There is an emerging interest in the aerospace industry to manufacture components with intricate geometries using discontinuous fibre carbon/polyether–ether–ketone moulding systems (obtained by cutting unidirectional tape into strands). This type of material system is termed randomly oriented strand composites and is appealing for structural applications as it bridges the gap between the lack of formability of continuous fibre composites and the lack of performance of short fibre composites. The objective of this study was to investigate mechanical properties (tensile, compressive, shear and fatigue) of randomly oriented strand composites and to quantify the effect of strand size on their properties. Overall, properties were found to be highly variable and dependent on the strand length. Interestingly, tensile, compressive and shear strength had similar magnitudes and exhibited the same failure mechanisms (strand fracture and debonding). This experimental work expands the knowledge base for randomly oriented strand composite materials.

A comprehensive review of the mechanical characterisation and modelling of randomly oriented strand material architecture and their hybrids with laminates is presented. Randomly oriented strand composites are long discontinuous fibre systems that exhibit excellent formability characteristics and stiffness properties comparable to quasi-isotropic laminates, allowing their use in the manufacture of intricate geometric parts for automotive and aerospace industries. Randomly oriented strand architecture complements out-of-autoclave methods such as compression moulding, thermostamping and resin-transfer moulding leading to cost-reduction. Their applicability is limited to non-structural and low-load bearing applications due to their low strength properties. Continuous fibre aerospace preforms exhibiting excellent mechanical performance possess low formability characteristics and are confined to simple shell-like geometries with minimal curvatures, while incurring high-costs and long manufacturing times. Hybridisation of randomly oriented strand and with other material architectures represents trade-off solutions of formability and performance characteristics often yielding synergistic effects. Seemingly simple, randomly oriented strand architecture is a complex material system that poses several structural and process challenges. This work categorises the pertinent research work from the literature on the mechanical characterisation into the framework of a formal characterisation environment of composite structures that includes the coupon, the part and structural levels. The manufacturing, dispersion methods and measurement techniques are qualitatively assessed with reference to the mechanical properties. The discussion on modelling involves the identification of crucial characteristics of significant analytical and numerical modelling techniques devised for the prediction of the mechanical behaviour of randomly oriented strand composites and their hybrids. Emphasis is on the methods of stiffness and strength prediction. Our perspectives on the effective use of randomly oriented strand composites and their hybrids are discussed. Process characterisation and process modelling of randomly oriented strand composites and their hybrids are beyond the scope of this article.

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There is an emerging interest in the aerospace industry to manufacture components with intricate geometries using discontinuous-fibre carbon/polyether-ether-ketone moulding systems (obtained by cutting unidirectional tape into strands). Great formability and high modulus can be achieved with this type of composites, but the high variability of measured properties can have a detrimental effect on the design allowables. When it comes to prediction of mechanical properties, it is important to capture the average strength and modulus as well as their statistical variability. This article proposes a stochastic finite element technique that uses the concept of randomly oriented strands to model variability, and the application of Hashin's failure criteria and fracture energies to estimate strength. Overall, the model matches the trends observed during experiments and shows that strength of randomly oriented strand composites is significantly lower than that of continuous-fibre laminates due to the 'weakest-link' principle.