A comparative study of fatigue properties of two face-centered-cubic metals
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THIS is a continuation of previous work by Bassim et al. It is well documented in the literature that fatigue is the cause of failure of many structures in service. Although fatigue of fcc metals is well reported for different conditions, the low-cycle fatigue behavior of fcc metals is not well understood. Extensive experimental studies on aluminum and nickel have revealed these materials produce different results for both the macro- and microscopic investigations in terms of cyclic deformation and dislocation structure, respectively. In aluminum, it was found that the cyclic stress strain curve (CSSC) exhibited a linear relationship as the plastic strain amplitude increased with the saturation stress. Nickel exhibited a linear relation in two stages. It showed in the middle stage a plateau plastic strain amplitude that increases while saturation stress remains constant. Fatigue starts normally with the nucleation and accumulation of dislocations, which might form structures, such as the well-documented ladderlike structure in nickel observed by Blochwitz et al.[1] The ladderlike structure is found inside the persistent slip bands, which are very localized and carry most of the plastic deformation in a crystal under continued cyclic deformation. It was found that copper exhibited a similar CSSC to nickel, as observed by Morrison et al.[2] Polak et al.[3] concluded that polycrystalline copper and nickel exhibit similar cyclic deformation behaviors and demonstrated that similarities in the CSS behavior and evolution of dislocation structures exist. Marchand et al. and Tsou et al.[4,5] conducted fully reversed strain-controlled push-pull tests on polycrystalline commercial pure aluminum and studied the cyclic deformation behavior of pure polycrystalline aluminum over a range of strain amplitudes using conventional fatigue testing methods. The cyclic hardening response revealed a rapid increase in stress in the early stage of deformation and a gradual decreasing rate toward a steady state of stress A.O. MOHAMED and Y. EL-MADHOUN, Graduate Students, and M.N. BASSIM, Professor, are with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 2N2. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Fourth International Alloy Conference,’’ which occurred in Kos, Greece, from June 26 to July 1, 2005, and was sponsored by Engineering Conferences International (ECI) and co-sponsored by Lawrence Livermore National Laboratory and Naval Research Laboratory, United Kingdom. METALLURGICAL AND MATERIALS TRANSACTIONS A
amplitude response. In addition, the saturated stress amplitude increases with increasing plastic strain amplitude. The increase in stress was found to be faster at lower strain amplitude and became more gradual at higher strain amplitude. This observation has been made for most fcc polycrystals. Microstructural investigation showed that nickel consists of elongated dislocation structure. The objective of th
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