High strain rate characterization of woven carbon/epoxy composites

Woven composite materials are known to exhibit non-linear, rate-dependent behavior in both the normal and off-axis directions, which becomes extremely important in developing models capable of accurately predicting structural response to high velocity impact loadings. Predicting accurate structural response, in turn, relies on the development of reliable damage constitutive models, for which experimental data at high strain rates is required. This paper summarizes tensile and biased extension experiments, which were performed on two woven composite materials at quasi-static and high strain rates. 2×2 twill woven carbon/epoxy and plain woven carbon/epoxy composite specimens were machined to a dog-bone shape, and tested using a hydraulic test machine and Hopkinson bar set up. A specially developed fixture was designed and machined to house the dog-bone specimens to achieve strain rate tests of approximately 400/sec. The fixture features require that slippage and eccentric loading be minimized. Several samples were instrumented with strain gages to validate the strains calculated using the strain gages on the Hopkinson bars and to obtain the effective gage length of the dog-bone specimen. Experimental results show that for both woven materials the ultimate tensile stresses and ultimate shear stress increased approximately 15% and 65%, respectively, when the strain rate was raised from 10-3/sec to 400/sec.