To evaluate the stability and compactness of high-temperature underground construction, it is necessary to test the fracture toughness of surrounding rock (red sandstone) under real-time high temperature. In this paper, SCB specimens recommended by the International Society for Rock Mechanics are used to measure the mode I fracture toughness of red sandstone at real-time high temperatures. Also, to reveal its fracture characteristics and fracture mechanism, the fracture morphology observation (SEM experiment), XRD experiment, mercury intrusion porosimetry testing, and fractal measurement of fracture trajectory are carried out on the red sandstone specimens at various temperatures. The results show that (1) temperature may have a significant impact on the fracture toughness and fracture characteristics of red sandstone. On the whole, the fracture toughness values decrease with the increase in temperature, while the fractal dimensions of fractal trajectories increase with the increase in temperature. (2) Temperature has a significant influence on the fracture mode of red sandstone. At relatively low temperatures (20°C–400°C), the main fracture mode is transgranular failure. At relatively high temperatures (400°C–700°C), the fracture mode is mainly intergranular failure. (3) The weakening mechanism of red sandstone is mainly due to the effect of thermal dehydration when the temperature is between 100°C and 400°C. When the temperature is between 400°C and 700°C, the weakening mechanism is mainly due to thermal cracking and the
In the deep geological disposal of high-level radioactive waste such as nuclear waste, the design requirements of surrounding rock and artificial barrier system are very strict. It is necessary to consider the original strength of surrounding rock as well as the influence of thermal stress on its microstructures. According to the study of Zuo et al. [
In geotechnical engineering science, fracture toughness is a property which describes the ability of a rock to resist fracture and is one of the most important features of any solid material for many underground tunnel design applications [
It should be noted that, for some complex underground and tunneling projects, surrounding rock is often greatly affected by extremely high temperature. For examples, when the radioactive nuclear waste is deeply buried, a large amount of heat will be generated due to the decay of radioactive elements (137Cs, 60Co, 90Sr, etc.) [
As a typical sedimentary rock, sandstones are widely distributed in nature. Due to the influence of the geochemical environment, there are substantial differences in mineral composition, particle shape, and cement type. After treatment with high temperature, the minerals in the sandstone undergo a polycrystalline transformation, and their mineral composition and physical and mechanical parameters will correspondingly change. Previously, several scholars have concentrated on the study of the damage mechanism, failure criterion, microstructure, and texture evolution of rocks after treatment with high temperature and have obtained some useful research results. Zuo et al. [
In summary, under the action of high temperature, the minerals in sandstone will undergo physical and chemical changes such as dehydration, melting, decomposition, and phase transformation, which will lead to great changes in the crystal structure and composition of minerals. However, the above scholars have not revealed the thermal damage mechanism of rocks from the mineralogical direction, and the correlation between micro- and macromechanical parameters has not been discussed. Although the relevant scholars have qualitatively analyzed the changes of chemical and mineral composition using XRD and FT-Raman spectroscopy, a great number of research works still need to be done on the quantitative analysis of compositional variation in minerals, the relationship between the evolution process of mineral components and physical and variational trend of mechanical parameters, microfracture morphology, porosity, and other micro-macroparameters. Given the complicated temperature-related engineering problems, the fracture toughness and fracture characteristics of typical red sandstone at real-time high temperature are carried out in this paper. Besides, microscopic analysis such as quantitative XRD analysis (mineral composition), SEM fractographic analysis (fracture topography), mercury intrusion porosimetry measurement, and fractal measurement (fracture propagation trajectory) reveal the thermal fracture mechanism of sandstone and the correlations of micro-macromechanical parameters.
In fracture mechanics, there are three types of fracture: mode I (tensile fracture), mode II (shear fracture), and mode III (out-of-plane tearing fracture) [
SCB specimen geometry.
The SCB specimens.
The data obtained from the experiment are calculated and processed according to the ISRM recommended method. Fracture toughness
The geometric dimensioning of SCB specimens used in this paper is as follows:
Red sandstones are taken from the Taiyuan area, Shanxi Province. The main components of sandstone are albite and quartz. The specimens are light red and cemented with shale. The particle size is 0.1–0.7 mm, and the average particle size is about 0.4 mm (see Table
Compositions and contents of the tested specimens (mean value).
Chemical component | Quartz (SiO2) | Albite (Ca-rich ((Na,Ca)Al (Si,Al)3O8)) | Albite (NaAlSi3O8) | Anorthite (CaAl2Si2O8) |
---|---|---|---|---|
Content | 85.4% | 8.7% | 4.8% | 1.1% |
The sample preparation process can be divided into three parts: Place the sandstone on a flat platform. When the fixing is completed, the cylindrical cores are drilled at a speed of 10 mm/s using a drill with a diameter of 50 mm. The cores obtained are cut into the semidisc samples with a thickness of 25 mm by a cutter. Then, the ends of the tested samples are smoothed using 30-grit fine sandpaper. Clamp the semicircular specimen with pliers. After the fixing is completed, the crack prefabrication is performed on the tested samples by using the crack prefabrication equipment. The thickness of the saw blade in the crack prefabrication equipment is 0.4 mm.
During the thermomechanical loading, the experimental load-displacement curves were recorded automatically. By analyzing the experimental load-displacement curves, the fracture in compression is a complex process consisting of 4 stages of fracture development (as shown in Figure
Typical load-displacement curve of sandstone.
Since the force-displacement curves of sandstone specimens at the identical temperature have similar variational trend, the typical force-displacement curves of each condition are listed in this paper, as shown in Figure
Typical load-displacement curve of sandstone at various temperatures.
At real-time high temperature, the relationships between the mean of fracture toughness of sandstone specimens and temperature are shown in Figure
The relationship between temperature and the mean of fracture toughness.
Fracture toughness values of sandstone at various temperatures.
Temperature (°C) | #1 | #2 | #3 | #4 | #5 | #6 | Mean | Standard deviation |
---|---|---|---|---|---|---|---|---|
20 | 4.05 | 3.42 | 4.08 | 3.62 | 3.91 | 3.29 | 3.73 | 0.34 |
100 | 4.19 | 4.46 | 4.69 | 4.17 | 4.08 | 4.06 | 4.27 | 0.25 |
200 | 4.05 | 4.09 | 3.46 | 3.97 | 3.69 | 3.80 | 3.84 | 0.24 |
300 | 3.40 | 3.30 | 3.90 | 3.28 | 3.87 | 3.72 | 3.58 | 0.28 |
400 | 2.78 | 3.08 | 2.49 | 3.21 | 2.20 | 2.57 | 2.72 | 0.38 |
500 | 1.57 | 1.29 | 1.35 | 1.41 | 1.15 | 1.19 | 1.33 | 0.16 |
600 | 0.81 | 1.25 | 0.89 | 1.02 | 1.18 | 0.80 | 0.99 | 0.19 |
700 | 0.15 | 0.12 | 0.17 | 0.26 | 0.14 | 0.25 | 0.18 | 0.06 |
It is well known that the shape and trajectory of rock fracture are determined by its microfracture mechanism, which can directly reflect the macrofracture characteristics of the rock. Since the initiation cracks generally occur at the surface, subsurface, stress concentration zones, and internal defects of the specimen, the fracture propagation path of the specimen is tortuous and continuously changing. According to the previous studies, it is widely recognized that fracture modes can be classified into 3 categories: transgranular fracture, intergranular fracture, and mixed fracture, which can be presented on the surface of the specimen. According to the research results of Zuo et al. [
Fracture trajectory of red sandstone at 700°C.
The average of fracture deviations from the center line originating at the notch tip of sandstone specimens at the temperature ranging from room temperature (20°C) to 700°C is shown in Figure
The average of fracture deviation of sandstone at different temperatures.
The average of fracture deviation from center line of sandstone at various temperatures.
Temperature (°C) | Fracture deviation (left side) (mm) | Fracture deviation (right side) (mm) |
---|---|---|
20 | 2.6 | 1.9 |
100 | 2.4 | 1.8 |
200 | 3.5 | 2.8 |
300 | 3.8 | 4.5 |
400 | 4.3 | 5.2 |
500 | 5.7 | 6.4 |
600 | 6.8 | 7.5 |
700 | 7.9 | 9.6 |
According to the previous studies, the fracture trajectories of the specimens obviously have strong statistical self-affinity. Hence, the fracture propagation path on the sandstone surface can be described by fractal dimension [
Note that we register exactly the same trajectory of a specimen for three times and we calculate the fractal dimensions immediately. Note that six SCB specimens were used for exploring the fracture characteristics under each set of temperature. Therefore, 18 tests (the calculation of fractal dimension of fracture trajectories) were performed in total under each set of temperature. The average fractal dimensions and total errors of fracture trajectories at each temperature are shown in Table
Average fractal dimension values of crack development paths under different temperatures.
Temperature (°C) | Average fractal dimension ( |
Total error |
---|---|---|
20 | 1.19 | 0.03 |
100 | 1.12 | 0.07 |
200 | 1.26 | 0.04 |
300 | 1.32 | 0.04 |
400 | 1.39 | 0.02 |
500 | 1.43 | 0.09 |
600 | 1.68 | 0.11 |
700 | 1.84 | 0.06 |
The relationship between average fractal dimension and temperature.
The main mineral components of sandstone are quartz and albite. After treatment with high temperature, the pore structure, surface morphology, crystal structure, and mineral composition of the specimens have undergone a series of complex changes. This will strongly influence the fracture behavior of sandstone. As shown in Table
The mineral composition and mass content of sandstone at various temperatures.
Temperature (°C) | Mineral composition | Chemical formula | Mass content (%) |
---|---|---|---|
25 | Quartz | SiO2 | 67 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 9 | |
Albite | NaAlSi3O8 | 21 | |
Anorthite | CaAl2Si2O8 | 3 | |
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100 | Quartz | SiO2 | 67 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 9 | |
Albite | NaAlSi3O8 | 21 | |
Anorthite | CaAl2Si2O8 | 3 | |
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200 | Quartz | SiO2 | 52 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 14 | |
Albite | NaAlSi3O8 | 29 | |
Anorthite | CaAl2Si2O8 | 5 | |
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300 | Quartz | SiO2 | 47 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 27 | |
Anorthite | CaAl2Si2O8 | 19 | |
Laumontite | Ca [AlSi2O6]2∗4H2O | 7 | |
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400 | Quartz | SiO2 | 57 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 23 | |
Albite | NaAlSi3O8 | 15 | |
Anorthite | CaAl2Si2O8 | 3 | |
Anorthite, Na-rich | (Ca,Na) (Si,Al)4O8 | 2 | |
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500 | Quartz | SiO2 | 53 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 11 | |
Albite | NaAlSi3O8 | 26 | |
Anorthite | CaAl2Si2O8 | 6 | |
Anorthite, Na-rich | (Ca,Na) (Si,Al)4O8 | 4 | |
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600 | Quartz | SiO2 | 67 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 9 | |
Albite | NaAlSi3O8 | 18 | |
Anorthite | CaAl2Si2O8 | 4 | |
Anorthite, Na-rich | (Ca,Na) (Si,Al)4O8 | 2 | |
|
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700 | Quartz | SiO2 | 75 |
Albite, Ca-rich | (Na,Ca)Al (Si,Al)3O8 | 6 | |
Albite | NaAlSi3O8 | 14 | |
Anorthite | CaAl2Si2O8 | 3 | |
Anorthite, Na-rich | (Ca,Na) (Si,Al)4O8 | 2 |
It is worthy to note that chemical reactions between mineral particles (crystalline water removal, the formation of new materials, decomposition reaction, and lattice transformation) will lead to a series of physical changes, such as the increase in pore structure, the generation of new pore and fracture, and volume expansion. According to the experimental results, we can conclude that the higher the temperature is, the more complex the chemical reactions occur in the crystals of sandstone. For example, when the temperature of sandstone rises from room temperature to 700°C, quartz decomposition and synthesis, the transformation between quartz crystals, dehydration of feldspar, and synthesis and decomposition of feldspar will occur. Because the heating rate in this paper is relatively low, it means that the chemical reactions experienced by mineral crystals at 700°C include not only all chemical reactions from room temperature to 600°C, but also those occurring at 700°C. But for 200°C, only a small part of quartz and feldspar is decomposed or synthesized. Therefore, the macroparameter (i.e., fracture toughness) gradually decreases with the increase in temperature.
However, to accurately determine the relationship between the macro- and microparameters of the sample, the XRD experiment is obviously not enough. Because the analysis of mineral composition or XRD experimental study only shows the variation of parameters concerning temperature from a qualitative point of view, which does not quantitatively reflect the correlation between macroparameters and microparameters. To quantitatively explain the variation of fracture toughness of sandstone concerning temperature, mercury injection experiments are also needed for sandstone.
The variation of average pore size and porosity of sandstone with respect to temperature is shown in Table
Pore structure parameters of sandstone at various temperatures.
Temperature (°C) | Average pore size (nm) | Porosity (%) |
---|---|---|
20 | 38 | 3.7 |
100 | 26 | 3.5 |
200 | 48 | 4.2 |
300 | 81 | 5.3 |
400 | 176 | 8.6 |
500 | 284 | 13.5 |
600 | 372 | 18.2 |
700 | 449 | 24.8 |
The relationship between average pore size and porosity of sandstone and temperature.
Based on the load-displacement curves, it can be obtained that when the temperature is between 20°C and 400°C, the fracture mode of the sandstone is mainly characterized by brittle fracture. When the temperature is between 400°C and 700°C, the fracture mode of the sandstone begins to show some ductility. The macroscopic fracture mode and fracture trajectory of the rock are fundamentally determined by the microscopic fracture mechanism. Therefore, the SEM observation of the fracture surface is necessary.
On the whole, the fracture mode of the specimen at a relatively low temperature (20°C–400°C) is mainly characterized by transgranular fracture. With the increase in temperature, the mineral particles gradually become loose, and the opening degree of the interfaces between the crystals gradually becomes more considerable. Moreover, due to the influence of thermal stress and expansion force, a series of complicated cracks are randomly distributed in the samples. As a result, the fracture mode of specimens become more complex and diversified. The fracture mode of the specimen at this temperature stage (400°C–700°C) is mainly characterized by intergranular fracture. This is also consistent with previous scholars’ research results. It is worth noting that cracks tend to propagate and evolve along with the interface of crystals under these conditions. Besides, the fractures will also coalesce with the thermal cracks under loading. Hence, the roughness of the fracture surface at high temperature is more significant than that at low temperature. Therefore, the fracture trajectory of the sample becomes more tortuous, and the fractal dimension gradually increases.
Interestingly, however, there are some differences in the fracture morphology of the specimens at real-time high temperatures, which have not been found in the specimens after treatment with high temperatures (the results of Feng et al. [
Fracture morphology of sandstone at different temperatures (SEM images): (a) 20°C; (b) 100°C; (c) 200°C; (d) 300°C; (e) 400°C; (f) 500°C; (g) 600°C.
According to the variation of fracture toughness, fractal dimension, and surface morphology of sandstone with temperature, the weakening mechanism of sandstone at a real-time high temperature can be divided into 3 stages.
Stage I (25°C–100°C): at this stage, the fracture toughness of sandstone increases slightly with the increase in temperature. This is because the evaporation of free water and the closure of original pores and cracks occur at this temperature stage. According to the principle of effective stress, the evaporation of water leads to a decrease in pore pressure and an increase in effective stress, which causes the rock mass to be squeezed, and finally, the rock strength is increased. In addition, heating causes thermal stress inside the rock. When the temperature is relatively low, the thermal stress can only cause recoverable elastic deformation, which will not cause thermal cracking in the samples. Moreover, thermal expansion at relatively low temperatures will result in the closure of existing microcracks and inhibit the coalescence and interaction of existing cracks, which will lead to the enhancement of the overall structural strength of sandstone. Undoubtedly, the increase in rock strength will lead to an increase in resistance to intergranular and transgranular stress corrosion cracking and ultimately lead to an increase in fracture toughness. Since the transgranular fracture occurs mainly at this temperature stage, the deviation of transgranular fracture from the center line tends to be smaller compared with intergranular fracture, thus producing a relatively straight fracture trajectory. In summary, due to the volatilization of water and the closure of the primary pores, the fracture toughness of the specimens increased. However, since the thermal stress is small, the growth rate of the fracture toughness is relatively tiny.
Stage II (100°C–400°C): at this stage, the fracture toughness slowly decreases with the increase in temperature. On the contrary, as the temperature increases, the evaporation of water in the rock becomes more and more severe. Not only the bound water is fully evaporated but also the strong combined water and structural water are gradually lost. Dehydration has a direct effect on the structure of the rock. For example, a type I tensile crack occurs during the dehydration of serpentine (Jung et al. [
Stage III (400°C–700°C): there are three reasons for the weakening mechanism of specimens at this stage. The first reason is the decomposition of some minerals, the recrystallization of some minerals, and the formation of new minerals. In this way, the decomposition of minerals will produce a range of cracks with a broad distribution of dimensions (length and width), orientations, and shapes, which will result in a higher porosity and fractal dimension. Besides, according to the study of Yu et al. [
In this paper, the macroscopic and microscopic fracture behaviors of red sandstone under real-time high temperature are studied experimentally. The main conclusions are as follows: With the increase in temperature, the value of fracture toughness of sandstone first increases and then decreases; the value of fracture toughness reaches the maximum at 100°C. According to the relationships between fracture toughness and temperature (i.e., curve slope), the variational trend of the curve can be divided into 3 stages: (1) the slow increase stage (20°C–100°C). The curve slope at this stage is 0.0055. (2) The slow descent stage (100°C–400°C). The curve slope at this stage is 0.0048. (3) The rapid descending stage (400°C–700°C), in which the slope of the curve is 0.0087. The fractal dimension and the fracture deviation from the hypothetical fracture originating at the notch tip of sandstone specimens gradually increases with the increase in temperature. And, the values are smaller at relatively low temperatures (20°C–400°C) but larger at relatively high temperatures (400°C–700°C). The higher the temperature is, the more complex the chemical reactions occur in the crystals of sandstone. When the temperature of sandstone rises from room temperature to 700°C, quartz decomposition and synthesis, the transition between quartz crystals, dehydration of feldspar, and synthesis and decomposition of feldspar will occur. And, the average pore size, total volume, and porosity of sandstone first decrease and then increase with the increase in temperature. When the temperature is between 100°C and 700°C, these parameters of sandstone gradually increase with the increase in temperature, which indicates that the micropores gradually change to mesopores and macropores. On the whole, the variational trend of mesoparameters is basically consistent with that of fracture toughness. There are some differences in the fracture morphology of the specimens at real-time high temperatures, which have not been found in the specimens after treatment with high temperatures. Some cracks appear on the fracture surface at 400°C and the density, number, and aperture of the fractures increase with the increasing temperature. Some of the crystals in sandstone are molten under real-time high-temperature conditions, and the mechanical properties are similar to those of soft rock, so the sandstone tends to deform plastically. Plastic deformation can cause tearing cracks in the fracture surfaces of the specimen.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This paper was supported by the Natural Science Funds for Young Scholar (51504220) and National Natural Science Foundation of China (Grant no. 51574173), which the authors thank a lot.