Development Of Constitutive Equations For Creep Damage Behaviour Under Multi-axial States Of Stress

A generic formulation for creep damage constitutive equations for multi-axial states of stress is proposed. First, the requirements of lifetime consistency and creep deformation consistency have been specified. Then two functions of states of stress were introduced to depict the influences of multi-axial states of stress on rupture lifetime and strain at failure, respectively; this is similar to that of separate yielding function and admissible function proposed and employed in the development of the generalised plasticity theory. Previous formulation and its applications are based only on lifetime consistency, and the fundamental requirement of creep deformation consistency has not been addressed, which results in incorrect prediction of creep strain and damage evolution. Furthermore, previous formulation is unable to consider the effect of mean stress on lifetime which is controversy to general conclusion. This paper presents a theoretical development through new formulation. Its super effectiveness is demonstrated by and compared with previous formulation through predicted strain at failure and lifetime under bi-axial cases. It is further concluded that this new formulation and associated concept and methodology is generic and could be applied in the development of other damage constitutive equations.