Acceleration and localization of subcritical crack growth in a natural composite material

Abstract

Catastrophic failure of natural and engineered materials is often preceded by an acceleration and localization of damage that can be observed indirectly from acoustic emissions (AE) generated by the nucleation and growth of microcracks. In this paper we present a detailed investigation of the statistical properties and spatiotemporal characteristics of AE signals generated during triaxial compression of a sandstone sample. We demonstrate that the AE event amplitudes and interevent times are characterized by scaling distributions with shapes that remain invariant during most of the loading sequence. Localization of the AE activity on an incipient fault plane is associated with growth in AE rate in the form of a time-reversed Omori law with an exponent near 1. The experimental findings are interpreted using a model that assumes scale-invariant growth of the dominating crack or fault zone, consistent with the Dugdale-Barenblatt "process zone" model. We determine formal relationships between fault size, fault growth rate, and AE event rate, which are found to be consistent with the experimental observations. From these relations, we conclude that relatively slow growth of a subcritical fault may be associated with a significantly more rapid increase of the AE rate and that monitoring AE rate may therefore provide more reliable predictors of incipient failure than direct monitoring of the growing fault.

Record-breaking events during the compressive failure of porous materials

“…localization has also been seen in acoustic emission data during laboratory experiments loaded at constant strain rate [27]. The spatial localization is accompanied by a rapid increase of the average event size [11,12].…”

“…The modeling approach proved to be successful in reproducing several important observed features of crackling noise in porous materials [1][2][3]6,7,26]. Figure 1 demonstrates the breaking sequence of a single simulation of a system of 20000 particles where 1832 events are obtained up to macroscopic failure in comparison with data from a real experiment [27]. In the example the breaking process starts at the deformation ε ≈ 0.0019 where initially small events occur with size = 1.…”

Triggering Processes in Rock Fracture

“…This suggests that fracture phenomena can also give rise to triggering associated with aftershocks. The observation of triggering in uniaxial tests is surprising because triggering is absent during triaxial fracture experiments on homogeneous samples such as sandstone, for which AE activity increases as a nonhomogeneous Poisson process [25]. Similarly, numerical simulations of a model of sedimentary rocks with timeindependent rheology subject to uniaxial compression do not show such triggering behavior [26].…”

“…S2 and S3 [37] and Refs. [25,26]) but no macrocrack has formed yet. The importance of the interaction between large-scale imperfections and microcracks is conceptually consistent with process zone models [51][52][53].…”

“…More importantly, Refs. [25,26] both provide clear examples that spatial localization and an increase in activity rates do not necessarily imply triggering behavior associated with aftershocks. Another uniaxial compression experiment seems to suggest time-dependent deviations from the OU relation [27], while other studies suggest that the OU relation is a general or even universal signature of relaxation processes in rock fracture independent of the specific perturbation source [8,28].…”