Determination of edge strain in DP steel sheet for predicting edge cracking

Abstract

.This contribution is concerned with determining the edge strain in edges with different finish qualities associated with crack formation in the free edge of a deep drawing blank. The sheared edge quality is of major importance, particularly in high-strength steels which are used for safety components. The purpose of this study was to determine for various parting processes for sheet metal the limit strain which leads to cracking in the free edge. The parameter which was varied in this investigation was the parting process that produced the edge: shearing and machining. A special fixture was employed for measuring the strain value. The values were verified using FE modelling with AutoForm software. The surface finish qualities of the edges were compared using a special test procedure, the Diabolo test, which had been previously described in publications by Held [1] and Liewald and Gall [2]. In this test, the sheared edge is subjected to loading under plane strain conditions. Another test of susceptibility to edge cracking is the hole expansion test [3], in which crack is induced in a circular hole in the centre of a part. The limit edge strain can be identified using a DIC optical measuring system which monitors strain increments in the course of the test. Thanks to these results, the factors which affect the susceptibility of the sheared edge to edge cracking material have been identified and validated in the DP1000 material.