Influence of Longitudinal Fibre Wrinkling on the Compressive Strength of Carbon Fibre Reinforced Composites

Abstract

Carbon fibre reinforced polymers are a class of stiff, strong, and lightweight composite materials, commonly used in high-performance marine, automotive, and aerospace applications where high specific strengths are required. They typically contain fibre wrinkling defects where the longitudinal fibre reinforcements deviate out-of-plane from their straight, idealized, perfectly aligned directions. This fibre wrinkling is known to increase the susceptibility of the laminates to fibre kinking failure, resulting in reduced on-axis compressive strengths which must be accounted for in design. This research determines the influence of fibre wrinkling on the on-axis compressive strength of carbon fibre reinforced polymer laminates and identifies manufacturing processes which will reduce the severity of these defects. A full factorial experiment was designed to create a set of panels with a range of process induced fibre wrinkling. Twelve [90/0]3s panels were manufactured from a 300-gsm intermediate modulus carbon-epoxy prepreg and cured out-of-autoclave with a range of cure temperatures and vacuum consumables. The on-axis compressive strength of these panels was then determined through Combined Load Compression testing and the maximum fibre wrinkle angle measured using optical microscopy and automated fibre wrinkle measurement software. The maximum fibre wrinkle angle of the longitudinal wrinkling was found to have a statistically significant inverse effect on the on-axis unidirectional lamina compressive strength. For the range of wrinkle angles from 1.65° to 5.34°, laminates with a lower maximum wrinkle angle have a higher on-axis unidirectional lamina compressive strength and more consistent failures. Due to the scatter in the experimental data, both empirical and analytical models failed to accurately predict this on-axis unidirectional lamina compressive strength solely from the maximum wrinkle angle measured on a two-dimensional cross-section. The vacuum consumables in contact with the laminate during cure were found to have a significant effect on the severity of the process-induced fibre wrinkling. The use of carbon-epoxy caul plates in contact with the laminate was found to reduce the maximum fibre wrinkle angle to an average of 2.3°. This was found to be a 50% reduction in fibre wrinkling compared to laminates manufactured with standard E-glass chopped strand mat bleeders. The use of flexible cross-ply and rigid quasi-isotropic caul plates resulted in B-Basis on-axis unidirectional lamina compressive strengths of 1200 MPa and 1214 MPa respectively. This is up to a 9.07% increase in compressive strength, which allows the development of composite structures with a higher specific strength. As a result, it is recommended that carbon-epoxy caul plate tooling is adopted to reduce the fibre wrinkling and increase the on-axis compressive strength of carbon fibre reinforced polymer laminates.