Consideration of Stress State Influences in the Material Modelling of Creep and Damage

Abstract

Creep tests performed for many engineering materials show an independent behaviour on the kind of loading during the primary and secondary creep. In this case the creep curves obtained for tensile or compressive loads are approximately the same (only the sign is different), the equivalent strain vs. time curves for tension, compression and torsion are identical, the influence of the mean stress can be ignored, etc. From this follows that the classical von Mises concept can be used in the constitutive modelling. On the other hand, the tertiary creep is strongly influenced by the kind of stress state because, for instance, of the voids opening and closing.

Considering constant temperature, small strains and isotropic behaviour a set of constitutive and evolution equations describing the creep-damage including non-classical effects is introduced. The creep equations are based on a potential depending on three linear-independent stress tensor invariants. The damage is taken into account using the equivalent stress concept. The unknown coefficients in the equations must be identified by tests. The identification procedure is discussed, and as the result the coefficients as dependencies of the material properties are defined.