Evaluation of an Ultrasonic Method for Damage Characterization of Brittle Rocks

Abstract

An intact rock specimen, when subjected to uniaxial compression, experiences multiple stages of deformation. This begins with the nucleation of microcracks at low stresses (crack initiation—CI) and their subsequent transition into unstable crack propagation (crack damage—CD) close to the ultimate strength. In the present study, an active ultrasonic monitoring method was used during uniaxial compression testing of Lyons sandstone specimens to evaluate the potential of the technique for damage characterization. The sensitivity of ultrasonic monitoring in relation to the input excitation frequency was also analyzed using four ultrasonic transducers with different central frequencies. A LabVIEW-controlled active ultrasonic system was used to acquire active seismic waveforms, which were made to propagate perpendicular to the direction of uniaxial stress. With increasing deformation, the corresponding changes in amplitude, frequency, and velocity of the active seismic signals were analyzed to characterize the CI and CD stress thresholds. Using statistical analysis, it was concluded that the changes in the amplitude and frequency of the active signals could be potential indicators of CI and CD. The comparison of wave characteristics corresponding to different input excitations also indicated that appropriate selection of transducer frequency is crucial for a representative interpretation of damage processes.

Appendix

Wave transmission in a medium is dependent on (1) the mechanical stiffness of the cracks present in the medium (defined by the spatial distribution and amount of contact within a crack); and (2) on the frequency of the propagating wave, as cracks having higher mechanical stiffness are sampled by higher frequency components of the wave and vice versa (Pyrak-Nolte and Nolte 1992; Acosta-Colon et al. 2009). As similar dominant wave frequencies are transmitted across the volume of the rock specimens for all the input excitation frequencies (see Table 3; except for 1 MHz), it is reasonable to deduce that the spatial distribution and stiffnesses of the microcavities (cracks and pores) at the initial state of the specimens is consistent for all the rocks specimens, as only a particular component of the input excitation frequency propagated through the rock specimens (Table 3). A closer look at Fig. 14, which shows variation in the mean frequencies of the seismic waves at different levels of stress, demonstrates that the variation in the mean frequencies as sampled through the 5, 10, and 15 MHz input frequencies are sensitive to the damage due to the uniaxial loading. Figure 14 (5 MHz) shows an increase in the mean frequency of the waves below the stress level of 20–30% of UCS, which starts to decrease as the applied load on specimens is increased (30–70% of the UCS). The mean frequency begins to reduce sharply as the load applied on the specimens is increased beyond 70% of the UCS. At higher input excitation frequencies (10 MHz and 15 MHz in Fig. 14), the results are relatively less consistent. As the inherent microstructure of the specimens in their intact state is similar based on the seismic measurements (similar transmitted dominant wave frequencies for 5, 10 and 15 MHz wave as given in Table 3), the reason for such inconsistency can be associated with the differences in the near-field of the ultrasonic beams as generated by the three transducer types (Basu and Aydin 2006). The near-field is the region directly in front of the transducer where the signal is in a transient state due to the constructive and destructive interference of the multiple pulses originating from the source face of the transducer, as shown in Fig. 15 (Basu and Aydin 2006; Hedayat 2013). The transient nature of the seismic signal in the near-field reduces the sensitivity of the signals to detect changes in the propagating medium; accordingly, studies have also shown that it is favorable to perform the seismic measurements in the far-field where the seismic intensity is uniform (Fig. 15) (Ghaziary and Kirk 1991; Basu and Aydin 2006; Olympus Ultrasonic Transducers Technical Notes 2011; Hedayat 2013). The near-field length of the 10 and 15 MHz transducers is at least twice the near-field length of the 5 MHz transducers (Table 3), which explains the seismic measurements obtained using the 10 and 15 MHz transducers. Figure 13, which compares the changes in normalized amplitude with damage, also shows that the 5 MHz frequency input provides a more consistent mapping of the internal damage processes in the specimens in comparison to the results of 10 and 15 MHz input (S6 and S9 specimens follow a different trend). These observations are consistent with the fact that it is essential to minimize the length of the near-field for greater sensitivity in seismic measurements (Ghaziary and Kirk 1991; Basu and Aydin 2006; Hedayat 2013).