Quantifying the coalescence process of microcracks leading to a system-size failure

Ilya Zaliapin, & Yehuda Ben-Zion

Submitted August 15, 2017, SCEC Contribution #7830, 2017 SCEC Annual Meeting Poster #187 (PDF)

We analyze Acoustic Emission (AE) data with the goal of quantifying the transition from distributed events to a system-size failure. Several data sets from different experiments provide information on the failure process in different rock types (granite, sandstone), loading conditions (constant loading rate vs. servo-controlled to maintain about constant AE rate) and sample properties (pre-existing notch vs. intact sample). Each data set is considered a realization of a coalescence process where earlier failures tend to merge with later ones. The merging of two existing failures can only occur via a new failure that connects the two. Healing is ignored and a global coalescence failure structure progressively grows in size by merging with other failures. Mergers (failures that unify earlier failures) are defined as the nearest-neighbors of the coalescing events with respect to a given space-time-magnitude distance. A catalog of AE events is represented with this simplified framework by a time-oriented tree, whose vertices are the catalog events with the root in the last event. The number Z(t) of unmerged events at time t is a useful statistic for describing the coalescence dynamics. Preliminary results suggest the existence of four quantitatively defined stages of the failure process. Stage I – accumulation of distributed and largely uncorrelated events corresponding to a steady increase of Z(t). Stage II – approximately equal generation of new events and mergers corresponding to steady-state Z(t) with small fluctuations. Stage III – transition to an instability-prone state corresponding to strong fluctuations of Z(t). Stage IV – abrupt drop of Z(t) leading to a system-size failure. The stages durations differ by orders of magnitude, with stages I and IV being the longest and shortest, respectively. However, the distributions of seismic moment, Benioff strain, and event number are comparable (differ by 10-20%) among the four stages. The event and coalescence rates exponentially increase prior to the system-size failure (during Stages I-III). Updated results will be presented in the meeting.

Key Words
acoustic emission, coalescence

Zaliapin, I., & Ben-Zion, Y. (2017, 08). Quantifying the coalescence process of microcracks leading to a system-size failure. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)