Experimental and numerical investigation of thickness effects in plasticity-induced fatigue crack closure

The understanding of fatigue crack closure has been proved to be a challenging and controversial topic among the fatigue community over the last three decades. The effect of the specimen (or component) thickness has been shown to have a significant effect on closure behaviour and this seems to be related to the relative size of the plastic zone. Real cracks are inherently three-dimensional; plane stress-like behaviour is found close to the region where the crack front intersects the free surface, whereas most of the crack front will experience something close to plane strain. The aim of the present work is to investigate the influence of specimen thickness on closure behaviour (both close to and remote from the surface) and on fatigue crack propagation. The paper will present results from a simple experimental program, which consists of fatigue testing CT specimens with different thicknesses. Fatigue crack propagation is measured optically. Crack closure is assessed using traditional compliance techniques (clip gauge and back face strain gauge) and Digital Image Correlation methods. Experimental results are compared with two and three-dimensional simulations of plasticity-induced fatigue crack closure. The implications of thickness effects for predicting the propagation of three-dimensional fatigue cracks are discussed.