An integrated, collaborative project among the Ohio State University (OSU), Colorado School of Mines (CSM) and General Motors R&D (GM) will remove the principal obstacles to the adoption of advanced high strength steels (AHSS) for automotive application: the inabilty to predict forming and springback and the identification of processing technologies to produce new materials with strength/ductility combinations superior to those available with current AHSS.
Intellectual Merit: The project has three inter-related thrusts: 1. Process Development: Formability and springback measurement and simulation of existing AHSS and candidate alloys and microstructures devoloped in Thrust 2. 2. Material Development: Identification of new processing routes and microstructures upon which to base a 3rd generation of AHSS; validation of application readiness with methodologies developed in Thrust 1. 3. Educational Program: Education of promising and diverse students in AHSS-related technical areas at undergraduate and graduate levels; incorporation of Thrust 1 and Thrust 2 technical results into existing curricula.
Broader Impact: Broader impacts in four principal areas are anticipated: 1. Benefits to Society: AHSS offer major improvement in automotive vehicle performance, cost, safety (personal security), energy savings, emissions, reliability, and durability. Nonetheless, adoption is uncertain without fundamental understanding. 2. Learning / Broadened Participation: The technical research program will be integrated into existing and new educational activities at the two institutions 3. Dissemination of Results: Project results will be widely disseminated in peer-reviewed journals and other avenues.
A new family of materials, Advanced High Strength Steels ("AHSS" for short) exhibits a remarkable combination of very high strength (for crash safety) and very high ductility (for manufacturing by forming). In 2006, these materials were increasingly being selected to improve the safety, fuel efficiency, emissions, and performance of future cars and trucks. It was discovered during the first production trials, however, that the formability of these new materials was not predictable using standard methods. A new kind of failure called "shear fracture" was observed. This occurs in regions of sharp bending, in contrast to typical failure patterns of traditional steels. Figure 1 shows an example of completely unpredicted shear fracture in an automotive part, making the part useless and threatening to block the adoption of these materials. This figure is from an NSF-sponsored workshop organized by the PI in October 2006. Shear fracture was originally attributed to internal cracking of the special AHSS microstructures. A range of options for dealing with this issue were being pursued, including this project initially. This conventional wisdom was upended after shear fractures were reproduced in a laboratory setting using a newly-developed Draw-Bend Fracture (DBF) test. Three types of failure were produced and were categorized as shown in Figure 2: normal tensile fracture (Type I), shear fracture (Type III), and a rare transitional fracture (Type I). The initial results of the new test suggested a new hypothesis: that the cause of shear fracture is unrelated to the special microstructure but instead is caused by the high temperatures reached during forming in sharp-bending regions. These high temperatures are special and inherent to AHSS because of their combination of high strength and ductility. These preliminary results were confirmed in an extensive study involving simulation of forming and fracture and a new mathematical description of the behavior of AHSS alloys. New methods were developed to predict accurately the formability of these materials, making their widespread adoption possible. These materials are now being incorporated successfully in nearly every new car and truck designed and produced in the United States.
