Limits of isotropic damage models for complex load paths -- beyond stress triaxiality and Lode angle parameter

The stress triaxiality and the Lode angle parameter are two well established stress invariants for the characterization of damage evolution. This work assesses the limits of this tuple by using it for damage predictions via ductile damage models from continuum damage mechanics. Isotropic and anisotropic formulations of two well-established models are used to avoid model-specific restrictions. The damage evolution is analyzed for different load paths, while the stress triaxiality and the Lode angle parameter are controlled. The equivalent plastic strain is moreover added as a third parameter. These analyses reveal that this triple does still not suffice to uniquely define the damage state. As a consequence, well-established concepts such as fracture surfaces depending on this triple have to be taken with care, if complex paths are to be investgated. These include, e.g., load paths observed during metal forming applications with varying load directions or multiple stages.