Dynamic tensile testing of plastic materials

Abstract

Dynamic tensile tests were carried out using a servo-hydraulic machine on four representative plastic materials following a practice guideline by the Society of Automotive Engineers (SAE). The experimental results generated this way were ranked as “good” using the qualitative measure provided in the guideline. To advance our understanding of dynamic tensile tests, the validity of the dynamic tensile tests was investigated by examining the condition of dynamic stress equilibrium; a criterion used in split Hopkinson pressure bar (SHPB) tests. The results show that the quantitative criterion for a valid SHPB test is also applicable to dynamic tensile tests and it is an unbiased method as compared to the qualitative method used in the current practice. Another issue in dynamic material testing using a tensile testing machine is system ringing. The testing system was analyzed using a basic vibration model. The analytical solutions were obtained for the one-degree freedom spring-mass model with and without a damper. The mathematical model provided descriptions about the influence of the loading rate and the natural frequency of the testing system on the magnitude of system ringing and its decay rate. It illustrated that the maximum strain rate at which a testing system can be used to generate acceptable data are limited by the natural frequency of the testing system. The analysis results agreed well with the empirical relationships established based on experimental evidence. For the first time, this work advances the dynamic tensile testing technique from its current empirical stage to an analysis-based level.