Quality control in plane-strain bead-forming processes through the load–displacement curve

Abstract

A theoretical and experimental analysis of a sheet edge-curling operation under plane-strain conditions with translating tools is presented. Special attention is given to describing how preliminary operations influence the final curling when high accuracy is required. Regarding such preliminary operations, it is necessary to carry out a plane-strain bead-forming operation to pre-bend the sheet edge in order to prevent the phenomena of instability from arising in the still-straight edge during the bending operation. Pre-bending is carried out by means of conventionally bending the edge in a V-die and removing the excess material at the extremity. These operations, the first in particular, are difficult to perform within strict dimensional tolerances and, at the same time, to affect the bead-forming process well beyond what a classical theoretical analysis succeeds in bringing to light. Recognition of the characteristic events of bead-forming due to the bead's deformative history is, therefore, made through finite-element analysis of the entire succession of operations considered, with the aim of pointing out the influence of the pre-bending parameters on the final product geometry and recognizing how these parameters affect the load–displacement diagram, thus developing a way to detect `in process' the final product geometry without the need of any measurements. An experimental investigation on the product geometry, carried out on specimens with different pre-bending conditions, confirms the interpretative validity of the theoretical results.