著者

皮籠石 紀雄 仮屋 孝二 中村 祐三 永野 茂憲 岩本 竜一

出版者

一般社団法人 日本機械学会

雑誌

日本機械学会論文集 (ISSN:21879761)

巻号頁・発行日

vol.83, no.855, pp.17-00274-17-00274, 2017 (Released:2017-11-25)

参考文献数

23

被引用文献数

2

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Fatigue cracks in some aluminum alloys propagate not in a typical tensile mode with striations but in a macroscopic shear direction with ductile facets under specific conditions. In the previous paper, it was shown that this unique propagation of a shear mode crack occurred in high humidity under rotating bending fatigue and under ultrasonic loading fatigue irrespective of humidity. Two types of mechanism for the shear mode crack were proposed: one is a case that a slip deformation to one direction in cross slip in formation process of striation was enhanced by hydrogen generated in corrosion process of Al alloy in high humidity and the other is that a slip deformation of one direction was inhibited by the re-welding of crack faces in vacuum condition. In the present study, fatigue tests of Al alloys of extruded and drawn 2017-T4 and extruded 7075-T6 were conducted under rotating bending and ultrasonic loading conditions in relative humidity environments of 25% and 85% and in nitrogen gas, to verify the proposed mechanism experimentally and investigate influencing factors on the shear mode propagation. In addition, the effect of change in the testing condition on the propagation behavior of a crack was investigated. Although the microscopic shear mode propagation occurred within grains in both of the extruded and the drawn alloys, the crack propagated to a specific shear direction macroscopically in the extruded alloy related to the strong texture, and the crack propagated in a tensile mode macroscopically in the drawn alloy possessing no texture. These differences in macroscopic crack propagation mode, thus, were caused by the degree of texture. The proposed mechanism was verified by some experiments in various environments. Furthermore the effect of change in testing conditions influencing on the shear mode crack was explained by the results under the constant conditions.

著者

皮籠石 紀雄 中村 祐三 仮屋 孝二 永野 茂憲 岩本 竜一

出版者

一般社団法人 日本機械学会

雑誌

日本機械学会論文集 (ISSN:21879761)

巻号頁・発行日

vol.83, no.855, pp.17-00273-17-00273, 2017 (Released:2017-11-25)

参考文献数

21

被引用文献数

1

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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.