Strip yield models of creep crack incubation and growth

Abstract

The strip yield model of Dugdale and Bilby, Cottrell and Swinden is applied to analyse the incubation and growth of creep cracks. This extends previous work by Vitek (on incubation) and by Heaton and Chan (following To) on growth. Following both Vitek and Heaton and Chan, the main criterion of crack growth used is a COD-based one. The crack is assumed to grow at constant crack opening displacement (COD) φ=φc and the creep strain ∈t at the crack tip is assumed to be φ/h, where the gauge length is a material constant. The COD-based theory indicates that the incubation and failure timest i,t fare controlled by the stress intensity factorK at large stresses, but by the net stress at very low stresses. A weakness of the theory is that it neglects primary-creep effects: these have to be absorbed by modifying the secondary-creep index. If this is done, then comparison with two independent experiments shows moderately good agreement with the COD-based theory for botht iandt f.