THE EFFECT OF OBSERVED CLIMATIC CONDITIONS ON THE MOISTURE EQUILIBRIUM LEVEL OF FIBER-REINFORCED PLASTICS

Calculations have been made to determine the moisture absorption behavior of fiber-reinforced epoxy-matrix composites after exposure to prescribed outdoor climatic environments. A review of world-wide meteorological conditions has been made and six environments were chosen to represent a variety of regions. The results suggest that the level of moisture currently assumed to be absorbed by a composite during the service life of an aircraft is too low. Since the absorption kinetics of resin matrices differ widely and also change with physical ageing, the validity of specifying a moisture level to define the degree of environmental degradation in structural assessment is questioned. An alternative criterion, a constant relative humidity environment that will produce a representative moisture level in all parts of the structure and for all matrices currently in use, is proposed. Using this philosophy it is suggested that the world-wide worst environment might best be simulated by a constant humidity of 87%.