In-plane biaxial compression and tension testing of thin sheet materials

Abstract

a b s t r a c tDetailed experimental data on the behavior of textured sheet metals under compressive loading is important to describe their tension-compression asymmetry. This is particularly needed for materials that exhibit a strength-differential effect, or in cases where the Bauschinger effect occurs. So far, there is no systematic work describing the third quadrant in the 2D stress space under biaxial compressive loading. This paper presents a new device for biaxial, compressive in-plane testing of thin sheets. Biaxial and uniaxial compression experiments are carried out in the strain controlled device, analyzing the behavior of deep drawing steel sheets with and without skin-pass treatment. Moreover, in order to allow for the experimental description of the yield surfaces, biaxial tensile tests are performed. Detailed numerical validations and experimental strain analysis both for the new specimen for biaxial compressive testing and for the cruciform specimen for biaxial tensile testing show that reasonably homogeneous strain distributions can be achieved. The combined experimental and numerical method presented here allows to evaluate the tension-compression asymmetry of thin sheet materials. The results for the skin-passed condition clearly exhibit a tension-compression asymmetry, which highlights the necessity of biaxial compression tests already in the as-received material condition. The biaxial compression test opens a pathway to a more detailed analysis of the flow behavior of thin sheets under biaxial compression loading.

On the identification of an effective cross section for a cruciform specimen

“…(a) Geometry of the cruciform specimen used in this study . For a sheet thickness of 1 mm, the smallest cross section is 14.58 mm 2 .…”

“…For the biaxial tensile tests with our specimen (Figure a), the set‐up described in Zillmann was used. Each of the four drives of the Zwick/Roell electromechanical testing machine is equipped with a load cell (maximum load capacity 20 kN).…”

“…We however highlight again that the subsequent calculation of A eff was based on the comparison of the initial yield strength (a material parameter used as input for the numerical simulation) and the necessary external forces divided by A eff , not on a comparison between experimental data and simulation results. To account for the different stress states (uniaxial vs. biaxial loading), we used the method of equivalent plastic work as a generalised criterion to determine initial yield stresses; details on this well‐known approach have been published in Zillmann et al…”

“…One approach to estimate A eff is based on an analysis of uniaxial tests using material from a pre‐deformed cruciform specimen . Equi‐biaxial testing, for example, in layer compression tests or bulge tests, has also been used to estimate an effective cross section, as have extensive finite element (FE) simulations that provide the most detailed information about the inhomogeneous stress and strain fields in different specimen types subjected to complex load cases. FE analysis has, for instance, been used in previous studies to design and optimise cruciform specimens, with the main goal of providing a large area in the centre of the specimens that exhibits an (almost) homogeneous strain distribution.…”

A generalized thermodynamically consistent distortional hardening model for Mg alloys

“…; ; Shimizu (2011);Zillmann et al (2015). Following , cast AZ31 alloy with specimens of 8mm×8mm×7mm were investigated.…”