It is well known that the modal properties of a structure are directly related to its mass, stiffness, and damping properties. In fact, the modal parameters (eigenvalues and eigenvectors), are solutions to the differential equations of motion, which are written in terms of the mass, stiffness, and damping. This paper focuses on the determination of the functional relationship between variations in the mass, stiffness, and damping, and variations in the modal properties of structures. For small changes, this "sensitivity function" reduces to a very simple function of variations in the modal frequencies and damping only. This makes it possible to defect, locate, and quantify structural faults by monitoring frequency and damping only. This finding was previously reported by Stubbs et.al. [4], [5], [6]. In this paper the complete sensitivity functions for mass, stiffness, and damping changes are derived, and the validity of the stiffness sensitivity for small changes is verified. It is also pointed out that the Structural Dynamics Modification (SDM) technique can be used to determine the additional terms for the complete sensitivity formulas.
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