The effect of variable fibre diameters in unidirectional fibre-reinforced bundles on stress redistributions around fibre breaks

Finite element modelling is conducted to simulate the stress redistribution around a broken fibre (BF) in a bundle with experimentally measured fibre diameter distributions (FDD), followed by a parametric study of the influence of the FDD coefficient of variation on the stress concentration factor (SCF) and ineffective length (IL). Two variants of the SCF definition are considered: based on average and maximum stress in the fibre cross-section. Results demonstrate that bigger fibre diameters show higher SCF and clustering of such fibres increases SCF in nearest neighbour fibres (NNFs). Critically, maximum stress-based SCF (maxSCF_max) significantly exceeds average stress-based SCF (maxSCF_avg), with differences about 40 to 75% in NNFs for FDD bundles compared to FCD bundles. This emphasises the necessity of prioritizing maximum stress criteria over conventional average stress models in failure predictions. The findings challenge benchmark models that rely on averaged SCF values, offering critical insights for improving accuracy in predicting fibre break propagation and composite strength.