Using Strain Gages as References to Calculate Free-Free Frequency Response Functions

Abstract

The objective of many modal tests to experimentally measure free-free modes is achieved by suspending the test article on soft supports such that the resulting rigid body modes are adequately decoupled in frequency from the flexible body. The soft supports can also be placed at the nodal locations of the primary free-free modes to provide further decoupling. Oftentimes, however, the rigid body modes cannot be separated from the flexible body modes for a variety of reasons, such as the flexible modes being very low in frequency, the test article being so large that it is not cost effective to build a suspension system, or a combination of both. In these cases, the rigid body and flexible modes are intermingled, which means that effort must be spent updating the model of the suspension system instead of the test article itself. Sometimes, one may also want to use the experimentally measured free-free modes directly in an analysis without building an analysis model. This paper proposes a method that uses strain gage measurements at the structure’s boundary as references for calculating frequency response functions. The resulting frequency response functions, associated with a free-free structure, can then be used to estimate modes.