Improving the performance of discontinuous fibre reinforced composites

Student thesis: PhD Thesis

Abstract

The automotive industry is under increasing pressure to produce light weight and fuel-efficient vehicles to meet stringent global emission targets. Discontinuous carbon fibre composites with thermoset resin are being favoured in the industry due to their potential for easy automation, short cycle times, and overall cost efficiency. The complex interactions of material, process, and microstructural parameters can significantly affect the mechanical, damping, and impact properties of the composites. Thus, the aim of the current study is to study the influence of these parameters and tailor to improve the composite performance. The primary parameters studied include the size effect in the reinforcing fibres, the types of fibres used, flow-induced fibre orientation, as well as impact resistance and post impact residual properties of the composites.
A study of size effect, i.e., fibre length vs. tensile strength, in range of carbon fibre grades demonstrated that high performance (both PAN and pitch-based) fibres exhibit lower defect population, thus, lower strength reduction at longer length than intermediate grades. However, composite strength was seen to increase monotonously with fibre length up to ~50 mm, beyond which no gain was observed for all the fibre types studied. Composite stiffness exhibited increasing trend with increasing fibre stiffness while composite strength with increasing fibre strength showed greater discrepancy with prediction in composites with higher fibre-matrix modulus ratio. This is perceived to be due to high interfacial and fibre stress concentrations which act as failure initiation site, as supported by literature.
The effect of hybridization of different fibre types has been studied using jute-carbon hybrids as a novel investigation of high-performance composites with reduced carbon footprint suitable for high volume applications. The results showed that hybrids with jute skin layers have higher tensile and damping performance due to the blocking effect of carbon layers and excellent vibration dissipation capability of natural fibres. On the other hand, as expected, higher flexural stiffness and strength were obtained when the stiffer and stronger carbon layers were used as external skin layers. The Cost-Performance Ratios (CPRs) have been used to evaluate the hybrids against their carbon and jute benchmarks.
In addition, in light of increased susceptibility to impact induced damage in automotive industry, a study has been conducted to investigate the impact behaviour and residual properties under different loading conditions. Notably, the impact response exhibited greater sensitivity to changes in composite thickness. Moreover, post impact compression loading resulted in greater strength reduction and stronger correlation between impact damage and post impact failure initiation site compared to tension loading.
Finally, experimental studies showed that longer fibre tows exhibit greater resistance to flow induced fibre alignment during compression moulding of carbon fibre Sheet Moulding Compounds (SMCs). Furthermore, a skin-core layered structure was observed, where the core maintained the initial fibre orientation relatively while high flow induced fibre orientation was seen on the surfaces. Moreover, by encouraging charge flow in the preferential alignment direction of the preform, high tensile stiffness and strength in the preferred direction were achieved (56.7 GPa and 238.9 MPa respectively) at 30% fibre volume fraction. Additionally, circular statistics parameters were employed for the interpretation of fibre orientation distributions, as a more detailed alternative to orientation tensor components.
Date of AwardMar 2024
Original languageEnglish
Awarding Institution
  • University of Nottingham
SupervisorXiaosu Yi (Supervisor) & Robert Samuel Pierce (Supervisor)

Keywords

  • Sheet Moulding Compound (SMC), Carbon fibre, Weibull effect, Residual strength, Fibre orientation, Charge flow

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