To investigate the mechanisms involved during semi-brittle flow, we deformed Carrara marble over a confining pressure (Pc) range of 10-300 MPa and room temperature to ≈10% strain. We apply triaxial loading to intact Carrara marble and collect mechanical, ultrasound pulsing, and acoustic emission (AE) data during pressurization and deformation stages. The pulsing and AE waveforms are recorded using a pair of piezoelectric sensors. At lower Pc, microcracking is the dominant deformation mechanism, whereas at higher Pc, crystal-plastic mechanisms such as twinning and dislocation glide are favored. These changes in the activity of defect populations are manifested in changes in mechanical properties, velocity variations, and AE characteristics. Samples at higher Pc exhibit higher strength and require more work for fault-development. Transition from localized faulting to distributed barreling is observed between 50 and 100 MPa Pc. We track precise velocity variations from the pulsing waveforms using correlation-based methods. During the pressurization stage, the velocity increases logarithmically with Pc between 0-100 MPa, followed by a linear increase at higher pressures. During the deformation stage, the compressional wave velocity initially increases before the yield point due to closing of crevices, and then decreases exponentially after the yield point. The rate of this velocity decay is smaller as Pc increases, owing to reduced microcracking with very little change at Pc ≥ 200 MPa. AE data show that individual defect types emit characteristic patterns. Twinning produces repetitive patterns of low amplitude, short signals localized in frequency space whereas microcracks are more energetic, emit over a much broader frequency range, and show more variation in signal shape and duration. The AE spectra shift from ≈ 500 kHz to ≈15 MHz mean frequency as Pc increases, which is associated with increasing twinning activity. This acoustic data agree with microstructural observations of microcracks and crystal-plastic deformation in the samples. By joint-analyzing the stress-strain and velocity evolutions with AE observations, we obtain detailed changes in the micro-mechanisms accommodating strain in the Carrara marble and constrain the deformation modes as it goes through the brittle-plastic transition.
