THE EFFECT OF HYDROSTATIC-PRESSURE ON TRANSVERSE STRENGTH OF GLASS AND CARBON-FIBER EPOXY COMPOSITES

An attempt was made to measure indirectly the transverse tensile strengths of uniaxially aligned fibre pultrusions by the diametral compression of disc-shaped samples using concave loading anvils. Two types of composite were investigated, containing ∼60%Vf of either type AS carbon fibres (CFRP) or S glass fibres (GRP), both in an epoxy resin matrix. Testing was carried out at atmospheric and under superposed hydrostatic pressures, -H, extending to 300 M Pa. The resultant principal stresses at the disc centre were δ1 = δA +H; δ2 =H; δ3 = -3δA +H, where δA = 2 P/πdt for a disc of diameter, d, and thickness, t, subjected to a load P. Deviations from linearity in the load-deflection response were detected throughout the pressure range at ã70% and ã90% of the failure load for CFRP and GRP, respectively, and these were associated with resin yielding. The pressure dependence of δA, approximately -0.1 H, was consistent with a two-parameter yielding criterion predicting hypothetical yield stresses in simple tension and compression of ã81 and -109 MPa, respectively, for both matrix materials. Irrespective of pressure eventual fractures took place along the loading diameter, but in the CFRP specimens tested under pressure initial cracks at the disc centres were at ã45 ° to the loading axis, i.e. on the plane of maximum shear stress. Fractographic observations were consistent with transverse failure taking place by fibre-matrix decohesion in GRP and by resin fracture in CFRP. Other than the atmospheric datum point for CFRP, the pressure dependence of δA for failure, δF, was also approximately -0.1 H. Of the various stress, strain and strain energy criteria for failure examined, only critical tensile strain was found consistent with this pressure dependence. © 1990 Chapman and Hall Ltd.