On the Origin and Handling of the Force Oscillation Phenomenon in Tensile Impact Testing of Polymer Materials

Abstract

Stress respectively force oscillation in mechanical testing is a phenomenon that can be monitored independent of material, stress state and test setup. This study looks at the phenomenon of force oscillation in tensile impact testing on viscoelastic materials and its dependency on longitudinal stress wave propagation. In addition to the extensive research on stress wave propagation and the force oscillation phenomenon, published within the last decades, the Society of Automotive Engineering (SAE) also gives valuable recommendations regarding tensile impact testing on polymer materials. Existing limitations to tensile impact testing reported earlier can be overcome using the approach outlined in this paper. Furthermore, we give an analysis to stress wave propagation in tensile impact testing and its correlation to force oscillations: We find that (1) longitudinal stress wave propagation in tensile specimens cannot be considered to be the origin of the force oscillation phenomenon - even more so it can be neglected, (2) specimens in tensile impact testing are exposed to a nearly oscillation free loading which is presented for haul-off velocities of up to 10 m/s and (3) it is not possible to achieve force equilibrium in the load train conducting tensile impact tests. In addition, we present an approach to measure a nearly oscillation free force signal in tensile impact testing on polymer materials up to 10 m/s. For this purpose a modification of a standard dumbbell specimen is introduced. Measurements presented in this paper are conducted using state of the art 3D high-speed deformation measurement technology based on Digital Image Correlation (DIC).