Size Effect on Cleavage Crack Initiation Sites and Fracture Toughness. 1st Report. Result on Ductile-Brittle Transition Temperature Region of SM400C Steel.

Abstract

Fracture toughness tests were performed at -50°C in the ductile-brittle transition temperature region for a low-strength steel SM400C using 0.3T, 0.5T, 0.8T and 1T compact specimens. All the locations of the cleavage crack initiation sites and fibrous crack shapes were investigated in detail from the fracture surface using by a scanning electron microscope and a binocular stereoscopic microscope. The probabilistic characteristics of cleavage fracture toughness and the thickness effect of cleavage crack initiation sites were analysed systematically. All cleavage crack initiation sites were found within a certain distance from the specimen surfaces. By locating all the sites and investigating fibrous crack shapes, the plane strain regions in specimens were deduced. The criterion for obtaining plane strain at the midplane of the specimens is given by B≥0.004(KC(J)/σys)2+0.01. This criterion indicates the specimen thickness necessary for the plane strain condition to be fairly smaller than the size requirements in ASTM Standard E399.