Abstract

Realizing the lack of material data of plastic materials at higher loading rates, a study was conducted to develop experimental method(s) in which the impact properties of plastics can be properly characterized. The method chosen was instrumented impact testing where a traditional impact testing apparatus was outfitted with instruments to measure load time history during impact. Three types of instrumented impact tests were performed: Tension impact, Izod impact, and Charpy impact tests.

The tension impact and Charpy impact tests proved to be a convenient method of characterizing the impact properties, i.e., yield strength, modulus of elasticity, modulus of resilience, and modulus of toughness of plastics. The Izod impact, however, proved to be unsuitable due to inherent difficulty in gripping the plastic specimen rigidly in the tester. Specimen slippage in the grips resulted in a deflection of the specimens that were erroneously high.

In all, six grades of structural plastics were tested. They were two grades of polycarbonate (PC), two grades of acrylonitrile-butadiene-styrene (ABS), and two grades of PC/ABS blends. They were tested at impact loading rates of 1.52 m/s (5 ft/s), and 3.35 m/s (11 ft/s). The properties of the plastics tested were indeed sensitive to loading rate. The amount and trend of this rate sensitivity, however, differed from one plastics or different grades of the same plastic to another. For the PC and ABS tested, the modulus of elasticity, yield strength, and the modulus of resilience always increased with the increase loading rates while the strain at failure did not change significantly with changing loading rates. For the PC/ABS blends, however, the materials undergo a ductile-brittle transition at the impact loading rates studied. As a result, there was a dramatic reduction in strain at failure and the modulus of toughness as the loading rate increases. The changes in material behavior with impact loading conditions illustrates the importance of testing plastic material at loading rates which the material may experience in actual usages.

Last Modified 
Jun 2004