Development of “Dropkinson” Bar for Intermediate Strain-rate Testing

Abstract

A new apparatus – “Dropkinson Bar” – has been successfully developed for material property characterization at intermediate strain rates. This Dropkinson bar combines a drop table and a Hopkinson bar. The drop table is used to generate a relatively long and stable low-speed impact to the tensile specimen, whereas the Hopkinson bar principle is applied to measure the load history with accounting for inertia effects in the system. In addition, pulse shaping techniques were applied to the Dropkinson bar to facilitate uniform stress and strain as well as constant strain rate in the specimen. The Dropkinson bar was used to characterize 304L stainless steel and 6061-T6 aluminum at a strain rate of ~600 s−1. The experimental data obtained from the Dropkinson bar tests were compared with the data obtained from conventional Kolsky tensile bar tests of the same material at similar strain rates. Both sets of experimental results were consistent, showing the newly developed Dropkinson bar apparatus is reliable and repeatable.

Review of Intermediate Strain Rate Testing Devices

“…In recent years, very little effort has been made to develop special apparatus and devices that will allow drop towers to perform tests under various loads using accurate load measurement techniques such as the special piezoelectric load cell and Hopkinson bar technique [62,63]. In 2018, a new tensile testing device, the "Dropkinson" bar, inspired by the drop tower, was developed by Bo Song and his collaborators [42], which is capable of testing at an intermediate strain rate shown in Figure 3. Unlike the other devices explained above, the Dropkinson bar system is the same hybrid test rig.…”

“…Over the last decade, some researchers have developed specialized test machines capable of performing intermediate strain rate tests to study material characterization, such as special commercial machines designed by Instron, Shimadzu, etc. [37,38]; elastic-bar-type systems (ISO 26203-1: 2018) [39], modified servo-hydraulic load frames (ISO 26203-2:2011) [40], hybrid testing apparatus [41], drop tower [42], flywheel device [43], and flywheel wedge [44].…”