V. Giurgiutiu, B. Lin, University of S. Carolina, Columbia, SC

Summary:

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 phased-array 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.

The purpose of this paper is to explore the durability and survivability issues associated with various environmental conditions on PWAS for impedance-based structural health monitoring. Our approach consists of several stages. First, we put the PWAS into oven to determine how it behaves in the temperature range associated with operational usage. PWAS was placed in an oven and exposed to a certain temperature and a heating cycle. Second, we put the PWAS in the outdoor environment to determine how it behaves in different weather conditions (sun, rain, etc.). Third, we determine how PWAS behaves when exposed to water and various maintenance fluids (hydraulic fluids and lubrication oils). The electrical impedance of the PZT material can be directly related to the mechanical impedance of a host structural component where the PWAS is attached. Characterization of the free PWAS and bonded PWAS on aluminum structures was recorded and compared before, during, and after the various conditions are applied. Most of our experiments proved that PWAS have a good record of environmental survivability. However, a couple of situations were encountered were failure was observed. One such situation was the failure of a PWAS submerged in saturated saline solution after a few weeks of exposure. Another such situation was that of a bonded PWAS that failed under temperature cycling after 9 months (>1500 cycles) exposures. Details of these experiments and interpretation of the results will be presented.