http://www.memagazine.org/backissues/membersonly/february97/features/steelcar/steelcar.html
“STRENGTH OR STIFFNESS?
Underlying these decisions regarding material selection is the overall design goal for the particular body structure component. “You’re designing either for strength or for stiffness,” Fekete said. “That’s what’s driving the thickness of the material.” The thickness of steel car parts is usually determined by the degree of required stiffness, but in about 20 percent of the applications the important property is strength. “If you’re designing for stiffness, an increase in strength level won’t give you anything because the elastic modulus of the material doesn’t change,” Fekete said.
“When you design for strength, you’re primarily trying to handle crash loads,” said Bruce Emmons, president of Autokinetics, an engineering services firm in Rochester Hills, Mich. “Suspension loads are also a concern, but they tend to be much lower magnitude than crash loads,” he said. “Generally, we want to use higher-strength steels in crash-sensitive parts such as impact beams, bumper bars, rockers, and B-pillar reinforcements. More recently, however, the focus has moved toward greater stiffness, which is desired for improved ride, vibration, and harshness [NVH] quality.” Higher stiffness targets are something relatively new in American car design, he said, joking that structural stiffness levels used to be set just high enough “to allow the car doors to close.””
