A Microcrack Description of Creep Damage in Crystalline Solids with Different Behaviour in Tension and Compression

A three-dimensional creep damage model for polycrystalline materials with parallel flat microcracks is presented, employing a damage vector. This model describes simultaneously damage induced anisotropy, different damage in tension and compression, and different creep properties in tension and compression. The equivalent stress in the creep potential contains joint invariants of the stress tensor and the damage vector. The proposed constitutive equation for creep together with the damage evolution equation take into account the primary, secondary, and tertiary creep. It is shown how three damage dependent functions in the proposed model can be determined from a series of basic experiments. The theoretical results have been compared with the experimental results for two- and three-dimensional stress states of various materials.