Modeling the anisotropic material response of paper accounting for damage progression coupled to plasticity

Paper is gaining more and more importance in numerous technically relevant applications, in particular in the packaging industry, because it is a renewable raw material and easily recyclable. Even though paper has been in use for around 2000 years, the material behavior and its modeling are still not sufficiently understood. The challenge in the material modeling of paper is its multiscale nature. The intrinsic microstructure, which consists of a fiber network, has a significant effect on the macroscale material response. The elastic-plastic behavior of the fibers, the breaking of individual fibers as well as of fiber bonds and friction between fibers result on the macro-scale in a combination of elasto-plasticity and progressive damage. In addition, due to the non-uniform fiber distribution these phenomena are anisotropic.The aim of this project is to understand the damage mechanisms evolving in paper and to develop a numerical modeling strategy that enables the description of the complex material behavior taking into account both anisotropic plasticity and anisotropic damage progression in a coupled manner. Further, the formulation has to be valid for large deformations, because those can occur during forming processes of paper.First, experimental investigations will be performed to characterize the material, where several new test set-ups need to be developed. Thereafter, a model for elasto-plasticity will be formulated for the large deformation regime. Then, a new anisotropic continuum damage model will be developed and calibrated to the experimental data. Moreover, the delamination between fiber layers will be described by a new cohesive law, which accounts for the fiber bridging effect. Finally, the modeling approach will be validated via comparison of experimental and numerical results for forming processes such as creasing of paper.