Estimation of characteristic stresses in granite through acoustic emission monitoring of microcrack fracture mode evolution

Abstract

Characteristic stresses are critical indicators for microcrack initiation and propagation in rock, a process intrinsically linked to fracture mode. To investigate fracture mode evolution and its feasibility for estimating characteristic stresses, this study conducted uniaxial compression and cyclic loading-unloading tests on fine- and coarse-grained granite with acoustic emission (AE) monitoring. Cyclic target stresses were set within intervals determined by characteristic stresses. Analysis using the AE parameters AF-RA revealed that fracture mode evolution correlates with damage level, and shear microcrack propagation primarily governs macroscopic failure. A characteristic stress estimation method was developed by mapping key points on the shear crack proportion curve: crack closure stress (transition between fluctuating and stable segments), crack initiation stress (inflection point of curve rise), and crack damage stress (slope change point in ascending segment). Comparative analysis with the crack volumetric strain method validated the proposed method. The influences of fracture mode dividing line and statistical interval were discussed, with practical recommendations provided. Compared to conventional AE parameters, the fracture mode proportion exhibits lower sensitivity to AE parameter variations, enabling more reliable identification of characteristic stress points. Furthermore, it directly reflects microcrack evolution behavior, enhancing interpretability and providing a novel perspective for AE-based characteristic stress determination.

1. Introduction

The failure of hard brittle rocks under loading constitutes a progressive process accompanied by microcrack closure, initiation, propagation, localized fracture development, and gradual cohesion loss, ultimately leading to macroscopic failure [1,2]. According to the extent of microcrack evolution, the pre-peak failure process of rocks can be divided into four distinct stages: crack closure stage, elastic deformation stage, stable crack growth stage, and unstable crack growth stage. In rock mechanics, the stress values demarcating these different stages are conventionally termed ''characteristic stresses'' or ''stress thresholds'' [3,4], including crack closure stress (rcc), crack initiation stress (rci), and crack damage stress (rcd). The term ''characteristic stresses'' is consistently used in this study for standardization. Beyond theoretical investigations into failure stages, characteristic stresses also serve as crucial evaluation indices for stability analysis in rock engineering. For instance, the crack initiation stress can be utilized to estimate rock spalling strength [5], while the crack damage stress provides an important indicator for assessing the long-term strength of engineering rock masses [6]. Consequently, research on characteristic stresses, especially in accurate determination, is of significant theoretical importance and practical utility.

Scholars have conducted extensive research on the estimation of characteristic stresses. Early studies primarily employed the stress-strain relationship (the ''Stress-Strain Method'') as an indicator of rock failure. The method's foundation lies in the principle that microcrack evolution changes the rock's elasticity modulus and Poisson's ratio, resulting in distinct features on the axial stress-strain curve. The linear segment is bounded by the crack closure stress and crack initiation stress [7]. While methodologically straightforward, this approach has limitations in precision and objectivity, prompting the development of more advanced techniques such as acoustic emission monitoring.