Compression moulding of hybrid carbon fibre composites for structural applications

Corbridge, David Michael (2018) Compression moulding of hybrid carbon fibre composites for structural applications. PhD thesis, University of Nottingham.

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Automotive manufacturers are receiving pressure from customers and regulators to reduce emissions. Reducing the weight of the vehicle through the use of carbon fibre is seen as one of these mechanisms. The challenge is to develop suitable manufacturing processes that can offer appropriate cycle times to meet demand and deliver materials with adequate mechanical properties for structural applications.

Compression moulding of discontinuous fibre moulding compounds with local continuous fibre inserts provide better production rates and part complexity compared to the autoclave components and higher performances than injection moulding. However, combining a unidirectional carbon fibre (UD) material with a random short fibre orientation sheet moulding compound (SMC) that flows heterogeneously will lead to degradation in the properties of the continuous reinforcement. This work aims to demonstrate a hybrid of continuous and discontinuous fibre compounds in a single moulding operation with increased stiffness and determine if the surface distortion of the reinforcement can be used to predict local stiffness.

A benchmarking study was carried out with UD and the SMC followed by hybridisation. This was non-destructively tested for flexural moduli providing a localised map of stiffness which was compared with a theoretical value. This work demonstrated that simply placing unidirectional (UD) prepreg with the SMC caused significant distortion and migration of the reinforcement in a one-dimensional flow scenario. Resin tended to bleed out of the hybrid reinforcement, causing a resin rich area at the UD ply drop off point. This resin bleed was more prominent at the ends of the UD fibres.

The resin system in the UD was staged by partially curing it to a controlled level through the measurement of the storage modulus, and showed that flow could be dramatically reduced. This was determined by rheology and inter-laminar shear tests to measure material degradation from staging to improve flow control. It was found that for flow control of the reinforcement staging beyond gelation was required. The inter-laminar shear strength of UD is significantly higher than the SMC, and found that even with 50% staging properties were still higher.

Where there were high levels of flow resistance in compression moulding, staged hybrids resulted in to two moulding defects; a dry region on the SMC under the reinforcement and rippling outside the reinforcement, which reduced the stiffness by nearly 50% in the affected areas.

Staging accompanied with charge layout design of the UD to 902/0 showed markedly reduced flow in one, two and three dimensional scenarios, almost completely resisting the flow of the SMC. In the 2D flow scenario where the SMC charge coverage was 60% compared to the manufacturers’ recommended 80%, flow was limited to 3% and the stiffness could be locally predicted to an accuracy of 16%.

By controlling the level of staging and careful consideration of the charge design, hybrid components can be manufactured repeatedly with increased accuracy in stiffness prediction and demonstrated an improved flexural strength and modulus increase of >44%, increasing the potential use to a wider range of complex geometry structural applications.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Harper, Lee T.
De Focatiis, Davide S.A
Keywords: Compression moulding, Sheet moulding compound, SMC, Staging, Hybrid composite, flexural modulus
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 49219
Depositing User: Corbridge, David
Date Deposited: 13 Jul 2018 04:40
Last Modified: 08 May 2020 08:31

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