The role of fibre alignment in the axial compressive failure of carbon-fibre polymer composites

• Main Author: Creighton, C.
• Published: University of Cambridge 2000
• Subjects: 668.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598144

The axial compressive strength of long-carbon fibre reinforced polymer matrix composites is known to be much lower (60% or less) than the tensile strength. The poor compressive strength is strongly associated with fibre misalignments which inevitably occur during the manufacturing process. Therefore, it is essential to be able to quantify the fibre alignment distribution in these materials. In the light of the fact that existing methods of alignment characterisation are very small scale procedures, extracting data from relatively few fibres, a new technique is presented with which information can be obtained on a much larger scale, yet within a reasonable time. A programme of experiments has been completed in order to determine the compressive strength of carbon-fibre reinforced polymer composites. Materials with varying degrees of fibre alignment and porosity have been investigated. The design of a new compression rig was required, the validation of which was accomplished using experimental results and numerical methods. The work described in this thesis addresses the observed failure mechanisms in well-aligned pultruded material and laminated composites with a wider distribution of fibre misalignments. It has been shown that, where measured fibre misalignments are very small, failure is due to fibre kinking, a localised instability (associated with the shear yielding of the matrix on planes parallel to locally misaligned fibres) as opposed to fibre crushing - governed by the intrinsic strength of the fibres themselves. The difficulty of testing representative volumes of material has been discussed but, when done so, it has been shown that elongated pores (approximately 2-3 fibre diameters in width) strongly influence the compressive strength. Numerical modelling has shown that local stress concentrations about such pores might be expected to act as sites for preferential nucleation of kinkbands.