Sorin Teles

Rapid reconstruction of the transmission coefficient (TC) of a fluid-loaded plate across large frequency and phase-match angular ranges is achieved by performing a synthetic aperture scan with a pair of highly focused transducers in a transmission arrangement. The mode cutoffs are easily mapped out with the transducers at low incident angles. This method is successfully demonstrated on both isotropic and anisotropic plates to reconstruct and use the TC to estimate the elastic property of the plate materials.

A method is devised and demonstrated for the detection of disbonds in E-glass-epoxy/end-grain balsa core composites. Guided acoustic waves are exploited in an approach that leads to positive identification of disbonds in these complex marine composites. The method uses the fact that a propagating Lamb wave in the 3-mm glass-epoxy facesheet composite will exist only when a disbond (even a disbond in partial contact) is present at the location of the incident acoustic beam. The resulting leaky wave field in the fluid is then detected and monitored over the scanned area, producing a C-scan of the interface with clear discrimination between bonded areas and disbonds. Numerical evaluations of the received transducer voltage corresponding to different interface conditions are compared with the experimental measurements. A theoretical calculation based on prior work predicts accurately the results of several experiments on a variety of samples with different disbond conditions.

The method developed here exploits the wide angular range of focused acoustic probes and the large synthetic aperture of scanned transducers to permit a rapid and reliable estimation of material properties in thin plates. It is found in several tests with various materials that estimates of elastic behavior using this method agree with contact measurements to within less than 5%. The method utilizes transmission (or reflection) coefficient reconstruction for an infinite thin plate, across a wide range of frequency and wave number, from which elastic property estimates are made. Data collected over a large synthetic acoustic aperture are processed with temporal and spatial Fourier transforms applied to change the acquired data from the coordinate and time domains to the wave number and frequency domains. Extrinsic real-beam effects on the data are accounted for with a complex transducer point analysis. Transmission measurements yield reconstructed data extending to the mode cutoffs, permitting easy and nearly unambiguous estimation of a subset of the elastic stiffnesses. For anisotropic plates, elastic stiffnesses are estimated with an inversion procedure that uses only limited data carefully selected from different portions of the measured scattering coefficient. Estimates are made by reconstructing in a stepwise fashion, based on sensitivity studies, where only one stiffness is estimated from the data at any one time, restricting the optimization to a robust one-dimensional search.