Noise isolation with phononic crystals to enhance fatigue crack growth detection using acoustic emission

The background noise imposes difficulties in monitoring crack activities in real-time monitoring of structures using the acoustic emission (AE) method. Here, a two-dimensional phononic crystal (PC) is designed to enhance fatigue crack growth detection capability of acoustic emission technique in plate-like structures, such that a single AE sensor is sufficient to monitor the crack activity. PCs, also known as acoustic metamaterials, are artificial composite, man-made periodic systems creating new responses through physical constraints in the constituent materials such that certain frequencies are prohibited by local the resonance of the periodic system. The integration of PC design with AE method can be utilized to block unwanted noise signals and enhance the crack detection ability of the AE method. The proposed PC structure to block certain frequencies is numerically studied using COMSOL Multiphysics software through applying two approaches: the unit cell analysis and the transmission loss. It is concluded that the primary reason of the band gap formation through the designed PC structure is the local resonance phenomenon. The final geometry is selected to provide a band gap near 150 kHz, which is a common frequency to monitor crack growth in metallic structures. The stubs forming the PC structure are placed on the compact specimen periodically, except along the crack growth path and sensor position, to block the friction emissions coming from the grip locations. The performance of the designed PC structure is demonstrated experimentally through the fatigue testing of aluminum standard compact tension specimens. The cumulative numbers of hits and AE energy obtained from the specimen integrated with the PC structure demonstrate the successful implementation of noise blocking in the AE testing of detecting active cracks.