On the blocking effect of grain boundaries on small crystallographic fatigue crack growth

Abstract

Small fatigue crack growth is often characterized by anomalous acceleration and deceleration growth patterns due to the presence of microstructural barriers such as grain boundaries. A straightforward method of calculating local Schmid factors in front of small crystallographic fatigue crack tips as they approach and cross grain boundaries has been developed. The method is based on an analysis of the stress distribution ahead of a crack, in conjunction with two further factors: the bulk Schmid factor and the crystallographic misorientation across the grain boundary. The crystallographic features of small crack growth and their influence on deceleration when a crack approaches a grain boundary were determined experimentally. It was found that the difference between the Schmid factors of the slip system containing the crack and the most favourably oriented slip system in the neighbouring grain plays an important role in crack growth retardation as the crack approaches the grain boundary. The experimental results also indicated that cracks prefer to propagate along slip systems with high Schmid factors when transferring from one grain to another. The predictions of the analysis are compared with experimental observation and good agreement is achieved.