The ultrasonic pulse-echo technique as applied to adhesion testing

Flaws are commonly detected by their ability to reflect a burst of ultrasound back to a piezoelectric transducer, and a flaw is taken to be found if an ultrasonic reflection is detected. In this form, relying simply upon the existence or otherwise of a signal, the pulse-echo technique is not suited to the detection of weak bonding between an adherend and an adhesive, because both perfect and imperfect interfaces between dissimilar materials always reflect ultrasound. In 1971 Rolls-Royce sponsored an investigation into a highly developed form of pulse-echo technique, in which all the information from the reflected pulse is used, expressed in terms of accurate amplitude and phase measurements. It has been found possible to detect `slackness' at an interface, in situations where this slackness is not severe enough to amount to an actual unbond.