Measurement and modelling of near-tip displacement fields for fatigue cracks in 6082 T6 aluminium

Recent work by de Matos and co-workers has used digital image correlation to examine the surface displacements in the near-tip region of a propagating fatigue crack. The primary purpose of the experiments was to investigate crack closure, and successful measurements were made. However, the data collected can also be used to investigate the more general in-plane displacement field in the neighbourhood of the crack tip and to compare this with alternative models for near-tip displacements. Experiments were undertaken using CCT specimens manufactured from 6082 T6 aluminium alloy. Three different specimen thicknesses were examined (3, 10, and 25mm), in order to examine the effect of this parameter on the measured displacements, closure levels, and crack propagation rates. The current paper re-analyses the experimental measurements by evaluating crack face displacements as a function of loading for a region close (i.e. within 500 μm) to the crack tip. These provide an excellent means of validating proposed models of crack tip stresses, strains, and displacements. The measurements will be compared to the predictions of classical crack tip models (such as the Westergaard solution, together with a recent elastic-plastic model, proposed by Pommier and co-workers, which partitions the field into elastic and plastic components. The results of these comparisons are discussed and recommendations made for future experimental work.