Strain-gauge compliance measurements near the crack tip for crack closure evaluation: applicability and accuracy.

The ‘‘plasticity-induced crack closure’’ phenomenon is the leading mechanism which controls the main effects on fatigue crack growth (e.g. stress ratio and load interaction effects) in metallic materials. Experimental tests, consisting mainly in global and/or local compliance measurements of the considered specimen, are usually carried out to quantify the physical phenomenon, but some aspects concerning the elaboration of acquired local compliance signals are not yet clear. From the analytical point of view, the so-called ‘‘Strip-Yield’’ model has proven to be the most versatile and powerful tool for estimating crack closure levels, but its application to structural steels is not yet straightforward due to the delicate calibration process. The present work tries to add some new ideas on the elaboration of local compliance experimental data, obtained from a M(T) specimen, simulating the measurements by means of an optimised Strip-Yield model implementation enriched by a novel module based on the Westergaard’s elastic complex potentials. The application of the method to the calibration of the Strip-Yield model has been already successfully faced elsewhere, so here the analytical results gave the possibility, together with dedicated FEM analyses, to investigate some of the different parameters and to state some conclusions about the reliability and applicability of local compliance measurements.