Investigating slow shock in low-impedance materials using a direct impact Hopkinson bar setup

Puneeth Jakkula,Stefan Hiermaier,Georg Ganzenmüller,Samuel Beisel,F Gálvez Díaz-Rubio,D.A Cendón Franco

doi:10.1051/epjconf/202125006009

Abstract

This work implements a direct impact Hopkinson bar, suitable for investigating the evolution of dynamic force equilibrium in low-impedance materials. Polycarbonate as the bar material favours for a long pulse duration of 2.6 ms for an overall length of only 5 m, allowing to compress large specimens to high strains. This setup is applied to polyurethane foams with different densities ranging from 80 - 240 kg/m3. Dynamic compression tests are performed at strain rates of 0.0017, 0.5 and 500 /s on the foams at room temperature. Depending on density, they show a saturation in increase of yield strength at strain rates of 500 /s, or even show a negative strain rate sensitivity for the lowest density. This behaviour is explained by comparing the dynamic force equilibrium to a phenomenon similar to shock in solid materials: For low densities and high rates of strain, homogeneous compression is replaced by a localized collapse front with a jump in stress across the front. Digital image correlation is performed to analyse elastic and plastic compaction waves by means of Lagrange diagrams.

Highlights

Dynamic material testing of low density soft materials has traditionally been challenging [1]
This behaviour is explained by comparing the dynamic force equilibrium to a phenomenon similar to shock in solid materials: For low densities and high rates of strain, homogeneous compression is replaced by a localized collapse front with a jump in stress across the front
The Symmpact setup presented here is useful for investigating the evolution of dynamic force equilibrium

Summary

Introduction

Dynamic material testing of low density soft materials has traditionally been challenging [1]. During the loading of a specimen, the stress state in the specimen requires a certain time to propagate through it, because the loading is only from one side. It reaches its limits when applied to low impedance materials: it is challenging to resolve the force amplitude due to low specimen strength using metallic bars, and it takes long time to reach dynamic force equilibrium because of the large impedance mismatch between specimen and the bar material. These problems can be addressed by switching to polymer bars. The smaller impedance mismatch increases the sensitivity of force measurement

Methods

Results

Conclusion