Tensile impact testing on polymer materials considering the force-oscillation phenomenon

Abstract

Thermoplastic materials are extensively used as a light weight replacement for metal alloys, especially in automotive applications. Furthermore polymer materials can be used to enhance the safety of passengers and pedestrians in a car accident. The state of the art design process for plastics parts is based on Computer Aided Engineering (CAE). Using Finite Element Analysis (FEA), highly dynamic systems can be simulated with a very high accuracy. Considering crash applications, the influence of strain rate on the mechanical behaviour is of paramount importance. To determine the effect of strain rate on the mechanical behaviour, tensile impact tests are conducted at different haul-off velocities. With an increase of the haul-off velocity, the force-oscillation phenomenon becomes significant. The force-oscillation phenomenon is an artifact caused by the dynamic load application in highly dynamic tests. It can be monitored as a superimposition of measured force data with oscillations which increase in amplitude to higher impact velocities.This study looks at the phenomenon of force oscillation in tensile impact testing on viscoelastic materials and its dependency on longitudinal stress wave propagation. Aside a detailed analysis of its origin, a new approach is presented to measure a nearly oscillation free force signal in tensile impact testing on polymer materials, basically independent of the considered haul-off velocity. For this purpose a modification of a standard dumbbell specimen was designed.