The coupling effect of freeze-thaw (F-T) temperature and dynamic load on the dynamic mechanical properties and fracture mechanism of saturated yellow sandstone was experimentally investigated in this research. The dynamic compression tests on the specimen after different F-T temperatures (i.e., −5°С, −10°С, −15°С, −20°С, −30°С, and 20°С) have been carried out with split-Hopkinson pressure bar (SHPB) setup under eight F-T cycle numbers. The density and
The mechanical properties and fracture behavior of intact rock have a big influence on engineering reliability and stability in mining and civil engineering projects [
The change in mechanical behavior is caused by the rock pore structure deterioration under F-T conditions. In previous works, some attempts have been made to investigate the degradation and fracture behavior of F-T treated rocks [
As shown in Figure
Cold region distribution in China.
Based on the above research interests, the mechanical and fracture behavior of yellow sandstone under the coupling effect of F-T temperatures and impact load is investigated through the split-Hopkinson pressure bar (SHPB) at eight F-T cycles. A high-speed camera is used to capture the fracture process of the specimens. The macroscopic and microscopic fracture morphology and the mechanical properties under different F-T temperatures are discussed.
As shown in Figure
Yellow sandstone samples in this study.
Physical and mechanical properties of yellow sandstone (50 mm × 100 mm).
Samples |
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1# | 2330 | 2450 | 34.32 | 4.64 | 1.31 |
2# | 2324 | 2463 | 31.01 | 4.21 | 1.28 |
3# | 2332 | 2441 | 33.46 | 4.35 | 1.35 |
Average value | 2329 | 2451 | 32.93 | 4.40 | 1.31 |
Uniaxial compression stress-strain curve of yellow sandstone.
Moisture content is an important factor affecting the physical and mechanical properties of rock mass during F-T process [ First, the Then, the air pressure is adjusted in the sealed tank to the atmospheric pressure and kept for 12 hours. It can be considered that rock samples are fully saturated as the saturated moisture content was found to be 3.17%. The saturated samples are placed in the F-T cycle tank (Figure Finally, the
Testing systems in this work. (a)
Temperature profile of the yellow sandstone samples undergoing the F-T cycle.
As shown in Figure Firstly, the impact tests of specimens under normal temperature are carried out by using the SHPB test system (Figure Then, a series of impact tests are carried out to study the dynamic mechanical and fracture behavior of saturated yellow sandstone after F-T. During the impact test, the striker bar velocity is measured by a speedometer at different F-T temperatures; meanwhile, the strain rates of specimens are obtained using a three-wave method [ Finally, the specimen fragments are collected after impact tests and then the particle size is also counted by using a classifying screen (Figure
Structural characteristic of the SHPB.
Typical shaped waveform.
Velocity of striker bar (
Sample |
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Each sample | Average value | Each sample | Average value | ||
1 | 20 | 9.53 | 9.51 | 74.34 | 74.74 |
2 | 9.69 | 76.98 | |||
3 | 9.31 | 72.87 | |||
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4 | −5 | 9.25 | 9.49 | 71.47 | 74.57 |
5 | 9.49 | 74.88 | |||
6 | 9.72 | 77.37 | |||
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7 | −10 | 9.81 | 9.51 | 78.69 | 74.89 |
8 | 9.33 | 72.16 | |||
9 | 9.39 | 73.81 | |||
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10 | −15 | 9.54 | 9.33 | 74.23 | 72.88 |
11 | 9.19 | 71.59 | |||
12 | 9.27 | 72.82 | |||
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13 | −20 | 9.71 | 9.67 | 76.80 | 75.97 |
14 | 9.77 | 77.61 | |||
15 | 9.54 | 73.50 | |||
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16 | −30 | 9.71 | 9.54 | 73.92 | 73.60 |
17 | 9.74 | 75.26 | |||
18 | 9.17 | 71.63 |
Figure
Relationship between physical parameters and F-T temperatures.
The dynamic stress-strain curves under various F-T temperatures are shown in Figure An obvious dynamic compaction stage after F-T treatment. As the F-T temperature is decreased from −5°С to −30°С, the percentage of dynamic compaction stage gradually increases in the prepeak region of the curve. However, there is no obvious dynamic compression under the normal temperature (20°С). It maybe because the internal cracks and pores of specimens after F-T treatment gradually increase compared with the normal temperature, resulting in a relatively obvious dynamic compression stage. The research results in Section The phenomenon of postpeak strain softening appears for all the curves when the specimens are exposed to various F-T temperatures. However, the stress-strain curves from 20°С to −20°С drop rapidly after dynamic hardening stage. When the F-T temperature arrived −30°С, the attenuation slope of the postpeak stress strain curve slightly decreases. It means that the ductility of the yellow sandstone gradually increases. The strength parameters of the rock mass, such as dynamic Young’s modulus (
Dynamic stress-strain curves under various F-T temperatures.
Typical dynamic stress-strain curve and fracture process of yellow sandstone.
As shown in Figure
Dynamic elastic modulus (
Specimen |
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Sample | Average | Sample | Average | Sample | Average | ||
1 | 20 | 36.71 | 38.29 | 72.06 | 74.02 | 0.0062 | 0.0064 |
2 | 38.42 | 73.94 | 0.0068 | ||||
3 | 39.73 | 76.05 | 0.0063 | ||||
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4 | −5 | 37.21 | 34.00 | 67.73 | 70.49 | 0.0072 | 0.0075 |
5 | 30.79 | 72.88 | 0.0075 | ||||
6 | 35.82 | 70.86 | 0.0079 | ||||
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7 | −10 | 28.74 | 27.57 | 62.10 | 61.97 | 0.0087 | 0.0091 |
8 | 29.78 | 57.38 | 0.0101 | ||||
9 | 24.19 | 66.52 | 0.0085 | ||||
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10 | −15 | 17.13 | 19.07 | 53.78 | 53.74 | 0.0089 | 0.0093 |
11 | 20.37 | 57.55 | 0.0086 | ||||
12 | 19.72 | 49.90 | 0.0104 | ||||
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13 | −20 | 12.67 | 12.43 | 45.58 | 46.44 | 0.0108 | 0.0104 |
14 | 13.04 | 45.26 | 0.0110 | ||||
15 | 11.58 | 48.49 | 0.0093 | ||||
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16 | −30 | 6.85 | 6.62 | 36.35 | 37.51 | 0.0147 | 0.0142 |
17 | 6.52 | 37.12 | 0.0136 | ||||
18 | 6.49 | 39.07 | 0.0142 |
The dynamic elastic modulus reflects the deformation resistance of the specimen exposed to F-T treatment. In general, there are many various methods to determine elastic modulus, which mainly include secant modulus, tangent modulus, average modulus, and so on. In this work, we take the straight line slope of the stress-strain curve in elastic stage as the elastic modulus of the specimen. The methods have already been used in other papers [
Relationship between dynamic elastic modulus (
The dynamic peak strength (
Dynamic peak strength (
The lumpiness distribution reflects the macroscopic fracturing of the specimen under different F-T temperatures. The lumpiness distribution can be expressed by the average particle size of the specimen. The typical lumpiness distribution and cross-section morphology of the yellow sandstone under different F-T temperatures are shown in Figures
Typical lumpiness distribution characteristic of yellow sandstone under different F-T temperatures.
Typical cross-section morphology of yellow sandstone after impact fracture. Fragment I, double-cone-failure; fragment II, single-side slope failure; fragment III, double-side slope failure; fragment IV, split failure.
Mass percentage of different particle size fragments under different F-T temperatures.
Figure
To quantify the influence of the F-T temperature on the fracture degree of the yellow sandstone, the particle size coefficient
Table
Particle size coefficients under different F-T temperatures.
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20 | −5 | −10 | −15 | −20 | −30 |
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15.27 | 14.24 | 12.55 | 8.72 | 7.46 | 6.15 |
Relationship curve between particle size coefficient and F-T temperatures.
Actually, the trend of particle size coefficient with increasing F-T temperature is different from the mechanical parameters (
Microstructural characteristics of yellow sandstone under different F-T temperatures.
Figure
Relationship among particle size coefficient and elastic modulus as well as peak strength.
Relationship between particle size coefficient and peak strain.
The mechanical properties and macrofracture characteristics of the rock mass are closely related to its microstructure. The scanning electron microscope (SEM) is used to reveal the microstructural characteristics of the yellow sandstone subjected to the different F-T temperatures from 20°C to −30°C. Figure At the normal temperature, some initial pores, microcracks, and inconspicuous defects, which are closely related to the natural environment of yellow sandstone, are distributed within the rock. The internal matrix of the specimen is relatively dense. After the F-T treatment, some obvious defects (including pore cluster, larger holes, and interconnected cracks) are found in the interior of specimens that results in the strength weakening of rock as the density decreased. SEM tests (magnification 1000) indicate that the F-T temperature has substantial influence on the microstructure of specimens. As the F-T temperatures drop from −5°C to −30°C, the internal defects of the specimens obviously increase, and the diameter of pores gradually expands (Figures
As the F-T temperature decreases, the internal damage degree of the sample increases, which worsens its integrity and weakens the mechanical properties of rocks. The lumpiness distribution is also closely related to the specimen integrity. The higher the integrity of the sample, the less the number of small size particles. It means that the particle size coefficients gradually increase with the increasing F-T temperature, which is consistent with the conclusion in Section
Based on the one-dimensional stress wave theory and the uniform stress assumption [
Then, the incident energy (
The incident energy (
The incident, reflected, transmitted, and dissipation energies of the yellow sandstone subjected to different F-T temperatures can be obtained by combining equations (
Variation regularity of energy dissipation with different F-T temperatures.
In addition, as the F-T temperature declined, the inoculation, germination, extension, and penetration of the microcrack inside the sample accelerate the damage degree of the specimen, which leads to the degradation of the physical and mechanical properties of the rock. The propagation of initial cracks, expansion of pores, and the generation of new cracks consume energy to develop fracture and promote the expansion of internal cracks. It should be noted that the energy dissipation decreases with an increasing porosity of the specimen [
In this work, a series of dynamic compression tests on the saturated yellow sandstone subjected to different F-T temperatures (i.e., −5°С, −10°С, −15°С, −20°С, −30°С, and 20°С) are carried out using SHPB equipment with eight F-T cycle numbers. The coupling effect of the F-T temperature and impact load on the mechanical properties, lumpiness distribution, microstructural feature, and energy dissipation of the specimens is discussed. From the investigation, the following conclusions can be drawn: Under the same strain rate, as the F-T temperature gradually decreased, both the When the strain rate is in the range of 72.88 s−1 to 75.97 s−1, the fracture modes of the yellow sandstone mainly include the double-cone failure pattern, single-side slope failure pattern, double-side slope failure pattern, and split failure pattern. The particle size coefficient markedly decreased from −5°С to −20°С with a high gradient while decreased gradually from 20°С to −5°С and −20°С to −30°С with a lower speed rate. Hence, the effect of the F-T temperature on the macroscopic fracture degree of the yellow sandstone is significant, particularly in the range of −20°С to −5°С. SEM analysis shows that the F-T temperature is able to contribute to the internal damage degree of the specimen and produces a high number of freeze-swell holes, interconnected cracks, pores cluster, and other defects inside the specimen. Both the pore diameter and crack width gradually increased with the decrease of the F-T temperature. The fragment size caused by the impact tests gradually decreased. This is consistent with the macrofracture characteristics of the yellow sandstone. As the F-T temperature decreased, both the incident and reflected energies remain approximately constant whereas the transmitted energy increased linearly and the dissipation energy decreased linearly.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the Fundamental Research Funds for the Central Universities (2018BSCXB20) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_1968).