Abstract
This paper proposes a three dimensional model to explain microscopic behavior of cleavage crack propagation in steel. This model is based on a fracture mechanics model, which assumes a crack propagation along the cleavage plane with the highest tensile normal stress among three {100} planes in a grain. For calculating the normal stresses on the {100} planes, local stress intensity factors need to be calculated. The model considered three factors for estimating the local stress intensity factors, i.e., non-straight crack front, irregular crack surface and unfractured ligament between the grains and approximate calculations were carried out. The results are compared with observations of Charpy impact specimen fracture surfaces by conventional SEM as well as 3D SEM. The comparison showed good agreement between the calculations and the experiments. It was demonstrated by the present model that grain size and critical value of shear fracture had influence on cleavage crack propagation direction, discontinuous boundary and tear ridge formation.
