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01/09/2007
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The correlation between computer simulations and real-life testing is improving all the time, writes Roger Bishop. But a remaining area of weakness concerns adhesive joints, rivets, spot welds and laser-welded seams for which conventional simulations do not allow for failure. They are usually assumed to be infinitely loadable. Given that a mid-size car uses about 5,000 spot welds and more than 120m of adhesive joints as well as rivets, it is a problem.
Research scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg have succeeded in reliably simulating what actually happens. “We have developed a simplified spot weld model for the crash simulation which reproduces the properties of joints, including their failure,” says Silke Sommer, who heads the project.
Engineers first examined individual joints in a tensile testing machine. For example, 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. Each joint model was then inserted into the whole vehicle crash model.
Once it has been determined where the seams will rupture in a collision, the joining methods used and body-in-white design can be adapted. This is particularly interesting in the context of lightweight construction. If the gauge sheet of steel is reduced, for example, high-strength steel has to be substituted. But the stronger the steel the more difficult it is to weld, increasing the risk of joint failure.
So far the laboratory has completed modelling and initial verification for joints covering spot welding, riveting and adhesive bonding.
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Author
Roger Bishop
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