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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

A plane bending fatigue test was conducted on Ti-6Al-4V, and the crystal grains where the fatigue crack initiated were identified from the crystal orientation map obtained by using the EBSD method. In addition, the slip system, which generated the fatigue crack initiation, was identified by calculating the out-of-plane component of the Schmid factor (SF) of each slip system. As a result, fatigue cracks were found to occur owing to basal slip. In this study, the out-of-plane component of the SF was mapped by considering the effect of the crystal grain size. The results clarified that the number of crystal grains where fatigue crack may initiate can be limited by using the out-of-plane component of the SF and the grain size.

DOI： 10.1080/02670836.2022.2127982

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

In recent years, glass-fiber-reinforced plastic (GFRP) applications have expanded. This study elucidates the fatigue fracture mechanism of GFRP with different fiber orientations and experimentally clarifies the relationship between fiber orientation and fracture mechanism by using specimens with unidirectional simple fiber orientations. First, Static tensile and fatigue tests are performed on two types of specimens with fibers oriented in a single direction (parallel to the load axis and orthogonal to the load axis), and it is found that specimens with a single fiber orientation parallel to the load axis shows higher strength in both tests. Next, In the case of GFRP with fibers oriented parallel to the load axis, it is found that fatigue loading causes fatigue crack initiation at the fiber ends, followed by fiber localized debonding due to crack propagation at the fiber-matrix interface and that the crack propagation rate gradually decreases and stops. Therefore, this suggests that the increase in fiber length of fibers oriented parallel to the load axis in GFRP leads to an increase in the crack propagation life at the fiber-matrix interface, which leads to an increase in fatigue life.

DOI： 10.1002/app.53088

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The mechanism of residual stress relaxation for carburized steels was elucidated and their fatigue limit was estimated. The compressive residual stress in the carburized steel after rotating bending fatigue tests is mainly influenced by the relaxation during the compressive loading phase, although it increases at the early stage. The fatigue limit of carburized steels can be estimated according to the modified Goodman diagram considering the residual stress relaxation. The proposed equation shows the maximum value of the fatigue limit (1013 MPa) of carburized steel and the maximum value of the compressive residual stress contributing to the increased fatigue limit.

DOI： 10.1016/j.ijfatigue.2022.106846

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Language：English   Publisher：SPRINGER HEIDELBERG  

The welded areas of spot welding of actual vehicles are confirmed welding defects such as blowholes between steel plates. However, most of studies on the strength reliability of spot welded structures do not consider welding defects, and it is necessary to clarify the effects of weld defects on the strength reliability of spot welded structures. This study investigated the effect of the size and location of through holes simulating welding defects on the fatigue properties of tensile-shear-type spot welded joints using automobile steel sheets. Moreover, the effect of the loading type both tensile-shear and cross-tension type on the fatigue properties of the spot welded joints was investigated, and a unified evaluation of the fatigue life for different loading type joints was established. The size and location of a welding defect do not affect the fatigue strength or the fatigue fracture morphologies of tensile-shear-type spot welded joints. This is because the slit tip of spot welded joints is in the singularity stress field under mixed-mode condition, regardless of the welding defect of the joint. Therefore, the equivalent stress intensity factor at the slit tips of joints can uniformly evaluate the fatigue life of joints with different loading types.

DOI： 10.1007/s40194-021-01238-5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS & MATERIALS  

In this study, to clarify the weld area shape that achieves excellent fatigue properties in laser welded joints, an phi-shaped weld, which is a combination of a circular weld and a linear weld, was selected for evaluation. Joints with a linear weld having three different linear weld angles with respect to the load axis were fabricated and fatigue tests were conducted. The results showed that the effect of the linear weld angle was remarkable in a relatively high loading amplitude range, and the fatigue life increased as the angle increased. In addition, three-dimensional observation of fatigue cracks was carried out to investigate the fracture mechanism and the factors causing the difference in fatigue strength. It was clarified that the crack propagation life is increased because the crack propagates in the thickness direction not only at circular weld area but also at the linear weld area, when the linear weld angle respecting to loading direction are 45 degrees, 90 degrees. Furthermore, three-dimensional FEM analysis was carried out to recognize the stresses field around the linear weld area. It was clarified that the stresses around the weld area were distributed as the linear weld angle increased, which delayed crack initiation, leading to excellent fatigue properties. Therefore, the fatigue life in the high test-force amplitude region can he improved by providing a weld area shape that distributes the stress and allows the crack to propagate in the thickness direction.

DOI： 10.2320/matertrans.MT-M2021223

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS & MATERIALS  

The structures of a hydraulic excavator with one-sided welded joints are investigated in this study. We investigated the fatigue lifetime evaluation method using S-version FEM. Especially we considered combination of cracks from multiple crack origins. Moreover, to achieve a more precise evaluation, we obtained the crack growth rate of the excavator weld using the unloading elastic compliance method instead of some engineering standard values. Results show that, the fatigue lifetime obtained from the analysis of all width crack; type1 - all cracks named in this study is extremely short compared with test results. Considering the combination of cracks from multiple crack origins, the S-version FEM provides an evaluation similar fatigue lifetime of a test piece imitating the excavator.

DOI： 10.2320/matertrans.MT-Z2022003

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

The fatigue life and fracture mechanism of friction stir spot welded tension-shear joints using 590-MPa class steel as a base material under constant-amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.

DOI： 10.1111/ffe.13332

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In this study, the bending fatigue properties of an oxide dispersion-strengthened platinum–rhodium alloy subjected to axial stress at 1400 °C were evaluated, and the effects of the axial stress and life prediction methods were examined. When the axial stress increased, the fracture mechanism changed from fatigue fracture to creep fracture, thus reducing the fracture time. The creep rupture time due to the stress generated at the center of the thickness direction was compared with the fatigue failure time due to the stress amplitude of the surface; the shorter one corresponded to the main fracture mechanism. Creep rupture was the main factor affecting the stress levels of the actual equipment, and its life should be predicted according to the creep rupture time.

DOI： 10.1016/j.ijfatigue.2020.105808

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

The objective of the study described in this article is to evaluate the effect of shakedown of surface compressive residual stresses introduced by shot peening on fatigue limit of stainless steel. First, the tension-compression fatigue tests were conducted on ASTM CA6NM specimens under controlled load and displacement conditions to acquire a fatigue limit diagram under various compressive mean stress. The results showed that shakedown of negative mean stress occurs under controlled displacement. We then carried out in-plane-bending fatigue tests under controlled load conditions on welded ASTM 309 stainless steel specimens with surface compressive residual stress introduced by ultrasonic shot peening. The results provide a fatigue limit of 415 MPa, which agrees with the value of 404 MPa calculated based on a modified Goodman line considering shakedown. Therefore, it is suggested that the surface layer is restricted by the internal bulk that creates controlled displacement conditions and the shakedown of surface compressive residual stress occurs.

DOI： 10.1111/ffe.13097

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG SINGAPORE PTE LTD  

We performed in-plane bending fatigue tests under controlled load conditions on ASTM 309 stainless steel specimens with surface compressive residual stress. The results provide a fatigue limit of 415 MPa, which agrees with the value of 404 MPa calculated based on a modified Goodman line considering shakedown. Therefore, the surface layer was clearly restricted by the internal bulk and was relieved under controlled displacement, after which shakedown occurred. Then, the fatigue limit of the metal material with surface compressive residual stress should be estimated by using a modified Goodman line considering shakedown to avoid unexpected fractures that could occur by following a modified Goodman line that does not consider shakedown.

DOI： 10.1007/978-981-15-0054-1_8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS & MATERIALS  

In order to investigate the effectiveness of ultrasonic shot peening treatment (USP) as repairing method for SCS6 material with surficial fatigue crack for hydraulic turbine runner, plane bending fatigue tests were carried out for USP treated SCS6 containing a surface fatigue crack with 1 mm in length and the fatigue crack propagation after USP was observed by a plastic replica method. As a result, the fatigue crack propagation life of SCS6 containing a surface fatigue crack was dramatically improved by USP treatment. Furthermore, the initial effective stress intensity factor ranges were calculated in the USP treated and untreated SCS6 containing a surface fatigue crack, respectively. According to the calculation, it was clear that the surficial fatigue crack could be harmless under the condition that the calculated initial effective stress intensity factor range considering the stress opening a fatigue crack, which was acquired by the unloading elastic compliance method, was less than the threshold of effective stress intensity factor range. Therefore, USP treatment is effective for repairing method of SCS6 containing surficial fatigue crack for hydraulic turbine runner.

DOI： 10.2320/matertrans.Z-M2020829

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The influence of microstructure on crack initiation behavior of Ti-6Al-4V was investigated by using atomic force microscope (AFM) and electron backscattered diffraction (EBSD). This research considers the effects of both the Schmid factor (SF) value and the slip direction of the grain on fatigue crack initiation sites. The result shows that fatigue cracks tend to form basal slips in primary alpha grains. In addition, these cracks are generated from primary alpha grains with relatively high basal SF values. However, it was found that fatigue cracks do not necessarily originate from basal planes of primary alpha grain with the maximum SF value. Finally, a new parameter that combines SF and the slip angle Omega (slip angle Omega is the angle between the specimen surface and the slip direction) was suggested and that parameter enables to predict fatigue crack initiation sites, since it was revealed that fatigue cracks initiated from primary alpha grains with high SFsin Omega values.

DOI： 10.1016/j.ijfatigue.2018.07.039

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：E D P SCIENCES  

This study considers shakedown in evaluating the fatigue limit of metals with compressive residual stress at the surface. We begin by applying tension-compression fatigue tests to ASTM CA6NM under conditions of controlled load and displacement to obtain fatigue limit diagram in compressive mean stress. The results imply that shakedown occurs under the condition of controlled displacement, therefore, shakedown should be considered when evaluating the fatigue limit of metals with compressive residual stress at the surface.

DOI： 10.1051/matecconf/201816518006

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In order to investigate the effect of an ultrasonic shot peening (USP) treatment on the fatigue characteristics of structural materials for hydroelectric facilities, plane bending fatigue tests were carried out using USP treated ASTM CA6NM stainless cast steel. The results showed that the fatigue limit of the USP material was approximately 60% higher than that of the untreated material. Haigh's diagram evaluation suggested that an increase in crack initiation resistance associated with the high hardness and compressive residual stress by USP treatment was the main reason for the improvement in fatigue strength. Furthermore, the simulation of fatigue crack propagation implied that compressive residual stress leads to a low crack propagation rate compared with the untreated material. Consequently, it is clear that the USP treatment is an effective process for improving the fatigue properties of CA6NM.

DOI： 10.1179/1743294414Y.0000000308

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Publishing type：Research paper (international conference proceedings)  

In this paper, in order to investigate the effect of the ultrasonic shot peening (USP) treatment on fatigue characteristics of the structural materials for hydroelectric facilities, plane bending fatigue tests were carried out using stainless cast steel ASTM CA6NM performed by USP treatment. The fatigue test results showed that the fatigue strength of the USP materials was approximately 60% higher than that of the untreated materials. In order to examine the reason for that, the effect of USP treatment on fatigue crack initiation behavior was evaluated based on the Haigh's diagram. This evaluation implied that increasing the crack initiation resistance associated with high hardness and high compressive residual stress at surface by USP treatment is the major cause for the improvement of fatigue strength. Furthermore, the effect of USP treatment on fatigue crack propagation behavior was also examined by simulating the crack growth rate considering the stress intensity factor at crack tip. Crack propagation simulation results suggested that compressive residual stress leads crack growth rate slow compared with untreated material. © (2014) Trans Tech Publications, Switzerland.

DOI： 10.4028/www.scientific.net/AMR.891-892.649

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

In order to estimate the fatigue limit of carburized steels having surface compressive residual stress and surface texture generated by peening, we investigated the effect of surface texture and residual stress relaxation on the fatigue limit of carburized steels. In this study, rotating bending fatigue tests were conducted for the carburized steels treated with electrochemically polishing, fine particle peening and shot peening prior to gas carburizing. In the proposed fatigue limit estimation method for carburized steels, the surface texture formed by peening and the sum of mean stress and compressive residual stress experiencing relaxation are considered. The method shows that the maximum value of fatigue limit of carburized steels increased with decreasing the waviness in the surface. Additionally, the lower the waviness in the surface, the lower the maximum value of compressive residual stress contributing to the increased fatigue limit of carburized steel. These results indicate that fatigue limit of the carburized steel depends on the surface texture formed by peening.

DOI： 10.1299/jsmemm.2021.os0818

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

In order to investigate the fatigue properties under varying loading conditions in a heat welding joint by friction stir spot welding of aluminum alloy and CFRP, this study was carried out repeated two-step loading test. Moreover, this study was established a fatigue damage evaluation method with safety and high accuracy under two-step loading conditions. As a result, a fatigue life of joints under constant force amplitude conditions was 2.85 times the difference between the regression line by the least-squares methods and the fatigue life having the failure probability from 10 to 90 %. Moreover, the cumulative damage of the joints based on the Miner’s law was much lower than 1, the effect of load interaction on fatigue life of the joints was confirmed under varying loading conditions. Therefore, the fatigue damage evaluation of the joints with the safe side and high accuracy under varying loading conditions could be achieved by considering a variation of the fatigue life of the joints using confidence intervals and correcting the effect of load interaction using Corten-Dolan method.

DOI： 10.1299/jsmemm.2021.os0826

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Language：Japanese   Publisher：The Society of Materials Science, Japan  

In order to investigate the effectiveness of ultrasonic shot peening treatment (USP) as repairing method for SCS6 material with surficial fatigue crack for hydraulic turbine runner, plane bending fatigue tests were carried out for USP treated SCS6 containing a surface fatigue crack with 1 mm in length and the fatigue crack propagation after USP was observed by a plastic replica method. As a result, the fatigue crack propagation life of SCS6 containing a surface fatigue crack was dramatically improved by USP treatment. Furthermore, the initial effective stress intensity factor ranges were calculated in the USP treated and untreated SCS6 containing a surface fatigue crack, respectively. According to the calculation, it was clear that the surficial fatigue crack could be harmless under the condition that the calculated initial effective stress intensity factor range considering the stress opening a fatigue crack, which was acquired by the unloading elastic compliance method, was less than the threshold of effective stress intensity factor range. Therefore, USP treatment is effective for repairing method of SCS6 containing surficial fatigue crack for hydraulic turbine runner.

DOI： 10.2472/jsms.68.897

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Other Link： http://id.ndl.go.jp/bib/030144549

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

In this study, the effect of ultrasonic shot peening (USP) treatment on the fatigue strength of stainless cast steel SCS6 for hydraulic turbine runner was investigated carrying out plane bending fatigue tests for USP treated material and untreated material. According to the fatigue test results, it was clear that the fatigue strength of SCS6 was improved by USP treatment, especially, the fatigue limit was 60 % improved. The reasons of the fatigue strength enhancement are thought to be the effects of high hardness and compressive residual stress generated by USP treatment. In addition, the effectiveness of USP treatment on the fatigue strength of SCS6 having the fatigue crack was also examined, The specimen was made by performing USP treatment after creating the fatigue crack of about 1 mm by using the plane bending fatigue test machine. The results of the fatigue tests indicated that USP treatment delayed the fatigue crack propagation compared with the case of USP untreated, The main reason of above results was that the fatigue crack propagation resistance was increased by the compressive residual stress generated by USP treatment, in other words, the stress intensity factor K was decreased by that factor. According to these results, it can be said that USP treatment is very effective method to improve the fatigue strength of SCS6.

DOI： 10.1299/jsmemecj.2014._g0310601-

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

This study aimed to examine the effect of ultrasonic shot peening (USP) process on the fatigue strength of stainless cast steel JIS SCS6 for hydraulic turbine runner. Fatigue test was carried out in laboratory air room temperature using the specimens which was treated USP (USP specimen) and was not (Virgin specimen). It was found that the modified layer formed by USP process had high hardness and compressive residual stress. The fatigue strength of USP specimen rose about 60% in comparison with Virgin specimen. The fatigue crack in each type specimen was observed in detail by using plastic replica technique. The fatigue crack initiation life of USP specimen was more extension than that of Virgin specimen. The fatigue crack propagation rate of USP specimen was lower than that of Virgin specimen. The modified layer formed by USP process affects the fatigue crack initiation life and the propagation rate. The two reasons are the factor of the improvement of the fatigue strength by USP process.

DOI： 10.1299/jsmemm.2012._os2110-1_

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