TODAY Fault Mechanic Webinar Series: What does a Brittle-to-Ductile Transition Sound Like? - Pec (MIT)

What does a Brittle-to-Ductile Transition Sound Like?

Matej Pec, Massachusetts Institute of Technology

Deformation of all materials necessitates the collective propagation of various microscopic defects. On Earth, fracturing gives way to crystal-plastic deformation with increasing depth resulting in a “brittle-to-ductile” transition region that is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. Here, we show that the crossing of a brittle–to–ductile transition in marble is accompanied by profound changes in the frequency of acoustic emissions suggesting changes to the size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle-ductile transition. As pressure increases, long-period signals are suppressed and short-period signals become dominant. At highest explored pressures signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types. Complex mixed-mode events indicate that interactions between the defects are common over the whole studied pressure range in agreement with post-mortem microstructural observations. Our measurements provide first experimental data on in-situ microscale dynamics of a brittle-to-ductile transition that can inform micromechanical models for semi-brittle deformation. [more info ] [register]


Looking inside granular materials

Karen Daniels, North Carolina State University

Granular materials such as those found near the Earth’s surface are inherently heterogeneous, and continuum models of properties such as the shear modulus and sound speed often fail. One promising alternative is to build an understanding of bulk behaviors from measurements at the particle scale. Our lab’s experiments make use of idealized, optically birefringent materials to quantify the interparticle forces - and thereby directly measure the stress tensor at the particle scale - within compressed or sheared granular materials. I will describe both these methods, and the heterogeneous network of forces that they reveal. Through our experiments, I will talk about several frameworks capable of connecting the internal structure of disordered materials to their rigidity and/or failure under loading, and describe how we apply these ideas. [more info] [register]


June 9 - Marie Violay, EPFL