New Methods for Fracture Detection of Automotive Steels

Abstract

The VDA 238-100 tight radius bend test has received significant attention from industry over the past decade because it provides a proportional plane strain–plane stress state until fracture. It will be demonstrated that the adoption of the vertical punch force as the unique metric for failure detection can lead to false positives. The punch force will reduce at large bend angles due to the mechanics of the test even in the absence of material failure. Two novel detection methods based on the nominal principal stress and the plastic work were evaluated on six different steel grades with a nominal ultimate tensile strength of approximately 590 MPa. Three 590R advanced high-strength steels provided by three different suppliers and three variants of the Ductibor^® 500 are studied. The stress-based metric can be seen as an improvement over the VDA 238-100 load methodology since it accounts for thinning of the specimen cross-section and can identify false positives when the material forms a full bend in the absence of fracture but drops at bend angles greater than 160°. For performance ranking of materials with the same strength level, the plastic work metric is found to be sufficiently sensitive to distinguish between various 590 MPa steel grades since the material hardening rate is directly embedded.