A three-dimensional, elastic-plastic, finite-element analysis of crack growth and closure under cyclic loading was developed and used to study the behavior of a crack in a finite-thickness middle-crack tension specimen. The finite-element model was composed of eight-noded isoparametric, hexahedron elements. The material was assumed to be elastic-perfectly plastic. Two load histories, constant-amplitude loading and a single-spike overload, were considered. The crack front was extended one element length per cycle when the applied stress reached the maximum level. The crack was assumed to grow as a straight-through crack.
Results show that during unloading the crack front closes first on the exterior (free) surface of the specimen and closes last in the interior. Under constant-amplitude loading with a maximum applied stress level Smax of 0.25 times the yield stress of the material, the crack-opening stress levels reached a peak value of 0.34 Smax and 0.6 Smax at the interior and exterior regions of the crack front, respectively. The application of a single-spike overload caused an immediate drop in the opening stress level, but as the crack grew, the opening levels (interior and exterior) rapidly rose to values higher than those calculated under constant-amplitude loading. The calculated crack-opening stresses for the exterior and for the interior of the specimen were in quantitative agreement with previous finite-element calculations under plane-stress and plane-strain conditions, respectively. The crack-opening stresses for the interior were much lower than the exterior values.