The Flexibility of a Tubular Welded Joint in a Vehicle Frame

Automotive frames frequently consist of thin-tube members thick enough for much of the structure to be modeled adequately by bar elements. However, previous results show that a typical welded joint cannot be handled by the classical “rigid joint” assumption of frame analysis. Those results include tests of a joint type common in passenger car frames, and establish errors of over 60% in analytical predictions for some of the lower natural frequencies. The present paper attempts to see how much improvement we can achieve by simply accounting for the actual tubular shape in the vicinity of the joint, without allowing for the flexibility of the weld line itself. The study uses the NASTRAN computer program. The joint region is treated as a small substructure in a model otherwise composed of bar elements. This procedure is economical because only those portions which really have to be analyzed using plate elements are so treated. Parameters investigated include joint length, and two ways of attaching a shell to adjacent bar elements. The present results reduce the worst two frequency errors, 38% and 60%, to less than 7% and 11%, respectively. This is good enough for many purposes. Residual discrepancies are believed to be due, in part, to actual weld line flexibility. In vibration modes involving a particular kind of bending deformation, the slightly larger discrepancies are also tentatively attributed to a nonlinear effect, i.e., changes of cross-section shape in the relatively thin rectangular tubing used.