A new approach for characterization of local elastic anisotropy has been developed and applied to various materials including long- and short-fibre reinforced polymer composites as well as wood and paper. The method is based on the excitation of plate waves by weakly focused air-coupled ultrasound. It uses "resonance" transmission of an acoustic wave in air through the sample due to flexural plate mode excitation and re-radiation, which depends on the angle of ultrasound injection. The value of the resonance angle enables to calculate the velocity of the plate wave. By measuring the azimuth dependence of the flexural wave velocity, the in-plane elastic anisotropy is determined. An automated measurement system which comprises an angular scanner, computer-controlled data acquisition and processing modules is developed and applied for mapping of local elastic anisotropy and fibre orientation in composite materials.
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