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

纸在许多技术相关的应用中越来越重要，特别是在包装行业，因为它是一种可再生的原材料，易于回收。尽管纸已经使用了大约2000年，但材料的行为及其建模仍然没有得到充分的理解。纸的材料建模的挑战在于其多尺度特性。由纤维网络组成的本征微结构对宏观尺度材料响应具有重要影响。纤维的弹塑性行为、单个纤维的断裂以及纤维结合和纤维之间的摩擦在宏观尺度上导致弹塑性和渐进损伤的组合。此外，由于纤维分布的不均匀，这些现象是各向异性的。本项目的目的是了解纸上的损伤机制，并制定一个数值模拟策略，使复杂的材料行为的描述，同时考虑各向异性塑性和各向异性损伤进展耦合的方式。此外，该公式必须适用于大变形，因为这些变形可能发生在纸的成型过程中。首先，将进行实验研究以表征材料，其中需要开发几种新的测试装置。此后，将制定一个模型的弹塑性大变形制度。然后，一个新的各向异性连续损伤模型将开发和校准的实验数据。此外，纤维层之间的分层将由一个新的内聚定律来描述，该定律解释了纤维桥接效应。最后，通过比较纸张压痕等成形过程的实验和数值结果，验证了建模方法的有效性。

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.