CRACK CLOSURE IN SMALL FATIGUE CRACKS - A COMPARISON OF FINITE-ELEMENT PREDICTIONS WITH IN-SITU SCANNING ELECTRON-MICROSCOPE MEASUREMENTS

A three dimensional, elastic-plastic, finite element analysis of. fati gue crack growth and plasticity-induced crack closure has been perform ed for a range of small, semi-circular cracks. Predicted crack opening displacements have been compared with data obtained from in-situ SEM measurements for a coarse-grained aluminium alloy 2024-T351. The magni tude of fatigue crack closure measured from in-situ SEM measurements w as consistently higher than that predicted from the finite element ana lysis. It is deduced that the higher closure stresses obtained from in -situ SEM measurements are due to the contact of asperities on the fat igue crack surfaces. A simple mathematical model is suggested to descr ibe the fatigue crack closure stress caused by the combination of both a plastic wake and asperities on the fatigue crack surfaces. The pred icted fatigue crack closure stresses and their dependence on crack siz e are consistent with experimental measurement.