Acoustic emission (AE) is the phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a material undergoes irreversible alterations in its internal structure, for example as a result of crack formation or plastic deformation due to aging, temperature gradients or external mechanical forces. Alternatively, AE can be described as the elastic waves produced by the abrupt release of stress in a solid body caused by an internal deformation in the constituent materials or the micro-fracture events in the bulk. As such, AE are important as they can be analyzed to extract information about the position of the AE source, event duration, and defect type. Therefore, they present an effective non-destructive technique for inspecting the condition of structures and real-time monitoring of machining processes. A review of published literature reveals that to date, piezoelectric type AE sensors are still the most established owing to their high sensitivity, reliability, and relatively easy installation.
AE sensor mounting is of critical importance as it directly affects performance. Related literature further shows that typical mounting methods of AE sensors include bonding to the target surface with glue or fastening to the target with a screw. Unfortunately, these methods for securing the AE to the target surface presents significant challenges particularly when the surface is deformable. Worse off, they can result in detection of false signals or omission of the correct signal owing to bad contact. To address these issues, researchers from the Department of Mechanical Engineering at the National Chung Cheng University in Taiwan: Professor Guo-Hua Feng and Dr. Cheng-Yen Chiang developed a stretchable and flexible acoustic emission sensor composed of patterned upper, lower piezoelectric film foils, magnets and stereolithographic structures. Their work is currently published in the research journal, Smart Materials and Structures.
In their approach, they employed a flexible and stretchable lower piezoelectric foil into the proposed AE sensor that was comprised of three major components to perform its unique functions. Overall, they presented piezoelectric sensing structures with magnetic repulsion that was effective for AE signal coupling. In their setup, the lower piezoelectric serpentine shaped AE detection structure with stretchable and flexible characteristics was included as it allowed for sensing of various curved surfaces.
The authors reported that the magnetic-repulsion-enhanced AE sensor exhibited a better bandwidth compared to that with only a lower AE sensing structure. In addition, the two co-authors reported that when the fabricated sensor was subjected to a sensing target force ranging from 4.98 to 14.85 mN, it resulted in a frequency change of the piezoelectric sensing beam of the upper foil from 20.073 to 20.135 kHz.
In summary, the new magnetic-repulsion-coupled AE sensor was successfully fabricated, and tested. Specifically, the magnetic repulsion was implemented between a lower serpentine-shaped foil and an upper bridge foil integrated with magnets. Remarkably, the proposed piezoelectric-sensor-actuator pair on the bridge structure of the upper foil demonstrated a frequency change as a function of the static force, when subjected to various tests. In a statement to Advances in Engineering, Professor Guo-Hua Feng emphasized that the developed sensor could be used to monitor AE waves as it uses the effect of the action at a distance. Further, their approach could open a new research field for non-contact AE wave detection.
Guo-Hua Feng, Cheng-Yen Chiang. Magnetic-repulsion-coupled piezoelectricfilm-based stretchable and flexible acoustic emission sensor. Smart Materials and Structures, volume 29 (2020) 035027 (12pp).