A statistical model for low temperature cleavage fracture in mild steels

Abstract Low temperature cleavage fracture of mild steels involves nucleation of cracks in grain boundary carbides and their propagation into the ferrite matrix. Recent models, for instance that of Lin, Evans and Ritchie [J. Mech. Phys. Solids34, 477 (1986)], regard crack propagation as the main controlling mechanism, and attribute the observed increase of fracture toughness with increasing temperature to the development of an elastic-plastic stress field at the crack tip. An alternative explanation is proposed here, namely that the controlling fracture mechanism changes from crack nucleation in the very low temperature range (−200 to −140C) to crack propagation in the higher range (−100 to −80C). A statistical model is developed separately for the controlling conditions of nucleation and propagation. Good agreement is found between the variation of fracture toughness with temperature experimentally determined by Lin et al. for a low carbon steel, and that predicted by the present model.