Specimen Size and Environmental Exposure Effects on Initial Diffusion in E-Glass/Vinylester Pultruded Composites

This paper studies the effect of specimen size on the moisture uptake characteristics of pultruded E-glass/vinylester composites exposed to conditions of immersion and 99% RH over a range of temperatures. Four different specimen sizes representative of sizes commonly used for material characterization (tension, short-beam-shear, and dynamic mechanical thermal analysis) as well as moisture uptake are included. It is shown that both exposure conditions and geometry significantly influence uptake behavior, and that the differences, in general, can be elucidated through consideration of surface-to-edge area ratios of the specimens. For the current study, the ratio extends from 2.528 at the lowest level for the short-beam-shear specimens to 16.979 at the highest for the tensile specimens. The overall levels of uptake in the period of exposure, the levels of transition uptake, and the diffusion coefficients are noted to increase with a decrease in the ratio, suggesting an increased influence of the edge effect, which is further enhanced with an increase in temperature. Levels of normalized transition uptake for the specimens with the lowest surface-to-edge area ratio are 12.5 and 8.2 times higher than those for the specimens with the highest ratio at the two extreme temperatures, respectively, when exposed to 99% RH, and are 7.2 and 15.3 times, respectively, under conditions of immersion. Activation energy calculations also highlight differences based on specimen size and the condition of exposure with immersion leading to a lower activation energy than exposure to 99% RH when considering the initial linear regime with the specimens having the largest surface-to-edge area ratios showing 11.3-13.5% higher levels due to exposure to 99% RH, whereas the two specimens with the smaller ratios show a 4.9% increase. The findings highlight the importance of specimen size and exposure conditions and emphasize that the commonly used assumptions could lead to inaccurate results especially when extrapolated. The use of the immersion condition as a means of accelerating field conditions of humidity could significantly overestimate effects. Further, the direct use of uptake characteristics from specimens at one size, or surface-to-edge area ratio, could lead to inaccurate conclusions if extrapolated to specimens that are significantly different leading to design and durability prediction implications.