Air-Coupled Ultrasonic Characterization of Composite Plates

摘要

Air-coupled ultrasonic materials characterization has been accomplished in composite plates through accurate reconstruction of the transmission coefficient using leaky guided waves. The spatial and frequency dependence of the transmitted leaky waves have been measured for a variety of composite plates including aluminum-aramid and graphite-epoxy. A general method has been introduced to reconstruct the transmission coefficient experimentally from a one-dimensional position scan of the relative transducer separation. This method utilizes a synthetic aperture technique to reduce the measurements by synthesizing an effective wide-angle focused aperture. This synthetic focused aperture is obtained by summing measured signals coherently for many transmitter-receiver separations and incoherently for many transducer angles with respect to the plate normal. These results have been employed to deduce, with high precision, the transmission coefficient of the plate over the frequency and angular band-widths covered by the measurements. These experimental results have been compared to three-dimensional theoretical calculations with good agreement. The high degree of accuracy in the measured transmission coefficient has been exploited to construct an efficient inversion algorithm to estimate the viscoelastic material properties and/or sample thickness.