Crack Initiation (CI) Thresholds from Acoustic Emission and Stress-Strain Data: A Comparison of Values Measured for Rocks from Montana and Idaho

ABSTRACT: Rock mechanics researchers have defined crack initiation (CI) and crack damage (CD) thresholds as the points at which microcracks begin to propagate and coalesce under increasing stress levels. CI and CD correspond to cracking events and can be identified using Acoustic Emissions (AE). AE monitoring is a passive, nondestructive testing method which records the elastic energy released during loading. In this study, CI is determined using the Cumulative Acoustic Emissions Hits (CAEH) method (Zhao et al, 2015), which involves graphing cumulative counts vs. axial stress, implemented as a MATLAB® v7 module. The CAEH method was designed to make the determination of CI less subjective. Augmenting previous studies by others, this study involves using unconfined compression tests in conjunction with AE monitoring to identify CI and CD thresholds for specimens of several different rock types from Montana and Idaho. The unconfined compression tests were performed on a TerraTek load frame using strain rate control, and the AE data were collected and recorded using a Mistras' Express-8 AE system and AEwin Express-8 software. CI values determined from the stress-strain curves ranged between 34% to 60% of the UCS for all but 2 of the specimens. The CAEH CI values were generally higher than the stress-strain CI values, and were sensitive to the selection of the initial point. Data from two AE sensors mounted on the same specimen yielded surprisingly different CI values. 1. INTRODUCTION As compressive stress increases in rock, cracks initiate and coalesce, leading to nonlinear behavior and eventually to failure. Stress and strain data from a traditional unconfined compression test on rock provide values of yield stress and unconfined compressive (peak) strength (UCS) as the specimen is loaded past the point of linear elastic behavior. Rock mechanics researchers have also defined crack initiation (CI) and crack damage (CD) thresholds as the stress levels at which microcracks begin to propagate and coalesce under increasing compressive stress, originally associated with the point of departure from linear on axial stress vs. lateral strain (for CI) and on axial stress vs. axial strain (for CD) plots (Ghazvinian et al., 2012, referring to work by Brace et al., 1966, Bieniawski, 1967, and Lajtai & Lajtai, 1974). Identifying CI and CD can be valuable for understanding the behavior of rock under load, so efforts have been made to associate them with more definitive observable points on the stress-strain data plot. Due to the subjectivity involved in selecting some of these points combined with large variations in behavior observed in rocks with drastically different porosity values, this process is not necessarily a straightforward one.