Crack deflection: Implications for the growth of long and short fatigue cracks
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I.
INTRODUCTION
CHARACTERIZATION of fatigue crack advance under Mode I loading conditions using the stress intensity factor range is based on the premise that the path of the crack is linear and that its plane of growth is normal to the loading axis. It is, however, well known that on a microscopic level, cracks seldom propagate in a linear fashion. In addition to the undulations in the fatigue crack path caused by the particular growth mechanism, pronounced deflection or branching of the crack can occur due to factors such as stress state, ~ environment, 2 load excursions, 3'4 and local microstructural discontinuities 5'6 like second phase particles and grain boundaries. There is a growing interest in the mechanics associated with the nonlinearities in crack path, 7-~~and an understanding of the crack deflection characteristics is essential for a variety of applications including toughening of brittle solids, n improving the fatigue resistance of commercial alloys, 5'~2 environmentally-assisted fracture, ~'~3and variable amplitude loading. 4'~4 Nonlinearities in crack path are generally ignored in the characterization of fatigue behavior because of the difficulties in incorporating crack meandering effects in estimates of the stress intensity factor range. In addition, local variations in the path of a long crack are normally overcome during subsequent propagation over a short distance. Existing evidence from both theoretical 7-~~ and experimental 4'~4 studies, however, indicates that deflections in crack profile result in marked alterations in fracture behavior in cyclic loading situations (e.g., in near-threshold and post-overload crack growth regimes), where crack growth along the deflected trajectory occurs over several thousand cycles. Severe crack deflection can occur due to mixed-mode cyclic loading, t5-~8Contradicting experimental results reveal S. SURESH, formerly with the University of California, Berkeley, is now Assistant Professor, Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted March 25, 1983. METALLURGICALTRANSACTIONSA
that the propagation rates of a Mode I fatigue crack can decrease, remain unchanged, or increase, when biaxial stresses are superimposed externally, is Several recent studies have attempted to characterize the fatigue crack propagation behavior of engineering materials under externally-imposed mixed-mode fatigue conditions involving either initially inclined cracks ~9'2~ or multiaxial loadsJ 5-~8 However, the important problem of local (near-tip) mixedmode growth of (nominally) pure Mode I fatigue cracks, induced solely by crack deflection, has received very little attention. 22'23It is the purpose of this paper to examine the role of crack deflection in influencing the growth of long and microstructurally-short fatigue cracks subjected to tensile (Mode 1) far-field loading conditions. II.
BACKGROUND
Figure 1 schematically shows two-dimensional models of typical fatigue crack deflection processes resulting in kinked, forked, multiple-kinked (
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