The second author and colleagues developed an elastic identification method based on model data obtained experimentally from free transverse vibration of impacted freely supported plates. Application of the method to thin, square isotropic metal and composite plates is described in this paper. For these modes, some additively and subtractively coupled modes arising from diagonal symmetry of geometry and boundary conditions (thus termed diagonal modes) are important. The focus of this paper is on how the number of modes used to represent the vibration affects accuracy. One-, three-, and six-term optimized Rayleigh displacement expressions are used for plate vibration, including the diagonal modes. Sensitivity analysis of the frequencies to each elastic constant is included. Results show that the optimal three-term approximation gives results comparable to the normal 36-term Rayleigh expansion, while the optimal six-term compares favorably with the 64-term Rayleigh expansion.
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