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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Water Immersion and Impact Damage Effects on the Residual Compressive Strength of Composites

Author: BERKETIS, Kyriakos

Year: 2006

Supervisor(s): Paul Hogg

This thesis investigates the combined effects of long-term water immersion and impact damage on the residual compressive strength of Glass Fibre Reinforced Polymer composite plates.

Long-term immersion of GFRP composites in water can result in a significant physical and possibly chemical degradation of the composites. The degradation rate at ambient temperature can be very slow so accelerated degradation procedures were used in this project. Three water temperature levels were employed for specimen immersion, at 43°C, 65°C and 93°C. Increasing the water immersion temperature caused an increasing degradation rate. Dynamic Thermal Mechanical Analysis was performed that has shown matrix plasticizing effects at 43°C and some post-curing effects at 65°C and 93°C. Existing impact damage on plates immersed in water did not cause any marked change in the maximum water absorption level or at the absorption rate.

The impact damage susceptibility and tolerance of three different E-glass reinforced composite plate types were studied. Two Non-Crimped Fabric and one plain woven fabric reinforcement were examined. Non-penetrating, low-velocity impact at three energy levels of 2.5 J, 5 J and 10 J was employed. A focus point of this study was to investigate the differences in behaviour, such as in terms of impact energy absorption, when impact preceded or followed water immersion. Impact energy absorption after water immersion was found to increase as a function of the immersion time. This finding was used to formulate an Environmental Damage Accumulation Metric.

Non-Destructive Techniques were applied Ultrasonic instrumentation to obtain impact damage sizes. The use of Air-coupled ultrasonics has been demonstrated for damage detection in wet composites. A novel application of the Air-coupled ultrasonics for degradation monitoring has been demonstrated.