MULTIAXIAL CYCLIC RATCHETTING UNDER MULTIPLE STEP LOADING

Strain ratchetting responses of 1070 steel are reported for multiple s tep cyclic loading histories. The stress amplitude and mean stress are varied between loading steps in multiple step loading. Experimental r esults reveal that the material exhibits a strong memory of the previo us loading history, and such memory plays a discerning role on the sub sequent ratchetting. The material could ratchet in the opposite direct ion to the mean stress or could reverse its ratchetting direction with time. The origin of the ratchetting transients has been linked to the variation of the plastic modulus within the loading cycle for proport ional loading and the noncoincidence of the plastic strain rate direct ion and yield surface translation direction for nonproportional loadin g. Many of the constitutive relations proposed for cyclic loading are not designed to handle the ratchetting evolution. Based on the Armstro ng-Frederick hardening algorithm, the model forwarded by Bower can qua litatively predict the ratchetting directions for certain multiple ste p loading cases, but the predicted ratchetting rates differ from the e xperimental values. The Ohno-Wang model, which introduces threshold le vels of dynamic recovery in nonlinear hardening, can simulate negative ratchetting under positive mean stress, or vice versa, as well as the ratchetting direction reversal during step loadings. This model can p rovide results that agree with experimental observations for a class o f nonproportional cases, where the plastic strain rate direction and y ield surface translation direction are noncoincident. Its performance deteriorates for proportional loading.