This paper presents a method for identifying the free-free modes of an unconstrained structure by utilizing the available vibration data of the same constrained structure. In modal formulations for flexible body dynamics, modal data are primary known quantities that must be obtained either experimentally or analytically. For vibration measurements, the flexible body may be constrained differently than its actual conditions in the multibody system. To apply the experimentally obtained modal data to the flexible dynamic model, the available vibration data must be properly identified.
For a flexible body model in multibody systems, we may need a set of free-free modal data or constrained modal data depending upon the dynamic formulations. The identification of constrained modes from the free-free vibration data is straight forward, but the reverse process to determine the free-free modes from the constrained vibration data is difficult and expensive. The aim of this study is to extract, based on experimentally obtained vibration data, the necessary free-free frequencies and the associated modes for flexible bodies in multibody systems. In this paper, two different forms of constrained vibration data are considered. The first form is for structures supported by springs, and the second form is for structures supported by kinematic joints. The available vibration data may be either a complete set or an incomplete set.