Ductile Fracture from Spot Weld and Flange Edge in Advanced High Strength Steels

A simple testing method is proposed in order to investigate ductile fracture in crashed automotive components made from advanced high strength steels. This type of fracture is prone to occur at spot-welded joints and flange edges. It is well known that the heat affected zone (HAZ) is a weak point in high strength steel due to the formation of annealed material around the spot-welded nugget, and the flange edge also has low ductility due to the damage caused by shearing. The proposed method is designed to simulate a ductile fracture which initiates from a spot-welded portion or a sheared edge in automotive components which are deformed in a crash event. Automotive steel sheets with a wide range of tensile strengths from 590MPa to 1470MPa are examined in order to investigate the effect of material strength on fracture behavior. The effects of material cutting methods, namely, machining and shearing, are also investigated. A digital imaging strain analysis system is applied into the fracture test to analyze the strain distribution at the spot weld and the flange edge. The experimental results indicate the criterion for fracture in spot-welded specimens. Based on the experimental results, a FE simulation is carried out considering the change in material properties of the heat-affected material around spot weld. The FE simulation showed good agreement with the experimental results in the force-stroke curve and the strain history around the spot weld. Further study is expected to improve the accuracy of the fracture prediction based on the experimental results obtained by the proposed simple experimental method.