The car crashes into the wall with a bang, the dummies are thrown forward in the seat belts. Crash tests are obligatory for any new automobile model before it is allowed on the road. Most of these crashes destroy the vehicle and are therefore an expensive exercise. This is why car manufacturers replace crash tests with computer simulations wherever possible. But this alternative has its downside: Conventional simulations do not allow for the fact that car body parts and the connections between them – adhesive joints, rivets, spot welds or laserwelded seams – can rupture. The assumption is made that the connections are infinitely loadable. The number of joints is large, a mid-size automobile is held together by about 5,000 spot welds and more than 120 meters of adhesive joints as well as numerous rivets. If they split open, the obstacle in the collision can penetrate deeper into the vehicle and increase the risk to the occupants. What loads can the joints withstand? When do they fail? Research scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg have for the first time succeeded in reliably simulating what actually happens. “We have developed a simplified spot weld model for the crash simulation which reproduces the properties of the connections – including their failure,” explains Silke Sommer, who heads the project at the IWM. To set up the simplified model the research engineers first had to take a step back and examine individual joints in a tensile testing machine. For instance, they investigated a spot weld under tensile, shear, bending and torsional loading. “From these experiments, we determine characteristics and use them to create a suitable simplified model for each type of joint,” says Sommer. The research engineers then insert the various joint models into the crash model representing the entire automobile, which can be used to easily simulate various crash situations. Once it has been determined where the seams will rupture in a collision, the joining method used and the design of the body in white can be adapted. This is particularly interesting in the context of lightweight construction: If the thickness of the steel sheets is reduced, they have to be made of high-strength steel in order to provide adequate protection to the occupants. The stronger the steel, the more difficult it is to weld and the higher the risk that the connections will fail. “We have almost completed the modeling and initial verifications for spot welds, rivets and adhesive joints,” says Sommer.
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