The effect of variability in the microstructure of tow-based discontinuous composites on their structural behaviour

Abstract

Tow-Based Discontinuous Composites (TBDCs) consist of a network of randomly positioned and oriented carbon fibre tows, embedded in a polymeric matrix. These materials provide a solution for combining the manufacturability and mechanical performance for high volume production of structural components, e.g. in automotive/sporting industry. The growing application of TBDCs in structures, in the meanwhile, urge the need for an efficient design tool to account for their discontinuous nature and the heterogeneous microstructure. This work therefore aims to deepen the understanding on the mechanical response of TBDCs and develop design tools for TBDC structures. An experimental study was conducted to compare the mechanical properties and failure mechanisms of randomly-oriented TBDCs to their equivalent ply-by-ply laminate counterparts. The results validated the equivalent laminate analogy used in literature for modelling TBDCs, while measuring the 30% strength knock-down in TBDC due to the intrinsic variability in the microstructure of TBDCs and hence highlighted the need to account for this variability in modelling TBDCs. To account for the intrinsic variability in TBDCs, an analytical strength model based on stochastic equivalent laminate assumption was developed. This model predicts the strength of TBDCs with good agreement to the experimental data, and is able to capture the change in the strength of TBDCs with changing tow dimensions, and therefore, can be used to aid material design and microstructural optimisation. The analytical strength model developed is then integrated with a finite element design framework to account for variability in TBDCs in structural components. By conducting a case study on the engine bonnet lid of Lamborghini Huracan PERFORMANTE, this design framework is shown suitable for large structures. The results from this case study show that the variability in TBDC has signi ficant effect on the critical load and failure initiation location of a TBDC structure. Overall, this work develops an efficient design tool to account for the intrinsic variability in the TBDC on the mechanical response of TBDC structures, which allows for more efficient structural design with these materials.