Experimental Determination of Lamb-Wave Attenuation Coefficients

This work determined the attenuation coefficients of Lamb waves of ten engineering materials and compared the results with calculated Lamb-wave attenuation coefficients, alpha-S and alpha-A. The Disperse program and a parametric method based on Disperse results were used for calculations. Bulk-wave attenuation coefficients, alpha(L) and alpha(T), were required as input parameters to the Disperse calculations. The calculated alpha-S and alpha-A values were found to be dominated by the alpha(T) contribution. Often alpha-A(o) coincided with alpha(T). The values of alpha(L) and alpha(T) were previously obtained or newly measured. Attenuation measurement relied on Lamb-wave generation by pulsed excitation of ultrasonic transducers and on surface-displacement detection with point contact receivers. The frequency used ranged from 10 kHz to 1 MHz. A total of 14 sheet and plate samples were evaluated. Sample materials ranged from steel, Al, and silicate glass with low attenuation to polymers and a fiber composite with much higher attenuation. Experimentally obtained Lamb-wave attenuation coefficients, alpha-S and alpha-A, for symmetric and asymmetric modes, were mostly for the zeroth mode. Plots of alpha-S-o and alpha-A(o) values against frequency were found to coincide reasonably well to theoretically calculated curves. This study confirmed that the Disperse program predicts Lamb-wave attenuation coefficients for elastically isotropic materials within the limitation of the contact ultrasonic techniques used. Further refinements in experimental methods are needed, as large deviations often occurred, especially at low and high frequencies. Methods of refinement are suggested. Displacement measurements were quantified using Rayleigh wave calibration. For signals below 300 kHz, 1-mV receiver output corresponded to 1-pm displacement. Peak displacements after 200-mm propagation were found to range from 10 pm to 1.5 nm. With the use of signal averaging, the point-contact sensor was capable of detecting 1-pm displacement with 40 dB signal-to-noise ratio and had equivalent noise of 4.3 fm/root Hz. Approximate expressions for alpha-S-o and alpha-A(o) were obtained, and an empirical correlation was found between bulk-wave attenuation coefficients, i.e., alpha(T) = 2.79 alpha(L), for over 150 materials.

Automated summary

This study determined the attenuation coefficients of Lamb waves for ten engineering materials and compared them with calculated values. The Disperse program and a parametric method were used for calculations. The results showed good agreement between the experimental and theoretical values, confirming the predictive capability of the Disperse program. Further refinements in experimental methods are needed to reduce deviations.