The application of ultrasonic guided waves (UGW) to the field of preventive maintenance of composite structures is in continuous increase. Today, UGW is taking an important economical place, especially in the fields of transport and nuclear technology, where the safety of individuals is of higher importance than financial cost.

Ultrasonic guided waves and composites

UGW are mechanical waves that propagate along an elongated structure while guided by its boundaries, such as a tube, a plate, a bar, a rod, a rail, etc. Commonly, these waves are generated at frequencies above the range of human audible frequencies (i.e. 20 kHz), the reason for which they are qualified by ‘ultrasonic’. Thanks to their capacity to travel along a long distance with little loss in energy (in some cases more than one hundred meters), their use is nowadays largely sought, especially for the inspection of metallic pipelines, vessels, cables and metal plate structures.

Generally, to test such a structure, two possible sensing measurements can be used, as shown in Fig. 1. In the pulse-echo measurement, the transducer plays a dual role: emitter and receiver. Alternatively, in pitch-catch measurements, two transducers are needed, and should be placed on either side of the area to be tested. This arrangement may not be suitable in testing context because it needs baseline data, which are not easy to get in most cases. Nonetheless, it is very useful in structural health monitoring SHM (i.e. sensors are attached to the structure to be monitored and live permanently with), and can be complementary to the pulse-echo arrangement when a defect cannot undergo an echo strong enough to be caught by the actuator/sensor.

This article appeared in the March/April issue of Reinforced Plastics. Log in to your free materialstoday.com profile to access the article.

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