Deformation, yield and fracture of unidirectional composites in transverse loading 2. influence of fibre-matrix adhesion

The influence of the adhesion between fibre and matrix on the transverse properties of unidirectional composites was studied using a combination of experimental and numerical analyses. The interface is modelled on a nano(metre)-scale and the aim is to investigate its local influence on the ultimate macroscopic transverse properties. Fibre-to-matrix stress transfer (i.e. fibre-to-matrix surface interaction) is simulated by introducing elastic interface springs. Since these elastic springs represent the chemical (covalent) bonds formed at the interface as a result of oxidative chemical surface treatment, the micromechanical model can be directly related to the effects of this treatment. For the verification of the numerical analyses, the influence of the interface is determined experimentally by transverse testing of carbon fibre reinforced composites, using fibres that were subjected to different levels of surface treatment. A direct relation between the oxygen concentration on the surface of the fibres, the interfacial bond strength and the resulting transverse strength was found. The interface strength required to obtain perfect bonding was found to be dependent on the fibre volume fraction and at increased fibre volume fractions a higher level of adhesion is required.