Abstract
Passive ultrasonic guided waves offer a promising solution for the inspection of thin structures such as aircraft fuselage, pipes, or ship hulls, as they are not affected by ambient noise but on the contrary make use of it as a source of mechanical excitation. To that extent, Electro Magnetic Acoustic Transducer (EMAT) sensors appear to constitute an interesting instrumentation solution, because of the large choice of designs and associated modal selectivity and directivity properties they offer, and because passive applications eliminate the need for actuation, which is the main difficulty with EMAT instrumentation. In this study, we design and test EMATs with various coil geometries and analyze their influence on modal selectivity and directivity. Experimental tests are carried out, using a thin piezoelectric disc for wave generation at first, validating the properties of the receiving EMATs, and then with an air stream as the only excitation source for the full passive configuration. The influence of the coil design on the proper recovery of the ultrasonic signals from the ambient noise but also directivity, modal selectivity and overall sensitivity is discussed. Such sensors coupled with passive techniques could be embedded in structures, enabling their Structural Health Monitoring (SHM).
