Osmotic cracking in unsaturated polyester matrices under humid environment

Class fiber reinforced polyester composites are widely used in marine appli cations where they may undergo chemical and physical aging. For this kind o f laminates, the durability is governed mainly by the matrix degradation. T he mechanical property reduction is not induced only by hydrolytic aging bu t rather by the formation of cracks or blisters. That is the reason why it became of major interest to understand the microcrack nucleation mechanism in neat matrices, to study the crack propagation, and to evaluate the incid ence of cracking on the tensile property loss. We first put forward a nucle ation mechanism based on polyester-organic molecule phase separation, and d rew a simple kinetic model to take into account the existence of a crack in duction time that is dependent on temperature and matrix structure. Crack p ropagation results from the build up of an osmotic pressure in microcavitie s, which is proportional to solute concentration. This second step has been studied in terms of crack density evolution and crack propagation rate. Fi nally, there is a good correlation between tensile mechanical properties va riation and crack parameters. (C) 2001 John Wiley & Sons, Inc.