Near threshold fatigue crack growth behaviour of a high strength steel: the effect of prior austenitic grain size

Near threshold fatigue crack propagation behaviour of a high strength steel under different prior austenitic grain sizes with constant tensile properties was investigated. Attention has been paid to the observation of facture modes at thresh-; old in different grain sizes. The observations indicate that three distinct modes of fracture can occur at threshold depending on the resistance offered by the grain boundary to the plastic relaxation of crack tip stress state by transgranular crystallographic slip. A low value of threshold was encountered in fine grain sizes due to limited slip distance where; as higher thresholds were seen in coarse grain sizes where dislocation pile-up and extensive plastic relaxation a t the: crack tip can occur to give rise to crack path tortuosity and crack blunting respectively.