Creep Crack Initiation and Propagation: Fracture Mechanics and Local Approach

Both the initiation and the propagation of macroscopic creep cracks have been studied in 316-L austenitic stainless steel, and, for comparison purposes, in 2219-T851 aluminum alloy. These alloys are, respectively, creep-ductile and creep-brittle. This difference in behavior is explained in terms of fracture mechanics concepts applied to creeping solids. The inability of fracture mechanics in providing unique correlations with K, C*, etc. . . for all the stages of both creep crack initiation and propagation is pointed out. Life prediction schemes using local rather than global fracture criteria are presented. A model based on creep ductility exhaustion concepts and the stress fields obtained by fracture mechanics is shown to provide good predictions for 316-L. Finite element analysis coupled to continuum damage mechanics is found to describe creep crack initiation in 2219-T851.