Environmental effects on the durability of advanced composite materials

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Advanced composite parts are fabricated by stacking plies of resin-impregnated fibres and curing under high temperature and pressure. Since the number and orientation of the plies are design variables, the part and the material are being designed simultaneously. Composites offer new possibilities to manufacturers to fabricate high performance, corrosion resistant, lightweight structures, but they have not yet reached their full potential in structures due to lingering uncertainty about their degradation and failure mechanisms. Research is paramount to support the development of the multi-billion dollar composite industry. The overall objective of the proposed research program is to develop a comprehensive understanding of the moisture degradation of composites and to predict its long-term effects on the integrity of structures in real world applications. Understanding the deterioration of composites is important to ensure proper inspection and maintenance of aging structures currently in service and to adopt the best possible methodologies in future design. Some aspects of moisture degradation have been investigated in our previous work such as the mechanics of moisture absorption, moisture effects on the properties of epoxy, and moisture effects on delamination growth in composites. More avenues must be explored to fully understand progressive damage in composites exposed to wet environments. In service, low-velocity impacts from blunt objects, like a dropped tool, can create internal damage to a laminated composite that is invisible from the surface. Subsequent loading can cause damage growth leading to delamination, which may progress undetected until complete failure of a structure. Models have been proposed to predict damage induced by low-velocity impact but the combined effects of impact and moisture exposure, and their consequences on the residual strength and life of a composite structure remain vastly unexplored. In this program, a methodology will be developed to understand and predict impact damage formation and growth in wet composites. Impact damage may involve fibre breakage, matrix cracking and/or fibre-matrix debonding. Moisture, by altering the matrix and the fiber-matrix interface, may change the fracture process and adversely affect the damage tolerance of a composite. Impact damage will be examined by thorough inspection of laboratory produced damaged samples. Failure criteria will be evaluated to determine which ones better predict damage initiation and progression. It is intended to combine damage mechanics, failure criteria and fracture mechanics in numerical models that will simulate the complete composite failure process from impact to final fracture with the inclusion of moisture effects. The results of this research will benefit the aerospace industry by paving the way to the implementation of a damage tolerance philosophy to composites.