Some observations on the assessment of short, sharp cracks at the roots of blunt notches, and their effect on brittle fracture

Abstract

The paper is concerned with effects of short, sharp quench cracks at the roots of blunt notches on cleavage fracture in a coarse-grained martensitic microstructure. At test temperatures between −196 and −140°C, such cracks control cleavage fracture. The values of local stress intensities calculated by loading such short cracks with the stress field ahead of the blunt notch are closely similar to the values of K IC measured in long-crack fracture toughness tests. At the test temperature of −100°C, such sharp cracks are blunted out (by ductile crack growth) and appear to play no part in the subsequent failure mechanism, which is now better characterised by values of the local cleavage fracture stress ω F * , below the notch. At the test temperature of −120°C, a ‘transition’ region is seen with a large observed scatter in faulure load. In a fine-grained martensitic microstructure tested in the ST-L orientation brittle plates of manganese sulphide inclusions have been shown to promote cleavage fracture at −196°C.