Identification of Coupled Degrees of Freedom at the Interface between Sub-Structures

A crucial step in sub-structuring, vibration source characterisation and transfer path analysis is to correctly define the interfaces between coupled sub-structures. In particular, it is necessary to identify the degrees of freedom through which coupling occurs. Even for experienced engineers, identification of the degrees of freedom required to fully describe interactions at the interface is not a trivial problem. One approach is to include all degrees of freedom, i.e. three translations and three rotations at every connection point, but this is not always practical because the measurement of in-plane and rotational degrees of freedom is required. There are also questions about how continuous interfaces should be represented by discrete data. The paper therefore addresses the question of to how to determine the set of degrees of freedom at an interface that can be said to fully characterise the coupling. A case study is presented that highlights the importance of appropriately describing interface degrees of freedom, and an experimental method for identifying those which should or should not be included is outlined. This method, named the Interface Completeness Criterion (ICC), is similar to the Frequency Response Assurance Criterion (FRAC), in that it provides a rating (between zero and one) that quantifies the completeness of a measured dataset for predicting structure borne noise and the passive dynamic properties of coupled assemblies. The ICC has potential applications in sub-structuring, in-situ blocked force measurement and transfer path analysis.