Effect of Loading Conditions and Material Properties on the Relation between Crack Opening Distribution and Fatigue Crack Growth Rate

Fatigue crack growth tests were carried out on an annealed, pre-strained or quenched-and-tempered 0.45% carbon steel under rotating bending or reversed torsion. Crack opening displacement ranges (ΔCOD's) along a crack were measured in the neighborhood of the crack tip from the SEM photographs of the plastic replicas taken under the maximum and minimum stresses. The ΔCOD distributions under the same crack growth rate are almost the same in bending fatigue and torsional fatigue. Therefore, the ΔCOD distribution near a crack tip can be considered as a fundamental factor controlling consistently the crack growth rate, independent of loading conditions. The ΔCOD distribution under the same crack growth rate is the smallest in quenched-and-tempered steel, the middle in pre-strained steel and the largest in annealed steel. This suggests that the higher yield stress results in the smaller crack opening displacement under the condition of the same crack growth rate.