Solving the RDoF problem in experimental dynamic substructuring

Taking into account Rotational Degrees of Freedom (RDoF) in experimental Dynamic Substructuring is crucial for a successful coupling. The experimental determination of these RDoF is however very difficult. In this paper a solution is therefore presented where the RDoF information is only implicitly incorporated in the subsystem interface description. The method consists of measuring the subsystem's interface at multiple nodes in multiple directions. The coupling of the subsystems' interfaces is now performed on multiple nodes. As the number of FRFs used in this kind of coupling corresponds to the number of DoF describing the interface, rotational information can be implicitly accounted for. Indeed, a minimum of 6 coupling DoF at three nodes suffices to describe all motions of a rigid interface. Taking into account more interface DoF allows the description of more complex interface deformations. Notice that in many cases interfaces are rather rigid, meaning that the interfaces' local flexibility is commonly described quite well already by 6 rigid body motions only. A projection of the measured FRFs on such local rigid modes can therefore be shown to reduce measurement errors and enhance coupling results. Of course, this method can be extended to include other interface flexibility modes in case these contribute to the coupled interface deformation. The method is illustrated on the coupling of vehicle rear axle component models to the experimentally determined mech+anic-mechanic and mechanic-acoustic bodywork FRFs.