Crack initiation and stable crack growth resistance in A508 steels in relation to inclusion distribution

Abstract

Several heats of A508 steel containing different volume fraction of MnS inclusions and different inclusion size have been investigated. Fracture toughness tests have been performed at 100°C using CT specimens cut in different orientations in order to determine the quantities relevant to the macroscopic characterization of crack initiation and stable crack growth, i.e. COD, J 1C and dJ da . The microscopic parameters characterizing inclusion distribution were determined using quantitative metallography. Large variations in the resistance to crack initiation and stable crack growth are found. These differences are empirically related to the inclusion distribution. A modified cavity growth model is utilized to describe the variations observed at crack initiation. This model is partly based on the results obtained on axisymmetric notched specimens which were used to evaluate the critical cavity growth at rupture. The results of numerical and analytical studies published in the literature are equally used to calculate cavity growth in front of a blunted crack. Finally a statistical approach incorporating the distribution of inclusions located in the vicinity of a crack tip is proposed.