Numerical and experimental analysis of the mechanical response of thin-ply cross-ply composites

Examensarbete för masterexamen
Master Thesis
Hagvall, Robin
Johansson, Simon
One of the major strivings of today is to reduce the emission of green house gases. Within the transport vehicle industry this can be achieved by reducing their structural weight, which lowers the fuel consumption. That strive motivates the use of fibre reinforced polymers, which offers an increased weight to stiffness ratio compared to metallic materials. Moreover, it has been shown that further advantages can be achieved if the thickness of certain plies in a composite is reduced. More specifically, if the thickness of a transversely loaded ply embedded in a multidirectional laminate is reduced, the onset of transverse cracks can be substantially delayed. This is called the in-situ effect. The aim of this project is to, with one experimental study and one numerical study, investigate this in-situ effect for thinner plies for which it has not yet been fully established. The thickness range considered is between 20 μm and 240 μm. For the experimental study test specimens are manufactured and subjected to a tensile load while the edges are inspected for cracks. The numerical analysis is conducted within the framework of Abaqus, where a cohesive zone model is developed in combination with extended FEM. The outcome of the experiments show a small delay in the onset of transverse cracks when the thickness of the 90°-ply is reduced. For some cases it is also seen that the crack density decreases when the thickness is reduced. Finally, the stiffness of the adjacent plies does not seem to have a large impact on the in-situ effect. However, difficulties related to manufacturing complicates the interpretation of those results and their validity can be questioned. The numerical study shows an in-situ effect in a model in which no in-situ properties are used. For the thinner case, the result aligns well with analytical models. It is shown that an extremely fine mesh is necessary in order to resolve the crack zone correctly, and the dependency of input variables such as the critical energy release rate and interface strength is demonstrated. It is also shown that for one of the studied material a transition from stable to unstable crack growth occurs at the thickness 80 μm, where a crack grows stably for thinner plies and unstably for a thicker plies. Taking into account the manufacturing complications, the results still, on the whole, indicate that the in-situ effect exist for the thin-ply composites studied in this project. This further motivates the development and research connected to thin-ply composites.
Materialvetenskap, Produktion, Maskinteknik, Materialteknik, Materials Science, Production, Mechanical Engineering, Materials Engineering
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