The growth of short fatigue cracks under compressive and/or tensile cyclic loading
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INTRODUCTION
IT has been known for some time that nonpropagating fatigue cracks, of the order of a millimeter in length, can form in the plastic zone at a notch root under cyclic compressive loading, m The driving force for the initiation of these cracks is the residual tensile stress that develops in the plastic zone in the rising portion of the loading cycle, i.e., upon unloading from the far-field compressive stress. Since the residual tensile stress field which is developed on the first unloading from the maximum compression load fails off with distance from the notch, this decrease might account for the eventual nonpropagation of the crack. This view was supported by Hubbard t2] who used a photoelastic technique to study the extent of the residual stress fields around notches. On the other hand, it is now known that in the short crack regime, crack closure develops with an increase in crack length, t3-91 and since this crack closure reduces the effective range of the stressintensity factor, arrest might occur simply because the continual increase in closure level with crack advance finally reduces the driving force for fatigue crack propagation to the threshold level. An example of the influence of closure has been provided by S u r e s h [41 who studied fatigue crack growth under cyclic compression loading in ASTM A542 class 3 steel. The fatigue cracks were formed at sharp notches (notch length 17.5 mm and notch tip angle 60 deg) in both compact-type (CT) and center-cracked-tension (CCT) specimens at an R (the ratio of the minimum to maximum stress in a loading cycle) value of I0. Crack closure was measured using compliance measurements obtained from strain gages mounted near the notch tips. The closure measurements indicated that the closure level increased up to 90 pet of the applied load range just prior to crack arrest. This work was extended by Holm e t a / . t91 to include the aluminum alloy 7075-T351. With crack advance, there was again a progressive increase in crack closure and a corresponding decrease in the extent of residual stress at the crack tip. For small cyclic load ranges, crack arrest occurred when the range of loading over which closure occurred approached the applied load range. It was also noted that the extent of crack growth increased with the increase in load range. A.J. McEVILY, Professor, and Z. YANG, Research Associate, are with the Metallurgy Department and Institute of Materials Science, University of Connecticut, Storrs, CT 06268. Manuscript submitted August 27, 1990. METALLURGICAL TRANSACTIONS A
As in tension-tension cycling, closure in compressioncompression cycling of a ductile material is due to fracture surface roughness augmented by oxidation and debris particles in the plane strain region and to plastic deformation in plane stress regions. However, in compression-compression loading, plasticity-induced closure might be more properly termed "plasticity-induced opening." For example, in tension-tension loading, the crack length at the surface usually lags that in the inte
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