THE EFFECTS OF OVERLOADS AND THEIR INTERACTION ON A MILD STEEL

Abstract

Tensile and compressive overload tests were performed on a CSA G40.21 mild steel. A tensile overload (OR=2) arrested a crack at the near threshold region, whereas, at a high stress intensity only crack growth retardation, was observed. A compressive overload significantly decreased crack closure and caused a dormant crack to begin growing again. A compressive overload applied before a tensile overload had little effect on the tensile overload retardation. However, a compressive overload applied immediately after a tensile overload completely eliminated the tensile overload retardation. Periodic tensile overloads decreased crack propagation rates and increased the threshold stress intensity factor. Periodic compressive overloads increased crack propagation rates and decreased the threshold stress intensity factor. The crack propagation rate under periodic tensile-compressive overloads was higher than that under periodic compressivetensile overloads. The effect of both tensile and compressive overloads was most pronounced at low stress intensities and disappeared at high stress intensities. Fracture surface examination showed stretched tearing edges resulting from periodic tensile overloads, and flattened facets due to periodic compressive overloads, which are probably related to roughness induced crack closure.