Gas coupled ultrasonic measurement of stiffness moduli of polymer composite plates

We describe a gas coupled ultrasonic technique for determining the stiffness moduli of unidirectional fiber-reinforced polymer composite plates. Resonant excitation of the first anti-symmetric (AO) Lamb mode in the long wavelength region is exploited for this technique. In our experiments, broadband electrostatic transducers are used to measure sound transmission through a plate specimen as the incidence angle is varied. A 1 mm thick plate is used as the sample. The AO Lamb mode phase velocity is inferred from the incidence angle at maximum transmission, which is precisely determined when the coupling fluid is air. A least squares algorithm is used to determine the stiffness moduli E₁₁ , E₂₂, shear modulus G₁₂, and Poisson´s ratio v₂₃ (fibers in “1” direction) from the resonance data