Limit strains in the processes of stretch-forming sheet metal

The process of the loss of stability is analysed for sheet metal subjected to biaxial tension when the ratio of the principal stresses 0.5 ⩽ σ2/σ1 ⩽ 1. The loss of stability manifests itself by a groove running in a direction perpendicular to the larger principal stress. In this groove local strains begin to concentrate gradually. In the initial stage of the process the deepening of the groove is associated with a gradually fading strain in the regions adjacent to the groove. This fading strain attains a certain limiting value ε∗. This paper contains both experimental results and a theoretical analysis of the process of the generation of the groove based on anisotropic plasticity theory. The system of equations derived was solved numerically with the aid of a computer, which enabled the limiting strain of the sheet metal to be determined as a function of the following properties of the material: (i) Initial inhomogeneity of the sheet metal, (ii) exponent of the strain-hardening function, (iii) coefficient of normal anisotropy, (iv) initial plastic strain, (v) strain at which the fracture occurs. The results are discussed and the properties are described that influence the drawability of sheet metal used in the stretch-forming process.