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A method was developed for testing and characterizing composite materials in compression at strain rates up to approximately 500 s−1. The method utilizes a thin ring specimen, 10.16 cm (4 in.) in diameter, 2.54 cm (1 in.) wide, and six to eight plies thick. This specimen is loaded by an external pressure pulse applied explosively through a liquid in a specially designed fixture. Pressure is measured by means of a calibrated steel ring instrumented with strain gages. Strains in the composite specimen are measured with strain gages applied to the inside surface. Strains in the calibration and specimen rings are recorded with a digital processing oscilloscope. The equation of motion is solved numerically and the data are processed by the microcomputer attached to the oscilloscope. Results are obtained and plotted by an x-y plotter in the form of a dynamic stress-strain curve. Unidirectional 0-deg and 90-deg graphite/epoxy rings were tested at strain rates up to 510 s−1. Times to failure ranged between 85 μs and 130 μs. The short duration of the loading results in material failure prior to buckling. The 0-deg properties, which are governed by the fibers, show some increase in initial modulus over the static value but no change in strength. The 90-deg properties, which are governed by the matrix, show much higher than static modulus and strength. In all cases the dynamic ultimate strains are lower than the static values by as much as 33%.
composite materials, graphite/epoxy, dynamic response, compression test methods, strain rate effects, characterization, data processing, dynamic compression
Professor and Director, Illinois Institute of Technology, Chicago, Ill.
Broutman and Associates, Chicago, Ill.