Three-Orthogonal-Direction Stress Mapping around a Fatigue-Crack Tip Using Neutron Diffraction
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NTRODUCTION
PREVENTING fatigue-damage failure is extremely important, especially in many engineering applications. Mughrabi[1] concluded that the dislocation activity is the dominant mechanism for the cyclic-loading-induced damage. Guinier and Julien[2] addressed the importance of the microcracks during fatigue degradation. Kuhlmann-Wilsdorf,[3] Hansen,[4] and Vasudevan and Sadananda[5] pointed out that the atomic structures responsible for various fatigue damages are related to the microstructure evolution subjected to various cyclic loadings. One of the key parameters to describe the cyclic loading effect is the fatigue-crack behavior.[6–10] It is reported that the formation of a fatigue crack might begin as early as 5 pct of fatigue lives.[7,9,11] Although people noticed the importance of investigating the fatigue degradation mechanisms during the past decades, when and how the fatigue cracks exactly affect the fatigue behaviors is still an emergent issue for both scientific research and daily applications. Among the fatigue-crack research, the fatigue-crackpropagation phenomena and their theoretical models have been widely investigated.[12–17] Upon the practical E-WEN HUANG, Assistant Professor, and KUAN-WEI LEE, Graduate Student, are with the Department of Chemical and Materials Engineering and Center for Neutron Beam Applications, National Central University, Jhongli City, 32001 Taiwan, ROC. SOO YEOL LEE, Postdoctoral Fellow, is with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada, and is also with the Canadian Neutron Beam Centre, National Research Council Canada, Chalk River, ON K0J 1J0, Canada. Contact e-mail: [email protected] WANCHUCK WOO, Instrument Scientist, is with the Neutron Science Division, Korea Atomic Energy Research Institute, Daejeon 305-353, South Korea. Manuscript submitted March 31, 2011. Article published online September 17, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
applications, the stress and strain distribution around the crack tip is the most critical issue, because it is strongly nonlinear due to large deformation.[13,14] Because of the nonlinear strain distribution around the crack tip, the direct experimental investigation of the strain/stress distribution around a fatigue crack tip is necessary for further research. A variety of nondestructive diffraction techniques (e.g., the X-ray diffraction and tomography[18–23] and neutron diffraction[16,17,24– 30] ) were used to directly measure internal strain/stress fields in the bulk sample during fatigue. Many efforts were applied to investigate the deformation mechanisms and the local microstructure around the crack tip. Lavine et al.[12] showed the displacement field surrounding the crack tip within several millimeters. Theoretically, Buehler et al.[13–15,31] modeled the strain distribution in the plane of the crack in a range between Angstrom and submicrometer. They also demonstrated the importance of considering the grain orientation at the crack tip. So far, the measurements
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