Elsevier

Engineering Geology

Volume 249, 31 January 2019, Pages 172-185
Engineering Geology

Progressive failure behaviors and crack evolution of rocks under triaxial compression by 3D digital image correlation

https://doi.org/10.1016/j.enggeo.2018.12.026Get rights and content

Highlights

  • 3D-DIC system with 6 cameras was applied to triaxial compression test of rock.

  • The effect of σ3 on progressive failure process was analyzed.

  • Full-field strain on the specimen surface under triaxial compression was obtained.

  • The stress-strain responses inside and outside the localization zone was discussed.

Abstract

In the study, a series of monotonic triaxial compression tests were carried out on Emochi andesite to evaluate the effects of confining pressure on progressive failure behaviors. The three-dimensional digital image correlation (3D-DIC) system consisting of six cameras was used to capture the images based on the transparent pressure cell. The effects of confining pressures on stress thresholds, energy evolution, field strain patterns and crack evolution were investigated qualitatively or quantitatively. The crack initiation (σci), crack damage (σcd) and peaks stress (σc) increased with confining pressure and the ratios σci/σc and σcd/σc respectively varied from 57.74% and 75.11% to 64.02% and 85.33%. Total energy U, elastic energy Ue, and dissipation energy Ud at different characteristic points increased with confining pressures. Field strain patterns indicated that localization developed rapidly in the post-peak region due to the acceleration of crack propagation and damage evolution. The failure mechanisms of Emochi andesite under the uniaxial and triaxial condition are respectively tension-shear destruction and shear destruction. Furthermore, the stress-strain response inside and outside the localization zone are characterized by strain accumulation or inelastic unloading. Finally, a method of confirming the stress level of localization initiation is proposed based on the DIC technique. The stress level decreases with confining pressure. The study provides a useful reference for understanding the failure process of rocks and improving the stability and safety of underground structures.

Introduction

The investigation on the progressive failure of rocks is crucial for the design of large underground structures under different loading conditions because the damage around the excavation may impair stability and safety (Liu et al., 2017; Feng et al., 2018).”

Fractures of brittle rocks have been extensively investigated. The stress-strain curve of rocks during compression is usually divided into five stages (Fig. 1): (I) crack closure stage, (II) linear elastic deformation stage, (III) crack initiation and stable crack extension, (IV) crack damage and unstable crack growth, and (V) failure and post-peak behavior (Bieniawski, 1967a, Bieniawski, 1967b; Brace et al., 1966). The stress thresholds between every two adjacent stages are σcc (crack closure stress), σci (crack initiation stress), σcd (crack damage stress), and σc (peak stress). Wen et al. (2018) summarized existing methods for determining these thresholds.

These thresholds are the key parameters for estimating progressive fractures of rocks in engineering projects. For example, the lower limit of spalling rock mass strength was reliably estimated with crack initiation stress in uniaxial compression tests (Diederichs et al., 2004; Martin and Christiansson, 2009). Turichshev and Hadjigeorgiou (2016) indicated the difference between the estimations of spalling rock mass strength based on UCS alone and that based on crack initiation stress. Munoz et al. (2016a) obtained a strong correlation between the crack damage stress, the peak stress and several brittleness indices. In addition, the lower and upper limits of long-term strength can be obtained with crack initiation stress and crack damage stress and the lower limit is usually used to analyze spalling in tunnels (Xue et al., 2014). These thresholds can be employed as warning indicators of rock mass damage and breakouts (Pepe et al., 2017). The thresholds and crack evolution of rock materials under unconfined conditions had been explored in details. Pepe et al. (2017) and Xue et al. (2014) reviewed the commonly used methods and the data of rock materials for determining the thresholds under unconfined conditions in recent decades.

Lee and Rathnaweera (2016) reported the progressive fracturing-combined acoustic emission and petrographic analysis method under uniaxial and triaxial stress conditions. Turichshev and Hadjigeorgiou (2016) explored the influences of mineral veins on the behaviors of rocks and discussed related implications. Yang (2016) found that the peak stress and crack damage parameters of hollow sandstone were affected by confining pressure and hole diameter. Wen et al. (2018) proposed a new method to determine crack initiation stress and indicated that the ratio of crack initiation stress to peak strength was approximately 0.55 under the triaxial condition. Chen et al. (2016) investigated the pre-failure damage characteristics of granite under triaxial compression by experiments and numerical simulation and indicated that the damage increased slowly before the reversal of volumetric strain and increased remarkably afterwards. However, compared to the previous results obtained under uniaxial conditions, the effects of confining pressures on progressive failure of rocks were seldom explored (Taheri and Munoz, 2016). Furthermore, the evolution of field strain and localization of rocks under triaxial compression was seldom reported.

In this paper, a series of conventional triaxial compressive tests were carried out on Emochi andesite samples to explore the effects of confining pressure on progressive failure behaviors of rocks. Based on the experimental results, the effects of confining pressure on the stress thresholds and energy evolution during crack extension were discussed. A novel three-dimensional digital image correlation (3D-DIC) instrument with six cameras was used to capture the full-field strain patterns. Damage and crack evolution under confining pressure were presented and discussed. Furthermore, the stress-strain response inside and outside the localization zone were discussed based on the measured results of virtual extensometers by DIC technique. A method of confirming the stress level of localization initiation is proposed based on DIC measurements.

Section snippets

Specimen descriptions and preparation

Emochi andesite is a kind of pyroxene andesite produced in Fukushima Prefecture of Japan. It is homogeneous, dense and fire-resistant and usually used as the building material in Japan. Its main mineral components include plagioclase, ordinary pyroxene, purple augite, magnetite and a small amount of biotite. The uniaxial compressive strength is about 68 MPa. The Young's modulus is about 8.9 GPa. The Poisson's ratio is 0.25 and the density is 2.21 g/cm3. All the specimens were cored in the same

Stress/strain data

Axial stress σ1, axial strain ε1, lateral strain ε3 and confining pressure σ3 can be directly obtained from the experiment. Then volumetric strainεv, elastic volumetric strain εve, and crack volumetric strain εvc can be calculated as follows (Martin and Chandler, 1994).εv=ε1+2ε3.εv=εve+εvcεve=1vEσ1σ3

Based on the σ1-ε1curve, εv-ε1curve, and εvc-ε1curve, the thresholds of crack initiation stress σci, crack damage stress σcd, and peak stress σc can be calculated according to the method of Martin

Effects of confining pressure on thresholds

Typical results of Emochi andesite under different confining pressures are shown in Figs. 8(a~d). Axial, lateral and volumetric strains vs deviator stress curves are presented. Under uniaxial compression, the strength of andesite specimen fast decreases to the lower stress and the failure occurs in the post-peak region. When the confining pressure increases, the axial stress-axial strain curves become more and more flat in the post-peak region. The volumetric strain-axial strain curve and the

Discussion

Although DIC method had been widely applied in rock mechanics (Dautriat et al., 2011; Munoz and Taheri, 2017; Patel and Martin, 2017; Yang et al., 2015) and could be used to measure the full-field strain, the field strain patterns and crack evolution were different under different observation directions and the 3D-DIC system with two cameras was not applicable to observe the whole deformation pattern of specimen in the test. Moreover, the previous studies on the triaxial condition are less than

Conclusions

In the study, a series of triaxial compressive tests with Emochi andesite were performed. With 3D-DIC technique and transparent triaxial cell, the progressive failure behaviors of Emochi andesite were investigated. The characteristic thresholds, energy evolution, full-field strain pattern and crack evolution in different stages of rock failure were analyzed under different confining pressures. The conclusions are drawn below.

  • 1)

    In the range of tested confining pressures, σci, σcd, and σc of Emochi

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51434003), Basic and Frontier Research Projects of Chongqing (cstc2016jcyjA0117), and the Fundamental Research Funds for the Central Universities (106112017CDJQJ248825).

References (36)

  • C.D. Martin et al.

    The progressive fracture of Lac du Bonnet granite

    Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

    (1994)
  • C.D. Martin et al.

    Estimating the potential for spalling around a deep nuclear waste repository in crystalline rock

    Int. J. Rock Mech. Min. Sci.

    (2009)
  • B. Menéndez et al.

    Micromechanics of brittle faulting and cataclastic flow in Berea sandstone

    J. Struct. Geol.

    (1996)
  • A. Ord et al.

    Shear band formation in Gosford Sandstone

    Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

    (1991)
  • R.A. Stirling et al.

    The application of digital image correlation to Brazilian testing of sandstone

    Int. J. Rock Mech. Min. Sci.

    (2013)
  • A. Turichshev et al.

    Triaxial compression experiments on intact veined andesite

    Int. J. Rock Mech. Min. Sci.

    (2016)
  • T. Wen et al.

    Evaluation of methods for determining crack initiation stress under compression

    Eng. Geol.

    (2018)
  • H. Xie et al.

    Energy analysis and criteria for structural failure of rocks

    J. Rock Mech. Geotech. Eng.

    (2009)
  • Cited by (79)

    View all citing articles on Scopus
    View full text