Low frequency ultrasonic method for characterizing ultra-thin bi-layer composite

A low-frequency ultrasonic (Lamb-like) wave method is used for characterizing the shear, longitudinal wave velocities, the thickness, and the density of ultra-thin two-layered bonding composite. The layer thickness is only a fraction of the ultrasound wavelength so that the echoes from the front and back surfaces cannot be separated in the time domain. The least square method and the secant algorithm are applied to determine the governing parameters basing on the measured dispersion curve of the low frequency Lamb wave propagating along the composite. The reasons for evaluation error of the parameters and the sensitivity of the method for different parameters are analyzed. The technique is employed experimentally to characterize the ultra-thin aluminum and brass layers of the aluminum/brass composite with thickness down to five percent of the ultrasonic wavelength by using a pair of broadband longitudinal wave transducers of 2 MHz. It is observed that the nominal value agrees well with the estimation.