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Abstract: The stress redistribution after a single fibre break is a fundamental issue in longitudinal strength models for unidirectional composites. Current models assume hexagonal or square fibre packings. In the present work, random fibre packings were modelled using 3D finite element analysis and compared to ordered fibre packings. Significant differences in the stress redistribution are found. Compared to square and hexagonal packings, random fibre packings result in smaller stress concentration factors for fibres at the same distance from the broken fibre. These random packings, however, also show higher maximal stress concentration factors. The influence of the fibre breakage is more localised, which results in lower ineffective and overload lengths. The presence of fibres at smaller distances from the broken fibre explains these phenomena. For an accurate representation of the stress redistribution after a fibre breakage, random fibre packings should be used. [Copyright &y& Elsevier]

Multiscale models for fibre-reinforced polymer composites currently lack experimentally validated microscale damage descriptors as input parameters. This work demonstrates the occurrence of strain localisation phenomena at the fibre/matrix level using nanoscale digital image correlation. Unidirectional carbon-fibre reinforced epoxy and glass-fibre reinforced PMMA composites were loaded in transverse compression in a scanning electron microscope. Radial and shear strain maps were extracted and compared with finite element simulations based on a conventional elastoplastic model. Near the interface, an interphase layer is present in the matrix, presumably due to locally different polymerisation conditions. A skin-core structure was found in carbon fibres, corresponding to an increased transverse modulus towards the interface. [Display omitted] • Measurement and simulation of radial strain fields in fibre-reinforced polymers. • Quantitative width measurement of the skin-core structure in carbon fibres. • Fibre/matrix interphase-like response partly due to thermal residual stresses. [ABSTRACT FROM AUTHOR]

Hybrid composites combining brittle and ductile reinforcing fibres can be designed to have pseudo-ductile behaviour by controlling their failure mechanisms. The recently developed carbon-fibre/self-reinforced polypropylene (SRPP) hybrids have demonstrated a unique combination of stiffness (7 GPa), toughness (>6% strain) and density (±1000 kg/m³). Here, the influence of a circular notch on their failure mechanisms is investigated. Discontinuous carbon-fibre/polypropylene composites retained 69% of the gross strength and 83% of the net strength. The strength retention of SRPP was highly influenced by the processing parameters, and a notch-insensitive composite could be produced. Pseudo-ductility was obtained even with the notched hybrids. Hybrids that do not achieve the optimal ductility, still exhibit high gross and net strength retentions and can be classified as notch-insensitive. These results illustrate the significant potential of this new hybrid composite for applications with notches or holes. [ABSTRACT FROM AUTHOR]

Tow-Based Discontinuous Composites (TBDCs) combine high manufacturability with good mechanical properties. This work presents a novel 3D approach for the numerical modelling of TBDCs. The framework generates 3D orientation tensors by adding a stochastic component to the orientation tensors deterministically predicted by a process simulation. The actual TBDCs are thus idealised as Equivalent Laminates (ELs), resulting from these stochastic tensors. A physically based 3D failure criterion is presented for the prediction of failure initiation of the ELs. The consequent stiffness reduction is captured by a ply-discount method. The approach is validated for two TBDCs materials, with two different moulding conditions and different amounts of in-mould flow. The proposed approach accurately predicts the characteristic variability of these materials. As a consequence, the predicted strength was in good agreement with the experimental results of both materials tested. [Display omitted] [ABSTRACT FROM AUTHOR]

Many models for prediction of longitudinal tensile failure of unidirectional (UD) composites have been developed in the last decades. These models require significant assumptions and simplifications, but their consequences for the predictions are often not clearly understood. This paper therefore presents a blind benchmark of seven different models applied to two virtual materials. Reliably capturing the localisation of stress concentrations was vital in predicting the effect of matrix stiffness and strength on composite failure strain and strength as well as fibre break and cluster development. Although the models have different assumptions regarding stress redistributions around fibre breaks, the 2-plet (clusters of two fibre breaks) development was similar. Distance-based criteria were shown to be inadequate for monitoring cluster development. The discussions provide detailed insight into how the model assumptions are linked to the differences in the predictions. Image 1 [ABSTRACT FROM AUTHOR]