Characterizing plasticity and fracture of sheet metal through a novel in-plane torsion experiment

V Grolleau,C C Roth,D Mohr

doi:10.1088/1757-899x/651/1/012101

Abstract

In-plane shear specimens typically feature free gage section boundaries along which the state of stress deviates from that of pure or simple shear. As a consequence, the geometry of in-plane shear specimens needs to be carefully chosen to avoid any early fracture initiation from the free boundaries, before the actual failure strain for shear is reached at the specimen center. From this perspective, disc specimens for in-plane torsion experiments offer a significant advantage: they do not feature any free boundaries. However, detailed analysis suggests that circular groove need to be introduced (local thickness reduction) to ensure a strain localization away from the clamped specimen shoulders. In most existing in-plane torsion tests, the specimen is clamped on the inner diameter by applying out-of-plane compression to avoid any slipping. In such configurations, it is impossible to monitor the entire sheared circumference with cameras for digital image correlation. It is the goal of the present work to develop in-plane torsion test using grooved specimens with full optical access to the specimen for 2D or 3D DIC measurements. Furthermore, the experimental set-up will be designed for plasticity and fracture characterization at strain rates of up a few 100/s. Its main feature is a new clamping technique. After identifying a suitable specimen geometry through finite element simulations, experiments are performed on specimens extracted from aluminum alloys and steels sheets. The experimental campaign includes proportional loading, reversed loading and strain rate jumps. The full optical access to the sheared gage section area also enables the discussion of the effects of plastic anisotropy on the strain fields in in-plane torsion experiments. The results from the in-plane torsion experiments are also compared with the fracture strain measurements from in-plane shear experiments performed in a conventional uniaxial loading frame.

Highlights

There is a constant quest for reliable experimental data characterizing the plastic and fracture response of materials
The geometry of in-plane shear specimens needs to be carefully chosen to avoid any early fracture initiation from the free boundaries, before the actual failure strain for shear is reached at the specimen center
It is impossible to monitor the entire sheared circumference with cameras for digital image correlation. It is the goal of the present work to develop in-plane torsion test using grooved specimens with full optical access to the specimen for 2D or 3D DIC measurements

Summary

Introduction

There is a constant quest for reliable experimental data characterizing the plastic and fracture response of materials. Another path is pursued by in-plane torsion experiments, in which a gage section clamped between an inner and an outer grip is deformed by rotational loading (Fig. 1b-d). The main drawback of the standard in-plane torsion test of flat specimen is that shear strain localization may occur at the inner clamping diameter To overcome this drawback and to reduce the sheared section size and the loading torque, some researchers machined annular slits on the specimen (Fig. 1c) [4], thereby accepting inhomogeneous strain fields and possible fracture from the free edges. The results from the in-plane torsion experiments are compared with fracture strain measurements from recently developed shear experiments performed in a conventional uniaxial loading frame (see Roth and Mohr [7] for details on the technique)

Design of in-plane torsion experiment

Experiments

Findings

Discussion and conclusion