A large amount of experimental studies have shown significant dependence of strength of ductile metals on stress state and stress history. These effects have to be taken into account in constitutive models and corresponding numerical analysis to be able to predict safety and lifetime of engineering structures in a realistic manner. In this context, the present paper deals with numerical analysis of the influence of the load path on damage and fracture behavior of aluminum alloys. A continuum damage model is discussed taking into account the effect of stress state and loading history on damage criteria and on evolution equations of damage strains. Experiments with the biaxially loaded H-specimen have been performed and different preloading histories have been taken into account. Evolution of strain fields is monitored by digital image correlation, and fracture modes are visualized by scanning electron microscopy (SEM). In addition, numerically predicted stress states are used to explain occurrence of different stress-state- and preloading-path-dependent localization behavior in critical specimens areas, as well as damage and fracture modes, revealed by SEM. The experiments with newly developed biaxially loaded specimens and corresponding numerical simulations show that the preloading history remarkably affects the occurrence of width and orientation of localized strain fields, as well as evolution of damage mechanisms and fracture modes. Therefore, characterization of materials must be based on an enhanced experimental program including biaxial tests with different loading histories. The observed damage and failure behavior can be predicted by the proposed continuum model taking into account stress-state-dependent damage criteria and damage strains.

The present paper comprises the experimental and numerical analysis of damage and fracture behavior of ductile metals under non-proportional loading with compression and shear preloading. For this purpose, biaxial experiments with the H-specimen using a pneumatic downholder for compression loading followed by failure under a tension stress state and corresponding numerical simulations are performed and analyzed. A thermodynamically consistent anisotropic continuum model is presented. It takes the effects of micromechanical damage mechanisms on the macro scale behavior as a motivation for a phenomenological description of damage behavior as a function of stress state. The experimental findings are compared with the results of tests with proportional load paths and corresponding numerical simulations. Thereto, strain fields of the critical areas of the specimens are analyzed by means of digital image correlation (DIC) technique. Compression and shear preloading can lead to significantly reduced ductility of the material with initiation of failure processes in the small damage strain range. Evolution of numerically predicted plastic and damage equivalent strains illustrates their stress state dependence. The numerical results are also confirmed by images of fracture surfaces taken by scanning electron microscopy (SEM). This experimental-numerical methodology is therefore an efficient tool to develop and validate general modeling approaches, and it is characterized by almost arbitrarily definable load paths with changes in stress states occurring in forming processes.