The tensile properties of pultruded GRP tested under superposed hydrostatic pressure

Abstract

The failure mechanisms in waisted tensile specimens of pultruded 60% volume fraction glass fibre-epoxide were investigated at atmospheric and superposed hydrostatic pressures extending to 350 MN m⁻². The maximum principal stress at fracture decreased from ∼1.7 GN m⁻² at atmospheric pressure to ∼1.3 GN m⁻² at 250 MN m⁻² superposed pressure and remained approximately constant at higher pressures, as had been observed with carbon fibre reinforced plastic (CFRP) and a nickel-matrix carbon fibre composite. In the high-pressure region the failure surfaces were fairly flat, consistent with the fracture process being solely controlled by fibre strength. Pre-failure damage, in particular debonding, was initiated at ∼0.95 GN m⁻² at atmospheric pressure and this stress rose to ∼1.2 GN m⁻² at 300 MN m⁻² superposed pressure, i.e. by about 9% per 100 MN m⁻². Unlike the pressure dependence in CFRP, this contrasts with the pressure dependence of the resin tensile strength, about 25% per 100 MN m⁻², but can be associated with that of the fibre bundle/resin debonding stress, about 12% per 100 MN m⁻² superposed pressure. Consistent with this interpretation, glass fibres of the failure surfaces were resin-free, again in contrast to CFRP.