Plane strain tension fracture at high strain rate

Morwan Adlafi,Laurent Mahéo,Dirk Mohr,Vincent Grolleau,Christian C Roth,Bertrand Galpin,F Gálvez Díaz-Rubio,D.A Cendón Franco

doi:10.1051/epjconf/202125001020

Abstract

Under plane stress conditions, most micromechanical and phenomenological models predict a minimum in ductility for plane strain tension stress state. Therefore, the stress state of plane strain tension plays a crucial role in many forming and crash applications and the reliable measurement of the strain to fracture for plane strain tension is particularly crucial when calibrating modern fracture initiation models. Recently, a new experimental technique has been proposed for measuring the strain to fracture for sheet metal after proportional loading under plane strain conditions. The basic configuration of the new setup includes a dihedral punch which applies out-of-plane loading onto a Nakazima-type of discshaped specimen with two symmetric holes and an outer diameter of 60 mm. In the present work, the applicability of the test is extended to high strain rates. High strain rates of about 100/s to 200/s are obtained using a drop weight tower device with an original sensor for load measurements. Quasi static tests are also performed for comparison, keeping the same specimen geometry, image recording parameters and set-up. The effective strains at fracture are compared from quasi-static to high strain rate loading for three different materials, i.e one aluminium alloy and two steels.

Highlights

The plane-strain-tension stress state plays a crucial role in many drawing applications as well as in crash events
Most micro-mechanical and phenomenological models predict a minimum in ductility for a plane strain tension stress state
We present the use of the dual hole plane strain tension mini Nakazima test with dihedral punch (DMN) adapted to a drop-tower apparatus to perform experiments at high strain rates in the range of several 100/s

Summary

Introduction

The plane-strain-tension stress state plays a crucial role in many drawing applications as well as in crash events. The dual-hole mini-Nakazima test with a dihedral punch (Fig. 1a), creates a constant stress state [2] It is composed of a disc-shaped specimen (component 1 Fig. 1) with an outer diameter of 60 mm and two 8 mm diameter circular cut-outs (2) that form a 10 mm long central ligament. With a so-called inverse configuration, where the specimen (component 1) is pushed against the stationary dihedral punch (component 3), the displacement field of the specimen surface can be monitored using Digital Image Correlation (DIC) This test leads to high major-tominor strain ratios (above 20) and a homogenous plane strain tension stress state along 80% of the ligament length. We present the use of the dual hole plane strain tension mini Nakazima test with dihedral punch (DMN) adapted to a drop-tower apparatus to perform experiments at high strain rates in the range of several 100/s. We compare fracture strain measurements from high and low strain rate experiments on a 1.5 mm thick AA2024 aluminium alloy and two DP steels, a 0.8 mm thick DP450 and a 1.5 mm thick DP600

Experimental set-up

Experimental results

Conclusion