In this paper, we focus on the ACM2 (Area Contact Model 2)model as a simplified spot weld model suitable for vibrationanalysis and reveal its dynamic characteristics. First, thetheoretical background of the multi-point constraint (MPC)used in the ACM2 model is explained. Next, we examine indetail the effect of the mesh pattern surrounding a spot weldon the modal properties (natural frequency and mode shape)of a spot welded structure. Finally, an appropriate mesh sizein the area of the spot weld for the ACM2 model is presented.As an example, we used two steel plates joined by three spotwelds.The results show that the configuration and size of the patch(group of shell elements in the ACM2 model) significantlyaffect the modal properties of the model. When the centers ofthe patch and the solid element that represent the spot weldare coincident, the natural frequencies monotonically increasewith the patch size. When the centers of the patch and thesolid element are not coincident and the shell element size islarge, the natural frequencies vary widely. When the patch iscomposed of shell elements whose size is the same or smallerthan the solid element size determined by the spot welddiameter, the variation in the natural frequency is small.
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