

The Implementation of Piezoelectric Wafer Sensors for Acoustic Emission Sensing in Aluminum
Abstract
Piezoelectric wafer sensors have been used extensively for the guided wave ultrasonic method in order to detect discontinuities at the aircraft aluminum components. This study shows the implementation of the piezoelectric wafer sensors for detecting propagating elastic waves in thin aluminum due to crack growth under fatigue loading using acoustic emission (AE). The responses of piezoelectric bulk and wafer type sensors to detect damage in a thin aluminum under fatigue loading are compared. The first comparison is conducted on a 1-mm thick plate made of 7075-T6 aluminum alloy using a piezoelectric actuator in order to control the frequencies of the propagating elastic waves. The actuator inputs are 3-cycle sine waves with sine envelope with 50 kHz frequency increments starting at 100 kHz to 400 kHz. In overall frequency range, piezoelectric wafer sensor is 9 dB less sensitive than conventional AE sensor. In the fatigue testing, the threshold levels of two sensors (conventional AE sensor as 50 dB, piezoelectric wafer sensor as 41 dB) are set up such a way that their dynamic sensitivity levels are similar. The AE hit rate histories of two sensors show a similar pattern, which indicates that both sensors detect the same events. Two piezoelectric wafer sensors placed at two ends of the sample can locate the damage source using the linear location algorithm. The sensors detect and locate the AE events earlier than visual observation. The cumulative AE event decreases when the sample is close to the fracture, which agrees with the theory that the volume of relaxing of material decreases due to the formation of dislocation arrays. The reduction in the AE hit rate and event accumulation after the cluster of events is detected can be considered as the proximity of the fracture. In the paper, in addition to the time based comparison of the sensors (e.g. hit envelope, amplitude, source location), waveform signatures and frequency contents of crack growth emissions detected by two sensor types are evaluated. The responses of the sensors are compared with the dynamic strain measurement at the vicinity of the hole. In summary, the piezoelectric wafer sensors show good promise to be used as the AE sensors in thin aluminum plates.