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

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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.