Characterization of the hygric behavior of single fibers

Hygrothermal effects in composites are induced by temperature and moisture concentration. The moisture concentration causes hygric strain of the lamina (matrix and fiber) and residual stress in the composite laminate. Owing to the linearity of the molecular structure, most engineers concur that the longitudinal hygric strain in a fiber as a result of moisture absorption is insignificant or even non-existent, and hence not easily measurable. This investigation adopts a simple technique to measure the hygric strain of a single fiber for the first time. Carbon fibers (AS4, IM6, and IM7, Hercules, Inc.), Kevlar 49 fiber (aramid 965, DuPont Co.) and natural silk (produced by silk worms) are tested. Unexpectedly, the test results indicated that the hygric strains of carbon and Kevlar fibers are negative and are not in fact too small to be measured.