Effect, on moisture absorption experiments, of failure to dry specimens prior to exposure

Abstract

A simple theoretical example is used to illustrate the effect of allowing composite coupon specimens, intended for moisture absorption experiments, to lie in the laboratory awaiting test without drying them prior to exposure. Clearly in these circumstances, the equilibrium moisture concentration will be underestimated and any ‘as manufactured’ values obtained from the measurement of other properties may in fact be partly degraded. The diffusion coefficients obtained by matching theoretical calculations to practical results will also be affected. It is shown that these apparent coefficients may be considerably in error, particularly if the variation of the equilibrium concentration with humidity is small. This distortion is at its maximum when the laboratory exposure time is fairly low, and the deviation from the actual coefficient is of a form which suggests that the diffusion coefficient is increasing with concentration and/or time, although a constant coefficient was assumed in the example used. The theoretical example is based on parameters pertaining to one particular graphite/epoxy composite, but the problem may easily occur with other materials absorbing a penetrant in a near-Fickian manner.