Measurement of directional anisotropy coefficients for AA7020-T6 tubes and prediction of forming limit curve

Abstract

The mechanical properties of tubes in hoop directions can be measured with ring hoop tensile test (RHTT). This test involves placing a ring sample of tube over the two D-shaped mandrels which can be parted with a testing machine. In this article, the goal was measuring the directional anisotropy coefficients of hydroformed AA7020-T6 tubes (r0, r45, and r90) to predict forming limit diagram (FLD). Due to deriving anisotropy coefficients in different directions, various tensile tests should be performed for the tube. For longitudinal and circumferential samples, a simple tensile test and ring hoop tensile test were applied to obtain r0 and r90, respectively. Additionally, for testing in 45° to the rolling direction, a new testing method which is inspired by RHTT was used to find r45. Afterwards, forming limit diagram of this aluminum alloy was predicted theoretically based on the Marciniak-Kuczynski (M-K) model and Voce hardening law. Since, one of the important factors in gaining accurate FLDs is yield function. Two advanced yield functions, BBC2008 and YLd2011, were selected. These advanced yield functions can describe material’s anisotropy behavior better than the classical ones. The importance of the directional anisotropy coefficients and other data obtained by tensile tests was in calibration of yield functions. Good agreement between the FLD derived under the influence of a calibrated yield function and the experimental FLD supports the present calibration.