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

纸张在许多与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，很容易回收利用。尽管纸已经使用了大约2000年，但人们对纸的材料行为及其模型仍然没有足够的了解。纸张材料建模的挑战在于它的多尺度特性。由纤维网络组成的本征微观结构对材料的宏观响应有很大的影响。纤维的弹塑性行为、单个纤维的断裂以及纤维之间的粘结和摩擦在宏观尺度上导致弹塑性和渐进损伤的组合。此外，由于纤维分布不均匀，这些现象是各向异性的。本项目的目的是了解纸张中的损伤演化机制，并开发一种能够同时考虑各向异性塑性和各向异性损伤演化的复杂材料行为的数值模拟策略。此外，该公式必须对大变形有效，因为这些变形可能发生在纸的成形过程中。首先，将进行实验研究以表征材料，其中需要开发几个新的测试装置。然后，建立了大变形区的弹塑性模型。然后，建立了一个新的各向异性连续介质损伤模型，并对实验数据进行了标定。此外，纤维层之间的分层将用一个新的内聚力定律来描述，该定律解释了纤维的桥联效应。最后，通过对纸张折痕等成形过程的实验结果和数值结果的比较，验证了该建模方法的有效性。

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.