Creep crack growth behaviour and theoretical modelling

Abstract

The creep crack growth behaviour of structural alloys along with the theoretical developments in crack growth modelling are critically examined. When crack growth occurs predominantly by a deformation-controlled process, the accumulated results indicate that the growth rates may be a unique function of the energy-rate parameter, J* integral, independent of material and temperature. Significant deviations in the crack growth rate v. J* relations result either when environmental effects become rate controlling or when the material is so ductile that crack-tip stress relaxations occur rapidly before any significant crack growth. Implications of these results in terms of the existing theoretical models are presented. Analysis of load effects and transient effects, etc., indicate that it is appropriate to consider crack growth as a result of a kinetic balance of two or more competing processes: processes that contribute to crack growth such as void nucleation and growth, grain-boundary sliding, diffusion, creep and embrittlement due to environmental effects, and processes that inhibit the growth such as crack-tip blunting, stress relaxation by matrix deformation, formation of fissures, oxide strengthening, etc. The establishment of a steady-state condition, under which J* parameter can characterize the growth, then corresponds to a dynamic equilibrium state where stress and strain rates reach steady-state values for any given crack length. The present analysis indicates that future modelling should involve consideration of the existence of non-linear elastic stress field ahead of a creep crack.