Evolution model: an anisotropic hardening model for the description of orthotropic sheet metal materials

Abstract

The accuracy of material models is of significant importance as the use of such modelling techniques are fast becoming a cornerstone in the design process. This is particularly true for sheet metal forming operations where the traditional methods of assessing formability for new designs by the use of prototypes adds substantial cost and lead time to the end product. The competitive advantage to be gained by accurate FE modelling of forming operations is then clear.

For sheet metals, modelling of the anisotropic hardening behaviour is a particular challenge inhibiting accurate simulation. Many models have been proposed that attempt to capture the hardening behaviour of orthotropic sheet metal materials, however the often cumbersome mathematical representations make such approaches unsuitable for implementation in FE packages tailored to sheet metal forming simulation. As a result commercially available packages generally implement simpler, generic models such as isotropic and kinematic hardening.

An alternative approach to modelling the hardening observed for sheet metal material is presented. Experimental data from four different test methods (Uniaxial tension, hydraulic bulge, uniaxial compression and plane strain compression) conducted on both as-received aluminium alloy 7075-O and sheet prestrained in uniaxial tension to four level of prestrain (1%, 2%, 5% and 10%) is presented. The data indicated a specific material response. The near-isotropic initial sheet material developed orthotropic characteristics due to the prestrain. This resulted in the yield surface evolving at different rates along each of the orthogonal axes.

From consideration of the mechanics of the material behaviour a concise method for determining large strains at any angle from the orthogonal axis is first presented. An approach to modeling the hardening behaviour is then developed. The proposed approach is based on the strain relations from the orthogonal frame and the use of readily available test data for the calibration of anisotropic constants in the yield criterion for a given plastic strain. The approach is first implemented using an equivalent hardening curve and is then further developed to include unique hardening curves for the calibrated orientations.

The efficacy of the proposed theory is illustrated through comparison of the model with results of the experimental programme.

Date of Award	Sept 2014
Original language	English
Sponsors	Letterkenny Institute of Technology & Department of Education and Learning (NI)
Supervisor	Alan Leacock (Supervisor) & Desmond Brown (Supervisor)