Interfacial micromechanics in thermoplastic and thermosetting matrix carbon fibre composites

Abstract

The interfacial micromechanics of carbon fibres in thermoplastic matrices [poly(methyl methacrylate) (PMMA) and polycarbonate (PC)] has been investigated by determining the distribution of interfacial shear stress along fibres in single-fibre model composites using Raman spectroscopy. The variations of fibre strain with position along the fibre in these composites are almost linear, indicating that stress transfer from matrix to fibre in the system is predominantly by frictional shear. It was found that the maximum values of interfacial shear strength for the PMMA and PC model composites are 3.1 MPa and 8.4 MPa, respectively, and much lower than the value of 30 MPa obtained for the same fibres in a thermosetting epoxy resin matrix. These low values of interfacial shear stress in thermoplastic systems can be explained by the lack of chemical bonding between the fibre and matrices, and possibly the effect of residual solvent. The interfacial adhesion in the systems stems primarily from mechanical interlocking, which can be enhanced by preparing the composites at higher temperatures. It is shown for PMMA that the maximum interfacial shear stress correlates very well with the radial pressure on the fibre as a result of thermal mismatch between the fibre and matrix.