Title: Structural Health Monitoring with Piezoelectric Wafer Active Sensors

Piezoelectric wafer active sensors (PWAS) are inexpensive, non-intrusive un-obtrusive devices that can be surface-mounted on existing structures, or inserted in a new composite structure. The PWAS can be used in both active and passive modes. In active mode, the PWAS generate Lamb waves that can exist as either traveling waves or standing waves. As traveling waves, PWAS-generated Lamb waves can be used with the pitch-catch, pulse-echo, or phasedarray methods that arrow far-field and some medium-field damage detection. As standing waves, PWAS-generated Lamb waves can used in conjunction with the electro-mechanical (E/M) impedance technique that allows near field and some medium-field damage detection. This paper presents new results obtained in the use of PWAS for the structural health monitoring of aerospace vehicles. One set of results to be presented will refer to experiments performed on detection the crack propagation in a fracture-mechanics panel with a stressconcentration precrack in the center. The crack was propagated using cyclic fatigue loading in a large testing machine. The crack was imaged with the PWAS EUSR method, which is the guided Lamb wave equivalent of the P-wave C-scan, only that it is produced from a single location and does not need a raster x-y coverage like the conventional P-wave Cscan. PWAS EUSR measurements were taken while the testing machine was running, thus simulating inservice monitoring conditions. Another set of results refers to the PWAS monitoring of disbonds in an spacecraft panel. The E/M impedance technique is shown to distinguish clearly between a bonded and disbonded part. The E/M impedance of bonded part measured at different locations is shown to be consistently the same, with the actual curves overlapping, thus confirming that the impedance changes noticed at the disbonded location are solely due to disbonding. PWAS Piezoelectric wafer active sensors (PWAS) are inexpensive, small, and unobtrusive transducers that operate on the piezoelectric principle by coupling the electrical and mechanical energy fields (Figure 1). For embedded NDE applications, PWAS couple their in-plane motion with the Lamb-waves particle motion on the material surface. The in-plane PWAS motion is excited by the applied oscillatory voltage through the d31 piezoelectric coupling. PWAS act as both exciters and detectors of ultrasonic Lamb-wave through in-plane strain coupling. Embedded ultrasonic NDE is achieved by using the pitch-catch, pulse-echo, and phased-array NDE methods with in-situ PWAS transducers. A comprehensive study of the state of the art in embedded ultrasonic NDE was published by Giurgiutiu and Cuc (2005).