The role of continuum damage in creep crack growth

Theories that have been used to predict the rate of growth of cracks due to creep are reviewed and assessed. The need is expressed for a sounder understanding of the mechanisms by which creep crack growth takes place. The aim of this paper is to answer the question: can continuum damage mechanics provide the mechanism by which cracks grow by creep? The paper reports the results of theoretical and experimental studies on internally and externally cracked, plane strain, tension members, in an aluminium alloy, in copper and in 316 stainless steel, all of which undergo high temperature creep rupture under steady loads. Theoretical predictions of lifetimes, expressed as a representative rupture stress, of damage fields and of crack growth are made by using a previously developed finite element system (Hayhurst, Dimmer & Morrison, Phil. Trans. R. Soc. Lond . 311, 103 (1984)) based on the theory of continuum damage mechanics. The theoretical predictions are shown to be in close agreement with experimental observations. The effect of the growth of continuum damage is to produce considerable stress redistribution and to cause the nullification of stress singularities. The multi-axial stress rupture criterion of the material plays an important role in the determination of lifetimes and of the planes upon which crack propagation takes place. The numerical solution procedure is automatic but requires that the constitutive equations model the elastic response, the creep strain rates, including tertiary behaviour, and the multiaxial stress rupture criterion of the material at the appropriate stress levels. Continuum damage mechanics theory is shown to be capable of modelling the propagation of cracks through material which has suffered relatively low damage.