Non-contact passive sensing of acoustic emission signal using the air-coupled transducer

Abstract

Structural health monitoring (SHM)/nondestructive evaluation (NDE) is an emerging multi-disciplinary field that aims at detecting/characterizing structural damage and providing diagnosis/prognosis of structural health status in a real-time or on-demand manner. It can reduce maintenance costs, shorten the machine service downtime, and improve the safety and reliability of the engineering structures. Acoustic emission (AE) is one of the SHM/NDE methods by means of detecting elastic waves due to dynamic motions at AE sources, such as cracking, delamination, cleavage, and fretting in a material. The acoustic emission inspection technique relies on the AE sensors to collect the AE signals from the structure to monitor the structural health. Conventionally, these AE sensors need to be permanently attached to the structure through the bonding adhesive layer which may introduce contamination to the structure. In this work, the research is focused on investigating non-contact passive sensing of acoustic emission (AE) signals using an air-coupled transducer (ACT). The well-acknowledged pencil-lead-break method has been used to simulate the AE source. A resonant type ACT is used to passively sense the AE signals, which leaves the testing object intact and provides a non-intrusive sensing method. The non-contact AE test on a thin aluminum structure as well as a thick steel structure is first conducted. Next, the investigation is extended to composite structures. Both single-layer composite structure and bonded composite structure are investigated. The results successfully demonstrate the capability of non-contact passive sensing of the AE signals using the ACT method.