Development and Validation of a Hopkinson Bar for Hazardous Materials

R.M Quinn,J.P Dear,P.A Hooper,D Townsend,G Aldrich-Smith,T Cartwright,M.J Cox,L.H Zhang

doi:10.1007/s11340-020-00638-w

Abstract

BackgroundThere are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure.ObjectivesA Split-Hopkinson pressure bar (SHPB) testing technique is presented which has been specifically developed for the characterisation of hazardous materials such as radioactive metals. This new SHPB technique is validated and a comparison is made with results obtained at another laboratory.MethodsCompression SHPB tests are performed on identical copper specimens using the new SHPB procedures at Imperial College London and confirmatory measurements are performed using the well-established configuration at the University of Oxford. The experiments are performed at a temperature of 20 ∘C and 200 ∘C. Imperial heat the specimens externally before being inserted into the test position (ex-situ heating) and Oxford heat the specimens whilst in contact with the pressure bars (in-situ heating). For the ex-situ case, specimen temperature homogeneity is investigated both experimentally and by simulation.ResultsStress-strain curves were generally consistent at both laboratories but sometimes discrepancies fell outside of the inherent measurement uncertainty range of the equipment, with differences mainly attributed to friction, loading pulse shapes and pulse alignment techniques. Small metallic specimens are found to be thermally homogenous even during contact with the pressure bars.ConclusionA newly developed Hopkinson bar for hazardous materials is shown to be effective for characterising metals under both ambient and elevated temperature conditions.

Highlights

There are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure
In addition to describing and validating the newly developed procedures, this study aims to provide some useful insight into the inter-laboratory variance which might be expected in Hopkinson bar results
Compression specimens for non-dynamic testing would typically be of a different geometry and of much greater size [26], but in this study, the same specimen geometries were used for consistency

Summary

Introduction

There are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure. High strain rate experimental data is required to calibrate material models which describe the behaviour of dynamic processes. The compression split-Hopkinson pressure bar (SHPB) test is not subject to any agreed standards regarding the operation or design of the apparatus [1]. This leads to an unknown degree of variance in results attained by different laboratories. The measured flow stress is known to be influenced by factors which include specimen size and specimen aspect ratio [3], interfacial friction [4, 5] and data processing/interpretation techniques [6]. The bar diameter and length is important as the pressure wave undergoes a process known as dispersion; whereby different frequency components exhibit different propagation velocities, resulting in a pressure wave whose form is evolving with respect to time [8, 9]

Objectives

Methods

Results

Conclusion