CONSIDERATION OF PRIMARY CREEP AT A STATIONARY CRACK TIP - IMPLICATIONS FOR THE CT PARAMETER

A finite element creep analysis of a center crack specimen has been carried out under small scale to extensive creep conditions. The crack was assumed to be stationary. Several constitutive models were used; these consisted of elastic, power-law creep with and without rate-independent plasticity, as well as one which also included primary creep. The mechanics basis of the C//t parameter, which has been proposed for correlating creep crack growth behavior under conditions ranging from small scale to extensive creep, is explored. For the aforementioned specimen geometry, consideration of primary creep seems to explain the differences between the measured and previously calculated load line deflection rates based on power-law creep only. It is also concluded that in small scale creep, C//t does not characterize the instantaneous crack tip singular stress field, but it accurately characterizes the rate of expansion of the crack tip creep zone regardless of whether primary or secondary creep is occurring. This result provides a rationale for using C//t to correlate creep crack growth rates even in the presence of significant primary creep deformation.