Abstract
The cyclic freezing-thawing action in cold regions leads to the deterioration of rock damage, resulting in local damage and further threatening the safety of engineering. In order to study the degradation characteristics of green sandstone and yellow sandstone under freeze-thaw cycles from macroscopic and microscopic aspects, the sandstone of a mining area in Inner Mongolia was used as experimental material. The freeze-thaw cycles were divided into 20 times, 30 times, and 40 times. NMR images and mechanical test results of two different rock samples were analyzed by binarization, NMR, and mechanical test. The test results show that, except that the mass change is less than that of yellow sandstone, the physical index degradation degree of green sandstone is higher than that of yellow sandstone, and the frost resistance is less than that of yellow sandstone. The change of acoustic emission event rate of green sandstone is mainly in the elastic deformation stage and stable crack propagation stage, and the change of acoustic emission event rate of yellow sandstone is concentrated in the crack closure stage. In the loading process, the energy release trends of the two sandstones are similar; the 30 freeze-thaw cycles are the boundary of brittle-plastic transformation of green sandstone, and the increase of cumulative energy is the most obvious. The research results provide a theoretical basis for studying the rock failure mechanism and improving the stability of rock engineering in cold regions.
1. Introduction
In recent years, with the increase of construction efforts in Inner Mongolia, engineering activities have increased year by year, and there are many geological problems in cold regions. Among them, the most representative and key problem is rock freeze-thaw degradation, resulting in a series of disasters in cold regions. Different degrees of freeze-thaw disasters such as frost heaving cracking, freeze-thaw sliding, and slope instability bring great challenges to construction safety. Therefore, it is of great significance to study the damage and deterioration mechanism of freeze-thaw rock mass in Inner Mongolia to prevent further deterioration of engineering rock mass and understand the development law of freeze-thaw rock mass. The freeze-thaw cycle test of rock is one of the important methods to study the damage mechanism of freeze-thaw [1, 2]. Many scholars have made important contributions to the damage and deterioration of rock under the effect of freezing-thawing. In order to study the influence of freezing-thawing cycles on the dynamic tensile properties of sandstone, scholars used φ50 mm split Hopkinson pressure bar and three average loading rates to conduct dynamic splitting tensile tests on sandstone with different freezing-thawing cycles, and analyzed the failure characteristics and dynamic splitting tensile strength of sandstone [3, 4]. Jiang Haibo [5] in order to incorporate the freeze-thaw cycle test into the diversion tunnel project, under the conditions of the lowest freezing temperature of −40°C and the melting temperature of 20°C, two groups of different state andesite rock samples were tested, and the internal microstructure of rock samples under different freeze-thaw cycles was detected by magnetic resonance imaging. Zhou et al [6] combined macroscopic and microscopic methods with statistical methods to quantitatively analyze the damage degree of rock under freeze-thaw cycles and loads and established a fractal damage constitutive model considering the residual strength of rock. Liping et al. [7] studied the physical parameters and triaxial compression mechanical properties of intact hard rock under different freeze-thaw cycles, with fine sandstone (UCS = 114.8 MPa) and coarse sandstone (UCS = 104.1 MPa) as representatives. In the work by Fang et al. [8], under freeze-thaw and loading conditions, the analytical expressions of model parameters and characteristic parameters in stress-strain curves under specific freeze-thaw cycles were established, and the attenuation model conforming to Newton ‘s law of material cooling was introduced. The relationship between elastic modulus, peak stress, and freeze-thaw cycles was developed. Si et al. [9] conducted a series of uniaxial compression tests on sandstone after freeze-thaw treatment. The purpose is to more quantitatively express the initial damage of rock after freeze-thaw treatment, study the influence of freeze-thaw damage on the crack propagation process of rock, and provide certain reference for the stability evaluation of rock engineering in cold regions. Qiao et al. [10] carried out uniaxial compression and acoustic emission synchronous test to study the deformation and failure characteristics of rock bridge of central locked specimen drilled in plateau in China. The effects of freeze-thaw cycles and rock bridge angles on fracturing and acoustic emission patterns were characterized. Based on orthogonal test method, Zhou et al. [11] designed a series of large-scale triaxial tests of soil-rock mixture under freeze-thaw environment. The influence of orthogonal test factors and confining pressure on the static characteristics of soil-rock mixture was analyzed by range and variance analysis. Lei et al. [12] studied the effect of freeze-thaw through a large number of shear tests. The effect of cyclic induced joint shear degradation on joint cohesion and friction Angle was considered. The variation law of shear strength parameters under freeze-thaw cycle was analyzed, and the evolution model of joint shear under freeze-thaw cycle was established. The cyclic property and joint durability of the model were further analyzed, and the feasibility of the model was also analyzed.
Based on the contribution of predecessors, this paper selects green sandstone and yellow sandstone in a mining area in Inner Mongolia as raw materials for this test specimen, and the geographical location of the mining area is shown in Figure 1. Through the core technology, the unified sample treatment and grinding are equivalent to the cylinder to form the A-F group. The samples are processed from the macro- and microaspects by the binary method, nuclear magnetic resonance (NMR), mechanical test, acoustic emission, and other methods, and the degradation mechanism of green sandstone and yellow sandstone under the action of freeze-thaw cycle is obtained, which has a certain reference value for preventing the further deterioration of rock mass in Inner Mongolia mining area and reducing the risk of engineering in cold regions.
2. Specimen Preparation and Experimental Scheme
The specimen of this test selects green sandstone and yellow sandstone from a mining area in Inner Mongolia as raw materials. Complete raw rock is selected on-site. Under the condition of avoiding its obvious cracks, a vertical core-taking machine is used to take out 50 mm diameter cores, and the raw materials are cut and processed by TY-450 stone sawing machine.The raw materials were uniformly processed into 50 mm × 100 mm cylindrical samples, and the end face was finally ground with a double-end grinding machine to ensure that the end face unevenness error was less than 0.05 mm. The green sandstone and yellow sandstone specimens were divided into three groups and numbered.
According to the actual working conditions of the mine in the cold area, the specimens of the two lithologies were put into the vacuum pressurized saturation device filled with water, frozen at −40°C for 6 hours, and then dissolved at 40°C for 6 hours. The period from the beginning of freezing to the end of dissolution was recorded as a cycle, with each cycle duration of 12 hours, as shown in Figure 2. The specimens were divided into 6 groups, with 3 specimens in each group. The freeze-thaw cycles mentioned above were carried out for 20, 30 and 40 times respectively, and the group numbers were marked as group A-F. Nuclear magnetic resonance system was used to measure each group of specimens after a freeze-thaw cycle, as shown in Figure 3.
3. Analysis of Test Results
3.1. Appearance Deterioration Characteristics of Specimens
After the six groups of specimens completed 20, 30, and 40 freezing-thawing cycles, their appearance deterioration degree was observed and analyzed, respectively. The freezing-thawing cycle results of yellow sandstone are shown in Figure 4, and the freezing-thawing cycle results of green sandstone are shown in Figure 5. Image binarization is a commonly used method in digital image processing. Through an appropriate threshold, the image contains only the target and background so that the changes in the appearance of time can be observed more intuitively. The local amplification results of the corresponding binarization samples are shown in Figure 6.
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Figure 4 shows that there is no obvious deterioration in the appearance of yellow sandstone during 40 freeze-thaw cycles. According to Figure 5, when freezing-thawing cycles were 20 times, the appearance of group A specimens did not change, which was basically the same as that after processing, indicating that freezing-thawing damage did not affect the outer surface of sandstone specimens. After 30 freezing-thawing cycles, the specimens in group B showed mild deterioration, and the surface of the specimens began to show microcracks and a small extent of skin peeling. Combined with Figure 6(a), it can be seen that the B-1 specimen produces horizontal cracks and body expansion, indicating that the pore water inside the specimen is frozen to generate ice crystals, and the pore space becomes larger when the ice crystal freezes and expands. However, the surface of the specimen is strongly bound, and the deterioration caused by the internal expansion changes does not seriously affect the surface, so only mild deterioration occurs. At the end of the freeze-thaw cycle, the specimens in group C showed a variety of deterioration forms and obvious grid-like staggered cracks. Combined with Figures 6(b)–6(d), it can be seen that both specimen C-1 and specimen C-3 have horizontal cracks, vertical cracks and reticular cross cracks, and there are a large number of microcracks in the specimen itself. The deterioration of freezing and thawing is intensified, and the particles with weak cementation on the microcracks are constantly falling off. The microcracks are continuously developed and expanded, and then developed to the surface of the specimen to form macro cracks. Small block shedding and oolitic structure appear in specimen C-2, which is considered to be due to the increase of freeze-thaw cycles, the continuous shedding of rock particles with weak cementation, and the formation of annular cracks. The deepening of circular cracks leads to the poor bonding ability between small block rock and the whole specimen, which leads to the shedding phenomenon. The shedding of rock particles near the crack has a sequence. The particles with poor cementation are preferentially shedding, and the particles with good cementation are subsequently shedding, so the oolitic phenomenon occurs near the crack of the specimen.
3.2. The Change of Specimen Quality and Porosity
The macroscopic physical properties of each specimen reflect the changes of the mesoscopic structure inside the specimen to a certain extent [1, 13, 14]. The quality of the specimen is one of the most important physical parameters. Under the action of freeze-thaw cycles, the change of specimen quality can be used as an important manifestation of specimen deterioration.
The average quality of the specimen is taken in each freeze-thaw cycle. In order to observe the overall law of the quality change of the specimen under different freeze-thaw cycles more intuitively, the test data of each specimen are statistically summarized, and the experimental data are fitted to obtain the relationship curve between the quality change rate and the number of freeze-thaw cycles. It can be seen from Figure 7 that the average mass of the two sandstones increased by about 1.6%. After 40 freeze-thaw cycles, the more the freeze-thaw cycles were, the more the water absorption of the specimen showed a continuous growth trend, and the mass change rate also increased. The tangent slope of the fitting curve of the mass change rate of the specimen increases with the increase of the number of freeze-thaw cycles, indicating that the water absorption of the specimen is less in the early stage and more in the later stage. The tangent slope of the fitting curve of the mass change rate of yellow sandstone decreases with the increase of the number of freeze-thaw cycles, indicating that the water absorption in the early stage of yellow sandstone is much larger than that in the later stage, and the average mass change rate of yellow sandstone is slightly larger than that of cyan sandstone.
Due to the different properties, shapes and particle sizes of rock particles, pores are formed in the space not filled by cement between particles. When the specimen is frozen, liquid water freezes into solid water, and the pore volume becomes larger due to frost heaving. When melting, solid water melts into liquid water. Because the original pore volume increases, the specimen will absorb more liquid water. In this way, the porosity increases continuously. However, due to the slight differences in the composition and properties of different specimens, the influence of freeze-thaw cycles on the porosity changes is also different.
With the above method, the pore change rates of the two sandstone specimens are fitted, as shown in Figure 8. Combined with Figure 8, it can be seen that with the increase in the number of freeze-thaw cycles, the porosity of sandstone shows an overall upward trend, with an increasing trend. After the freeze-thaw cycle, the maximum variation of porosity of green sandstone is 2.06%, while that of yellow sandstone is only 0.55%. The porosity growth of the two sandstone samples shows a fluctuating upward trend, and is not completely positively correlated with the number of freeze-thaw cycles. With the increase of freeze-thaw cycles, the average porosity change rate will tend to be stable. From the change trend, the porosity will be stable at a relatively fixed value, which shows that the increase of rock porosity caused by freeze-thaw cycles is limited. The average porosity change rate of yellow sandstone is still slowly increasing, showing a linear growth trend, indicating that the freeze-thaw cycle can continue to increase the porosity of yellow sandstone.
3.3. NMR T2 Spectrum Analysis
Nuclear magnetic resonance (NMR) technology reflects the pore structure by measuring the NMR relaxation signal of hydrogen atoms in pores. When the specimen is tested by nuclear magnetic resonance technology, the H proton in the rock pore fluid is subjected to an external magnetic field to make the atomic nucleus reach equilibrium and absorb electromagnetic energy. At this time, the H proton is excited by the radio frequency pulse to release the absorbed energy. This process produces a large number of energy signals, which is called relaxation. For fluids in pores, there are three independent relaxation types, namely volume relaxation, surface relaxation and diffusion relaxation. According to the above principle, the total transverse relaxation rate of NMR is expressed as follows:
Among them, freedom is free relaxation time of fluid (ms), pore surface area (cm2), is the relaxation intensity of the transverse plane (μm/ms), D is the diffusion coefficient, γ is the spin-magnetic ratio (rad/(s⋅T)), Gis the magnetic field gradient (Gs/cm), and is echo time (ms). If the fluid in the pore is unique, the surface relaxation is much larger than the other two relaxations. The above formula can be simplified as follows:
It can be seen from the above equation that the pore size is proportional to the distribution of T2. At present, most domestic scholars classify pores by T2 = 10 ms, pores corresponding to T2 < 10 ms are micropores, large pores corresponding to T2 > 10 ms, and the greater the signal strength, the more pores corresponding to T2. The T2 spectra of Group C and F sandstone specimens are shown in Figure 9.
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It can be seen from Figure 9 that the T2 distribution range of green sandstone rock samples is roughly between 10−1 ms and 104 ms, which is a state of coexistence of large and small pores. The T2 distribution range of yellow sandstone rock samples is concentrated between 10 and 103 ms, indicating that the internal pores are mostly large pores. With the increase of freeze-thaw cycles, the T2 distribution curves of green sandstone and yellow sandstone shifted to the right, but the right shift of green sandstone was more obvious than that of yellow sandstone, which indicated that the pores of green sandstone increased significantly under the influence of freeze-thaw cycles. The T2 distribution curves of green sandstone samples are obvious bimodal structure, the first peak position is less than 10 ms, which is small pore, the second peak position is about 102 ms, which is a large pore. The T2 distribution curve of yellow sandstone samples is a single peak structure, and the peak position is between 102 and 103 ms, which is large pores. According to the pore size of the spectrum peak position, the order is: yellow sandstone spectrum peak, green sandstone second spectrum peak, green sandstone first spectrum peak; the peak signal intensity of green sandstone sample is only close to 2000 n/a, while that of yellow sandstone sample is close to 5000 n/a. This shows that the proportion of pores of various sizes in the green sandstone sample is much lower than that in the yellow sandstone sample. With the increase of the number of freeze-thaw cycles, the signal strength of the first peak of the green sandstone decreased significantly, and the signal strength of the second peak increased significantly, which showed that the number of small pores decreased and the number of large pores increased with the increase of freeze-thaw cycles. The peak signal intensity of yellow sandstone increased, indicating that the pore size of yellow sandstone changed little, but the number increased significantly.
4. Deterioration Analysis of Sandstone Mechanical Properties
4.1. Experimental Design
After the surface moisture was wiped, two rubber bands were covered on the surface of the test rock sample to prevent sputtering after the rock sample was broken [15, 16]. When the rock sample is raised near the loading platform, the parameter setting of the testing machine is preloading 2 kN, and the loading rate is 0.1 mm/min. The uniaxial compression acoustic emission test is carried out by using SAS-2000 microcomputer controlled electro-hydraulic servo rock testing machine, as shown in Figure 10.
4.2. Analysis of Test Results
It can be seen from Figure 11 that with the increase of freeze-thaw cycles, the intersection of shear failure of the X-shaped conjugate inclined plane moves obviously to both ends after the failure of blue sandstone rock samples. During 20 freezing-thawing cycles, the failure mode of the sandstone sample was typical X-shaped conjugate inclined plane shear failure, but the intersection point moved up to 3/4 of the rock sample, and the upward movement was obvious. The failure mode of A-1 and A-3 specimens is still shear failure, but there is only one obvious main crack running through the rock specimen and obviously inclined, which belongs to typical single inclined shear failure, and a small number of short tensile cracks appear. It can be seen from Figure 11(b) that after 30 freeze-thaw cycles, the failure modes of specimens B-2 and B-3 are typical single-slope shear failure, but the number of tensile cracks increases, and their length also increases. The slope of the main crack of B-2 specimen increased slightly; the slope of the main crack of the B-1 rock sample is larger, and the tensile cracks appear more. The failure characteristics of the brittle rock are no longer obvious when the rock sample is destroyed. According to Figure 11(c), there was no obvious instability phenomenon in the whole loading process of the three rock samples after 40 freeze-thaw cycles, and there was basically no green sandstone particle powder after failure. At this time, the rock samples had shown plastic failure properties. The failure type is still single inclined plane shear failure, but the slope of the main fracture crack is very large, tends to be vertical, and accompanied by a large number of tensile cracks.
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In summary, with the increase of freeze-thaw cycles, the change trend of the failure mode of green sandstone is as follows: X-shaped conjugate inclined plane shear failure ⟶ X-shaped conjugate inclined plane shear failure ⟶ monoclinic plane shear failure ⟶ monoclinic plane shear failure, which indicates that freeze-thaw will make the internal structure of rock change to a certain extent and then change the failure mode of rock.
The failure modes of yellow sandstone specimens under different freeze-thaw cycles are shown in Figure 12. It can be seen from Figure 12 that the failure mode of the yellow sandstone specimen is relatively simple, which is similar to that of green sandstone in 20, 30, and 40 freeze-thaw cycles. They are all single inclined plane shear failures and are accompanied by a small number of tension cracks. However, with the increase in freeze-thaw cycles, the slope of the inclined plane increases to a certain extent.
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The axial stress-strain curve is an intuitive expression of the deformation law of rock specimen under external load. Figure 13 shows the stress-strain curves of green sandstone and yellow sandstone under different freeze-thaw cycles. According to Figures 13(a) and 13(d), the stress-strain curves of the two kinds of rock samples are similar and concentrated, and the discreteness is small, which shows that the mechanical properties of the rock samples are similar under the same number of freeze-thaw cycles. The peak strength of rock samples between different freeze-thaw cycles decreased to a certain extent, and the strain decreased slightly. It can be seen from Figure 13(b) that after 30 freeze-thaw cycles, the stress-strain curve of green sandstone is inconsistent, and the curve trend of B-2 and B-3 specimens is similar, but the curve trend of B-2 is compared with B-3. The axial stress only decreases slightly, but the strain increases significantly; the curve trend of B-1 is similar to that of 40 freeze-thaw cycles. Compared with B-2 and B-3, the peak stress is significantly reduced and the axial strain is significantly increased. Figure 13(c) shows that the rock samples of 40 freeze-thaw cycles all show typical plastic failure characteristics, and the discreteness of the stress-strain curves of the three rock samples decreases slightly. Combined with the stress-strain curve of green sandstone, the rock sample begins to change from typical brittle failure characteristics to plastic failure characteristics during 30 freeze-thaw cycles, and the mechanical properties of the rock sample begin to change from brittle to plastic. Therefore, 30 freeze-thaw cycles can be used as the mechanical property boundary of green sandstone freeze-thaw degradation. According to Figures 13(d)–13(f), with the increase of freeze-thaw cycles, the peak stress of yellow sandstone rock samples gradually decreases, but the strain slightly increases, and the growth rate is between 0.1% and 0.2%. The specimen shows brittle failure characteristics as a whole, but with the increase in freeze-thaw cycles, it tends to transform into plastic failure. The curve discreteness of rock sample increases with the increase of freeze-thaw cycles.
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It can be seen from Figure 14 that the overall trend of AE event rate of green sandstone rock samples after 20 freeze-thaw cycles is similar to the “U” type, that is, high event rate in the early stage, low event rate in the middle stage, and rapidly rising to high event rate in the late stage. After 30 freeze-thaw cycles, the whole loading process was a high event rate, and there was no obvious change trend. At this time, the stress-time curve was still the change trend of brittle rock, but the “U” type characteristics of AE event rate of rock samples after the first 20 freeze-thaw cycles had completely disappeared. After 40 freeze-thaw cycles, the AE event rate was opposite to the first 20 times. That is, the early low event rate, the middle high event rate, and the late event rate decreased gradually, showing a “convex “feature. With the increase in the number of freeze-thaw cycles, the AE events of stage II and III of Qingsha sandstone gradually increased, and the AE event rate increased, reaching the highest 120/s at 30 cycles (Figures 14(a) and 14(b)). The analysis is due to the continuous deterioration of freeze-thaw, the cementation between rock particles is weakening, the pores are more likely to rupture, and the microcracks are more likely to penetrate, resulting in more active acoustic emission activities. The internal deterioration of rock samples after 30 cycles was the most obvious; after 40 freeze-thaw cycles, the rock samples showed plastic failure characteristics. The AE event rate increases with the increase of stress, and the highest event rate is 118/s near stage IV.
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It can be seen from Figure 15 that after 20 freeze-thaw cycles, the AE event rate of yellow sandstone shows the “√” type change trend of mid-early event rate, mid-low event rate, and late high event rate; after 30 freeze-thaw cycles, the loading process was high event rate, no obvious change trend, the overall gradually presented “U” type; after 40 freeze-thaw cycles, the overall trend of AE event rate is approximately “U” type. The AE event rates in the II and III stages of yellow sandstone increased slightly with the increase of freeze-thaw cycles; the AE event rate of stage IV and the AE event rate before and after peak stress are very stable, about 130/s. After 20 and 30 freeze-thaw cycles, the peak event rate appears in stage IV, and in 40 freeze-thaw cycles, the peak event rate appears in stage I. After 30 freeze-thaw cycles, the event rate of stage I was 2 times higher than that of 20 freeze-thaw cycles, which was close to the event rate near the peak stress. The event rate was high throughout the loading process, and there was no obvious change trend. After 40 freeze-thaw cycles, the event rate of stage I continued to rise and was higher than that near the peak stress, which was approximately “U” type. After 20, 30, and 40 freeze-thaw cycles, the maximum values are about 70, 120, and 140/s, respectively. This is because under the effect of freeze-thaw cycles, the porosity of yellow sandstone rock samples increases, the number of macropores increases, and the phenomenon of pore or crack pressure sealing increases, resulting in more active AE activities. This also shows that the degradation of rock samples by freeze-thaw cycles is mainly concentrated on the role of pores. The local stress that the cementation strength between particles of rock samples can withstand is smaller and smaller, and the pores inside the rock samples are larger and larger, and the number of pores is also increasing.
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When the peak stress reaches, the two kinds of rock samples have an obvious AE low event rate missing phenomenon, which is a sign of the fracture precursor of rock samples, indicating that the acoustic emission activity is extremely active when the rock sample is damaged, and more macroscopic cracks appear at this time. However, with the increase of freezing and thawing times, the AE low event rate missing phenomenon of the two rock samples is not consistent. With the increase of cycles in the 20th and 30th freeze-thaw cycles, the area of low event rate missing is becoming smaller and smaller, and the high and low event rates coexist, and the failure mode is more complex. As shown in Figure 14(c), after 40 freeze-thaw cycles, the high event rate appears at the peak stress, and the obvious low event rate loss occurs at the same time. With the slow decline of stress, the event rate is also declining, the phenomenon of instantaneous instability of rock samples disappears, and the damage of pores and the development of microcracks are still emerging after the peak stress. However, the phenomenon of low event rate loss of yellow sandstone has no obvious change rule, and the area of low event rate loss has no obvious change. This is because the yellow sandstone rock samples in 40 freeze-thaw cycles, although there is a trend of ductile development, it did not reach the transition stage such as 30 freeze-thaw cycles of green sandstone. The brittle rock characteristics are obvious, and it is destroyed near the peak stress, forming a macroscopic crack that makes the rock samples unstable instantaneously.
In order to explore the effect of freeze-thaw on the acoustic emission energy release of rock samples, the stages of Figures 16 and 17 are divided, but most AE energy and cumulative energy-time curves only change significantly at the peak stress. If the crack volume strain method such as Figure 15 is continued to be used, the stage division will not be applicable. Because the energy released by the rock near the peak stress is much larger than in other time periods, the order of magnitude is obvious, so according to the different order of magnitude of AE energy rate, the energy release stage is divided into two categories, in which the stage of higher acoustic emission energy release near the peak stress is defined as the “explosion period,” and the stage of lower acoustic emission energy release is defined as the “stationary period.” The “stationary period” and “explosion period” division results of each rock sample are shown in Figures 16 and 17. Figure 16(c) shows that in addition to the rock samples being divided into “stable period-explosion period-stable period” three stages, the rock samples are only divided into “stable period-explosion period” two stages. This is because the physical and mechanical properties of the rock sample deteriorate after repeated freeze-thaw cycles, and the deformation characteristics of the lithology gradually change from brittleness to plasticity. After reaching the peak stress, it does not immediately lose stability as the brittle rock does, and it can still continue to bear pressure. After the peak stress, the rock sample gradually destroyed and continued to emit acoustic energy under the action of external load, but the order of magnitude of the released energy was small, reaching the second “stationary period”.
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After 20 freeze-thaw cycles, there is almost no obvious energy release in the “stationary period” of Qingsha sandstone samples, and the average energy rate in the “stationary period” is about 1/90 of the peak energy rate, indicating that the energy release is concentrated in the outbreak period. After 30 cycles, there is a higher energy release in the “stationary period,” and the average energy rate of the “stationary period” can reach about 1/10 of the peak energy rate, which shows that the intensity of acoustic emission activity of the rock sample after 30 cycles is larger and the energy release is more uniform. The deterioration of rock samples is the most obvious in the first 30 rocks with brittle characteristics; there are two “stationary periods” in the whole process of 40 cycles of rock samples, and the ratio of average energy rate and peak energy rate of “stationary period” decreases to 1/50, and the energy release is concentrated in the “explosive period.” At this time, the acoustic emission energy rate of the “stationary period” is similar to that of 20 cycles of the “stationary period,” and there is no obvious energy release, and the intensity of acoustic emission activity is much lower than that of 30 cycles. There was no obvious energy release during the 40 freeze-thaw cycles of yellow sandstone rock samples in the “stationary period,” but the ratio of the average energy rate and the peak energy rate in the “stationary period” increased from 1/600 to 1/30, indicating that the energy release in the “stationary period” gradually increased. As shown in Figure 17(c), the “stationary period” produces a small outbreak. It can be predicted that if the number of cycles continues to increase, the “stationary period” will erupt in higher energy release, and the change of cumulative energy and AE energy rate is close to the trend in Figures 16(b) and 16(c). With the increase in the number of freeze-thaw cycles, the number of pores in the rock sample increases, and the size of pores is also increasing. Under the action of external load, the pores continue to rupture, and the acoustic emission events increase, which is also accompanied by a large amount of energy release. Therefore, the energy release in the “stationary period” is gradually obvious, such as the whole process of the “stationary period” in Figure 16(b) and the small energy outbreak in the early stage of the “stationary period” in Figure 17(c). After 30 freeze-thaw cycles, the energy released by a single AE event in the green sandstone sample is more than that in the yellow sandstone sample. This reflects the freeze-thaw degradation of green sandstone is more serious than yellow sandstone. Under the action of the freeze-thaw cycle, the internal microcracks of cyan sandstone have a certain degree of initiation, which leads to the pore rupture of cyan sandstone accompanied by the initiation and penetration of microcracks under the action of external load, releasing a lot of energy; while the internal pores of yellow sandstone are broken, the initiation and penetration of microcracks are relatively small, thus releasing less energy. As shown in Figures 16 and 17, the AE energy “burst period” only exists near the peak stress. With the increase of the applied load, the pores inside the rock sample are continuously compacted, and the microcracks are also continuously pressed and sealed. During this period, huge energy is accumulated. When the peak stress is reached, a macroscopic fracture that makes the rock sample unstable instantaneously is formed. The accumulated energy is released instantaneously, and the AE energy rate increases instantaneously. At this time, the energy rate corresponds to the peak energy rate, and the accumulated energy is also rising linearly. After 40 freeze-thaw cycles, the peak energy rate decreased significantly, and the peak energy rate decreased significantly after the brittle-plastic transformation of rock samples. At this time, the release of energy increased and decreased with the increase and decrease of stress. This shows that the freeze-thaw cycle will reduce the energy released by acoustic emission activity because the freeze-thaw cycle weakens the internal structure of the rock, and the energy released by rock failure is also reduced.
5. Conclusions
(1)After several freeze-thaw cycles, the appearance of green sandstone samples showed obvious deterioration compared with yellow sandstone, and the degree of appearance deterioration also deepened. The average mass change rate of the yellow sandstone sample is slightly larger than that of the green sandstone sample; green sandstone absorbs water earlier, later more; the porosity of green sandstone is generally larger than that of yellow sandstone. The number of small pores in green sandstone decreases and the number of large pores increases, while the pore size of yellow sandstone changes little, but the number increases.(2)The discreteness of stress-strain curves of two kinds of rock samples increases with the increase of freeze-thaw cycles. After 40 freeze-thaw cycles, the stress-strain curve of yellow sandstone still maintained the typical brittle characteristics, but it had the trend of plastic transformation. The curves of B-1 and 40 freezing-thawing cycles show obvious plastic characteristics, and 30 freezing-thawing cycles can be used as the dividing line of mechanical properties for the freezing-thawing degradation of green sandstone. The peak stress of yellow sandstone has no obvious change, and the peak stress of green sandstone decreases obviously.(3)With the increase of cycles in the 20 th and 30 th freeze-thaw cycles, the area of low event rate missing is becoming smaller and smaller, and the high and low event rates coexist, and the failure mode is more complex. There is no obvious change in the low event rate loss phenomenon of yellow sandstone, and there is no obvious change in the area of low event rate loss. This is because the yellow sandstone rock samples in 40 freeze-thaw cycles, although there is a trend of ductile development, it did not reach the transition stage such as 30 freeze-thaw cycles of green sandstone. The energy released by a single AE event of the green sandstone rock sample is more than that of yellow sandstone. With the increase in freeze-thaw cycles, more and more energy is released during the stationary period.Data Availability
The data used to support the findings of this study may be released upon request to the corresponding author.
Conflicts of Interest
The authors declare no conflicts of interest in this article.
Acknowledgments
This study was supported by Qiankehe Foundation ([2020]1Z047).