Strain rate and thermal softening effects in shear testing of AA7075-T6 sheet

Taamjeed Rahmaan,Cliff Butcher,Ping Zhou,Michael J Worswick,E Buzaud,A Cosculluela,E Cadoni,H Couque

doi:10.1051/epjconf/201818302037

Abstract

Shear tests were performed at strain rates ranging from quasi-static (0.01 s-1) to 500 s-1 for AA7075-T6 sheet metal alloy at room temperature. A miniature sized shear specimen was used in this work to perform high strain rate shear testing. Digital image correlation (DIC) techniques were employed to measure the strains in the experiments. At maximum in-plane shear strains greater than 20%, the AA7075-T6 alloy demonstrated a reduced work hardening rate at elevated strain rates. At lower strains, the AA7075-T6 alloy showed mild positive rate sensitivity. The strain to localization (using the Zener-Holloman criterion), measured using the DIC technique, decreased with strain rate in shear loading. The strain at complete failure, however, exhibited an increase at the highest strain rate (500 s-1). The current work also focused on characterization of the thermal conditions occurring during high rate loading in shear with in situ high speed thermal imaging. Experimental results from the highest strain rate (500 s-1) tests showed a notable increase in temperature within the specimen gauge region as a result of the conversion of plastic deformation energy into heat.

Highlights

Vehicle weight reduction has been traditionally achieved by using advanced lightweight high strength metal alloys within automotive structures and body panels, while maintaining strength and crash performance
An isothermal assumption during plastic deformation is valid in the case of low strain rate experiments since the heat generated within the specimen is dissipated either through conduction to the grip regions or through convection to air [7]
El-Magd and Abouridouane [11] demonstrated that the flow curves at quasi-static condition increased continuously with increasing strain due to strain hardening, while at higher strain rates (≥ 1000 s-1) the flow curves increased only at lower strains and decreased beyond a stress maximum due to thermal softening caused by the adiabatic temperature rise in the specimen

Summary

Introduction

Vehicle weight reduction has been traditionally achieved by using advanced lightweight high strength metal alloys within automotive structures and body panels, while maintaining strength and crash performance. Conditions become more adiabatic at high strain rates since the flow stress is higher and the heat generated cannot be dissipated immediately [8,9]. Rahmaan et al [2] conducted shear tests on AA5182-O sheet metal alloys, and demonstrated a decrease in strain to localization with strain rate due to the effect of temperature rise. El-Magd and Abouridouane [11] demonstrated that the flow curves at quasi-static condition increased continuously with increasing strain due to strain hardening, while at higher strain rates (≥ 1000 s-1) the flow curves increased only at lower strains and decreased beyond a stress maximum due to thermal softening caused by the adiabatic temperature rise in the specimen. Only a limited number of studies are available on the effect of strain rate on the constitutive and fracture response of 7000-series aluminum sheet alloys under shear loading

Methods

Results

Conclusion