Closure of fatigue cracks at high strains

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Virginia Polytechnic Institute and State University

Experiments were conducted on smooth specimens to study the closure behavior of short cracks at high cyclic strains under completely reversed cycling. Testing procedures and methodology, and closure measurement techniques, are described in detail. The strain levels chosen for the study cover from predominantly elastic to grossly plastic strains. Crack closure measurements were made at different crack lengths. The study reveals that, at high strains, cracks close only as the lowest stress level in the cycle is approached. The crack opening was observed to occur in the compressive part of the loading cycle. The applied stress needed to open a short crack, under high strain was found to be less than for cracks under small scale yielding. For increased plastic deformations, the value of σopmax is observed to decrease and approaches the value of R. Comparison of the experimental results with existing analysis has been made and indicates the limitations of the small scale yielding approach where gross plastic deformation behavior occurs.