Stress concentrations in hybrid unidirectional fibre-reinforced composites with random fibre packings

Strength models for fibre-reinforced composites often rely on the calculation of the stress concentrations around a single broken fibre. This paper presents the first results for stress concentrations in unidirectional hybrid composites, more specifically around a broken carbon fibre. The centres of the carbon and hybridisation fibres are randomly placed in a two-dimensional packing. The common assumption that both fibre types have the same fibre radii, is proven to lead to significant errors. The relative ratio of the volume fraction of the two fibre types only has a minor influence on the stress redistribution. A small increase of the stress concentration factors on both fibre types is noted with decreasing carbon fibre content. The ineffective length, which is a measure of the length of the influenced zone, remains unaffected. A stiffer hybridisation fibre reduces the SCFs on the hybridisation fibre, while this influence on the SCFs on carbon fibres is much smaller. The influence of the hybridisation fibre on the ineffective length is again small. The differences with existing literature are explained based on the more realistic packings in this paper. These results should now be implemented in a model to predict the influence on the strength of hybrid composites.