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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.

New highly ductile AHSS steel grades with tensile strength greater than 980 MPa have been developed with the aim of combining high strength and excellent formability. The new jetQ-Family offers high local and global ductility while still fulfilling standards for resistance towards hydrogen embrittlement and weldability. These improved properties are based on their specifically engineered microstructure, which utilize the TRIP-mechanism in a strengthened matrix. This work shows how the microstructure plays a significant role for the tensile testing as well as hole-expansion. Based on the increased yield strength a better crash performance compared to conventional DP steel grades can be attained. The local ductility is demonstrated with excellent hole expansion ratios and high resistance to sheared edge failure. In combination with improved bending angles and thickness strain at fracture a robust process for manufacturing of components can be achieved.

A fundamental study on the ductility of high strength steels under impact deformation is carried out to investigate the effect of the local ductility of various materials on crash performance. In this study, newly developed 980 and 1180 MPa grade steels are investigated to clarify their advantages in term of crash performance compared to conventional DP (Dual Phase) steels. The features of the developed steel, named as jetQ are higher yield strength and higher local ductility due to an optimized microstructure by the quenching and partitioning process (QP) [1, 2]. The bending test according to VDA 238-100 is performed while observing the fracture propagation during the bending test. Fracture strain in the tensile tests is evaluated by a three-dimensional shape measurement technique for the fracture surface. Both three-point bending tests and axial impact tests are performed to evaluate the crashworthiness of different types of steel.