Experimental methodology for the measurement of plasticity on metals at high strain-rates

Alexander Sancho,Mike J Cox,Giles Aldrich-Smith,John P Dear,Paul A Hooper,Tim Cartwright,Catrin M Davies,E Buzaud,A Cosculluela,E Cadoni,H Couque

doi:10.1051/epjconf/201818302063

Abstract

An experimental methodology has been developed for the tensile characterisation of ductile isotropic metals at high strain-rate. This study includes the region beyond plastic instability or necking, which is rarely analysed for conventional applications. The research explores an imaging technique used to track the geometry of the specimen during tensile tests and calculate true local values of stress and strain by applying Bridgman theory [1]. To improve the quality of the images taken at high strain-rate an in-situ high speed shadowgraph technique has been developed, and to obtain better results from the images a sub-pixel accuracy edge detection algorithm has been implemented. The technique has been applied to an austenitic stainless steel. Its tensile behaviour has been assessed by testing round samples at strain-rates ranging from quasi-static to ~103 s-1. The results obtained with the proposed methodology have been validated by comparison with more conventional techniques such as video-extensometer and digital image correlation in the pre-necking region and good performance even at the highest strain-rate tested has been proved.

Highlights

The experimental measurement of tensile properties of metals is one of the most extended tests in materials engineering
When necking starts the video extensometer and digital image correlation (DIC) results are not representative though, while the proposed method shows the real increase in local stress and strain after necking and before fracture
To overcome the challenges that high speed imaging imposes on resolution and contrast of images an in-situ shadowgraph method combined with a sub-pixel accuracy edge detection algorithm have been implemented

Summary

Introduction

The experimental measurement of tensile properties of metals is one of the most extended tests in materials engineering. The aim of this work is to establish a set of experiments that can be applied for the tensile characterisation of metals at any deformation strain-rate. The results obtained can be used for improved plasticity models calibration. Many authors have addressed the tensile characterisation of metals at high strain-rate being some examples the work done by Johnson and Cook on copper, iron and steel [2,3], Stout and Follansbee on austenitic steel [4], Smerd et al on aluminium sheet [5] and El-Magd and Abouridouane on aluminium, magnesium and titanium alloys [6]. The experimental data obtained from these results is generally used to calibrate plasticity models that include the effects of strain-rate on the yield and hardening behaviour. Some of the most commonly used models for this purpose are those developed by Johnson and Cook [2], Zerilli and Armstrong [7], Bai and Wierzbicki [8] or the MTS (mechanical threshold stress) model [9]

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