Development of hybrid compounds for compression moulding for structural composites

Abstract

The automotive industry is targeting reduced vehicle weight to improve emissions or, in the case of battery electric vehicles, improve range. Polymer-based composite materials offer high specific properties which can lead to large weight savings for automotive components. The high cost of continuous fibre composites has resulted in a low uptake of composite materials in the automotive sector, with only 8% of the materials used in the transport sector being composites. Discontinuous fibre composites offer a lower-cost composite solution, however, these materials do not offer the large performance benefits of their continuous fibre counterparts. Hybridisation, through the combination of different fibre architectures and different fibre materials, presents an opportunity to maintain a competitive cost whilst improving the mechanical performance for specific applications, therefore, enabling further adoption of composite materials.

This thesis explores multiple discontinuous fibre composite materials with hybridisation used to improve mechanical performance. Experimental techniques were used to observe the tensile, compressive, low-velocity impact, and vibrational damping of various discontinuous fibre-based hybrid materials. Analyses were performed using density and fibre content measurement techniques such as acid digestion and with imaging through Scanning Electron Microscopy, ultrasonic C-scans and X-ray computed tomography. All hybrid materials tested showed advantageous properties in specific use cases.

It was found that hybridisation through different fibre formats, using a short fibre injection moulding compound and UD pre-preg, produced large improvements in tensile, and flexural properties, with benefits also observed in compressive modulus. A comparison of 1 and 2-step manufacturing processes revealed an increase in the fibre volume fraction of the final panels with the 2-step process, however, only a minor change in void content was observed compared to the 1-step process. Stochastic computational modelling of the short fibre injection moulding compound was achieved, accurately capturing the variability in tensile properties, thus allowing better design confidence when using these materials. Strain rate and temperature-dependent properties showed further advantages of the hybrid material. In compression, the short fibre material showed an increase in strength at higher strain rates, however, the hybrid material comprising of short fibres and continuous UD fibres showed an even greater improvement in strength with an increased strain rate. An increase in temperature resulted in an increase in matrix ductility and, therefore, a drop in compressive strength. At a higher strain rate, this was only observed for the hybrid material as the UD layers restricted crack propagation and final catastrophic failure, allowing time for ductile deformation to develop. Under tensile loading, the hybrid material showed no strain rate dependence as the behaviour was dominated by the UD fibres, whereas the short fibre material showed a significant increase in strength with an increased strain rate. An increased temperature resulted in a drop in strength for the quasi-static tests only, with a 14.3% drop in the strength of the short fibre composite from 23 to 85◦C. A large drop in performance of 58.7% was observed for the hybrid material due to the debonding of the UD fibres.

The vibrational damping of automotive components can have a large effect on passenger comfort and cabin noise. DMA testing indicated minimal viscoplastic damping from the PEEK and Phenolic resin systems tested, confirming friction in the fibre matrix interface as the main damping mechanism. The discontinuous fibre composites exhibited the lowest natural frequencies due to their low tensile stiffness. The weak interfaces resulted in high damping performance, with the largest damping observed for the short fibre injection moulding compound. The hybrid material showed similar flexural stiffness to the quasi-isotropic laminate in DMA storage modulus. However, at high frequencies, the hybrid material showed a significant improvement in damping performance. Therefore, the hybrid provided a cost-effective improvement in damping performance for a quasi-isotropic laminate of comparable flexural stiffness.

Impact performance was explored with hybridisation through different fibre materials with glass and carbon fibres used in an SMC material. The glass fibre SMC showed the highest impact tolerance due to the large strain to failure. The carbon fibre SMC showed good tensile properties, however, due to the brittle fibres, a large crack density was observed post-impact through X-ray CT. A hybrid of glass and carbon fibre SMC produced an intermediate performance in tension and under impact, with the hybrid material showing the largest compression after impact strength after 30 J impact due to the glass fibres acting as effective energy absorbers, protecting the carbon fibre layers. X-ray CT indicated the damage was localised to the interface between glass and carbon fibre layers, with significant cracking in the carbon closer to the back surface, where larger deformation was observed.

This investigation provides critical data on material properties for the future design of automotive components such as battery covers, wheel arches, and body panels. Reduction in weight of these components will improve vehicle range and therefore reduce the energy required for transport improving sustainability. In addition, strain and strain rate behaviour and an improved understanding of impact performance allow for a better understanding of crash structure and damping analysis allows for better control of noise, vibration and harshness.