Dislocation distribution and prediction of fatigue damage
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1. I N T R O D U C T I O N m R E L I A B L E and practical means for forecasting fatigue failure through the measurement of accumulated damage has been sought for many years. Despite the considerable effort devoted to nondestructive evaluation of fatigue damage, however, only limited progress has been made toward correlating the cumulative damage to the amount of fatigue life actually expended. Since X-ray diffraction analysis provides a sensitive indication of structural changes due to deformation, a variety of diffraction methods has been applied to study fatigued metals and alloys. Many investigators have reported changes in the X-ray patterns during the early stages of cycling, including the appearance of asterism, line shifts and peak broadening. 2-8 For much of the subsequent fatigue life, however, the diffraction patterns were found to remain virtually unaltered, until the initiation of macroscopic failure. As a result of the invariance of the X-ray patterns during a very long period of cycling, from approximately 20 to 90 pct of the life, attempts to predict failure from any stage prior to the terminal one were unsuccessful. The current study was focused on achieving two primary goals: 1) to identify the differences in deforR. N. PANGBORN, formerly Research Associate, Rutgers College of Engineering, is now Assistant Professor, Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802, S. WEISSMANN is Professor, Department of Mechanics and Materials Science, College of Engineering, Rutgers University, Piscataway, NJ 08854, and I. R. KRAMER is Technical Advisor, Materials, David W. Taylor Naval Ship R&D Center, Annapolis, MD 21402. Manuscript submitted November 26, 1979. METALLURGICAL TRANSACTIONS A
mation response between the surface layer and bulk material for both single and polycrystalline metals, by determining the dislocation density and distribution at various depths into the sample, and 2) to provide an assessment of the localized deformation incurred by the individual grains of a fatigued alloy, and at the same time, to evaluate the overall, average fatigue damage in the surface and bulk regions, respectively. The first objective derived from a number of previous investigations which had indicated that when a metal is stressed, a surface layer that extends about 100 to 300/zm in depth work hardens to a greater extent than the bulk material. 9 ~2 High resolution stressstrain analysis, etch pit density measurements and transmission electron microscopy were employed to provide evidence of preferential dislocation accumulation and pileup at the surface as compared to the bulk. It was shown further that this surface layer also formed in commercial alloys during push-pull fatigue. 13,14The long range stresses introduced as a result of the surface layer formation were considered to influence to a large degree the dislocation multiplication and interaction in the bulk. In the present investigation, a method based on X-ray doublecrystal diffractometry (DCD)
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