Effects of State Recovery on Creep Buckling Under Variable Loading

Structural alloys embody internal mechanisms that allow recovery of state with varying stress and elevated temperature; that is, they can return to a softer state following periods of hardening. Such material behavior is known to strongly influence structural response under some important thermomechanical loadings; for example, those involving thermal ratcheting. Here, we investigate the influence of dynamic and thermal recovery on the creep buckling of a column under variable loading. The column is taken as the idealized (Shanley) sandwich column. The constitutive model, unlike the commonly employed Norton creep model, incorporates a representation of both dynamic and thermal (state) recovery. The material parameters of the constitutive model are chosen to characterize NARloy-Z, a representative copper alloy used in thrust nozzle liners of reusable rocket engines. Variable loading histories include rapid cyclic unloading/reloading sequences and intermittent reductions of load for extended periods of time; these are superimposed on a constant load. The calculated results show that state recovery significantly affects creep buckling under variable loading. Failure to account for state recovery in the constitutive relations can lead to nonconservative predictions of the critical creep-buckling time.