Formulation of Cross-Hardening in Creep and its Effect on the Creep Damage Process of Copper

The present paper is concerned with the modelling of creep and creep damage in polycrystalline metals and the experimental evaluation of the proposed model. By ascribing the reduction of creep rates caused by the principal stress rotation (i.e., cross-hardening) to the intersection mechanism of dislocations on active slip planes in crystal grains, a constitutive equation of creep describing the cross-hardening is first formulated. Then, in view of the metallurgical observations on the nucleation and the growth of grain boundary cavities in the creep damage process, an evolution equation of anisotropic creep damage is expressed as a function of the stress, a second rank damage tensor and the creep rate of the material. Finally, the validity of the proposed theory is discussed by performing systematic creep damage tests of thin-walled copper tubes under nonsteady multiaxial states of stress at 250°C.