Santa Ana, CA—As its name implies, finite element analysis (FEA) performs mathematical analysis on "elements"—small sections of larger parts of a structure—so that designers can predict the behavior of the structure. When it comes to vehicles, FEA deals with large parts but some regions are hard to break down into accurate elements. For example, more than 4,000 spot welds connect some 300 body panels on a typical mid-sized car to form the basic vehicle structure, and more welds connect the body to the chassis. For accurate simulation of the vehicle as a whole, those welds have to be modeled accurately before running FEA.
Working with Volkswagen AG, MSC.Software has developed a new element representing spot welds that helps to automate the process of creating large-scale finite element models of entire cars. "Automotive body engineering departments have always focused their simulation research efforts on spot welds, because they control the bending and torsional modes, as well as noise, vibration and harshness of ride (NVH) behavior—all of which influence the durability, structural integrity and comfort of the vehicle," says Claus Hoff, manager, nonlinear technology for MSC.
In a model of half of the front section of a car, MSC. Software's new C-Weld element indicates spot welds used for mounting the suspension.
MSC reports that the new C-Weld element, by automating difficult parts of the full body FEA mesh (representing all the generated elements), saves more than 30% of design cycle time. It also overcomes analysis problems caused by singularities, rigid body invariance, and differing mesh densities—all of which can occur when modeling a vehicle that incorporates parts from a variety of suppliers using different design programs.
C-Weld is part of the company's MSC.AMS (automatic and manual shell modeling) full vehicle analysis (FVA) application that simplifies the MSC.Patran interface for automotive users. With C-Weld, body panels can be meshed keeping only the structural behavior of the model in mind. At the end of the meshing process, the spot weld data can be incorporated fully in the finite element model.
The accuracy of any finite element analysis depends on the accuracy of the mesh. In the case of welded vehicle models, meshes of different parts have to match at the spot weld connection points, and the spot weld grids must line up. If the meshes differ—which can happen in parts designed by different suppliers—the parts often have to be remeshed manually. In addition, numerical problems may be caused by underestimating the stiffness of large-diameter spot welds, or modeling the welds with rigid elements.
"C-Weld considers the weld area, enabling correct force transfer. It also models other types of connections, including seam welds, rivets, bolts, screws, and engine mounts," says Hoff. "In an area of complex connections, such as a C-Pillar upper and A-Pillar lower in the cowl area, most geometry has had to be falsified to connect parts—resulting in incorrect NVH, crash and durability information. The new weld element joins non-congruent meshes and takes the area or diameter of the spot weld into account to simulate the transfer force accurately."
MSC quantified the time savings at approximately 30% based on Volkswagen's experience using C-Weld for a recent body-in-white (BIW) project that had 3,712 C-Weld elements, of which 3.562 elements connected two parts, and 105 connected three parts. C-Weld spot weld elements made it possible for Volkswagen to eliminate the congruent flanges that used to be required for manual spot weld modeling—and save nearly a third of the design cycle time in the process.
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