This paper relates a further development of an earlier work by the first author and others, in which they presented a method of deriving the elastic constants of composite materials from resonance data obtained from the modal analysis of freely-supported plates made out of orthotropic or isotropic materials. In the present work, the diagonal modes are included in the optimized Rayleigh three-term displacement representation for the square isotropic or almost isotropic composite plates. The use of sensitivity analysis of the frequencies including the diagonal modes to each elastic constant is also investigated in the procedure of the identification of elastic properties. Comparison of results obtained using diagonal-inclusive modes with those excluding the diagonal modes suggest that lower diagonal modes have relatively higher sensitivities to the elastic constants and can significantly improve accuracy when included in the elastic identification method applied here to square isotropic or almost isotropic plates.

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