Neutron and X-ray Microbeam Diffraction Studies around a Fatigue-Crack Tip after Overload
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THE accurate understanding of the micromechanism for the load-interaction effects during fatigue crack growth is essential for the damage tolerance design and the development of the lifetime prediction model. One aspect that is still not fully understood is the overload effect and crack closure behavior in the structural materials subjected to cyclic loading. A variety of crack closure measurements have been used to investigate the crack growth retardation mechanisms for structural materials.[1–9] However, the various closure measurements between the surface and bulk resulted in different closure levels.[10] In addition, due to a lack of experimental capabilities to measure strain/stress fields within the bulk under the applied load, the relationship S.Y. LEE, Y. SUN, and L. LI, Graduate Research Assistants, P.K. LIAW, Professor and Ivan Racheff Chair of Excellence, and C. FAN, Research Professor, are with the Department of Materials Science and Engineering, The University of Tennessee, TN 37996. Contact e-mail: [email protected] R.I. BARABASH, Research Professor, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, and the Center for Materials Processing, The University of Tennessee, Knoxville. J.-S. CHUNG, Associate Professor and Department Head, is with the Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea and the Materials Science and Technology Division, Oak Ridge National Laboratory. H. CHOO, Associate Professor, is with the Department of Materials Science and Engineering, The University of Tennessee, and the Materials Science and Technology Division, Oak Ridge National Laboratory. D.W. BROWN, Instrument Scientist, is with the Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, NM 87545. G.E. ICE, Group Leader and Corporate Fellow, is with the Materials Science and Technology Division, Oak Ridge National Laboratory. This article is based on a presentation given in the symposium entitled ‘‘Neutron and X-Ray Studies for Probing Materials Behavior,’’ which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee. Article published online August 5, 2008 3164—VOLUME 39A, DECEMBER 2008
between overload and retardation has not been quantitatively established. Recently, a neutron-diffraction measurement was performed to probe the crack closure phenomena after an overload during fatigue crack growth.[11,12] The deep penetration capability of neutrons enables the nondestructive studies of the bulk crack closure behavior as compared to the surface crack closure phenomena observed using strain gauge.[4] Furthermore, the changes in internal strains can be measured in situ under the applied load using the load frame as a function of the distance from the crack tip. At the same time, the dislocation density can be carried out from
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