Three-Dimensional Visualization of the Interaction between Fatigue Crack and Micropores in an Aluminum Alloy Using Synch
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CAST-ALUMINUM alloys exhibit excellent casting characteristics, high strength, high ductility, and high corrosion resistance.[1,2,3] The performance of these alloys is usually limited by the presence of cast defects, such as porosity, which strongly influence the fatigue behavior of these materials.[4,5,6] For ductile metals and alloys, the fatigue damage process can be generally divided into three stages, i.e., crack initiation, shortcrack propagation, and long-crack propagation.[7] The acceleration of fatigue-crack initiation has been observed in some studies on high-cycle fatigue-life behavior, which is due to local stress concentration developed at pores and corresponding localized slip during cyclic loading, although the remotely applied stress is elastic.[8,9] Additionally, the existence of porosity could also cause the reduction of low-cycle fatigue life by strain localization near or between pores.[10,11] All these observations have indicated that acceleration in the crack-initiation stage due to the presence of porosity is an important factor influencing both high- and lowcycle fatigue lives. H. ZHANG, Research Fellow, H. TODA and T. KOBAYASHI, Professor, M. KOBAYASHI, Lecturer, and H. HARA, Master’s Student, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan. Contact e-mail: [email protected] or dhuizhang @yahoo.com D. SUGIYAMA and N. KURODA, Researchers, are with Topy Industries, Limited, Toyokawa, Aichi 442-8506, Japan. K. UESUGI, Researcher, is with the Japan Synchrotron Radiation Research Institute, Mikazuki, Sayo-gun, Hyogo 679-5198, Japan. Manuscript submitted October 3, 2006. Article published online July 6, 2007. 1774—VOLUME 38A, AUGUST 2007
Among the three stages of fatigue damage, smallcrack growth remains one of the most important research areas in fatigue research, because small cracks interact strongly with the microstructure during crack growth.[12,13,14] The existence of porosity makes the question of small-crack propagation more complicated. In cast aluminum, the acceleration or retardation of small-crack propagation is usually associated with microstructural constituents, such as eutectic silicon particles or porosity.[15,16] There have been few direct observations of the influence of porosity on small-crack propagation behavior. However, in some similar cases, acceleration of small-crack propagation has been observed near notches in high-cycle fatigue, due to accelerated damage in regions of plastic strain that develop near porosity.[17,18] Although acceleration of crack-initiation processes have been observed in both low- and high-cycle fatigue, investigations of high-cycle fatigue tests have indicated that porosity appears to have little influence on the propagation rate in the overall crack stage. This was explained as due to the increased length of the crack deflection toward porosity as well as the increased roughness-induced crack closure that results from crack deflection.[8,9,19] Pendse and R
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