Effects of Changing Seawater Temperature on Mechanical Properties of GRP Composites

Glass fibres were used to fabricate epoxy and unsaturated polyester laminated composites, which were then immersed in seawater. The specimens were immediately transferred from one seawater bath to another one at a different temperature to induce thermal shock, with concurrent hydrothermal ageing. Thermal shocks of two types, up-cycle (lower to higher temperature immersion) and down-cycle (higher to lower temperature immersion) were applied and their repetition induced thermal fatigue. The aged samples were subjected to 3-point short beam shear tests. The tests were performed at room temperature with 2 mm/min and 50 mm/min crosshead speeds. The weakening effects were sensitive to loading rate. The effect of fibre volume fraction was also investigated. The maximum interlaminar shear strength fell by 35% in glass/epoxy composites (in 55 and 65 weight% fibres) and by 33% in glass/polyester composites (in 60 weight% fibres) as a result of the most severe conditioning cycles. It was also observed that the reduction in shear strength in general was less at high crosshead speeds. This may have been linked to a reduction in matrix ductility.