Propagation characteristics of acoustic emission waves in liquid media in near-field

Acoustic emission (AE) detection through a liquid medium is a simple and effective method that can be widely applied to AE diagnosis for rotating machinery. As AE diagnosis is performed by correlating the signal intensity with the failure level and the frequency characteristics with the failure type, it is important to clarify the propagation characteristics of AE waves in liquid media to enable accurate AE diagnosis using a simpler system that utilizes AE propagation in a liquid medium. Therefore, in this study, systematic experiments were conducted focusing on the liquid medium type and the propagation distance as factors affecting the propagation characteristics of AE waves. As the area where AE waves propagate, we mainly focused on the near-field, where AE waves do not spread and are not attenuated with the propagation distance. First, we investigated the propagation and attenuation characteristics of AE waves in the near-field using an artificial AE source in liquid media such as purified water, cutting fluid solution, and lubricating oil. Differences in attenuation characteristics with the liquid medium were observed. Moreover, the propagation distance affected the attenuation characteristics, i.e., the signal intensity decreased with increasing propagation distance in a manner depending on the viscosity of the liquid medium, especially in the frequency bandwidth range from 1.5 to 3.0 MHz. Second, we theoretically verified some of the experimental results from the viewpoint of ultrasonic propagation in a liquid medium. As a result, the observed differences in attenuation characteristics with liquid medium were explained by the acoustic impedances of the liquid media, and the attenuation of the signal intensity of AE waves with increasing propagation distance was caused by the increasing absorption coefficient with increasing viscosity of the liquid medium.