This thesis investigates the application of hot stamped ultra-high strength steel (greater than 1000 MPa tensile strength) tailor-welded blanks in vehicle frontal crash energy management structures, as well as their potential for weight savings through sheet material thickness down-gauging. The ultra-high strength steels examined in this thesis are Ductibor® 1000-AS and Usibor® 1500-AS. The vehicle frontal crash structure of focus is the side frame member, which is typically comprised of various gauges of 590 MPa advanced high strength steel, such as JAC590R. The suitability of using hot stamping steels in the side frame member is assessed by comparing the frontal crash performance of a production side frame member (baseline front end module) to a side frame member comprised of tailor-welded hot stamped steels (tailor-welded hot stamped side frame member).

The crash performance of the driver’s side frame member in a commercial SUV is numerically evaluated in the US-NCAP Full Width Rigid Barrier frontal crash test configuration. A set of design requirements and constraints for the baseline front end module and tailor-welded hot stamped side frame member are developed from the production side frame member evaluation. The evaluation includes:​ matching the crush response (crush modes), deceleration profile, final crush distance, crush forces, resistance to passenger compartment intrusion and extent of spot weld failure.

A baseline front end module is fabricated from production components and houses the production side frame member. Its development is based on a set of design specifications established from consideration of the full-vehicle model. A key consideration when developing the baseline front end module is to include the least amount of body-in-white components in order to reduce the scope of fabrication and testing. Dynamic crash sled testing at 51 km/hr is used in conjugation with a calibrated numerical model to characterize the performance of the production side frame member. An adaptive test matrix is employed during the nine baseline front end module tests, meaning that test configuration and boundary conditions are changed between sequential tests. Due to these test configuration changes, three different crush responses are observed, only one of which matches the crush response of the side frame member in the full-vehicle model. When a similar crush response to the full-vehicle model is observed in the baseline front end module, it is demonstrated that the velocity history, crush loads, occupant compartment intrusion resistance and extent of spot weld failure meet the design specifications.

The tailor-welded hot stamped side frame member is designed using the boundary conditions developed from the baseline front end module. The high energy absorbing crush section is comprised of Ductibor® 1000- AS, while the high rigidity, anti-intrusion S-rail section is made from Usibor® 1500-AS. It is clear from the design process that using higher strength materials (Ductibor® 1000-AS) in the crush section requires topological changes to be made to the production side frame member design. In order to capture the desired crush response and reduce spot weld failure severity in the tailor-welded hot stamped side frame member, new enhanced fold initiators, as well as geometric changes to the crush tip spot weld flanges are required. Ultimately, a two-component, 5.5 kg tailor-welded hot stamped side frame member is developed, which demonstrates a 2.1 kg (27.6%) weight reduction compared to the 7.6 kg production JAC590R side frame member. The designed tailor-welded hot stamped side frame member matches the baseline front end module crush response, deceleration profile, crush forces and passenger compartment intrusion resistance, while also exhibiting good parent metal fracture resistance and relatively low severity spot weld failure. Inserting the tailor-welded hot stamped side frame member into a model of the commercial SUV, in place of the production driver’s side frame member, further verified the suitability of the proposed side frame member.

Designing and fabricating the tooling for the full-length tailor-welded hot stamped side frame member comes with many complexities. To reduce the complexity, only the crush tip section of the tailor-welded hot stamped side frame member is manufactured. Two crash forming tools are made to hot stamp the channel “main rail” section and a flat “enclosure panel” section. The hot stamping process consisted of soaking multigauge (1.0 mm and 1.2 mm) Ductibor® 1000-AS blanks in a 950 °C furnace for 6 minutes, transporting them to the tool for forming and quenching in chilled dies for 10 seconds. The average hardness of the 1.0 mm and 1.2 mm Ductibor® 1000-AS sections in the main rail part are 368.5 HV and 401.2 HV, respectively. In the enclosure panel part the average hardness of the 1.0 mm and 1.2 mm Ductibor® 1000-AS sections are 412.8 HV and 392.8 HV, respectively. A model of the hot stamping process is used to map the predicted thinning due to forming onto a crash suitable mesh.

The crash performance of the tailor-welded hot stamped crush tip is evaluated to determine whether it is a suitable simplification to the full-length tailor-welded hot stamped side frame member. A crash model of the first 491 mm of the full-length tailor-welded hot stamped side frame member is constructed to represent the crush tip section. The inclusion of thinning predictions from the hot stamping model into the tailor-welded hot stamped crush tip crash model are shown to have only a small effect on the predicted crash performance of the crush tip. Evaluating the tailor-welded hot stamped crush tip against the baseline front end module and full-length tailor-welded hot stamped side frame member demonstrates that the crush tip provides valuable information on crush response, parent metal fracture resistance, crush forces and extent of spot weld failure. The tailor-welded hot stamped crush tip is deemed a suitable simplification of the full-length side frame member when evaluating the suitability of Ductibor® 1000-AS in frontal crash applications.

This research to-date supports the use of Ductibor® 1000-AS and Usibor® 1500-AS, in the form of tailor-welded blanks, as a higher strength alternative for conventional 590 MPa strength materials in frontal crash energy management structures. Through sheet material thickness down-gauging significant weight savings are shown. The results support the use of Ductibor® 1000-AS in high energy absorbing frontal crush structures requiring sequential folding and Usibor® 1500-AS in high rigidity anti-intrusion structures. These findings are tempered by the fact that experimental testing of the crush tips is still pending and that additional load cases need be considered. In future work, crash tests on the tailor-welded hot stamped crush tip will be conducted to support the numerical crash simulations of this thesis.