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.