Examples of hybrid dynamic models combining experimental and finite element substructures

Abstract

Substructuring methods have been used for many years to reduce the size of FE dynamic models and maintain satisfactory response for a limited bandwidth of interest. However, experimental substructures have been used only in a very limited number of cases, generally where there was only a single connection point idealized to six connection degrees of freedom. This is because it is difficult to experimentally characterize the moments and rotations at multiple connection points. Mayes and Allen developed a method to practically characterize continuity and equilibrium at multiple connection points through the instrumentation of a flexible fixture. The instrumented fixture is transformed into a force and response sensor that is expressed in terms of the modal connection forces and modal connection responses of the fixture. The sensor is called the transmission simulator. Two previous results of the method for coupling R&D pieces of hardware are given. The primary emphasis of this paper is on a set of representative system hardware, for which two designs of a transmission simulator are compared with the view to discover design characteristics that optimize the sensor effectiveness. Frequency response functions (FRF) from the assembled system hardware are measured and designated as the truth against which to compare. The accuracy of the experimental substructure is evaluated by attaching it to a finite element substructure and predicting full system response FRFs which are compared against the truth FRFs. The result is that the transmission simulator design is relatively robust to the stiffness chosen, although the stiff transmission simulator appears to be a slightly better choice in the structure evaluated in this work. Substructure modes tend to maintain the shapes of the bare transmission simulator if it is stiff. A one piece transmission simulator design with no joints is easier to model accurately, which is of value in this methodology. ©2010 Society for Experimental Mechanics Inc.