High strain rate properties of unidirectional composites, part 1

Experimental methods were developed for testing and characterization of composite materials at strain rates ranging from quasi-static to over 500 s(sup -1). Three materials were characterized, two graphite/epoxies and a graphite/S-glass/epoxy. Properties were obtained by testing thin rings 10.16 cm (4 in.) in diameter, 2.54 cm (1 in.) wide, and six to eight plies thick under internal pressure. Unidirectional 0 degree, 90 degree, and 10 degree off-axis rings were tested to obtain longitudinal, transverse, and in-plane shear properties. In the dynamic tests internal pressure was applied explosively through a liquid and the pressure was measured with a calibrated steel ring. Strains in the calibration and specimen rings were recorded with a digital processing oscilloscope. The data were processed and the equation of motion solved numerically by the mini-computer attached to the oscilloscope. Results were obtained and plotted in the form of dynamic stress-strain curves. Longitudinal properties which are governed by the fibers do not vary much with strain rate with only a moderate (up to 20 percent) increase in modulus. Transverse modulus and strength increase sharply with strain rate reaching values up to three times the static values. The in-plane shear modulus and shear strength increase noticeably with strain rate by up to approximately 65 percent. In all cases ultimate strains do not vary significantly with strain rates.