5.1 INTRODUCTION

Dimensional instability caused by compositional changes was undoubtedly noticed by the earliest boat builders, who observed the effect of moisture on the swelling and shrinkage of wood and other organic structural materials. The hygroscopic nature of polymers and most organic materials continues to be a major concern with modern composites. For example, the vacuum-induced dryout of spacecraft composites [1] may easily cause defocusing of optical images in telescopes or distortions of microwave signals from antennas. Since moisture is the most common agent of nonthermal environmentally induced strains, we shall concentrate our discussion of dimensional changes in terms of H ₂O absorption and diffusion. The theory is sufficiently general to encompass the absorption of other substances, for example, benzene in the elastomeric matrix of a tire ply.

The concept of a coefficient of moisture expansion (CME) has been discussed since at least 1975 [2]. It has also been called the hygral expansion coefficient [3] and coefficient of hygroelasticity [4]. It could also be considered as a hygral compliance. The analytical treatment of moisture-induced expansion in polymer matrix composites has limited analogy to temperature effects. Both are anisotropic volume effects, e.g., higher mass-diffusion rates at the fiber-matrix interfaces and higher thermal diffusion along the axis of some fibers (graphite) than elsewhere in the composite. On the other hand, there may be permanent preferential absorption of a solute at the interface. Temperature affects the dilatation of all composite constituents, but moisture has little effect on the expansion of inorganic fibers.