Experimental and numerical damage investigations on the unidirectional layer-level of short glass fibre reinforced plastics under fatigue loading considering hysteresis behaviour and micro-mechanical modelling

短玻璃纤维增强热塑性塑料的特征在于异质材料结构。材料的机械性能是由纤维和基体性质的相互作用以及纤维的微观结构（质量分数、长度分布和取向）决定的。注塑成型过程中与加工和几何形状相关的不同流动条件导致厚度方向上的层结构具有不同的纤维取向，由此机械材料行为将变得各向异性。此外，热塑性基体的材料行为的特征在于时间和温度依赖性。因此，当施加外部载荷时，在微机械材料结构内形成复杂的应力状态。微观层面上的相互作用过程决定了宏观复合材料的行为。超过局部材料极限会导致微观机械损伤，从而逐渐降低机械性能。如果材料还承受疲劳载荷，则也会发生与周期相关的裂纹扩展。为了能够充分利用短玻璃纤维增强塑料的材料潜力，通常适用的和材料适当的失效模型是必要的。然而，对平行或连续损伤机制的描述，特别是在疲劳载荷的情况下，仍然是一个具有挑战性和未解决的任务。这正是计划中的研究项目发生的地方。为了更好地理解微观力学机制和相互作用，使用均匀高度取向的试样，其在整个厚度上表现出均匀的纤维取向。这意味着层间相互作用不再存在，并且可以在单向纤维层中研究损伤机制。此外，可以将宏观材料行为直接与微观机械应力状态相关联。因此，在本研究项目中，Wöhler试验是在均匀高度取向的试样上进行的，并为数值研究开发了具有相同材料配置的代表性体积单元。这使得它可以模拟局部应力，并在微观水平上研究材料损伤和粘弹性引起的相互作用过程。这为开发普遍适用和全面的材料和疲劳模型提供了基础。

Short glass fibre reinforced thermoplastics are characterised by a heterogeneous material structure. The mechanical material behaviour results from the interaction of the fibre and matrix properties as well as from the micro-structural fibre configuration (mass fraction, length distribution and orientation). The processing and geometry-related different flow conditions in the injection moulding process lead to a layer structure in thickness direction with differently developed fibre orientation, whereby the mechanical material behaviour will become anisotropic. In addition, the material behaviour of the thermoplastic matrix is characterised by time and temperature dependencies. Consequently, when an external load is applied, a complex stress state develops within the micro-mechanical material structure. The interaction processes on the micro-level determine the macroscopic composite behaviour. Exceeding local material limits leads to micro-mechanical damage, which gradually reduces the mechanical properties. If the material is also subjected to a fatigue load, cycle-dependent crack growth also occurs. In order to be able to fully utilise the material potential of short glass fibre reinforced plastics, generally applicable and material-appropriate failure models are necessary. However, the description of the parallel or successive damage mechanisms, especially in the case of fatigue loading, still represents a challenging and unsolved task.This is exactly where the planned research project takes place. In order to better understand the micro-mechanical mechanisms and interactions, a homogeneously highly oriented test specimen is used, which exhibits the uniform fibre orientation across the thickness. This means that the layer interaction is no longer present and the damage mechanisms can be investigated in the unidirectional fibre layer. Furthermore, it is possible to correlate the macroscopic material behaviour directly with the micro-mechanical stress state. Therefore, in this research project, Wöhler tests are carried out on homogeneously highly oriented specimens and representative volume elements are developed for numerical investigations, which have the same material configuration. This makes it possible to simulate the local stress and to investigate the interaction processes caused by material damage and viscoelasticity on the micro-level. This provides a foundation for developing generally applicable and comprehensive material and fatigue models.