The ILSS (Interlaminar short-beam shear) data, paints a different picture, with again a loss in property between dry and wet, but also a steady loss in strength with exposure time at all temperatures (Fig. 37.6). This behaviour ties in with the hypothesis that the thermoset resin may degrade and dissolve with exposure time. As the material displayed a loss in short-beam strength, discoloration and greater surface roughness following exposure, samples having undergone exposure to seawater for 245 days at all three temperatures were observed at high magnification under an SEM and compared to an unaged test piece (Fig. 37.7). Scratches and pits were seen on the surfaces of all samples, and the glass fibres themselves were often visible. The increase in surface roughness with exposure time and temperature appears to be linked to non-reversible chemical ageing. An increasing level of degradation was seen at the interface between the fibres and the matrix accompanied by a loss of fibre integrity with increasing test temperature. No cracking of the matrix material was observed in any of the samples. Therefore, it continued to protect the glass fibres within the bulk material hence maintaining mechanical properties after saturation. Only those glass fibres that were at the surface have been degraded. It can be assumed that exposure for longer periods of time (beyond the scope of this project) would cause further fibre degradation as seawater slowly attacked the protected fibres within the material bulk. For the above study, ILSS appears to be a good indicator of composite ageing. This mechanical method can be associated with other tools to produce a good picture of composite behaviour in aggressive fluids. However, reliable accelerated ageing methods and life prediction procedures are still not defined for these fascinating materials. 0.0 0.5 1.0 1.5 0 2 4 6 8 10 Mass Change from Dry [%] Exposure Time [days1/2] Thermoset/Glass Fibre 60°C 80°C 100°C Fig. 37.3 Change in mass as a function of square root of time and temperature Unaged 60 °C / 140 °F 80 °C / 176 °F 100 °C / 212 °F Fig. 37.4 Visual changes after 245 days exposure to synthetic seawater at vapour pressure and different temperatures; from left to right - Unaged, 60 C / 140 F, 80 C / 176 F, 100 C / 212 F 324 S. Munch et al.
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