In-Situ Imaging of Crack Growth with Piezoelectric-Wafer Active Sensors

Abstract

∗Piezoelectric-wafer active sensors (PWAS) are small, inexpensive, non-invasive, elastic wave transmitters/receivers that can be easily affixed to a structure. PWAS are wide-band nonresonant devices that can selectively tune in various Lamb wave modes traveling in a thin-wall structure. PWAS can be wired into sensor arrays and connected to data concentrators and wireless communicators. They have the potential to bring about a revolution in structural health monitoring, damage detection, and non-destructive evaluation just as significant as ultrasonic inspection did 50 years ago. However, its development is not yet complete, and a number of issues have still to be resolved. This paper will present results obtained in using a PWAS phased array to in-situ image crack growth during a simulated structural health monitoring test. The fatigue crack growth experiment was set up on a 1000 mm by 1000 mm 2024-T3 1-mm thick aluminum plate. An initial pre-crack of 30 mm was grown under fatigue loading to a final length of 60 mm over a total of 58 kilocycles run at 10 Hz frequency in an MTS-810 testing machine. During the test, insitu readings of the PWAS phased array were taken while the testing machine was running. The PWAS were excited with a 10-Vpp smoothed 3-count tone burst of 372 kHz, which excited the symmetric S0 Lamb-wave mode. To minimize the equipment requirement to just a single channel, the excitation of the PWAS array was performed in a round robin fashion. Simultaneously, all the PWAS were in term considered as receivers. In this way, an N x N matrix of signals was collected and stored. In the beginning, the received signals were unusable due to the high 10 Hz noise superposed by the testing process. Additional hardware was constructed to pre-filter the received signals prior to digitization. Consequently, usable signals with clear crack reflections were successfully collected. The received signals were post processed with the embedded ultrasonics structural radar (EUSR) algorithm to obtain a direct imaging of the crack in the test plate (similar to the C-scan from ultrasonic NDE, only that was obtained from a single location using a sweeping beam of focused Lamb waves. The imaging results were correlated with physical measurements of the crack size using penetrating liquid and a digital camera. Remarkable consistency was obtained. Subsequently, the results were used to predict further crack growth and make a prognosis of component failure using the structural health monitoring results.