiece system. As a result of the calculations and the experiments, we reached conclusions that two characteristic values of dynamic cutting process, standard deviation of dynamic thrust force normalized by the standard deviation of workpiece vibration displacement and coefficient of correlation between dynamic thrust force and vibration displacement, have the following properties. (i) The characteristic values remained almost constant with variation of feed rate of a cutting tool and vibration amplitude of a workpiece. (ii) On condition that vibration frequency of a workpiece is constant, the characteristic values obtained under different cutting speeds can be standardized by normalizing the width of flank wear to the cutting speed. (iii) Tendency of calculated characteristic values with increase of vibration frequency was partly disagree with that of experimental values.

DOI： 10.1299/kikaic.66.1700

Language：English   Publishing type：Research paper (scientific journal)  

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter, σ_a ⁿl (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanical parameters, σ_a ⁿl and ΔK. The applicable conditions of the two parameters were discussed and manifested.

DOI： 10.1007/s11661-000-0228-6

Language：English   Publishing type：Research paper (scientific journal)  

The fatigue crack growth behavior of Inconel 718 was investigated under rotating bending fatigue at room temperature, 300, 500, and 600°C in air. It has been found that the small crack growth rate could be evaluated by the small crack growth law at high stress levels, where the small-scale yielding conditions are exceeded and the Paris law is not applicable, irrespective of the temperature. The fatigue strength of plain specimens increased considerably in the long-life region at the elevated temperatures, because the early growth of a small crack in the range of 20-30 μm was suppressed. However, a crack grew faster at higher temperature after growing beyond about 50 μm due to the decrease of crack growth resistance. The fatigue life in the stable crack growth period can be predicted by the small crack growth law.

DOI： 10.1016/S0921-5093(99)00555-9

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/s0142-1123(99)00081-x

Language：English   Publishing type：Research paper (other academic)  

Rotating bending fatigue tests were performed for nickel-base superalloy Inconel 718 at room temperature, 300 degrees C, 500 degrees C, and 600 degrees C in air. The influence of temperature on the small fatigue crack behavior in the alloy was investigated. It has been found that the fatigue limit of a plain specimen is determined by the limiting stress for small crack growth and the fatigue strength in the long life region is higher at elevated temperatures than at room temperature, because the early growth of small cracks is enormously suppressed at elevated temperatures. In the steady-state growth process, however, a crack grows faster at higher temperature due to the decrease of fatigue crack growth resistance. The small crack growth rate can be evaluated by the small crack growth law at both room and elevated temperatures.

DOI： 10.1016/B978-008043011-9/50022-3

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The influence of cubic boron nitride (CBN) grinding on fatigue strength was investigated on an annealed carbon steel, a quenched and tempered carbon steel at room temperature, and a nickel-base superalloy, Inconel 718, at room temperature and 500°C. The results were discussed from several viewpoints, including surface roughness, residual stress, and work hardening or softening due to CBN grinding. The fatigue strength increased upon CBN grinding at room temperature, primarily because of the generation of compressive residual stress in the surface region. However, in the case of Inconel 718, this marked increase in the fatigue strength tended to disappear at the elevated temperature due to the release of compressive residual stress and the decrease of crack growth resistance at an elevated temperature.

DOI： 10.1361/105994999770346981

Language：Japanese   Publishing type：Research paper (scientific journal)  

Initiation and growth behavior of a fatigue crack in inconel 718 at elevated temperatures

DOI： 10.1299/kikaia.65.1954

Language：English   Publishing type：Research paper (scientific journal)  

The notched fatigue strength of nickel-base superalloy Inconel 718 has been investigated under rotating bending loading at room temperature and 500 °C in air. The applicability of linear notch mechanics to the evaluation of notched fatigue strength at elevated temperature was assessed in terms of the fatigue limit for crack initiation and that for crack growth. The effect of temperature on the fatigue strength and notch sensitivity of Inconel 718 was examined. The results were discussed from the viewpoints of the softening of matrix and surface oxidation at elevated temperature.

DOI： 10.1016/S0142-1123(99)00081-X

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out for Ni base super alloy Inconel 718 at room temperature and elevated temperatures of 300°C, 500°C and 600°C in order to investigate the effect of surface oxidation at the elevated temperatures on the fatigue crack initiation and its early growth process. The fatigue strength was higher at the elevated temperatures than at room temperature when stress levels were low. This phenomenon was caused from that the growth of a crack smaller than the size of a few grains was suppresed at the elevated temperatures, though the growth of a larger crack was accelerated. The main reason for the suppression of the early growth of a small crack was a formation of oxide films on the specimen surface at the elevated temperatures.

DOI： 10.1299/kikaia.64.839

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out for nickel-base superalloy Inconel 718 in air at room temperature and the elevated temperatures of 300°C, 500°C and 600°C using the 60 degree V-grooved specimen in order to investigate the influence of temperature on the notch sensitivity of the alloy. The notch sensitivity was assessed in terms of the limiting stress for crack initiation σ_w1 and that for crack growth σ_w2, based on the Linear Notch Mechanics (LNM). It was found that the fatigue limits σ_w1, σ_w2 of notched specimen can be evaluated by LMN at each temperature. Although the notch sensitivity of Inconel 718 to the two fatigue limits is relatively low considering its high static strength at all the temperatures, especially to the fatigue limit for crack growth, it becomes more sensitive at the elevated temperatures than at room temperature. The results were discussed with the effect of temperature on the arresting behavior of fatigue crack growth.

DOI： 10.1299/kikaia.64.2869

Language：English   Publishing type：Research paper (other academic)  

In this study, rotating bending fatigue tests were carried out for a nickel-base superalloy at room and elevated temperatures in order to investigate the influence of CBN (Cubic Boron Nitride) grinding on the fatigue strength. The results were discussed from the viewpoints of surface roughness, residual stress and work hardening or softening due to the CBN grinding and oxides formed at elevated temperature. The fatigue strength was increased considerably at room temperature because the crack propagation was effectively suppressed by the compressive residual stress generated in the CBN grinding process. However, this marked increase of the fatigue strength tended to disappear at elevated temperature owing to the decreases of both the compressive residual stress and the resistance to crack propagation.

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out for Inconel 718 at room temperature to investigate the growth characteristics of a small crack. It was found that: (1) The crack initiated at the grain boundary and then propagated as a transgranular crack. (2) When the stress level was higher than the fatigue limit, the fatigue life was mainly controlled by the growth life of a crack smaller than 1 mm. (3) The fatigue limit was a limiting stress for the crack propagation. (4) At low stress levels (σ_a/σ_0.2<0.5, σ_a: stress amplitude, (σ_0.2:0.2% proof stress), the crack growth rate was determined by the stress intensity factor range ΔK; and at high stress levels (σ_a/σ_0.2>0.6), it was determined by the small crack growth law.

DOI： 10.1299/kikaia.63.1398

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out on carbon steels and a nickel-based super alloy, Inconel 718, for investigation of the effect of CBN wheel grinding on the fatigue strength at room temperature and 500 °C. The results are discussed from several view points, including surface roughness, residual stress and work hardening or softening due to grinding. The fatigue strength increased upon CBN wheel grinding at room temperature, primarily because of generation of compressive residual stress due to the CBN wheel grinding. However, this marked increase in the fatigue strength due to CBN wheel grinding tended to disappear at elevated temperature.

DOI： 10.1299/kikaia.63.1844

Language：English   Publishing type：Research paper (scientific journal)  

Rotating bending fatigue tests were carried out for Inconel 718 at room temperature and elevated temperatures of 300 °C, 500 °C and 600 °C to investigate the influence of temperature on the fatigue crack growth resistance. The crack growth rate can be expressed by the small crack growth law, dl/dN = C₁σ_a ⁿl = C₂(σ_a/σ_B)ⁿl, at all the temperatures, where σ_B, σ_a and l are the tensile strength, the stress amplitude and the crack length, and C₁, C₂ and n are constants. Therefore, the resistance to crack growth was evaluated using the small crack growth law. The fatigue crack growth resistance decreased with the increase of temperature.

DOI： 10.1299/kikaia.63.2298

Language：English   Publishing type：Research paper (scientific journal)  

Because of its excellent mechanical properties at elevated temperature, a nickel-base superalloy has been widely used in the aerospace industry and nuclear power plants. In this study, rotating bending fatigue tests were carried out for Inconel 718 at room temperature and elevated temperatures of 300 °C, 400 °C and 500 °C to investigate the notch sensitivity at the elevated temperatures. It was found that the fatigue limits of plain specimens were higher at the elevated temperatures than at room temperature, which might be due to the suppression of the crack initiation at the elevated temperatures, however, the resistance to crack growth became smaller at the elevated temperatures than at room temperature. Therefore, the fatigue strength of a notched specimen, in which a crack initiated easily owing to the stress concentration introduced, was lower at the elevated temperatures than at room temperature. Consequently, the fatigue strength reduction factor was larger at the elevated temperatures than at room temperature.

DOI： 10.1299/kikaia.62.960

Language：English   Publishing type：Research paper (scientific journal)  

Event date： 2011.9

Language：English  

Venue：Dubrovnik   Country：Croatia  

Effect of high humidity on growth mechanism of a fatigue crack was investigated for an extruded bar of an age-hardened Al alloy 7075-T6 which had the marked texture of plane (111). Fracture in high humidity occurred by the growth of a shear mode crack under high stress levels, though a crack propagated in a tensile mode under low stress ones, macroscopically. Many voids and slip planes were observed on the fracture surface yielded by the shear mode crack. Especially most of the fracture surface was occupied by voids where the crack was small. The void percentage decreased with increasing in the crack depth. Fracture surface yielded by the shear mode crack was a plane (100). The growth direction of the shear mode crack to the cross section of specimen was about 55° corresponding to the angle composed by this plane and the texture of plane (111). These results suggest that the shear mode crack was related to microstructure, stress and environment. The growth mechanism of the shear mode crack assisted by hydrogen was proposed based on the results of the acceleration of crack growth and the formation of voids in high humidity.

Event date： 2008.9

Language：English  

Venue：Brno   Country：Czech Republic  

To investigate the effects of loading frequency on fatigue crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy 7075-T6 in ambient air and in N2 gas. The results obtained were compared with those in rotating bending fatigue. Fatigue strength was higher in ultrasonic fatigue than in rotating bending fatigue. This may be caused by the retardation of crack initiation and early crack propagation. In ultrasonic fatigue, the growth direction of cracks changed macroscopically from tensile mode to shear mode with the direction oblique about 35 degrees to the stress axis. Fracture mechanism involved changed from striations featured to transgranular facets and microvoids predominated. Crack growth rate at growth mode transition was ~3x10-9 m/cycle. The relation between applied stress, a, and crack depth, bT, at the transition sites can be approximated as anbT = constant. The results were discussed from viewpoints of crack tip plasticity, time dependent environment effect and the texture microstructure of the alloy.

Event date： 2007.8

Language：English  

Venue：Ann Arbor, MI   Country：United States  

Maraging steel is one of typical high strength steels used for light-weight mechanical structures due to its significantly high strength over 2000 MPa. It was melted in vacuum furnace in order to reduce the contents of unexpected inclusions so that higher mechanical properties can be achieved. After solution treatment, appropriate aging treatments were applied to some specimens to provide the desired strength levels. Ultra-long life fatigue properties were investigated by using both solution treated and aging hardened specimens under rotating bending fatigue. The effects of aging conditions were examined from the viewpoints of fatigue life, strength distribution, fatigue mechanism, and fracture morphology.

Event date： 2003.3

Language：English  

Venue：Crete   Country：Greece  

Rotating bending fatigue tests were carried out for a radical nitrided bearing steel in order to investigate the initiation and propagation behavior of an internal crack. Surface fracture occurred under high stress levels and fish-eye fracture happened under low stress levels so that the S-N curve appeared in the shape of two-step lines. The depth of the origin of internal fracture was confined in a relatively narrow region near the boundary between the hardened and non-hardened matrix. By two-step loading fatigue tests in the internal fracture region, a circular trace corresponding to the stress change was observed within the fish-eye. In the internal fracture region, a crack was initiated at the early stage of the fatigue process, that is, most of the fatigue life of internal fracture was occupied by the growth life of an internal crack.

Event date： 2002.5

Language：English  

Venue：Kitakyushu   Country：Japan  

Corrosion and fatigue are two types of damages found in aircraft and offshore structures. Corrosion fatigue processes are characterized by randomness due to the random nature of materials, initial defects, loading, and environmental condition. In this study, a mechanistically based probabilistic model for prediction of pitting corrosion fatigue life in aluminum alloys has been developed which captures the effect of the interaction of cyclic load and corrosive environment and all the stages of the corrosion fatigue process. Monte Carlo simulations were performed to define the failure probability distribution. Predicted cumulative distribution functions of fatigue life agreed reasonably well the experimental data.

Event date： 2001.6

Language：English  

Venue：Seville   Country：Spain  

In order to investigate the effect of radical nitriding on fatigue strength of die steel, rotating bending fatigue tests were carried out at room temperature and 450°C. Fatigue strength was increased by nitriding at both temperatures due to surface hardening and compressive residual stress generated in the surface region. Comparing with the non-nitrided steel in which the fracture origin was at surface slip bands or surface defects, the nitrided steel fractured with the origin at inner inclusions at both temperatures. In the nitrided steel, cracks initiated from inclusions in the non-hardened layer or near the boundary between hardened and non-hardened layers. The difference in fatigue strength between the two temperatures was discussed based on the feature of fish-eye.

Event date： 2001.6

Language：English  

Venue：Pittsburgh, PA   Country：United States  

The fatigue strength of a nickel-base superalloy, Inconel 718, was investigated at room temperature under an ultrasonic frequency (19.5 kHz) loading in ambient air environment. The endurance (S-N) data show that fatigue strength was enhanced at ultrasonic frequency as compared to those at conventional frequencies. Small crack initiation and propagation behavior was observed during the fatigue progress at both ultrasonic fatigue and conventional rotary bending fatigue. The effect of frequency on the fatigue fracture mechanism was examined from a microscopic viewpoint.

Language：English  

Country：Japan  

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter, σanl (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanical parameters, σanl and ΔK. The applicable conditions of the two parameters were discussed and manifested.

Language：Japanese  

Country：Japan  

Rotational bending fatigue strength of grinding materials of different notch sensitivity.

Language：Japanese  

Country：Japan  

High temperature fatigue strength of grinding materials of Ni base superalloy metal.

Language：Japanese  

Country：Japan  

Grinding effects on fatigue strength of Ni base superalloy metal.

Language：Japanese  

Country：Japan  

Fatigue Strength of Notched Specimen in Nickel-Base Superalloy at Elevated Temperatures
Because of its excellent mechanical properties at elevated temperature, a nickel-base superalloy has been widely used in the aerospace industry and nuclear power plants. In this study, rotating bending fatigue tests were carried out for Inconel 718 at room temperature and elevated temperatures of 300°C, 400°C and 500°C to investigate the notch sensitivity at the elevated temperatures. It was found that the fatigue limits of plain specimens were higher at the elevated temperatures than at room temperature, which might be due to the suppression of the crack initiation at the elevated temperatures, however, the resistance to crack growth became smaller at the elevated temperatures than at room temperature. Therefore, the fatigue strength of a notched specimen, in which a crack initiated easily owing to the stress concentration introduced, was lower at the elevated temperatures than at room temperature. Consequently, the fatigue strength reduction factor was larger at the elevated temperatures than at room temperature.

Language：Japanese  

Country：Japan  

Fatigue Strength of Notched Specimen in Nickel-Base Superalloy at Elevated Temperatures
Because of its excellent mechanical properties at elevated temperature, a nickel-base superalloy has been widely used in the aerospace industry and nuclear power plants. In this study, rotating bending fatigue tests were carried out for Inconel 718 at room temperature and elevated temperatures of 300 °C, 400 °C and 500 °C to investigate the notch sensitivity at the elevated temperatures. It was found that the fatigue limits of plain specimens were higher at the elevated temperatures than at room temperature, which might be due to the suppression of the crack initiation at the elevated temperatures, however, the resistance to crack growth became smaller at the elevated temperatures than at room temperature. Therefore, the fatigue strength of a notched specimen, in which a crack initiated easily owing to the stress concentration introduced, was lower at the elevated temperatures than at room temperature. Consequently, the fatigue strength reduction factor was larger at the elevated temperatures than at room temperature.

Language：Japanese  

Country：Japan  

Fatigue Crack Growth Resistance in Nickel-Base Superalloy at Elevated Temperatures
Rotating bending fatigue tests were carried out for Inconel 718 at room temperature and elevated temperatures of 300°C, 500°C and 600°C to investigate the influence of temperature on the fatigue crack growth resistance. The crack growth rate can be expressed by the small crack growth law, dl/dN=C₁σⁿ_al=C₂(σ_a/σ_B)ⁿl, at all the temperatures, where σ_B, σ_a and l are the tensile strength, the stress amplitude and the crack length, and C₁, C₂ and n are constants. Therefore, the resistance to crack growth was evaluated using the small crack growth law. The fatigue crack growth resistance decreased with the increase of temperature.

Language：Japanese  

Country：Japan  

Characteristics of Small Crack Growth in a Nickel-Base Superalloy
Rotating bending fatigue tests were carried out for Inconel 718 at room temperature to investigate the growth characteristics of a small crack. It was found that : (1) The crack initiated at the grain boundary and then propagated as a transgranular crack. (2) When the stress level was higher than the fatigue limit, the fatigue life was mainly controlled by the growth life of a crack smaller than 1mm. (3) The fatigue limit was a limiting stress for the crack propagation. (4) At low stress levels (σ_a/σ_0.2<0.5, σ_a : stress amplitude, σ_0.2 : 0.2% proof stress), the crack growth rate was determined by the stress intensity factor range ΔK ; and at high stress levels (σ_a/σ_0.2>0.6), it was determined by the small crack growth law.

Language：Others  

Country：Japan  

Fatigue strength of specimens ground by a CBN wheel (In cases of carbon steels and a nickel-base super alloy)
Rotating bending fatigue tests were carried out on carbon steels and a nickel-based super alloy, Inconel 718, for investigation of the effect of CBN wheel grinding on the fatigue strength at room temperature and 500 °C. The results are discussed from several view points, including surface roughness, residual stress and work hardening or softening due to grinding. The fatigue strength increased upon CBN wheel grinding at room temperature, primarily because of generation of compressive residual stress due to the CBN wheel grinding. However, this marked increase in the fatigue strength due to CBN wheel grinding tended to disappear at elevated temperature.

Language：Others  

Country：Japan  

Fatigue crack growth resistance in nickel-base superalloy at elevated temperatures
Rotating bending fatigue tests were carried out for Inconel 718 at room temperature and elevated temperatures of 300 °C, 500 °C and 600 °C to investigate the influence of temperature on the fatigue crack growth resistance. The crack growth rate can be expressed by the small crack growth law, dl/dN = C1σanl = C2(σa/σB)nl, at all the temperatures, where σB, σaand l are the tensile strength, the stress amplitude and the crack length, and C1, C2and n are constants. Therefore, the resistance to crack growth was evaluated using the small crack growth law. The fatigue crack growth resistance decreased with the increase of temperature.

Language：Japanese  

Country：Japan  

Characteristics of Small Crack Growth in a Nickel-Base Superalloy
Rotating bending fatigue tests were carried out for Inconel 718 at room temperature to investigate the growth characteristics of a small crack. It was found that: (1) The crack initiated at the grain boundary and then propagated as a transgranular crack. (2) When the stress level was higher than the fatigue limit, the fatigue life was mainly controlled by the growth life of a crack smaller than 1 mm. (3) The fatigue limit was a limiting stress for the crack propagation. (4) At low stress levels (σa/σ0.2<0.5, σa: stress amplitude, (σ0.2:0.2&#37; proof stress), the crack growth rate was determined by the stress intensity factor range ΔK; and at high stress levels (σa/σ0.2>0.6), it was determined by the small crack growth law.

Language：Japanese  

Country：Japan  

Fatigue Strength of Specimens Ground by a CBN Wheel(In Cases of Carbon Steels and a Nickel-Base Super Alloy)
Rotating bending fatigue tests were carried out on carbon steels and a nickel-based super alloy, Inconel 718, for investigation of the effect of CBN wheel grinding on the fatigue strength at room temperature and 500°C. The results are discussed from several view points, including surface roughness, residual stress and work hardening or softening due to grinding. The fatigue strength increased upon CBN wheel grinding at room temperature, primarily because of generation of compressive residual stress due to the CBN wheel grinding. However, this marked increase in the fatigue strength due to CBN wheel grinding tended to disappear at elevated temperature.

Language：Japanese  

Country：Japan  

Cutting mechanism in micro-cutting depth.

Language：Japanese  

Country：Japan  

Detection of tool wear by characteristic value in dynamic cutting process.

Language：Japanese  

Country：Japan  

Notch Sensitivity in Fatigue of Nickel-Base Superalloy at Elevated Temperatures
Rotating bending fatigue tests were carried out for nickel-base superalloy Inconel 718 in air at room temperature and the elevated temperatures of 300°C, 500°C and 600°C using the 60 degree V-grooved specimen in order to investigate the influence of temperature on the notch sensitivity of the alloy. The notch sensitivity was assessed in terms of the limiting stress for crack initiation σ_w1 and that for crack growth σ_w2, based on the Linear Notch Mechanics (LNM). It was found that the fatigue limits σ_w1, σ_w2 of notched specimen can be evaluated by LMN at each temperature. Although the notch sensitivity of Inconel 718 to the two fatigue limits is relatively low considering its high static strength at all the temperatures, especially to the fatigue limit for crack growth, it becomes more sensitive at the elevated temperatures than at room temperature. The results were discussed with the effect of temperature on the arresting behavior of fatigue crack growth.

Language：Japanese  

Country：Japan  

Effect of Oxide Films on a Fatigue Crack Initiation and its Early Growth of Ni-Base Super Alloy
Rotating bending fatigue tests were carried out for Ni base super alloy Inconel 718 at room temperature and elevated temperatures of 300°C, 500°C and 600°C in order to investigate the effect of surface oxidation at the elevated temperatures on the fatigue crack initiation and its early growth process. The fatigue strength was higher at the elevated temperatures than at room temperature when stress levels were low. This phenomenon was caused from that the growth of a crack smaller than the size of a few grains was suppresed at the elevated temperatures, though the growth of a larger crack was accelerated. The main reason for the suppression of the early growth of a small crack was a formation of oxide films on the specimen surface at the elevated temperatures.

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切刃の丸味による切削面の弾性回復と切削抵抗

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The influence of cubic boron nitride (CBN) grinding on fatigue strength was investigated on an annealed carbon steel, a quenched and tempered carbon steel at room temperature, and a nickel-base superalloy, Inconel 718, at room temperature and 500°C. The results were discussed from several viewpoints, including surface roughness, residual stress, and work hardening or softening due to CBN grinding. The fatigue strength increased upon CBN grinding at room temperature, primarily because of the generation of compressive residual stress in the surface region. However, in the case of Inconel 718, this marked increase in the fatigue strength tended to disappear at the elevated temperature due to the release of compressive residual stress and the decrease of crack growth resistance at an elevated temperature.

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The notched fatigue strength of nickel-base superalloy Inconel 718 has been investigated under rotating bending loading at room temperature and 500 °C in air. The applicability of linear notch mechanics to the evaluation of notched fatigue strength at elevated temperature was assessed in terms of the fatigue limit for crack initiation and that for crack growth. The effect of temperature on the fatigue strength and notch sensitivity of Inconel 718 was examined. The results were discussed from the viewpoints of the softening of matrix and surface oxidation at elevated temperature.

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The strength of Inconel 718 under rotary bending fatigue is investigated at room temperature, 300, 500 and 600 °C in air. It is found that in the long-life region, the fatigue strength of a plain specimen is much higher at elevated temperatures than at room temperature, though the static strength decreases with the increase in temperature. The effect of temperature on the fatigue strength is examined in terms of the initiation and early growth behaviour of a small crack. The results are discussed in relation to the competition between the softening of the nickel matrix (γ phase) and the surface oxidation at elevated temperatures.

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The fatigue crack growth behavior of Inconel 718 was investigated under rotating bending fatigue at room temperature, 300, 500, and 600°C in air. It has been found that the small crack growth rate could be evaluated by the small crack growth law at high stress levels, where the small-scale yielding conditions are exceeded and the Paris law is not applicable, irrespective of the temperature. The fatigue strength of plain specimens increased considerably in the long-life region at the elevated temperatures, because the early growth of a small crack in the range of 20-30 μm was suppressed. However, a crack grew faster at higher temperature after growing beyond about 50 μm due to the decrease of crack growth resistance. The fatigue life in the stable crack growth period can be predicted by the small crack growth law.

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618 Evaluation of Fatigue Damage Using Pattern Recognition of Occurrence of Acoustic Emission

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Estimation of Fatigue Damage Using Pattern of Occurrence of Acoustic Emission
Fatigue fracture is a serious problem in high-speed machines and lightened structures. Therefore, it is very important for fully automated machines to predict or estimate fatigue damage of the machines nondestructively. On the other hand, acoustic emission is the phenomenon that elastic waves caused by released elastic deformation energy propagate in materials, and initiation and propagation of cracks can be detected by using the acoustic emission.. Accordingly, fatigue damage of a specimen in plane bending fatigue test was tried to be estimated by using pattern of occurrence of AE per cycle of periodic change of stress. As a result, it was verified that proposal method can estimate fatigue damage, but with a little error.

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308 In-process Identification of Machined Surface Topography Using Vibration Analysis

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108 Fatigue Properties of Al Cast for Road Wheel

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1011 Effect of Grinding on Fatigue Strength off Nickel-Base Superalloy

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FATIGUE PROPERTIES OF NICKEL-BASE SUPERALLOY AT ELEVATED TEMPERATURES(ABSTRACTS OF DOCTORAL THESIS)

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707 Ultrasonic fatigue strength of Nickel-base superalloy
The fatigue strength of a nikel-base superalloy, Inconel 718, was investigated at room temperature under an ultrasonic frequency (19.5kHz) loading in ambient air environment. The endurance (S-N) data show that fatigue strength was enhanced at ultrasonic frequency as compared with those at conventional frequencies. Small crack initiation and propagation behavior was observed during the fatigue progress at both ultrasonic fatigue and conventional rotary bending fatigue. The effect of frequency on the fatigue mechanism was examined from a microscope viewpoint.

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Fatigue Properties of Ni-Base Superalloy in Long Life Region at Elevated Temperature
The fatigue properties of a nickel-base superalloy, Inconel 718, in long-life region up to 10⁸ cycles were investigated under rotating bending at room temperature and elevated temperature of 300°C, 500°C and 600°C. S-N curves at 500°C and 600°C showed the stepwise shapes, though those in cases of room temperature and 300°C were the conventional one-step ones. All the fractures originated from the specimen surface at room temperature and 300°C, while the internal fractures occurred in long-life region beyond 10⁷ cycles and the surface fractures at the stress levels over the horizontal line of S-N curves corresponding to the fatigue limit for the surface fracture at 500°C and 600°C. Intergranular crackings were observed at the origin of internal fracture.

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The fatigue properties of a nickel-base superalloy, Inconel 718, in long-life region up to 108 cycles were investigated under rotating bending at room temperature and elevated temperature of 300°C, 500°C and 600°C. S-N curves at 500°C and 600°C showed the stepwise shapes, though those in cases of room temperature and 300°C were the conventional one-step ones. All the fractures originated from the specimen surface at room temperature and 300°C, while the internal fractures occurred in long-life region beyond 107 cycles and the surface fractures at the stress levels over the horizontal line of S-N curves corresponding to the fatigue limit for the surface fracture at 500°C and 600°C. Intergranular cracking were observed at the origin of internal fracture.

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The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was > 107 cycles. A significant change in the slope of the S-N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 107 and 109 cycles are > 99&#37;. © 2002 Elsevier Science Ltd. All rights reserved.

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Effect of Peierls Stress on Mode I Crack Propagation

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Ultrasonic Fatigue Properties of a High Strength Extruded Al Alloy
In order to investigate the influence of loading frequency on the fatigue strength and fracture mechanism of a high strength Al alloy, ultrasonic fatigue tests were carried out for an extruded Al alloy 7075-T 6 and the results were compared with those of rotating bending fatigue tests. Fatigue strength of ultrasonic was higher than that of rotating bending. The crack initiation and the crack growth rate in ultrasonic fatigue were delayed compared with those of rotating bending fatigue. In case of ultrasonic fatigue, the direction of crack growth changed from tensile mode to shear mode and many voids were observed on the fracture surface in shear mode growth, whereas the crack propagated in tensile mode and striations were observed in rotating bending fatigue.

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In order to investigate the effect of load frequency on the fatigue strength and fracture mechanism of a nickel-base superalloy, Inconel 718, fatigue tests were carried out under an ultrasonic frequency (19.5 kHz) in ambient air environment. The results were compared with those obtained under conventional rotatary bending fatigue at a frequency of 50 Hz. Fatigue strength increased at ultrasonic frequency, which is mainly caused by the suppression of crack initiation and its growth at the early stage. Under both ultrasonic and rotary bending fatigue tests, most of fatigue life consumed in the growth of a crack smaller than 1 mm. In the ultrasonic fatigue, intergranular and cleavage crack propagations were observed in addition to striation, which was a dominant fracture mechanism in the conventional fatigue.

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In order to investigate the influence of loading frequency on the fatigue strength and fracture mechanism of a high strength Al alloy, ultrasonic fatigue tests were carried out for an extruded Al alloy 7075-T 6 and the results were compared with those of rotating bending fatigue tests. Fatigue strength of ultrasonic was higher than that of rotating bending. The crack initiation and the crack growth rate in ultrasonic fatigue were delayed compared with those of rotating bending fatigue. In case of ultrasonic fatigue, the direction of crack growth changed from tensile mode to shear mode and many voids were observed on the fracture surface in shear mode growth, whereas the crack propagated in tensile mode and striations were observed in rotating bending fatigue.

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Ultrasonic Fatigue Properties of Ni-Base Superalloy
In order to investigate the effect of load frequency on the fatigue strength and fracture mechanism of a nickel-base superalloy, Inconel 718, fatigue tests were carried out under an ultrasonic frequency (19.5kHz) in ambient air environment. The results were compared with those obtained under conventional rotatary bending fatigue at a frequency of 50 Hz. Fatigue strength increased at ultrasonic frequency, which is mainly caused by the suppression of crack initiation and its growth at the early stage. Under both ultrasonic and rotary bending fatigue tests, most of fatigue life consumed in the growth of a crack smaller than 1 mm. In the ultrasonic fatigue, intergranular and cleavage crack propagations were observed in addition to striation, which was a dominant fracture mechanism in the conventional fatigue.

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The effect of a lattice friction stress on the monotonic growth of a plane strain mode I crack under small-scale yielding conditions is analyzed using discrete dislocation plasticity. When the friction stress is increased from zero to half the dislocation nucleation stress, the crack tip stress field changes from one characteristic of a plastic solid to one characteristic of an elastic solid, and the crack growth resistance decreases substantially. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

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Corrosion and fatigue properties of aircraft materials are known to have a considerable scatter due to the random nature of materials, loading, and environmental conditions. A probabilistic approach for predicting the pitting corrosion fatigue life has been investigated which captures the effect of the interaction of the cyclic load and corrosive environment and all stages of the corrosion fatigue process (i.e. the pit nucleation and growth, pit-crack transition, short- and long-crack propagation). The probabilistic model investigated considers the uncertainties in the initial pit size, corrosion pitting current, and material properties due to the scatter found in the experimental data. Monte Carlo simulations were performed to define the failure probability distribution. Predicted cumulative distribution functions of fatigue life agreed reasonably well with the existing experimental data.

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The effects of pitting corrosion on very high cycle fatigue behavior of aluminum alloys were discussed. The results indicated that the presence of pre-existing corrosion pits, produced by 1-day, 4-day and 7-day immersion in salt water significantly reduces the fatigue life of the alloy by a factor of 10-100. It was also found that the pittings accelerated crack initiation and promoted multiple-site damage.

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Ultrasonic Fatigue Properties of Metals

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Ultrasonic Fatigue Properties of an Age-Hardened Al Alloy 6061-T6
In order to investigate the influence of loading frequency on the fatigue properties of Al alloy, ultrasonic fatigue tests were carried out for an age-hardened Al alloy, 6061-T6. The results were compared with those of rotating bending fatigue in view points of initiation and propagation behavior of a crack and fracture mechanism. Fatigue life was longer in ultrasonic fatigue than in rotating bending fatigue, which was mainly caused by the suppression of crack initiation. In ultrasonic fatigue, many fatigue voids and cleavage cracks were observed on the fracture surface in addition to the shear mode cracks, striations and dimples, which were typical fracture modes in rotating bending.

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In order to investigate the influence of loading frequency on the fatigue properties of Al alloy, ultrasonic fatigue tests were carried out for an age-hardened Al alloy, 6061-T6. The results were compared with those of rotating bending fatigue in view points of initiation and propagation behavior of a crack and fracture mechanism. Fatigue life was longer in ultrasonic fatigue than in rotating bending fatigue, which was mainly caused by the suppression of crack initiation. In ultrasonic fatigue, many fatigue voids and cleavage cracks were observed on the fracture surface in addition to the shear mode cracks, striations and dimples, which were typical fracture modes in rotating bending.

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The fatigue behavior of 7075/T6 and 6061/T6 aluminum alloys in the very long life regime was investigated by using piezoelectric accelerated fatigue tests at 19.5 KHz. The mechanism of fatigue crack initiation and propagation by void formation and growth was identified using scanning electron microscopy (SEM).

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Defective metallic components and structures are being repaired with bonded composite patches to improve overall mechanical and fatigue properties. In this study, fatigue crack growth tests were conducted on pre-cracked 7075/T6 Aluminum substrates with and without bonded Boron/epoxy patches. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of patch plies increased. The experimental results demonstrate that the patch configurations and patch thickness can enhance fatigue life by order of magnitude. Quantitative comparisons between analytical and experimental data were made, and the analytical model based on a modified Rose's analytical solution appears to best estimate the fatigue life.

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Fracture and small crack behavior in the very high cycle domain of 10 9 cycles were investigated with a nickel-based superalloy under ultrasonic fatigue in ambient air at room temperature. The influence of ultrasonic frequency is examined by comparing the results with those in conventional low frequency fatigue. It is found that fatigue strength increases as frequency is raised up to 19.5 kHz and the most of fatigue life is consumed in nucleating and propagating small cracks up to 100 μm. Transition of fracture mode from transgranular ductile fracture to cleavage-dominated fracture occurs beyond a critical stress intensity factor range of approximately 21MPam, leading to the catastrophic failure under ultrasonic fatigue. © 2005 Elsevier Ltd. All rights reserved.

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Ni基超合金の中高温疲労強度に及ぼす結晶粒径の影響

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107 Crack Growth Behavior of Al Alloy under Ultrasonic Fatigue
In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T6, and the results were compared with those in rotating bending fatigue. Fatigue strength in ultrasonic was higher than that in rotating bending. Growth direction of a crack changed from tensile mode to shear one in ultrasonic. The relation between an applied stress σ_a and a crack length at transition of growth direction b_T was expressed as σ_a ^nb_T=C.

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Effect of Loading Frequency on Fatigue Strength of Ni-Based Super Alloy
In order to investigate the growth mechanism of a fatigue crack of Ni-based super alloy at ultrasonic frequency, ultrasonic fatigue and rotting bending fatigue tests were carried out for Inconel 718 in ambient air and in N₂ gas using plain and notched specimens and the results in both tests were compared by focusing on the effects of strain rate and environment. In ambient air, fatigue strength was higher in ultrasonic fatigue than in rotating bending fatigue in plain specimen, while the results in notched specimen were the reverse. Many transgranular cracks and cleavage like cracks were observed on the fracture surface in ultrasonic fatigue in addition to striation and transgranular facets observed in rotating bending fatigue and there was no difference in the fracture mechanism between plain specimen and notched one. Moreover, these effects of frequency on the growth mechanism of a crack in ambient air were nearly the same in N₂ gas, though fatigue strength in N₂ gas was higher than that in air. The increase in fatigue strength in N₂ gas was mainly caused by the retardation of a crack initiation.

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Fatigue induced from various aeromechanical sources can reach gigacycle regime due to vibratory stress cycles at high frequencies. Composite repairs offer various advantages over mechanical fastening or riveting, including improved long-term durability behavior and reduced corrosion. In this study, by using ultrasonic frequency cyclic loading, fatigue crack growth tests in gigacycle regime were conducted on notched 7075 and 6061 aluminum alloy substrates (in T6 condition) with and without bonded Boron/epoxy patches. A considerable increase in the fatigue life was observed with more number of patch plies. The effect of patch configuration and thickness on enhancing the fatigue strength by orders of magnitude has been demonstrated. © 2006 Elsevier Ltd. All rights reserved.

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Very long life fatigue and near-threshold fatigue crack growth behaviour of 7075 and 6061 Al-alloys in T6 condition were investigated using accelerated fatigue testing at 19.5 kHz on a piezoelectric machine. Significant interfacial voids were observed in the two Al-alloys during crack initiation and early fatigue crack growth process. Fatigue failure in Al-alloys could be ascribed to: the formation of a number of fatigue voids, their growth and coalescence, and the subsequent formation and propagation of macroscopic cracks. Moreover, fatigue crack growth rates of small cracks in these alloys were found to be greater than those of large cracks for the same stress intensity factor range. © 2006 Elsevier Ltd. All rights reserved.

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In order to investigate the growth mechanism of a fatigue crack of Ni-based super alloy at ultrasonic frequency, ultrasonic fatigue and rotating bending fatigue tests were carried out for Inconel 718 in ambient air and in N 2 gas using plain and notched specimens and the results in both tests were compared by focusing on the effects of strain rate and environment. In ambient air, fatigue strength was higher in ultrasonic fatigue than in rotating bending fatigue in plain specimen, while the results in notched specimen were the reverse. Many transgranular cracks and cleavage like cracks were observed on the fracture surface in ultrasonic fatigue in addition to striation and transgranular facets observed in rotating bending fatigue and there was no difference in the fracture mechanism between plain specimen and notched one. Moreover, these effects of frequency on the growth mechanism of a crack in ambient air were nearly the same in N2 gas, though fatigue strength in N2 gas was higher than that in air. The increase in fatigue strength in N2 gas was mainly caused by the retardation of a crack initiation.

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In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T 6, in ambient air and in N2 gas, and the results were compared with those in rotating bending fatigue. Fatigue strength was higher in ultrasonic fatigue than in rotating bending one. This was caused by the retardation of a crack initiation and its early propagation. In ultrasonic fatigue, the growth direction of a crack changed macroscopically from a tensile mode to a shear one which the direction was about 35 degrees against to the specimen axis, and fracture mechanism also changed from striations to transgranular facets with voids microscopically. Crack growth rate at the transition of crack growth direction was about 3×10-9 m/cycle. The relation between an applied stress σa and a crack depth at the transition of crack growth direction bT was expressed by σanbT=constant. These results were explained from the time dependent environmental effect and texture of microstructure.

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Crack Growth Behavior of Al Alloy 7075-T6 under Ultrasonic Fatigue
In order to investigate the effect of frequency on the crack growth behavior, ultrasonic fatigue tests were carried out for an extruded age-hardened Al alloy, 7075-T 6, in ambient air and in N₂ gas, and the results were compared with those in rotating bending fatigue. Fatigue strength was higher in ultrasonic fatigue than in rotating bending one. This was caused by the retardation of a crack initiation and its early propagation. In ultrasonic fatigue, the growth direction of a crack changed macroscopically from a tensile mode to a shear one which the direction was about 35 degrees against to the specimen axis, and fracture mechanism also changed from striations to transgranular facets with voids microscopically. Crack growth rate at the transition of crack growth direction was about 3 × 10^-9 m/cycle. The relation between an applied stress σ_a and a crack depth at the transition of crack growth direction b_T was expressed by σⁿ_ab_T=constant. These results were explained from the time dependent environmental effect and texture of microstructure.

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Influence of shot peening on fatigue strength of maraging steels with different hardness
In order to investigate the hardness dependence on the fatigue strengths for surface fracture and internal fracture, rotating bending fatigue tests were carried out for maraging steels using specimens with different hardness obtained by heat-treating at different under-aging conditions. Fatigue limit for surface fracture was evaluated in electro-polished specimens and the one for internal fracture was in shot-peened ones, where fatigue limits were defined as fatigue strengths at 107 cycles for surface fracture and 108 cycles for internal fracture, respectively. Fatigue limit for internal fracture increased with increase in hardness similar to the one for surface fracture. However, the proportional relationship between the fatigue limit and the hardness existed till higher hardness in the fatigue limit for internal fracture than in the one for surface fracture. This difference in the hardness dependence on fatigue limit was discussed from the viewpoint of the influence of humidity on the fatigue strength. © 2007 The Society of Materials Science.

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506 System for Very High Cycle Fatigue Reliability Assessment of Nano/Micromaterials
A novel strain controlled, free standing tensile fatigue system was developed by using the combined PZT and micromachining technologies in order to conduct in-situ observation and assessment of high cycle fatigue behavior in LP-CVD poly Silicon thin films within a scanning electron microscope (SEM).

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117 Effect of microstructure on ultrasonic fatigue properties of Inconel 718

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Effect of Grain Size on Ultrasonic Fatigue Properties of Ni-Base Super Alloy Inconel 718
Ultrasonic fatigue tests of plain specimens with different grain sizes were carried out for Ni-base super alloy, Inconel 718, in ambient air in order to investigate the effect of grain size on fatigue properties. Fatigue strength was increased with decrease in grain size. The increase in fatigue strength by refining grain size was mainly caused by the suppression of crack initiation. That is, the effect of grain size on crack growth rate was hardly recognized, though crack morphology was rougher in the specimen of larger grain, meaning that the crack growth in the large-grained specimens was suppressed by roughness induced crack closure. On the other hand, however, more flat facets caused by twin boundary cracking were observed in the large-grained specimens, which inversely led to crack growth acceleration.

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Ultrasonic fatigue tests of plain specimens with different grain sizes were carried out for Ni-base super alloy, Inconel 718, in ambient air in order to investigate the effect of grain size on fatigue properties. Fatigue strength was increased with decrease in grain size. The increase in fatigue strength by refining grain size was mainly caused by the suppression of crack initiation. That is, the effect of grain size on crack growth rate was hardly recognized, though crack morphology was rougher in the specimen of larger grain, meaning that the crack growth in the large-grained specimens was suppressed by roughness induced crack closure. On the other hand, however, more flat facets caused by twin boundary cracking were observed in the large-grained specimens, which inversely led to crack growth acceleration.

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209 Fatigue properties of LP-CVD polysilicon

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Fatigue strength of age-hardened & extruded al alloy under high humidity (rotating bending and ultrasonic loading)
In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075T 6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%~70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.

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OS1501 Effect of Thickness on Tensile Properties of Parylene C Thin Films
Parylene C is an inert, hydrophobic, flexible and biocompatible polymer coating material, which has recently found broad MEMS and BioMEMS applications including microstructures, micro sensors and actuators. As a result, many researches have been conducted by focusing on the utilization of Parylene C thin films. Little attention, however, was paid to and rare was known about the mechanical properties of the thin films that may subject to severe tension, bending, twisting or the combination of these loading, especially in biomedical implants in which strength reliability must be secured during a reasonable lifetime. The objective of the present study, therefore, is to investigate the tensile properties of Parylene C thin films fabricated by micromachining process in the range of thickness from submicron to tens of microns. The effect of sample thickness is discussed.

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Effect of microstructure on fatigue properties of Al alloy 2017 in high humidity
In order to investigate effects of a texture and a slip characteristic on fatigue properties of Al alloys in high humidity, rotating bending fatigue tests were carried out using plain specimens of an extruded bar and a drawn one of age-hardened Al alloy 2017-T4 and an Al alloy annealed the extruded one in relative humidity of 25% and 85%. The extruded Al alloy and the annealed one had a marked texture of a (111) plane at the cross section of each bar, but a specified orientation was not observed in the drawn one. In addition, the aged-hardened Al alloys show a planar slip property and the annealed alloy shows a wavy one. In the all Al alloys, the obvious retardation of a small crack growth due to blocking effect of a grain boundary was observed in low humidity, though the retardation disappeared in high humidity. Moreover the acceleration of a crack growth was yielded due to the change in growth mechanism of a crack from a tensile mode in low humidity to a shear mode at high stress levels in high humidity in the extruded alloy, though the effect of humidity on the growth mechanism was very small in the drawn Al alloy and the annealed one. In the extruded Al alloy, the fracture surface yielded by the shear mode crack in high humidity was occupied by many slip planes and voids and was a (100) plane, though the fracture surface formed by the growth of the tensile mode crack in low humidity was covered with striations. In the drawn and annealed Al alloys, fracture surfaces were mainly occupied by striations irrespective of humidity and stress levels. © 2012 The Society of Materials Science, Japan.

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Effect of humidity on growth behavior of a fatigue crack of 7075-T6 extruded Al alloy
In order to investigate the effects of humidity change and loading frequency on growth behavior of a fatigue crack of an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out in relative humidity of 25% and 85% at frequencies of 50Hz and 6Hz. At 50Hz, macroscopic appearances of fractures were a shear mode at high stress levels in high humidity, though those were a tensile mode in other conditions of humidity and stress levels. The crack growth rate was accelerated by high humidity. By changing humidity, both of the growth rate and the growth mode of a crack were immediately changed to those corresponding to the changed humidity. That is, the growth behavior and fatigue life under humidity change may be estimated by those in constant humidity. Fatigue life in high humidity at 6Hz was longer than that at 50Hz in spite of accelerations of both of the crack initiation and its growth rate in the early growth process in high humidity at both frequency, though there was no or little influence of frequency on fatigue life in low humidity. Moreover, the crack propagated in the tensile mode even at high stress levels where the shear mode crack propagated at 50Hz. These results were explained by the effect of hydrogen on the crack growth behavior. © 2012 The Society of Materials Science, Japan.

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Aging condition for improvement of fatigue properties of maraging steel
The effect of aging condition on fatigue properties and the mechanism of decrease in fatigue strength in high humidity in 18% Ni maraging steel of grade 350 were investigated under rotating bending in relative humidity of 25% and 85%. Aging conditions investigated were under-, peak- and over-aging ones at the conventional aging temperature of 753K. In addition, double-aging treatments which were under- peak- and over-aging treatments at 673K and under-aging one at 473K after the peak-aging at 753K were also examined. Both of static and fatigue strengths were increased by the double-aging without any decrease in ductility and fatigue fracture toughness. Fatigue strength was markedly decreased by high humidity in all of the steels, and the decrease in fatigue strength was mainly caused by the accelerations of crack initiation and its growth at the early stage of fatigue process. The decrease in fatigue strength in high humidity was also suppressed by the double aging. A few facets comparable to a grain size of a prior austenite were observed at the fracture origins in high humidity but transgranular cracks by slip deformation were in low humidity. However most of the fracture surfaces were covered with lath boundary cracking regardless of the humidity and aging conditions. River pattern was observed in the facets, suggesting that the acceleration of crack growth in high humidity was a behavior related to hydrogen generated in cathode reaction. Based on the results, new aging treatment for improvement of fatigue properties of maraging steel was proposed. © 2013 The Society of Materials Science, Japan.

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3B2 Growth Mechanism of Fatigue Crack of High Strength Metals under Ultrasonic Loading

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GS04 Growth Properties of a Fatigue Crack of Extruded 7075-T6 in High Humidity
Effect of humidity on the growth properties of a fatigue crack in an age-hardened Al alloy was investigated in relative humidity of 25% and 85% at loading frequencies of 50 and 6Hz. Macroscopic growth mode of a crack was a tensile one in low humidity, while the mode changed from the shear one to the tensile one with increase in fatigue life in high humidity. The crack in low humidity propagated by forming striation. On the other hand, in high humidity, most of fracture surface was occupied by many slip planes in the shear mode crack, and many granular facets in addition to striations were observed in the tensile mode growth. The difference in growth mechanism was explained by microstructure and cyclic softening behavior of the alloy and hydrogen content.

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The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.

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Microstructural cyclic deformation mechanisms and their relation to small fatigue crack initiation and early propagation behavior were experimentally investigated in a rare earth-containing magnesium alloy (Mg-Gd-Y-Zr). The results indicate that basal slip is the dominant deformation mechanism, especially in coarse grains, and eventually leads to fatigue crack initiation. Early crack propagation behavior was strongly influenced by local microstructural heterogeneities in grain size and orientation. Three kinds of microstructures—favorably-oriented coarse grains, fine grain clusters and unfavorably-oriented coarse grains—are discussed in terms of their deformation mechanisms and resistance to crack propagation. These microstructural effects caused highly variable crack propagation rates within the first ∼200 μm of cracks.

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Corrosion and fatigue are two types of damages found in aircraft and offshore structures. Corrosion fatigue processes are characterized by randomness due to the random nature of materials, initial defects, loading, and environmental condition. In this study, a mechanistically based probabilistic model for prediction of pitting corrosion fatigue life in aluminum alloys has been developed which captures the effect of the interaction of cyclic load and corrosive environment and all the stages of the corrosion fatigue process. Monte Carlo simulations were performed to define the failure probability distribution. Predicted cumulative distribution functions of fatigue life agreed reasonably well the experimental data.

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In order to investigate the effect of radical nitriding on fatigue strength of die steel, rotating bending fatigue tests were carried out at room temperature and 450°C. Fatigue strength was increased by nitriding at both temperatures due to surface hardening and compressive residual stress generated in the surface region. Comparing with the non-nitrided steel in which the fracture origin was at surface slip bands or surface defects, the nitrided steel fractured with the origin at inner inclusions at both temperatures. In the nitrided steel, cracks initiated from inclusions in the non-hardened layer or near the boundary between hardened and non-hardened layers. The difference in fatigue strength between the two temperatures was discussed based on the feature of fish-eye.

DOI： 10.2495/SURF010201

Language：English  

The fatigue strength of a nickel-base superalloy, Inconel 718, was investigated at room temperature under an ultrasonic frequency (19.5 kHz) loading in ambient air environment. The endurance (S-N) data show that fatigue strength was enhanced at ultrasonic frequency as compared to those at conventional frequencies. Small crack initiation and propagation behavior was observed during the fatigue progress at both ultrasonic fatigue and conventional rotary bending fatigue. The effect of frequency on the fatigue fracture mechanism was examined from a microscopic viewpoint.

DOI： 10.7449/2001/superalloys_2001_573_582

Language：Japanese  

Improvement of fatigue strength of maraging steel by nitriding
Rotating bending fatigue tests were carried out to investigate the influence of radical nitriding on the surface integrity and the fatigue strength for a maraging steel. Fatigue strength increased by nitriding. Although the initiation site of fracture was the surface in the aged steel, it was the specimen suface at high stress levels and the interior at low stress levels in the nitrided steel. The main reason for the surface fracture was the surface embrittlement and the one for the interior fracture was the surfce hardening due to nitriding. As the results, S-N curves of the nitrided maraging steel showed a double staged curve.

DOI： 10.1299/kikaia.67.314

Language：English  

Detection of Tool Wear using Characteristic Values of Dynamic Cutting Process: 2nd Report, Adaptability of Characteristic Values to Variation of Measurement Conditions
In-process measurement of tool flank wear is very important for judgment of tool life with the object of running fully automated machine tools economically. The purpose of this study is to propose a new method of detecting tool flank wear by using characteristic values of dynamic cutting process during steady-state cutting in turning operation. In this report, a simple calculation model of dynamic thrust force was proposed to derive equations of characteristic values from it, and effectiveness of the equations was experimentally verified under many cutting conditions and excitation conditions of a workpiece system. As a result of the calculations and the experiments, we reached conclusions that two characteristic values of dynamic cutting process, standard deviation of dynamic thrust force normalized by the standard deviation of workpiece vibration displacement and coefficient of correlation between dynamic thrust force and vibration displacement, have the following properties. (i) The characteristic values remained almost constant with variation of feed rate of a cutting tool and vibration amplitude of a workpiece. (ii) On condition that vibration frequency of a workpiece is constant, the characteristic values obtained under different cutting speeds can be standardized by normalizing the width of flank wear to the cutting speed. (iii) Tendency of calculated characteristic values with increase of vibration frequency was partly disagree with that of experimental values. © 2000, The Japan Society of Mechanical Engineers. All rights reserved.

DOI： 10.1299/kikaic.66.1700

Language：Japanese  

Initiation and growth behavior of a fatigue crack in inconel 718 at elevated temperatures
Rotating bending fatigue tests were performed for Inconel 718 at room temperature and elevated temperatures of 300°C, 500°C and 600°C to investigate the influence of temperature on the fatigue crack initiation and its propagation. Although the propagation of a micro-crack smaller than about 20 um is suppressed at elevated temperatures, that of the larger-crack is accelerated inversely. These results were discussed from the viewpoints of the softening of matrix and the oxidation of surface caused by the high temperature.

DOI： 10.1299/kikaia.65.1954