Predictive Methods for Estimating the Durability of Simple Composite Materials

Interest in the use of polymer based composites, particularly polyester composites reinforced with E-glass fibres, has grown tremendously in recent years as a result of their attractive strength-toweight ratio and relatively low corrosion rates. Industry acceptance of advanced composite materials for marine use however is currently awaiting resolution of some major technological issues. Principal amongst these is the issue of reliability and in particular the current inability to predict the long-term degradation of FRP materials which are subjected to prolonged contact with moisture (Weitsman and Elahi (2000), Karbhari, et al. (2003)). Many factors are responsible for the current absence of durability models. Environmental aging mechanisms are complex and poorly understood. Durability studies by their nature require long term commitments of time which are often not practical. As a result durability data is sparse and findings from different studies are not easily compared thereby making the development of general models of the breakdown process difficult. Despite the paucity of available data some preliminary material has been published presenting methods by which the different aspects of the degradation process can be modeled. However there has been no attempt to pull the different strands together to generate a more general description of the breakdown process. The following paper outlines a method by which the moisture induced decline in mechanical properties of simple composite systems can be estimated using a combination of methods already available in the general literature. The hybrid model incorporates the effects of reversible damage to the matrix, debonding and stress corrosion of the reinforcing fibres. The aim of this paper is not so much to present a definitive model of the breakdown process but rather to present a preliminary modelling framework which allows the direction for future simulation and experimental studies to be assessed. The model comprises two parts; the evolution of the moisture distribution throughout the composite is first determined then the resultant changes to the mechanical properties of the composite are evaluated. For the purposes of this discussion the durability of a glass reinforced polyester composite will be examined.