The influence of plasticity in creep crack growth in steels

Abstract

Much of the reported work on assessing time-dependent (creep) crack growth has focussed on creep ductile materials, with the deformation conditions ranging from small-scale creep to extensive steady state creep. This previous research on crack initiation and growth has been mainly concerned with characterising creep crack growth rates using the C ∗ parameter. In this paper we interpret recent experimental results by exploring the contributions from elastic, plastic and creep deformation processes prior to and during crack growth. To assess the contribution of each component we use displacement partitioning. Elastic compliance functions are used to determine the elastic contribution. A key curve analysis is presented to determine the plastic contribution. The creep displacements are found by subtracting the sum of the elastic and plastic displacements from the total displacements. This approach is employed to explore the behaviour of a Carbon–Manganese (C–Mn) steel. Results show that after onset of crack growth the contribution of creep displacements is relatively minor for short-term laboratory tests, e.g. tests less than 2000 h. However, for longer-term tests it is evident that there is a greater contribution from creep. The implications of these findings are discussed in terms of using the C ∗ parameter. A simpler approach presented in this paper is to provide, irrespective of test duration, material resistance curves whereby material toughness is a function of time.