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

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

Deformation, yield and fracture of unidirectional composites in transverse loading 1: Influence of fibre volume fraction and test-temperature

The influence of the fibre volume fraction and test-temperature on the transverse tensile properties of glass fibre reinforced epoxy is studied using experimental and numerical techniques. The numerical analyses are based on micromechanical models with square and hexagonal fibre packings. Special attention has been directed towards the identification of the necessary failure criteria. Using a von Mises failure criterion, an increase in transverse tensile strength is predicted at higher fibre volume fractions with both models. This is in good quantitative agreement with experimentally determined transverse flexural strengths. With decreasing test-temperatures, higher transverse strengths are obtained. This is primarily caused by the temperature dependence of the yield stress of the matrix. The counteracting influence of the residual thermal stresses and the temperature dependent matrix ductility consequently proved to be less significant for the transverse strength

The influence of matrix plasticity on the failure strain of transversely loaded composite materials

The influence of the matrix on the transverse tensile properties of unidirectional carbon fiber-reinforced composites is investigated, based on epoxy matrixes with various failure strains. Finite element micromech. analyses are used to compare the deformation of the matrix in fiber-reinforced plastics with the deformation in rubber-toughened plastics. The stress situation leads to a brittle behavior in the former and favors a high failure strain in the latter with ductile matrixes. It is expected that the combination of rubber coated fibers with a ductile matrix yields composites with high transverse failure strain