This paper introduces SEMM: a method based on Frequency Based Substructuring (FBS) techniques that enables the construction of hybrid dynamic models. With System Equivalent Model Mixing (SEMM) frequency based models, either of numerical or experimental nature, can be mixed to form a hybrid model. This model follows the dynamic behaviour of a predefined weighted master model. A large variety of applications can be thought of, such as the DoF-space expansion of relatively small experimental models using numerical models, or the blending of different models in the frequency spectrum. SEMM is outlined, both mathematically and conceptually, based on a notation commonly used in FBS. A critical physical interpretation of the theory is provided next, along with a comparison to similar techniques; namely DoF expansion techniques. SEMM's concept is further illustrated by means of a numerical example. It will become apparent that the basic method of SEMM has some shortcomings which warrant a few extensions to the method. One of the main applications is tested in a practical case, performed on a validated benchmark structure; it will emphasize the practicality of the method.
As the number of models created in a modular fashion increase, the need for accurate identification of real joint dynamics rises. Since joint dynamics are a consequence of component-to-component interaction, they are only present in the assembled state. Yet, it is in the assembled state that measuring the interface degrees of freedom is practically infeasible. Nevertheless, the effects of the joint are present in measurements throughout the component, i.e. the joint dynamics are observable. In this work, system equivalent model mixing is used to expand an experimental measurement with interface degrees of freedom -either rotational or translational-extracted from a numerical model. Subsequently, joint dynamics can be obtained by applying classic frequency based decoupling methods. The strength of this method lies in the ability to test different interface configurations from a single measurement campaign, limited only by the the actual number of sensor or impact locations. The paper shows that an updating scheme can be used to identify joint dynamics without directly measuring interfaces.
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