COUPLANTS AND THEIR INFLUENCE ON AE SENSOR SENSITIVITY

The removal of air from the interface between a measurement surface and an AE sensor is crucial to the transmission of ultrasonic energy. The acoustic impedance of air is around 5 orders of magnitude lower than that of the two contacting surfaces, allowing for very little transmission of acoustic energy at the frequencies typical of acoustic emission (AE). The use of a couplant can greatly improve this transmission by around 2 times at 100 kHz and more than 10 times at 500 kHz; a typical gel-based couplant having a high acoustic impedance around 4 orders of magnitude higher than that of air. The effectiveness of a given couplant is dependent on its acoustic impedance, acoustic absorption, application thickness and viscosity. Each of these can have a strong influence on the sensitivity response of the sensor and can ultimately change the way the sensor responds to different wave modes. This paper considers a number of ultrasonic/AE couplants and compares the sensor response for each couplant to longitudinal and shear waves, demonstrating that a high-performance ultrasonic couplant can improve sensor sensitivity significantly above 400 kHz compared with a grease-type couplant.