Fichera, Maryann

The objective of this work is to characterize the moisture transport in a unidirectional,
transversely isotropic carbon/vinylester composite. Diffusion occurs when the material is
immersed in sea water and the moisture is transported through the voids, and interface gaps. This
uptake of moisture can cause problems including matrix degradation and swelling, reduction of
fiber/matrix interface strength, etc. To characterize water transport, three diffusivities are
required, D1, D2, and D3. However, transversely isotropic material can be characterized by two
diffusivities, along and transverse to the fibers (D1=DL, and D2=D3=DT). Composite materials
may absorb moisture along the fiber/matrix interface, especially if the ends of the fibers are
exposed. This mechanism of moisture transport inside the composite is known as “wicking”,
which would increase the value of DL. Wicking is promoted by voids and unbonded regions
between the fiber and matrix.
Experiments will be conducted on specimens made from vinylester resin reinforced with
unidirectional carbon fibers. A range of specimens will be prepared and immersed in 40°C sea
water. The moisture content will be monitored until maximum saturation. Previous studies
indicate that the Fickian diffusion model is a reasonable descriptor of the moisture absorption
process. From the plots of moisture content versus time1/2, the apparent diffusivity will be
reduced for the different size panels. The longitudinal and transverse diffusivities will be
determined using a Matlab algorithm. The amount of moisture absorbed into the composite due
to wicking will be quantified from mass balance analysis and related to the longitudinal and
transverse diffusivities.

The objective of this research was to characterize the seawater transport and its effect on the transverse tensile strength of a carbon/vinylester composite. The moisture contents of neat vinylester and unidirectional carbon/vinylester composite panels immersed in seawater were monitored until saturation. A model for moisture up-take was developed based on superposition of Fickian diffusion, and Darcy’s law for capillary transport of water. Both the predicted and measured saturation times increased with increasing panel size, however the diffusion model predicts much longer times while the capillary model predicts shorter time than observed experimentally. It was also found that the saturation moisture content decreased with increasing panel size. Testing of macroscopic and miniature composite transverse tensile specimens, and SEM failure inspection revealed more fiber/matrix debonding in the seawater saturated composite than the dry composite, consistent with a slightly reduced transverse tensile strength.