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.

This paper presents and demonstrates a noncontact method for measuring the Lamb wave dispersion spectrum of a plate. Noncontact air-coupled source and receive transducers are used with line-focus mirrors and 50-700 kHz broadband apparatus for simultaneous measurement over a broad spectrum of refractive angles and multiple guided modes. Broadband, wide-angle wave forms are measured as a function of position. The Fourier transform of these wave forms from the t -x domain to the v -k domain gives an approximate spectrum of the dispersion relation. We measure the dispersion spectra of Lucite™, aluminum, balsa wood, and a carbon fiber epoxy laminate, and show that the measured spectra agree well with the dispersion relation calculated from Lamb wave theory. This paper presents and demonstrates a noncontact method for measuring the Lamb wave dispersion spectrum of a plate. Noncontact air-coupled source and receive transducers are used with line-focus mirrors and 50-700 kHz broadband apparatus for simultaneous measurement over a broad spectrum of refractive angles and multiple guided modes. Broadband, wide-angle wave forms are measured as a function of position. The Fourier transform of these wave forms from the t -x domain to the -k domain gives an approximate spectrum of the dispersion relation. We measure the dispersion spectra of Lucite™, aluminum, balsa wood, and a carbon fiber epoxy laminate, and show that the measured spectra agree well with the dispersion relation calculated from Lamb wave theory.

No abstract

No abstract

Plate-wave dispersion spectra have been inferred directly in a single air-coupled coordinate scan using mirror-focussed acoustic beams on both transmitting and receiving transducers. The spectra of the leaky Lamb modes are extracted through a two-dimensional DFT of the measured broadband focused air-coupled signal, as has been demonstrated previously in water. To enhance the typically weak air-coupled signal, a novel signal coding scheme using a random-phase constant-amplitude analog burst 200 μs in length has been employed. Subsequent correlation yields the system impulse response, from which the processed wavenumber-versus-frequency plot is obtained by the two-dimensional DFT. Comparison of measured dispersion spectra with calculated dispersion curves based on nominal stiffness values shows agreement between the measured and calculated data.Quantitative evaluation of hidden defects in cast iron components using ultrasound activated lock-in vibrothermography Rev. Sci. Instrum. 83, 094902 (2012) Quantitative trap and long range transportation of micro-particles by using phase controllable acoustic wave J. Appl. Phys. 112, 054908 (2012) Nonlinear nonclassical acoustic method for detecting the location of cracks J. Appl. Phys. 112, 054906 (2012) Development of nondestructive non-contact acousto-thermal evaluation technique for damage detection in materials Rev. Sci. Instrum. 83, 095103 (2012) Grain structure visualization with surface skimming ultrasonic waves detected by laser vibrometry ABSTRACT. Plate-wave dispersion spectra have been inferred directly in a single air-coupled coordinate scan using mirror-focussed acoustic beams on both transmitting and receiving transducers. The spectra of the leaky Lamb modes are extracted through a two-dimensional DFT of the measured broadband focused air-coupled signal, as has been demonstrated previously in water. To enhance the typically weak air-coupled signal, a novel signal coding scheme using a random-phase constant-amplitude analog burst 200 µs in length has been employed. Subsequent correlation yields the system impulse response, from which the processed wavenumber-versusfrequency plot is obtained by the two-dimensional DFT. Comparison of measured dispersion spectra with calculated dispersion curves based on nominal stiffness values shows agreement between the measured and calculated data.