Development of constitutive equations for creep damage behaviour under multi-axial states of stress

The two fundamental requirements for phenomenological creep damage constitutive equations for multi-axial states of stress are lifetime consistency and creep deformation consistency. However, these were not satisfied in previous formulation and a number of its variations as creep deformation consistency was not considered.
This paper reports a new formulation in which the above deficiency is overcome. The development of a set of creep constitutive equation was considered as a two-objective optimisation [25]. Specifically, two separate functions were introduced to depict the influences of states of stress on creep strain rate and creep damage evolution. With the chosen appropriate specific forms for these two functions, the characteristics and effectiveness of new formulation are demonstrated through predicted creep curves, strain at failure and lifetime under various bi-axial states of stress.