Simultaneous changes in the stiffness and mass matrices of a dynamic structure can be detected by using reduced measurements. In the method proposed here, both, the stiffness and the mass matrices, are assumed to be incorrect. The missing measurements can be calculated by utilizing the connection between the measured and unmeasured quantities trough the iterative process of identification of the tested structure. The missing measurements and to some extend the partial measurements obtained during the tests are treated as hidden functions of the parameters of the structure. This makes the Euclidean norm of the appropriate matrix to be minimized a nonlinear function of the parameters. This norm may have more than one minimum. The changes in the stiffness and mass matrices must be identified by looking for the global minimum of the Euclidean norm. The global minimum can be found by changing the initial values of the parameters or by changing some special coefficients, or both. A discussion of the minimal number of necessary measurements in connection with the number of the parameters of the stricture is included.

In order to identify the stiffness and mass parameters of the structure the measured and the supplied missing quantities are forced to comply with the general laws for a linear structure. This compliance is achieved during any step of the iterative process.

The structure is idealized to be a linear dynamic structure without damping. The reduced measured quantities are measured during the tests at discrete points of the Frequency Response Function.

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