Stress generation during the processing of epoxy-carbon composites.

The stresses generated during the processing of carbon fibre/epoxy resin composite materials are predicted using the finite element method and classical lamination theory. Elastic material behaviour is assumed. Emphasis is placed on the residual microstresses which have been less-extensively studied than other aspects of the stress generation process.Temperature and stress distributions are modelled through the thickness of laminates assuming cooling from the cure temperature of 190°C. At the microlevel the effect of varying fibre volume fraction, interfibre distance, packing geometry and fibre diameter are studied. Random and regular fibre arrays are considered.It is found that the residual stresses are generated almost entirely due to the differing properties of the fibre and the matrix and the anisotropy of the fibres, rather than any temperature gradients within the materials. At the macrolevel maximum stresses (10-100 MPa) are calculated in the transverse layers of multidirectional laminates. At the microlevel maximum stresses (10-100 MPa) are predicted at the fibre/matrix interface. The exact values depend on the assumed laminate stacking sequence and distribution of fibres, respectively. The maximum values of the microstresses are found to be approximately inversely proportional to the minimum interfibre distance and proportional to the fibre diameter. This implies that, at the shorter minimum interfibre distances typical of more realistic random arrays, the maximum stress values are greater. When the macrostress and microstress fields are superimposed it is predicted that cracks will form at some of the fibre/matrix interfaces and propagate outwards into the matrix.Observations of laminate samples under the electron microscope show no such cracking to occur, rather in a few localised regions, cracking around the fibre/matrix interface is apparent. It is suggested that in these regions the interface is weak and fails due to the weaker radial stress. Otherwise it is suggested that cracking is not observed due to a visco-elastic/visco-plastic behaviour of the matrix, the presence of an interlayer at the fibre/matrix interface with properties different to that in the matrix away from the interface and a crack suppressing mechanism resulting from the interaction of adjacent plies. The latter effect is most significant for thin plies.It is proposed that regular packing of the fibres, which precludes low interfibre distances, will prevent microcracking. Hexagonal packing is preferred since this achieves the highest volume fraction and thus the highest strengths. Sizings applied to the fibres which improve the fibre/matrix adhesion, and react/diffuse into the matrix to produce a flexible interlayer, will improve the strength and impact resistance of these composites. In multidirectional laminates thin transverse layers, less than 0.5 mm are advised.