THE ISOTROPIC VISCOPLASTICITY THEORY-BASED ON OVERSTRESS APPLIED TO THE MODELING OF MODIFIED 9WT-PERCENT-CR-1WT-PERCENT-MO STEEL AT 538-DEGREES-C

The viscoplasticity theory based on overstress (VBO) is augmented to m odel the rate dependent behavior of modified 9wt.%Cr-1wt.%Mo steel at 538 degrees C. VBO is a unified state variable theory with growth laws for three state variables: the equilibrium (back) stress, the kinemat ic stress and the isotropic stress. The growth law for the equilibrium stress is formulated in a common hardening-recovery format. Its growt h is also indirectly influenced through the growth of the isotropic an d kinematic stresses which are made to depend on a newly formulated de formation history-modified accumulated inelastic strain. This variable allows modeling of permanent softening when loading takes place over a sufficiently long time. The present formulation models strain rate h istory dependence, cyclic softening which depends on rate, and seconda ry and tertiary creep at stress levels within the quasi-elastic region of the stress-strain diagram.