Abstract
Uni-axial compression tests of two plastics, polycarbonate (PC) and polyacetal (PA), were conducted at various strain rates from 2.2×10-4/sec up to the order of 103/sec by using short cylindrical specimens. Also the same tests were conducted under the static outer pressure of 200 or 400kg/cm2 applied to the circumferential surface of specimens. High strain rates of the order of 102∼103/sec were obtained by a Hopkinson pressure bar apparatus, and some discussions were made on the stress-strain relations obtained. As a result, the stress-strain relations for the materials used showed remarkable strain rate dependence. The ratio of the yield strength of PC without the static outer pressure at the high strain fate 360/sec to that at the low strain rate 2.2×10-4/sec was 1.65, and that of PA was 1.77 under almost the same condition as PC. The relation between the yield strength and the logarithmic strain rate for both PC and PA was found to be non-linear. The inclination in the relation was apt to increase as the strain rate increased. When the static outer pressure was applied to the specimen, the yield strength of the both materials at every strain rate was a little larger than that deduced from Tresca's or Mises' yield condition.
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