FE implementation of damage-coupled inelastic constitutive equation for P91 steel

The objective of this study is to numerically evaluate the cyclic softening behavior and creep-fatigue damage of P91 steel. To achieve this, a damage-coupled inelastic constitutive equation is formulated by coupling the Chaboche-type viscoplastic constitutive equation and the damage evolution equation, and aging stresses are considered with back stresses to describe the cyclic softening behavior of P91. In accordance with the damage mechanics concept, the damage parameter D is incorporated in the inelastic constitutive equation to represent the creep-fatigue damage progress. The discretization equations of the damage-coupled inelastic constitutive equation are derived according to the backward Eulerian method which was proposed by Kulling and Wippler (2006). A creep-fatigue analysis is performed on the one-element model using the developed finite element (FE) analysis software. It was confirmed that the hysteresis loops, stress relaxation curves and creep-fatigue life obtained from the FE analysis are in good agreement with the test results.