Rayleigh Wave Calibration of Acoustic Emission Sensors and Ultrasonic Transducers

Abstract

Acoustic emission (AE) sensors and ultrasonic transducers were characterized for the detection of Rayleigh waves (RW). Small aperture reference sensors were characterized first using the fracture of glass capillary tubes in combination with a theoretical displacement calculation, which utilized finite element method (FEM) and was verified by laser interferometer. For the calibration of 18 commercial sensors and two piezoceramic disks, a 90° angle beam transducer was used to generate RW pulses on an aluminum transfer block. By a substitution method, RW receiving sensitivity of a sensor under test was determined over the range of frequency from 22 kHz to 2 MHz. Results were compared to the sensitivities to normally incident waves (NW) and to other guided waves (GW). It was found that (1) NW sensitivities are always higher than RW sensitivities, (2) differences between NW and RW receiving sensitivities are dependent on frequency and sensor size, (3) most sensors show comparable RW and GW receiving sensitivities, especially those of commonly used AE sensors, and (4) the receiving sensitivities of small aperture (1 mm diameter) sensors behave differently from larger sensors.

Revisiting Piezoelectric Sensor Calibration Methods Using Elastodynamic Body Waves

“…When we place conical PZT sensors from the bottom surface to the top surface of the steel transfer plate where ball impact occurs, current methodology can be extended to calibrate these sensors but using "surface (or Rayleigh) wave calibration" instead of "body wave calibration". Although the Rayleigh wave calibration of PZT sensors is outside the scope of this study [39], we suppose the calculated instrumental response keeps almost the same as which we showed in Fig. 10 since same KRNBB sensors are utilized and the point contact assumption still stands.…”

“…If the wavelength is not significantly larger than sensor aperture, the amplitude of elastic waves will be averaged at the contact area while the true ground motion at the small tip is masked. In this case, aperture effect decreases the calculated instrumental response where the amount can be estimated by a Bessel function of wave speeds, frequency and sensor aperture [39,52]. This effect is weakest at the takeoff angle of 0 • (PZT sensor directly below the impact) since all waves arrive the sensor aperture almost in the same phases; and should be strongest at the large takeoff angle (PZT sensor on the bottom surface but has an incremental epicentral distance) until 90 • (strongest, PZT sensor on the top surface) where waves arrive out of phases.…”

“…hot glue, Vaseline) is used through this study. These sensors are sensitive to ground motion in the 3-direction over a wide frequency range (1 kHz to 1 MHz) and their spectral characteristics have been well documented [9,25,38,39]. A small account of historical development of the KRNBB sensor is given in Appendix B.…”

“…The derived ω max χ group corresponding to small ball diameters (< 2 mm) are out of order, suggesting that in the characterization experiments, we should keep the epicenter of PZT sensors close to the location of the ball impact. Note that we used conical shaped PZTs throughout these experiments; the corresponding analysis could be quite different for disk shaped PZTs with a large diameter [39,50].…”

Dimension Effects on the Acoustic Behavior of TRC Plates

“…The effect in the final signal acquisition, as acknowledged in these studies, is quite severe masking the initial characteristics of the AE source. In addition, a secondary effect, especially in plates, where the wave propagation is parallel to the plate and the sensor contact plane, comes from aperture effects or the interaction between the wavelength and the sensor's diameter that starts to become important [27][28][29][30], since the concept of point receiver is no longer valid. Therefore, in general, the upgrade from laboratory coupons to realistic geometries needs further investigation as the same classification between AE signals from different fracture modes, as examined in small coupons, may not apply for larger scale.…”