Variability of the effective strain rate in SHTB tests and related effects on the dynamic stress amplification

Giuseppe Mirone,Raffaele Barbagallo,E Buzaud,H Couque,A Cosculluela,E Cadoni

doi:10.1051/epjconf/201818302024

Abstract

Although the standard procedure for SHTB testing is based on the hypothesis that the strain rate is nominally constant during each test, it is known by many experimental evidences that the effective strain rate histories cannot be really constant, and this may cause the function expressing the dynamic amplification of the stress to substantially differ from that inferred under the hypothesis of strain rate constancy. This aspect is stressed out in this work by experiments and by numerical analyses, relating the variability of the strain rate to the finite rise time necessary in Hopkinson bars for reaching the plateau of the loading wave, and to the abrupt large increase in the effective strain rate naturally induced by the necking onset. Also the interplay between the achievement of the strain rate plateau and the achievement of the necking initiation strain is analysed, for assessing if and eventually how the same strain rate plateau achieved before or after the necking onset can affect the resulting dynamic amplification of the stress17 mm from the left and right page margins and justified..

Highlights

The effect of the strain rate on the elastoplastic response of metals consists of a dynamic amplification of the equivalent stress, usually modelled by monotonically increasing functions of the strain rate saturating at strain rate values within the range of those achieved in Split Hopkinson Tensile Bar (SHTB) tests, as in the formulations by Johnson-Cook, Cowper-Symonds and Zerilli-Armstrong
Osovski et al (2013) examined the effects of sample size and boundary conditions on the necking inception and development in dynamically stretched steel specimens concluding that the local ductility of the material is geometry independent, i.e. it is insensible to notches
An interaction between the necking, the effective true strain rate and the dynamic amplification was already identified in previous works (Mirone, 2013; Mirone et al, 2017) where it was found that the necking onset freezes the sensitivity of the stress-strain curve to the strain rate

Summary

Introduction

The effect of the strain rate on the elastoplastic response of metals consists of a dynamic amplification of the equivalent stress, usually modelled by monotonically increasing functions of the strain rate saturating at strain rate values within the range of those achieved in Split Hopkinson Tensile Bar (SHTB) tests, as in the formulations by Johnson-Cook, Cowper-Symonds and Zerilli-Armstrong. An interaction between the necking, the effective true strain rate and the dynamic amplification was already identified in previous works (Mirone, 2013; Mirone et al, 2017) where it was found that the necking onset freezes the sensitivity of the stress-strain curve to the strain rate. The necking onset and the post necking evolution are firstly investigated under different static and dynamic straining histories. Camera-assisted static/temperature/dynamic experiments by Split Hopkinson tensile bars (SHTB) with AISI304 steel are discussed for investigating how the dynamic amplification evolves beyond the necking

Stress-strain definitions and general hardening models

Criteria of instability and necking onset

Static Dynamics and mixed experiments with AISI 304

Strain

Findings

Conclusions