The effects of residual macrostresses and microstresses on fatigue crack propagation
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INTRODUCTION
Residual stresses may arise in a body from a wide variety of processing and/or service operations and are often present in materials. In structures undergoing fatigue loading, these residual stresses can alter the local loading conditions and affect fatigue behavior. Thus, many studies have been undertaken to examine the effects of residual stresses on fatigue crack propagation (FCP) of through-thickness cracks.[1–7] In general, compressive residual stresses are found to decrease FCP rates, while tensile residual stresses produce the opposite effect. This simple trend is not always observed, however, since residual stress effects depend critically on the type of existing residual stress field, its evolution during crack growth, and the material-dependent mechanisms driving fatigue. In polycrystalline and/or multiphase materials, residual stresses may be microstresses formed due to incompatibilities between grains or between phases and/or macrostresses formed by differential plastic deformation over a large scale relative to the microstructure.[8] Macrostresses are the stresses traditionally measured in techniques such as hole drilling and sectioning, while microstresses or the total (macro 1 micro) stress can only be determined through nondestructive techniques such as diffraction.[9] In steels, the individual contributions of macro- and microstresses on FCP have received little attention, because prior studies have often assumed, implicitly or explicitly, that the existing stresses were macrostresses. Bai-ping and Nian[10] observed increases in the fatigue threshold stress intensity DKth of martensitic steels with decreasing tempering time, which they attributed to the presence of beneficial microstresses within the martensite grains. However, these auJ.D. ALMER, formerly Research Assistant, Department of Materials Science and Engineering, Northwestern University, is Research Staff Member, Konstruktionmaterial IKP University of Linkoping, 581 83 Linkoping, Sweden. J.B. COHEN, Dean, Engineering, and Frank C. Engelhart Professor, Materials Science and Engineering, is with Northwestern University, Evanston, IL 60208. R.A. WINHOLTZ, Associate Professor, is with the Department of Mechanical Engineering, University of Missouri, Columbia, MO 65211. Manuscript submitted December 2, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
thors did not measure the macrostresses that might arise also during quenching and affect DKth, so it is difficult to unambiguously interpret their results. Microstresses have been found to affect mechanical behavior in many composite systems. For example, Shi et al.[11] found thermal residual microstresses cause asymmetric tension/compression deformation behavior in SiC-reinforced aluminum composites. In addition, microstresses have been found to influence microcracking in ceramics[12,13] and to increase fracture toughness in zirconia-based ceramics through stress-induced phase transformations.[14] Due to their long-range nature, macrostresses are typically treated as loading
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