As composite materials become more attractive for use in large Navy structures, the need to understand the mechanical response of composites greater than 6.4 mm (0.25 in.) in thickness becomes a necessity.
In this program, a compression test fixture that allows the testing of composites up to 25.4 mm (1 in.) in thickness and greater was designed and refined. This fixture was used to evaluate the effects of constituents, fiber orientation, and thickness on the compressive response of composite materials. In addition, the fixture was used to determine if the failure mechanisms observed for thick composites are similar to those that have been observed and reported for composite materials less than 6.4 mm (0.25 in.) thick.
The in-plane moduli, in-plane and through-thickness Poisson's ratios, compression strength, and failure mechanisms of the thick composites were shown to be independent of material thickness. The predominant failure mechanisms for both materials were kink bands and delaminations, and were identical in geometry to those that have been reported for composite materials in the range of 2.54 mm (0.1 in.) thick.
Although unchanged with thickness, the through-thickness Poisson's ratio for the carbon- and glass-reinforced laminates were found to be significantly nonlinear, resulting in changes in this property of up to 58% from the initial region of the strain-strain curve to the final region of the strain-strain curve.