An initial investigation of the large strain and fatigue loading behavior of piezoelectric wafer active sensors

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.