Air-coupled ultrasonic estimation of viscoelastic stiffnesses in plates

Materials characterization by means of megahertz air-coupled ultrasound is reported in this paper. In plates of composite laminates the reconstruction of a function closely related to the transmission coefficient has been accomplished using leaky guided waves. The spatial and frequency dependence of the transmitted leaky waves have been measured for a variety of plates, including aluminum-aramid and graphite-epoxy. These results are based on a general method, developed in this paper, to reconstruct a transmission function experimentally from scans both of receiver position and incident angle. A synthetic aperture technique is utilized to reduce the measurements by forming an effective wide-angle focused aperture. This synthetic focused aperture is obtained experimentally by summing measured signals coherently for many transmitter-receiver separations and summing incoherently for many transducer angles with respect to the plate normal. These results have been employed to deduce, with high precision, a function related to the plane-wave transmission coefficient of the plate over the frequency and angular bandwidths covered by the measurements. Calculation of this function is demonstrated, and the source of its deviation from the plane-wave transmission coefficient is explained. The experimental results show good agreement when compared to three-dimensional theoretical beam calculations. The high degree of accuracy in the measured transmission function has been exploited to construct an efficient inversion algorithm to estimate the viscoelastic material properties and/or sample thickness.