Use of focussed ultrasonic beams to characterize the interfacial region in bonded joints

A model is presented for an ultrasonic pitch-catch immersion measurement system for nondestructive characterization of the interfacial region in bonded joints using focused beams. The adhesion between the adhesive and the adherend is modeled by means of interfacial springs. The model predicts that the reflected signal becomes strongly frequency dependent as the aperture angle is increased for a fixed probe diameter. It is shown that the frequency dependence is due to interference between the various angular components of the beam. This phenomenon leads to a strong dependence in the spectral response of the system to the condition of the interfacial region