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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.

Ultra high strength steels have contributed to weight reduction in automobiles. However, due to the poor formability of these materials, it is difficult to manufacture automotive parts with complex shapes using conventional sheet metal forming techniques such as deep drawing, bulging, and stretch flange forming. In this study, a bend-forming technology which enables forming of complex shapes with ultra high strength steel sheets is investigated. The target part was a front side member extension (FSM EXT) of a white body. The part has a dogleg shape and must provide adequate crashworthiness. The basic structure of the FSM EXT produced by this bend-forming technology was proposed. A bend-forming process consisting of three stages was proposed from the standpoint of commercial production. FSM EXT prototypes showed excellent crashworthiness in FEM simulations and crash tests.

Initiation and propagation of ductile fractures in crashed automotive components made from high strength steels are investigated in order to understand the mechanism of fracture propagation. Fracture of these components is often prone to occur at the sheet edge in a strain concentration zone under crash deformation. The fracture then extends intricately to the inside of the structure under the influence of the local stress and strain field. In this study, a simple tensile test and a 3-point bending test of high strength steels with tensile strengths of 590 MPa and 1180 MPa are carried out. In the tensile test, a coupon having a hole and a notch is deformed in a uniaxial condition. The effect of the notch type on the strain concentration and fracture behavior are investigated by using a digital imaging strain measurement system.