Actual Geometry

Certainly one can reference the Critical Buckling chart and read off a certain t/b ratio to determine if a structure is geometry-limited. Real-world examples cannot be categorized this easily. As can be seen from the surfaces there are holes, formations etc. which complicate the choice of a simple t/b ratio. Vari-Form has done extensive CAE analysis on both production intent sheet metal and hydroform structures. Shown here are the results of a roof crush was performed on a structure and iterated for three material grades. Once the material strength approaches 1000 MPa (tensile strength) on the roof rail reinforcement, there is little performance difference in roof crush if the material properties are upgraded to UHSS grades. Yet, if you reduce the material strength, you see a proportional drop in load carrying capability that is directly proportional to the drop in yield strength.

So if you go back to the Critical Buckling stress curve, we believe that most of the structure is going to act in the t/b range of ≤.025. At this point, material > DP 980 strength is not fully utilized because the structure buckles before the full yield strength is reached.

Our experience over many similar roof studies show that materials beyond 1000MPa provide little roof crush improvements unless the geometry supports it.

While boron has good formability on its side, much of the potential strength of the material is never utilized. BIW engineers are well advised to explore technologies that allow geometric optimization and more material flexibility. That is why hydroformed tube structures are an excellent choice to maintain performance and be competitive in weight.