The goal of the research presented in this paper was to study the behavior of piezoelectric wafer active sensors (PWAS) under large strain and fatigue conditions. To test the characteristics of the PWAS under large strain conditions, the PWAS was bonded to an aircraft grade 2024 aluminum test specimen and subjected to tensile loading. The baseline impedance was recorded at zero strain and additional readings were recorded at 200 micro-strain intervals until failure of the PWAS occurred. Minimal changes occurred to the impedance signature until the value of 5000 micro-strain was exceeded. Eventually the PWAS failed in tension at approximately 7200 micro-strain. Theoretical data was developed to determine how the frequencies and resonance qualities change due to increased tensile loading to compare to the experimental data. For fatigue testing, the PWAS was again bonded to a 2024 aluminum test specimen and the specimen was loaded in fatigue. Appropriate mean loads and amplitudes were calculated to cause failure of the substrate at various values between 100 thousand and 10 million cycles. The baseline impedance reading was taken with the mean load applied at the beginning of the tests and at predetermined cyclic intervals. Small settle-in changes occurred in the impedance readings in the first 30 to 40 thousand cycles. Beyond this the PWAS readings were relatively unchanged until the metallic specimen finally broke under fatigue. The PWAS survived the fatigue failure of the metallic specimen.
A key question that needs to be addressed and answered with regard to successfully implementing Structural Health Monitoring technologies in Air Force systems involves the long-term operability, durability, and survivability of integrated sensor systems and their associated hardware. Whether a sensor system is fully integrated within a structural material, or surface-bonded to the structure, a number of environmental and system level influences will tend to degrade the sensor system’s performance and durability over time. In this effort, an initial sensor durability study was undertaken to better understand the performance and degradation of piezo wafer active sensor (PWAS) systems under adverse mechanical, temperature, and moisture conditions. A novel displacement-field imaging approach was utilized to understand the vibration characteristics of PWAS transducers placed in accelerated vibration, temperature-cycling, and moisture-cycling conditions. The results showed damage in the form of PWAS sensor cracking events, bond degradation and failure, as well as indications of performance variation and reduction due to the accelerated exposure levels. Future activities will focus on identifying critical durability and survivability issues through advanced sensor modeling and additional accelerated testing efforts, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.