Fatigue Crack Closure and Crack Growth Outside the Small Scale Yielding Regime
by
R. Craig McClung
Department of Mechanical and Industrial Engineering
Abstract
The changes in crack opening stresses and the effects of these changes on crack growth rates are investigated for several problems in fatigue cracking outside the regime where small-scale yielding assumptions are strictly valid. The primary tool used here for the determination of opening stress, Sopen, is an elastic-plastic finite element simulation of fatigue crack growth. Computations are performed on a CRAY X-MP/48 supercomputer. Several modeling issues are investigated, including the effects of mesh design, crack advance scheme, material properties, and constitutive model on the resulting values of Sopen. Special attention is given to the dependency of Sopen/Smax on Smax, the maximum stress. Crack opening behavior is interpreted and explored in terms of information about crack opening displacements, crack-tip plastic zones, and stress-strain histories. Parameters which might be used to correlate crack growth rates under these conditions are critically reviewed, and the potential role of closure in these parameters is explored. Fatigue crack growth data obtained under conditions of intermediate- and large-scale yielding and biaxial loading are successfully correlated only when closure-modified parameters are employed. The changes in Sopen for a crack growing from a hole are investigated. Simple models are developed to correlate this particular crack opening behavior and the associated accelerated crack growth rates.
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