In order to predict the strength of an adhesive bond between two metal sheets, it is necessary to measure the physical state of the adhesive layer that mechanically joins the two pieces of metal. This requires rapidly performing a detailed analysis of the ultrasonic echoes reflected from the entire structure when it is immersed in a water bath for a normal ultrasonic pulse-echo inspection. To achieve this result, computer-operated ultrasonic inspection systems have been assembled and equipped with special signal processing routines so that particular features of the ultrasonic echo in both the time domain and the frequency domain can be extracted in a time short enough to meet the requirements of a production inspection system. Such features as the relative amplitude of the signals reflected from the top and bottom of the adhesive layer and the frequencies for which standing waves are excited in the adhesive and in the metal adherends are of particular interest for making the strength predictions. It is also important that the interrogating ultrasonic pulse be of very short time duration so that the echoes from the various interfaces in the sandwich-like joint can be resolved in the time domain display. This requires the use of special high frequency pulse generators coupled to broad band transducers and amplifiers. Special procedures are also needed to insure the accuracy of the analog-to-digital conversion at the input to the computer and the subsequent transformations to and from the frequency domain
