AIR-COUPLED ULTRASONIC ESTIMATION OF VISCOELASTIC STIFFNESSES IN PLATES

Materials characterization by means of megahertz air-coupled ultrasoun d is reported in this paper. In plates of composite laminates the reco nstruction of a function closely related to the transmission coefficie nt has been accomplished using leaky guided waves. The spatial and fre quency dependence of the transmitted leaky waves have been measured fo r a variety of plates, including aluminum-aramid and graphite-epoxy. T hese results are based on a general method, developed in this paper, t o reconstruct a transmission function experimentally from scans both o f receiver position and incident angle. A synthetic aperture technique is utilized to reduce the measurements by forming an effective wide-a ngle focused aperture. This synthetic focused aperture is obtained exp erimentally by summing measured signals coherently for many transmitte r-receiver separations and summing incoherently for many transducer an gles with respect to the plate normal, These results have been employe d 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 i s demonstrated, and the source of its deviation from the plane-wave tr ansmission coefficient is explained. The experimental results show goo d agreement when compared to three-dimensional theoretical beam calcul ations. The high degree of accuracy in the measured transmission funct ion has been exploited to construct an efficient inversion algorithm t o estimate the viscoelastic material properties and/or sample thicknes s.