On the Experimental Detection of Necking in Stretch-Bending Tests

The standard ISO 12004-2 has recently established a methodology for estimating limit strains in Marciniak and Nakajima tests, that is, situations in which the strain gradient through the sheet thickness does not play an important role. This method, known as a "position-dependent method", is based on the analysis of the major strain profile just before fracture occurrence. The present paper proposes an alternative method for detecting the onset of necking and determining the forming limits, based on temporal analysis of the major strain. This time-dependent methodology is applicable to more general situations where strain gradients in the sheet cannot be neglected, such as severe stretch-bending tests, small radius punches, etc. A series of stretching and stretch-bending experiments, close to plane strain conditions, have been carried out to check the ability of a number of methods to predict the onset of necking. Finally, a comparison of these methodologies is presented and their advantages and limitations are discussed in detail.