Micromechanism of Fracture for Stage I Hydrogen-Induced Crack Growth

摘要

Definition of the micromechanism of fracture for stage I crack growth, and therefore threshold and the stage I/II transition, is of particular importance since this stage of crack growth is stress controlled instead of hydrogen transport controlled as often observed for stage II crack growth. Examination of hydrogen-induced crack growth data shows an average ratio of 1.38 exists between the stage I/II transition stress intensity (K/sub I/II/) and threshold (K/sub Th/). A quantitative model based on the stress intensity required for fracture nucleation along a single grain boundary, K/sub n/, and grain boundary orientation within the fracture process zone predicts the distribution of K/sub I/II//K/sub Th/ values in agreement with experimental data. The model shows that the fracture process zone size at the state I/II transition is twice that at threshold. Furthermore, it provides a basis for describing stage I crack growth in terms of the amount of fracture within the process zone. During stage I growth, the stresses are insufficient for fracture over two process zones (a prerequisite for crack advance). This is accomplished by an increase in the amount of fracture which shifts the stress distribution into the sample and promotes additional fracture within the process zone. (ERA citation 10:024033)