Nihon Kikai Gakkai ronbunshu (Oct 2017)
Effect of atmosphere on crack growth mode in age-hardened Al alloys (Rotating bending fatigue and ultrasonic loading fatigue)
Abstract
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.
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