Researchers at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, working together with their colleagues from the Laboratory for Material and Joining Technology LWF in Paderborn, and the Association for the Advancement of Applied Computer Science GFaI in Berlin, have developed a simulation model allowing the improved forecasting of rivet performance in crashes.
Manufacturers use welding equipment for cars made entirely of steel. However, for combining steel together with aluminum, for example, or steel with plastic materials, then conventional welding techniques are unsuitable. Automakers therefore resort to mechanical connections instead—such as rivets.
|A punch-riveted joint fails under bending load: the red areas were particularly seriously deformed. © Fraunhofer IWM. Click to enlarge.|
Very often, the connections are the weak points: in a crash, they are typically the first thing to fail. And since a car has about 3,000 to 5,000 joints, manufacturers strive to minimize this risk. This is why automakers use simulations to verify if the various connection points sustain these stresses in an accident.
However, it has been difficult to predict with great precision how much load rivets could tolerate. In many cases, calculations can clearly predict how the individual joining points will perform—but not for every type of strain.
If the joined components become bent (experts refer to this as a “flexural load” or “bending load”), then the simulations are quite often off the mark. For example, such computations could ascribe a greater load capacity than the rivets can actually bear under real emergency conditions.
The new, more advanced model now delivers more realistic projections.
We have further engineered a model that allows us to forecast rivet performance more reliably—both with slow and fast bending loads, as well as with pull and shear forces that emerge when the joined components become shifted, relative to each other.—Dr. Silke Sommer, Group Manager at IWM
For this purpose, researchers produced individual “sample components” from a variety of materials, connected them with rivets, and then applied stress. They bent them in a variety of directions, and pulled them and pushed them at varying speeds. They then integrated the performance of the rivet points into the mathematical equations, which contain various parameters to account for the different materials and their densities, Sommer said. The researchers at IWM and LWF studied about 15 different combinations of materials. Based on these data, their colleagues at GFaI prepared projections for other similar material and density combinations.