Influence of thermal treatment on mode I fracture toughness of certain Indian rocks
Introduction
The experimental study of the measurement of fracture toughness is useful in rock and material science for recognition of stress responses in rocks, metals, ceramics and concrete etc. From the point of view of rock engineering, the effect of thermal treatment in rocks plays a crucial role in different fields of engineering applications such as rock drilling, ore crushing, deep petroleum boring, geothermal energy extraction, deep burial of spent nuclear fuel, and waste disposal (Balme et al., 2004, Homand-Etienne and Houpert, 1989, Kranz, 1983, Nasseri and Mohanty, 2008, Ozguven and Ozcelik, 2014, Yin et al., 2015, Yin et al., 2012). The safe and successful effectuation of modern geotechnical engineering projects such as nuclear waste disposal (Sundberg et al., 2009), underground coal gasification (Roddy and Younger, 2010), CO2 sequestration (Rutqvist et al., 2002), geothermal heat energy (Zhao, 2000), construction in fire exposed rocks (Zhan and Cai, 2007), hydraulic fracturing for oil and natural gas recovery (Funatsu et al., 2014, Papanastasiou, 1999, Whittaker et al., 1992) requires knowledge of the thermo-mechanical behaviour of rocks. In recent years, many researchers have attempted to investigate the influence of the thermal environment or heating on the physico-mechanical properties of rocks, such as the modulus of deformation, Poisson's ratio, tensile strength, compressive strength, cohesion, angle of internal friction, thermal expansion coefficient and fracture toughness (Al-Shayea et al., 2000, Bazant and Prat, 1988, Brede, 1993, Dwivedi et al., 2008, Gautam et al., 2015, Heuze, 1983, Jaeger and Cook, 1976, Lakshmikantha, 2009, Liu and Xu, 2015, Meredith and Atkinson, 1985, Meredith, 1983, Ranjith et al., 2012, Rao et al., 2007, Tian et al., 2012, Vishal et al., 2011, Whittaker et al., 1992, Zhang et al., 2009). The geo-mechanical properties of a rock gradually decrease with increasing temperature of thermal treatment which induces micro-cracks due to differential thermal expansion. Other mineralogical factors such as grain expansion, dehydration, decomposition, phase transition and re-crystallisation add more complexities to the experimental observations (Yin et al., 2012).
There is a decrease in the fracture toughness of a rock when it is treated at elevated temperatures until it reaches an elasto-plastic transition phase. It is obvious that damage within rocks is introduced in the form of micro-cracks due to thermal treatment. However, the degree of influence on the mechanical properties are rock type- and temperature-dependent (Balme et al., 2004, Yin et al., 2012). Nasseri and Mohanty (2008) experimentally observed that in the temperature range of 250 °C to 850 °C, with gradual thermal treatment, the number and the average opening distance of micro-cracks gradually increase resulting in a decrease in the p-wave velocity as well as in the fracture toughness of Westerly granite. According to Yavuz et al. (2010), the p-wave velocity in carbonate rocks increases from room temperature to 100 °C, possibly due to the dilation of calcite grains, and then gradually decreases with an increase in temperature because of thermal damage above 150 °C. Funatsu et al. (2004) evaluated the fracture toughness of Kimachi sandstone and Tage tuff and observed that fracture toughness gradually decreases up to a temperature range of 75 °C, after which it gradually increases up to 125 °C. In the current study, an attempt has been made to demonstrate the influence of thermal treatment on the fracture toughness of certain Indian rock types.
Section snippets
Locations and rock description
Three different types of rocks were collected from different places for the thermal treatment experiments. These were Manoharpur Sandstone from the Manoharpur area in Sundargarh district in Odisha, Bellary Dolerite from the Bellary area in Karnataka, and Dholpur Sandstone from Dholpur in Rajasthan. The geological map of three different areas is shown in Fig. 1. Manoharpur sandstone belongs to the lower Kamthi formation of the Lower Gondwana super-group, Bellary Dolerite belongs to the Kolar
Preparation of CSTSCB specimen
Rock cores with diameters of 72 mm, 54 mm and 50 mm were obtained from blocks of rock 1, rock 2 and rock 3, respectively. The cores were sliced into discs of the required thickness that is shown in Table 1, according to the standards suggested by ISRM. The discs were cut into halves to form semi-circular bending (SCB) specimens. A straight notch that was perpendicular to the diametral core at the centre of each SCB specimen was then introduced using a diamond blade of a thickness of 1 mm with the
Determination of mode I fracture toughness using CSTSCB specimens
Fracture toughness is one of the most important parameters in the field of fracture mechanics and represents the energy or, in other words, the material resistance that is required to initiate brittle failure around the crack tip (Ayatollahi and Alborzi, 2013, Ayatollahi, 2015, Kanninen and Popelar, 1985, Liu, 1983). The fracture toughness of a given rock can be evaluated under any definite type and magnitude of loading because it can be explained in terms of the Stress Intensity Factor (
Experimental work
The experimental work included the determining of various geo-mechanical properties of the rocks mentioned above. The main objective of this study was to measure the variation of the fracture toughness of the rocks as a function of elevated temperature. In order to support the findings of the study of fracture toughness, petrographic analysis, XRD analysis, and SEM studies were also performed to provide a qualitative understanding of the associated mechanism.
Results and discussion
It can be inferred from the petrographic analysis of these rock types that when the temperature of treatment increases, different types of micro-fractures (intragranular, intergranular and transgranular) are being induced in these rocks on the basis of their mineralogical composition. In the case of rock 3, the number of induced fractures is less, which may be due to its mono-mineralic nature.
From the petrographic and XRD analyses of these rocks, it can be inferred that quartz, mica, feldspar
Conclusion
In this study, an attempt has been made to establish a relation between mode I static fracture toughness and elevated temperatures up to a range of 600 °C, using CSTSCB specimens of three types of rocks. The following important conclusions can be drawn:
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The fracture toughness of these rocks increases up to a temperature range of 100 °C, possibly due to the closure of pre-existing cracks and the desorption of the water that is present in these rocks.
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A gradual fall in the fracture toughness value up
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