A study of failure in the post yield regime using single edge-notched tension specimens

Abstract

Three post yield analyses have been performed on fracture data obtained from single edge-notched tension specimens of widths 25, 15 and 8 mm and constant thickness 25 mm made of a 1% Cr MoV high temperature bolting steel. The three methods were: the compliance technique, and two curve fitting procedures which utilize solutions to the Bilby, Cottrell and Swinden-Dugdale model and are based upon the load/gauge length displacement and the load/external crack opening displacement. All three methods gave results in good agreement with each other. The fracture toughness values determined increased with decreasing crack length, the toughness for the shortest crack length (2.5 mm) was twice that for the longest (10.5 mm). This is associated with an observed increase in ductility on the fractured surfaces which were still predominantly cleavage in appearance. This rise in toughness value occurs for crack sizes from 6 to 8 times the characteristic length {ie233-1}. In ferritic steels the results imply that post yield failure analyses which use valid toughness values for assessing the integrity of plant containing shallow cracks may underestimate critical defect sizes and fracture loads. Hence safe results will be obtained provided failure is predominantly by a cleavage mechanism. There is also the possibility that toughness values obtained from data on invalid specimens may be greater than the fracture toughness values obtained from valid tests at the same temperature.