Experimental procedure and results for the identification of elastic constants of thick orthotropic plates

Ability to determine six independent material parameters of thick orthotropic plates by a non-destructive numerical/experimental method is demonstrated. The parameters, which are the four in-plane elastic constants and the two transverse shear moduli, are deduced simultaneously from natural frequencies of the completely free plate by a nonlinear least squares approach. The main new contribution of the present paper is the consideration of higher mode natural frequencies of thick plates linked with an accurate numerical model based on a higher-order shear deformation theory for the theoretical predictions. This has enabled both transverse shear moduli to be estimated along with the in-plane parameters. Focus is here on the experimental technique and the application of the method in real tests. High quality frequency measurements are performed by the impulse technique employing a non-contacting microphone combined with advanced curve fitting of the frequency response function. The first 12 to 15 natural frequencies are typically considered. The different sources of errors associated with the method are discussed. The method is illustrated by evaluating the six elastic constants of an aluminium plate and of two different unidirectional composite plates. For the composites, close agreement is found between in-plane and out-of-plane shear moduli in the planes parallel to the fibres, confirming transverse isotropy.