Impact damage behaviour of composite materials after long-term exposure to a hygrothermal environment pp.913-932
Authors: (K. Berketis, D. Tzetzis, Spectrum Labs SA, Greece, and others)
Abstract: This paper analyses the changes in the impact damage behaviour of composites as a function of the degradation mechanisms caused by water immersion at elevated temperatures. The effect of different types of reinforcement architecture, woven and noncrimped glass fabric, in polyester matrix flat composite plates was evaluated experimentally. To accelerate the degradation rate, undamaged and impact damaged specimens were immersed in water baths for up to 30 months at 43° C, 65° C and 93° C. Low-velocity impact, at three energy levels of 2.5 J, 5 J and 10 J, was employed either preceding or following water immersion in order to investigate the differences of the impact energy absorption. The results have shown that increasing the water immersion temperature caused an increase in the degradation rate of all materials under study. Extended matrix dissolution and interfacial damage appeared on the specimens following prolonged water immersion while a multi-stage absorption profile was observed especially at 65° C regardless of the reinforcement architecture type. 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. Low velocity normal impact at various immersion time intervals did not increase considerably the in-plane delamination damage size but produced gradually a greater density of through thickness damage. The impact energy absorption after water immersion was found to increase as a function of the immersion time. An Environmental Damage Accumulation Metric (EDAM) was formulated linking the degradation state caused by a given water immersion period with the loss of the elastic impact component during the impact event, acting as a metric of the penetration limit.