Identification of elasto-plastic characteristics by means of air-bending test

Abstract

A new identification method of material elasto-plastic characteristics by means of simple testing is proposed. The outcome of the procedure is the true-stress versus true-strain curve. The result is proposed in analytical form, according to Swift's law. The method requires acquisition of actual force-penetration values, carried out during a standard air-bending test. One of the main advantage of this method is that it allows an immediate identification of the material behaviour just in-process, that is to say that material characteristics can be accounted differently for each sheet subjected to bending; this is a fundamental preliminary step to achieve a bending adaptive control of sheets. Input requirements for identification include the geometry of tools and sheet, and the friction coefficient between sheet and die which needs to be estimated previously. In the proposed method, at a first stage the bending moment–curvature curve for the sheet is obtained by the numerical computation of force-penetration data through a multi-joint model of sheet during air-bending. Then, a simplified bending model, characterized by linear strain distribution across the thickness and plane-stress assumption, is employed to calculate the stress–strain curve.