Abstract
A comprehensive novel investigation into the characterisation of interply friction behaviour of thermoset prepregs for high-volume manufacturing was conducted. High interply slipping rate and normal pressure typically used for high-volume manufacturing present challenges when preforming carbon fibre reinforced plastics. The study involved multiple reinforcement architectures (woven and unidirectional with the same rapid-cure resin system) which were characterised using a bespoke interply friction test rig used to simulate processing conditions representative to press forming and double diaphragm forming. Under prescribed conditions, woven and unidirectional prepregs exhibit significantly different frictional behaviour. Results demonstrated the unidirectional material obeys a hydrodynamic lubrication mode. For the woven material, a rate-dependent friction behaviour was found at low normal pressure. At higher normal pressure however, the woven material exhibited a friction behaviour similar to that of a dry reinforcement and significant tow displacement was observed. Post-characterisation analysis of test-specimens showed significant resin migration towards the outer edges of the plies, leaving a relatively resin-starved contact interface. The findings generate new knowledge on interply friction properties of thermoset prepreg for high-volume manufacturing applications, yet reveal a lack of understanding of the influence of tow tensions as well as the pre-impregnation level for a range of processing conditions.
Highlights
The increasing demand for fuel efficient vehicles has led to an increasing use of thermoset prepreg materials due to their high specific strength and stiffness
The prepreg materials investigated are based on the same epoxy rapid cure thermoset resin, which allows for a direct comparison of the reinforcement effect on the interply friction properties
No further compaction seems possible as the coefficient values plateau. This behaviour corresponds to the region characteristic of a hydrodynamic regime, where the friction coefficients decrease with increasing normal pressure and increase with increasing sliding speed
Summary
The increasing demand for fuel efficient vehicles has led to an increasing use of thermoset prepreg materials due to their high specific strength and stiffness This has created a need for more efficient, less labour and capital-intensive manufacturing processes, as opposed to the more traditional autoclave method. Similar to traditional manufacturing methods, one of the first steps of prepreg compression moulding (PCM) is the preforming of an initial flat blank of material into its final 3D shape. Preforming of thermoset material can be automated and is typically done using press-forming [4], or double diaphragm forming [5, 6], prior to the curing stage Both methods involve preheating a thermoset prepreg blank in order to lower the resin viscosity and to ease the deformation. The accuracy of preforming simulations rely on the accurate description and characterisation of all of these deformation mechanisms
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