Anomalous attenuation effect on reflectivity of an ultrasonic wave from a thin layer between dissimilar materials

Distinctive features of ultrasonic spectroscopy of adhesive joints of dissimilar materials (including anisotropic) are addressed both in theory and experiment. It is found that for dissimilar joining materials the depth of the reflection spectrum minimum depends nonmonotonically on attenuation in the layer and differs for measurements from opposite sides of the joint. It depends on both the attenuation in the adhesive and on adhesive/substrate impedance mismatches, resulting sometimes in zero reflectivity at nonzero attenuation in the adhesive. The effect is observed at normal and oblique incidence. A technique for measurement of the layer attenuation from amplitude and phase spectra is proposed. Experimental results show good agreement with theoretical predictions.