Microscopic defect dynamics during a brittle-to-ductile transition.

Area of Science:

• Geophysics
• Materials Science
• Rock Mechanics

Background:

• Material deformation involves microscopic defect propagation.
• The brittle-to-ductile transition (BDT) is crucial for understanding tectonic plate strength, earthquake cycles, and geothermal resource utilization.
• The BDT signifies a shift from fracturing to crystal-plastic deformation with increasing depth.

Purpose of the Study:

• To investigate the microscale dynamics of rocks crossing the brittle-ductile transition.
• To analyze acoustic emissions and waveform characteristics during deformation experiments.
• To correlate acoustic signals with defect behavior and micromechanical models.

Main Methods:

• Laboratory deformation experiments on marble at pressures from 10 to 200 MPa.
• Monitoring and analysis of acoustic emissions (AE) during rock deformation.
• Application of unsupervised learning to classify dominant AE waveform classes.
• Microstructural observations for postmortem analysis.

Main Results:

• A systematic increase in acoustic emission frequency was observed as rocks crossed the BDT.
• Unsupervised learning identified distinct AE waveform classes whose relative activity changed with pressure.
• Long-period signals were suppressed, and short-period, avalanche-like signals became dominant at higher pressures.
• Complex mixed-mode events indicated frequent defect interactions.

Conclusions:

• The study provides real-time microscale dynamics data across a broad pressure range.
• Changes in AE frequency and waveform characteristics signal a profound shift in defect size and propagation velocity at the BDT.
• The findings support the correlation of waveform classes with dominant defect types and inform micromechanical models for semi-brittle deformation.