THE INFLUENCE OF THE HARDENING STATE ON TIME-DEPENDENT DAMAGE AND ITSCONSIDERATION IN A UNIFIED DAMAGE MODEL

A model is presented for the prediction of the lifetime of metals in t he high-temperature range under arbitrary variable multiaxial load. Th e definition of an internal variable for damage in continuum damage me chanics is adopted, which allows indirect measurement of damage via th e deformation behaviour. To acquire some knowledge of damage evolution , damage is measured in two ways during uniaxial strain controlled cyc lic tests: (a) a change of the modulus of elasticity and (b) a decreas e of the peak stress. Surprisingly, both methods lead to results which are in goad agreement. A new damage law is then developed (with refer ence to known models and lifetime rules) which is a modification of th e creep damage law of Rabotnov that is extended by a dependence on the inelastic strain rate instead of the dependence on internal variables to take into account the hardening state. Uniaxial as well as multiax ial formulations of the new damage model (Inelastic Strain Rate Modifi ed (ISRM) model) are presented. The parameters of the ISRM model are d etermined with a view to applying them to AISI 316 L(N) austenitic ste el. Some of the parameters are derived from standard creep experiments . To determine further parameters, the ISRM model is applied to uniaxi al cyclic tests. Both failure behaviour and damage evolution are well described.