The structural inspection with guided waves described in this specification is an active ultrasonic method. For that purpose ultrasonic waves are introduced into the object being monitored by a transducer acting as an actuator. These waves pass through the test object, interact with defects to a different extent depending on the wave-length and wave mode and are detected by transducers which pose as sensors. The detected signals, characterizing the conditions of the structure, are then compared and signal changes are displayed via damage indicators. This method is also called “signature analysis” or “lamb wave analysis” and is known in English-speaking coun-tries as “acousto-ultrasonics”. However, guided waves can also be excited by processes in the structure (e.g. crack growth) and recorded by sensors. This passive measurement, also known as “acous-tic emission testing”, has already been described in technical guidelines and will not be the subject of this specification.
