How do thermally induced microcracks alter microcracking mechanisms in Hong Kong granite?
Introduction
Understanding the thermal-mechanical behavior of rocks is of great importance in the fields of earth sciences and geoengineering because the rock masses are often subjected to an ambient high temperature in scenarios such as volcanic evolution (Browning et al., 2015), magma transport in the crust (Gudmundsson, 2011, Gudmundsson, 2012), enhanced geothermal systems (Brudy and Zoback, 1999; Kitao et al., 1995), and nuclear waste disposal (Ranjith et al., 2012; Zuo et al., 2017).
The effects of thermal treatment (ThT) on the mechanical properties of rock such as uniaxial compressive strength (Liu et al., 2019; Shao et al., 2020; Wong et al., 2020; Wu et al., 2019a; Yang et al., 2017; Zhao et al., 2020), Tensile strength (Jin et al., 2019; Wang and Konietzky, 2019; Wu et al., 2019b; Zhao et al., 2018), mode I fracture toughness (Mahanta et al., 2016; Meredith and Atkinson, 1985; Nasseri et al., 2007; Zuo et al., 2017), shear strength (Chen et al., 2020b), dynamic strength (Fan et al., 2017, Fan et al., 2020b; Huang and Xia, 2015), and dynamic fracture toughness (Yin et al., 2012) as well as the micro-properties (Fan et al., 2018, Fan et al., 2020a; Freire-Lista et al., 2016; Shen et al., 2020) have been well studied. However, the cracking processes of thermally-treated rocks under elevated stresses, which have profound influences on the deformation and rupture of the rock masses in the field, have received much less attention (Guo et al., 2021; Zuo et al., 2017).
Preceding the initiation and propagation of a macrocrack (~mm) in rocks, microcracks (~μm) are known to have developed in front of the macrocrack tips, forming a damage zone called fracture process zone (FPZ) (Friedman et al., 1972, Wei et al., 2021). A thorough investigation of the microcracking mechanisms contributes to better predicting and understanding of the subsequent macroscopic cracking behavior. The nucleation and growth of microcracks are accompanied by the emanation of elastic waves, known as acoustic emission (AE) (Lockner, 1993, Lockner et al., 1991). To investigate the microcracking mechanisms in the laboratory scale, the AE signals recorded in rock fracturing test are analyzed by performing parameter analysis, source localization, and moment tensor inversion (Graham et al., 2010; Guo and Wong, 2020; Guo and Wong, 2021; Kwiatek et al., 2014a; Meng et al., 2016, Meng et al., 2019, Meng et al., 2020; Ohtsu, 1991, Ohtsu, 1995).
Upon thermally treating the polycrystalline rocks such as granite, thermal stress could be induced as a result of the spatial and temporal changes of temperature in the specimens because of the distinct and incompatible thermal expansion (contraction) among neighbor minerals with different thermoelastic moduli (Kranz, 1983). Once the thermal stress exceeds the local strength, thermal microcracks are produced. The pre-existing thermal microcracks have been recognized to play an important role in altering the mechanical properties of crustal rocks such as granite (Yin et al., 2012; Zhao et al., 2018; Zuo et al., 2017). Although the development of microcracks or FPZ of granite without ThT under mode I loading has been extensively studied using the AE technique (Kao et al., 2011; Li and Einstein, 2017; Nasseri et al., 2006), the knowledge of how thermal microcracks affect the microcracking mechanisms remains elusive.
In a previous study (Guo et al., 2021), we investigated the effects of ThT on the microcracking behavior of granite under mode I loading with the aid of the AE technique. The microcracking behavior transition phenomenon is identified as the granite is thermally treated to a high-temperature level where a large number of thermal microcracks appear. For the granite containing substantial pre-existing thermal microcracks, we find that the rapid development of microcracks begins at a relatively lower load level, and this period lasts for a longer duration as compared with the granite containing fewer pre-existing thermal microcracks. Lastly, a larger fully-developed FPZ with maximum microcrack density will form preceding the unstable propagation of macrocracks.
Additional to the spatial-temporal evolution features of AE events, information such as the source mechanisms (or microcrack type) and magnitude of AE events could be extracted from the AE signals (Li and Einstein, 2017; Wong and Xiong, 2018). In rock laboratory fracture tests, AE events are often classified into tensile, shear and mixed-mode types by conducting moment tensor inversion analysis (Ohtsu, 1995). The temporal evolution of event-type ratios (Li and Einstein, 2017; Wong and Xiong, 2018) and the energy budget of different types of events (Kwiatek et al., 2014b) are intriguing AE-derived microcracking mechanisms. However, very few studies have investigated the effects of thermal microcracks on these mechanisms.
To fill in the gap, we revisit our AE data collected in three-point bending tests on pre-notched semi-circular bend (SCB) Hong Kong granite specimens (Guo et al., 2021). We focus on the microcracking behavior of granite under mode I loading, because it is the most prevalent loading mode in nature (Gudmundsson, 2011). Different amount of thermal microcracks are introduced in the specimens which have been slowly heated to different temperature levels (50 °C, 100 °C, 150 °C, 200 °C, and 400 °C) before the loading tests. We conducted moment tensile inversion and calculated the magnitude of the AE events, which were detected before the initiation of macrocracks. The results of this study improve our understanding of microcracking mechanisms of granite with varying amount of thermal microcracks subjected to mode I loading.
Section snippets
Material and specimen preparation
The properties of studied granite and experimental setup have been described in detail in Guo et al. (2021). We briefly summarize the key information below for completeness. The Hong Kong granite is sourced from the northwestern part of Hong Kong Island with mineral sizes ranging from 0.15–7.9 mm and an average value of 1.32 mm. The granite consists of approximately 32.9% quartz, 35.1% K-feldspar, 29.1% plagioclase and 2.9% biotite according to the thin-section analysis. Its uniaxial
Typical spatial-temporal evolution of AE events
We summarize the typical spatial-temporal evolution characteristics of AE events of the SCB tests (Fig. 2). In the beginning, when the load is low, no AE events are detected indicating few microcracks nucleate during this period (Fig. 2a). As the load increases to a higher level, a number of AE events are observed. The AE event rate first remains at a low level and then increases sharply when the load approaches its peak indicating the rapid development of microcracks or FPZ. When the FPZ is
Conclusions
To investigate the effects of thermally induced microcracks on AE-derived microcracking mechanisms of Hong Kong granite under mode I loading, the acoustic emissions (AEs) recorded preceding the initiation of macrocrack are analyzed from different perspectives in depth. The magnitudes of AE events are calculated and the events are classified into tensile, shear and mixed-mode types by conducting moment tensile inversion. Combined with the results of a companion study (Guo et al., 2021), we draw
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the support from the National Natural Science Foundation of China (Grant No. 41877217), the Startup fund, Seed Funding Programme for Basic Research for New Staff at the University of Hong Kong, the General Research Fund (17303917) of the Research Grants Council (Hong Kong), the Hung Hing Ying Physical Sciences Research Fund 2017-18. The second author acknowledges the Postgraduate Scholarship at the University of Hong Kong. The authors would also like to thank Benjamin
References (59)
- et al.
Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude
Int. J. Rock Mech. Min. Sci.
(1999) - et al.
Reliable onset time determination and source location of acoustic emissions in concrete structures
Cem. Concr. Compos.
(2012) - et al.
Experimental investigation of thermal effects on dynamic behavior of granite
Appl. Therm. Eng.
(2017) - et al.
An investigation of thermal effects on micro-properties of granite by X-ray CT technique
Appl. Therm. Eng.
(2018) - et al.
Thermal stress-induced microcracking in building granite
Eng. Geol.
(2016) - et al.
Fracture-surface energy of rocks
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts
(1972) - et al.
Comparison of polarity and moment tensor inversion methods for source analysis of acoustic emission data
Int. J. Rock Mech. Min. Sci.
(2010) - et al.
Microcracking behavior of three granites under mode I loading: Insights from acoustic emission
Eng. Geol.
(2020) - et al.
Microcracking behavior transition in thermally treated granite under mode I loading
Eng. Geol.
(2021) - et al.
Rock burst process of limestone and its acoustic emission characteristics under true-triaxial unloading conditions
Int. J. Rock Mech. Min. Sci.
(2010)