Environmental and time dependence of fracture toughness and crack growth in glass-fibre reinforced polyester resins

Abstract

The validity of simple (maximum load) fracture toughness testing of glass-fibre reinforced polyester resin laminates has been examined for SEN specimens using “slow strain rate” test techniques developed for metallic materials. Laminates were tested both in air and whilst immersed in an acidic environment. In general, valid test conditions were only established at test speeds <10−2 mm min−1 which are slower than those generally reported in the past. Fracture toughness was found to be greatly reduced by the presence of the chemical environment, as would be expected from the known sensitivity of these materials to environmental stress cracking. The slow strain rate test technique indicated a threshold level of critical stress intensity for environmental stress cracking and hence could be useful in ranking materials. Environmental effects were also found to determine crack growth rates as measured on DCB specimens. Crack growth data was analysed in terms of initial and/or inherent flaws and its use in predicting creep-rupture life examined. In principle it was concluded that use of crack growth data could provide an alternative to the present qualitative approach to design and a more predictable alternative to long term creep-rupture testing.