Modelling of Creep Crack Growth using Fracture Mechanics and Damage Mechanics Methods

Methods for modelling of crack growth and prediction of the lifetime of structure operated at creep conditions are presented in the paper. Analyses were performed at fracture mechanics (FM) and damage mechanics (DM). In the case of FM, the C*-integral and C(t)-integral were used. Two models were considered: without and with damage development in the vicinity of crack tip. For DM models, the Kachanov equation was chosen as the basic one. To avoid the strain and damage concentration and thus the mesh-dependence of the solution, the non-local integral method was applied. Since the damage model is stress-dependent, strains cannot be used as an averaged parameter; nevertheless, damage increment was used. The simulations of creep crack growth were performed for a compact tension specimen made of 316 stainless steel at a temperature of 550°C and their results were compared with experimental results. The FM method in a damaged environment and the non-local DM method were identified as the most promising. As the results for time to failure were ambiguous, the safety margin and critical crack length were recognised as parameters that are useful in safety analysis in the case of creep crack growth.