The relationship of grain size to critical distance and hydrogen-induced crack growth thresholds

Abstract

IN903 thresholds were found to increase as grain size increased at a constant yield strength. This leads to the conclusion that grain size establishes the minimum critical distance over which the fracture stress must be exceeded for cracking to occur. A first order normalization with respect to yield strength shows that the effect of grain size on thresholds for IN903 and 4340 was the same although they have different crystal structures. It was also shown that a local critical stress model with the critical distance assumed to be equal to the grain size and reasonable assumed fracture stress values predicts the lower limiting threshold behavior found in alloys such as 4340 at high yield strengths. However, at lower yield strengths the critical distance extends several grain diamters. Under these conditions, the critical distance is no longer equivalent to a basic microstructural dimension and is dependent on other material properties such as yield strength and fracture stress. As a consequence, use of the grain size as the critical distance for predicting sustained crack growth thresholds gives reasonable values at high yield strengths and conservative values at lower yield strengths. Accurate modelling of thresholds at lower yield strengths, particularly from a predictivemore » standpoint, requires more theoretical and experimental work to define the relationship between the critical distance for sustained crack growth and material properties.« less