“Front-end design is tricky,” Pete Peterson, director-Marketing, Automotive, United States Steel Corp. (Pittsburgh) and a member of the Auto/Steel Partnership, points out with what can only be characterized as an understatement. A fundamental part of the trickiness is directly related to crash-energy management. Specifically, with passing the offset crash tests that are performed both in the U.S. and in Europe1. As he points out, the way these tests are conducted, 40% of the frame is required to take the full load. One of the ways that it would be conceivable to do a more robust design would be to make the front end of the car beefier. But that introduces another set of issues. Such as handling and braking problems resulting from the addition of mass. And when you consider the front-engine architecture of most vehicles, there isn’t a lot of desirability to having additional weight up front. As an extension of the UltraLight Steel Auto Body-Advanced Vehicle Concepts (ULSAB-AVC) program (see Better Vehicles Through Steel), a project was conceived and supported by the Department of Energy (DOE) to figure out how to do a better job of designing and producing front ends that could manage crash energy. While the initial idea was to simply design a front end and then attach it to a mule vehicle that would be run into an offset barrier, the program has expanded such that actual GMC vehicles have been contributed to the program, and the Big Three are all working with the Auto/Steel Partnership companies on this advanced high strength steel (AHSS)-based program2. (DOE is in for $1-million per year for two years.)
An important aspect of this program goes beyond the design of the front end. Peterson observes, “You can convince a design engineer how to design it. But in the end, someone has to manufacture it. If something creates enough problems in Manufacturing, then it’s not going to get very far.” (DOE got into this program because of its interest in technology being developed that would be applied, not book shelved.) AHSS materials form differently, so stamping is a slightly different proposition compared with traditional HSS materials. For example, Peterson points out, “These steels cold work like crazy. You need more ‘horsepower’ in the presses to overcome the strength in the steel.” And there are different springback characteristics than are typical of HSS materials, which have a variety of ramifications, from binder design to determining part fit up for subsequent assembly. Peterson suggests, for example, that laser welding could be a good method for joining AHSS body parts (for one thing, the flanges that are ordinarily made to accommodate spot welding guns aren’t necessary, which would save significant weight on a body structure), but it would be necessary to know the effects of springback. Consequently, a move toward using these materials means that there are a series of challenges that must be addressed, especially inasmuch as given the newness of these steels, there isn’t an abundance of information that can be readily consulted by engineers to determine the appropriate process parameters for manufacturing with AHSS.
1When there are full-width rigid-barrier tests, the entire front-end structure absorbs energy. Consequently, this test really matters vis-à-vis the safety cage and the passenger restraint systems. With the offset test, only a portion of the width takes the brunt of the crash energy, which is more demanding of the structure than in the full-width tests.
2Among the advanced high-strength steels are dual-phase (DP), transformation inducted plasticity (TRIP), and complex phase (CP) materials. The “advanced” nature of these materials is a result of their microstructures, which contain martensite, bainite, or retained austenite so that they are both strong and formable.