Embedded Ultrasonic Structural Radar with Piezoelectric Wafer Active Sensors for Damage Detection in Cylindrical Shell Structures

Structural health monitoring (SHM) is a major component of the vehicle health monitoring (VHM) concept currently considered for the civilian and military aerospace applications. Piezoelectric wafer active sensors (PWAS) are one of the candidate embedded sensors considered for SHM applications. PWAS are inexpensive, non-intrusive, un-obtrusive, devices that can be used in both active and passive modes. In active mode, PWAS generated Lamb waves that can be used for damage detection through pulse-echo or pitch-catch techniques. An efficient application of the pulse-echo method with PWAS technology is through the phased array technique. In the embedded ultrasonics structural radar (EUSR) concept, an array of closely spaced PWAS is used to detect structural cracks based on the scanning beam principle with guided Lamb waves. A scanning beam of ultrasonic Lamb waves works like “structural radar”. When encountering a crack, it generates echoes and backscatter. This concept was initially developed by the authors for guided Lamb waves traveling in flat plates. In this paper, the EUSR concept is extended to cylindrical shells. First, the theory of guided waves in cylindrical shells, with the associated modes and dispersive frequencies is reviewed. It is shown that cylindrical shells accept three types of guided ultrasonic waves: longitudinal, flexural, and torsional. The first and second of these ways can be associated with the Lamb waves in flat plates, while the third can be related to the shear-horizontal guided waves in flat plates. Subsequently, the paper describes validation experiments performed on cylindrical shells of various curvatures. It is shown that the EUSR concept works on cylindrical shells with curvatures representative to actual aircraft structure just as well as it works on flat plates.