Influence of Heat Treatment on the Mechanical Properties of Fine-Grained Granite under Dynamic Impact Loading

Abstract

In order to study the influence of heat treatment on the dynamic properties of fine-grained granite, an improved split Hopkinson pressure bar (SHPB) system was used to conduct impact compression tests on the granite specimens treated at 20~1000 °C under three loading rates. The experimental results show that the shape of the impact stress–strain curve is affected by the loading rate and heat treatment temperature. Under the same loading rate, the average strain rate, peak strain, and maximum strain of granite specimen exhibit a trend of “slow increasing (20~200 °C)—slow decreasing (200~400 °C)—slow increasing (400~500 °C)”. The peak stress and elastic modulus show the opposite trend. The average strain rate, peak strain, and maximum strain of the granite specimen treated at 600 °C increase significantly. The peak stress and elastic modulus decrease significantly. Within the heat treatment temperature range of 600~800 °C, the dynamic properties of granite deteriorate slowly. The average strain rate, peak strain, and maximum strain of the granite specimens treated at 900 °C and 1000 °C increase sharply, while the peak stress decreases sharply. Within the heat treatment temperature range of 600–1000 °C, the elastic modulus of the granite specimen shows an approximately linear decreasing trend. There are no changes in the mineral composition of granite within the heat treatment temperature range of 20–1000 °C. After heat treatment at 600 °C, the width of internal cracks in granite increases significantly. The width of internal cracks in the heat-treated granites at 900 °C and 1000 °C increases sharply. The change in the dynamic properties of granite is determined by the internal microstructure of the heat-treated granite at different temperatures.

1. Introduction

Temperature has a significant influence on the mechanical properties of rocks. During the extraction of deep resources, deep disposal of nuclear waste, utilization of geothermal resources, underground gasification of coal seams, construction and protection of stone buildings, repair and reinforcement of tunnels surrounding rocks after fires and other areas, the influence of temperature on the mechanical properties of rocks must be considered. In addition, the mechanical properties of rocks are also affected by the loading rate [1,2]; that is, the mechanical properties of rocks exhibit a time-dependent relationship, such as strength, deformation, stiffness, and fracture characteristics. In the fields of penetration, blasting, mechanized drilling, and collapse collision, rocks are often subjected to impact loading. Therefore, it is essential to research the dynamic properties of rocks after high-temperature treatment, providing a scientific basis for the evaluation and prediction of construction design, stability, and safety of rock mass engineering.

The emergence of the split Hopkinson pressure bar (SHPB) system provides a reliable testing platform for studying the mechanical properties of brittle materials such as rock and concrete under impact loading. Wang et al. [3,4] conducted dynamic compression tests on the heat-treated Huashan granite at 20–900 °C using an improved SHPB system. They found that there was a temperature threshold for dynamic properties between 500 °C and 700 °C at the same impact velocity. The heat-treated granite had maximum and minimum energy dissipation densities at 300 °C and 700 °C, respectively. Huang et al. [5] established an empirical equation based on X-ray micro-computed tomography (CT) to quantify the correlation between damage variables and dynamic compressive strength of Longyou sandstone after heat treatment. Liu et al. [6] found that the growth factor DCF of dynamic compressive strength of Qinling biote granite showed a trend of gradually increasing with increasing temperature and the dynamic compressive strength decreased significantly after 600 °C. Yang et al. [7] used fractal dimension and average fragment size to evaluate the fragment characteristics of granite after heat treatment under impact compression loading. In addition, the crack propagation law during dynamic compression was studied based on numerical simulation results.

In addition, Wang et al. [8,9] conducted dynamic splitting tensile tests on the heat-treated Huashan granite at 100–900 °C using the SHPB system. The relationship between dynamic splitting tensile strength, splitting tensile fracture characteristics, degree of fragmentation, and energy dissipation with loading rate and heat treatment temperature were studied. Yao et al. [10] conducted dynamic splitting tensile tests on the heat-treated Longyou sandstone at 150–850 °C and established an empirical equation between dynamic tensile strength, loading rate, and heat treatment temperature. Yin et al. [11] studied the influence of loading rate and heat treatment temperature (25–850 °C) on the fracture toughness of Laurentian Granite using an SHPB system. Wang et al. [12,13] studied the degradation law of dynamic mechanical parameters, the cumulative damage evolution law, the crack propagation process, and the energy dissipation law of the heat-treated Huashan granite at 20–600 °C under cyclic impact loading using an improved SHPB system.

However, the degradation mechanism of dynamic properties of rocks influenced by heat treatment was not fully studied in the aforementioned literature. In this paper, an improved SHPB system was used to conduct impact compression tests on the granite specimens treated at 20–1000 °C under three different loading rates. Variation law of impact stress–strain curve, average strain rate, peak stress, peak strain, elastic modulus, and maximum strain were analyzed based on loading rate and heat treatment temperature. In addition, the X-ray diffraction (XRD) and Tescan MIRA4 thermal field emission scanning electron microscope (FESEM) were used to reveal the deterioration mechanism of the dynamic properties of the heat-treated granite at different temperatures.

2. Heat-Treated Granite Specimen Preparation, Experimental Apparatus, and Experimental Scheme

2.1. Heat-Treated Granite Specimen Preparation

The granite material used for heat treatment and dynamic impact compression tests was obtained from the Luotian area of Hubei Province, China. The size of mineral particles in granite ranges from 0.1 to 1.2 mm, belonging to fine-grained granite. At room temperature, its density is 2610 kg/m³, longitudinal wave velocity is 3920 m/s, uniaxial compressive strength is 291.57 MPa, elastic modulus is 78.06 GPa, and splitting tensile strength is 8.39 MPa.

When using the SHPB system for impact compression tests, there are certain requirements for the size selection of rock specimens. If the rock specimen is too long, it will result in a smaller number of stress waves reflecting back and forth inside the specimen, which is not conducive to achieving stress balance at both ends of the rock specimen [14]. If the rock specimen is too short, it will cause a transverse inertial effect [15,16] and an end friction effect [17]. Additionally, due to the heterogeneity of rock materials, the size of rock specimens should be at least 10 times the average size of mineral particles [18,19]. The optimal length-to-diameter ratio of rock specimens in SHPB tests should be 0.5, comprehensively considering the end friction effect, transverse inertial effect, and uniformity assumption [20]. A length-to-diameter ratio of 0.5–1 was recommended by Zhou et al. [18] and Gray et al. [21]. Therefore, the granite specimens were processed into a cylinder with Φ50 mm × 25 mm in SHPB impact compression tests in this paper.

The heating equipment used for heat treatment is the SX2-5-12TP furnace, which consists of a furnace chamber, heating elements, and a temperature control system. Its maximum heating temperature is 1200 °C, as shown in Figure 1. To investigate the influence of heat treatment on the dynamic properties of granite, eleven heat treatment temperature levels were set at 20 °C, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, and 1000 °C. The granite specimens were placed in the furnace, heated at a rate of 2 °C/min to the corresponding temperature level, and maintained at a constant temperature for 2 h [6,22]. The granite specimens were slowly cooled to room temperature in the furnace to avoid crack formation caused by thermal shock [23]. The granite specimens treated at different temperatures for dynamic impact compression tests are shown in Figure 2.

2.2. Experimental Apparatus

The improved SHPB system was used to carry out the impact compression tests on the heat-treated granite. This system is composed of the air chamber, striker bar, incident bar, transmission bar, damper, data acquisition system, and data processing system, as shown in Figure 3. Compared with the traditional SHPB system, the greatest feature of this system is the use of a striker bar with a tapered structure. It can produce a stable semi-sinusoidal incident wave without pasting a pulse shaper [24]. The incident wave ε_I(t), reflected wave ε_R(t), and transmitted wave ε_T(t) signals are measured by strain gauges attached to the incident and transmitted bars. According to the “three-wave method” [19], the strain, strain rate, and average stress of rock specimens can be obtained using the following formulas.

$${\epsilon }_s\left(t\right)={\displaystyle \frac{C_e}{L_s}}{\displaystyle {\int }_0^{\tau }\left[{\epsilon }_{\mathrm{I}}(t)-{\epsilon }_{\mathrm{R}}(t)-{\epsilon }_{\mathrm{T}}(t)\right]}dt$$

(1)

$${\dot{\epsilon }}_s\left(t\right)={\displaystyle \frac{C_e}{L_s}}\left[{\epsilon }_{\mathrm{I}}(t)-{\epsilon }_{\mathrm{R}}(t)-{\epsilon }_{\mathrm{T}}(t)\right]$$

(2)

$${\sigma }_{\mathrm{s}}\left(t\right)={\displaystyle \frac{E_e{A}_e}{2A_s}}\left[{\epsilon }_{\mathrm{I}}(t)+{\epsilon }_{\mathrm{R}}(t)+{\epsilon }_{\mathrm{T}}(t)\right]$$

(3)

If the uniformity assumption (ε_I(t) + ε_R(t) = ε_T(t)) is applied to the rock specimen, then the above formula can be transformed into the following “two-wave method” formula [25].

$${\epsilon }_s(t)=-2{\displaystyle \frac{C_e}{L_s}}{\displaystyle {\int }_0^{\tau }{\epsilon }_{\mathrm{R}}(t)dt}$$

(4)

$${\dot{\epsilon }}_s(t)=-2{\displaystyle \frac{C_e}{L_s}}{\epsilon }_{\mathrm{R}}(t)$$

(5)

$${\sigma }_s(t)={\displaystyle \frac{E_e{A}_e}{A_s}}{\epsilon }_{\mathrm{T}}(t)$$

(6)

Here, A_e and A_s are the cross-sectional areas of the elastic bar and rock specimen, respectively; E_e and C_e are the elastic modulus and longitudinal wave velocity of the elastic bar, respectively; and L_s is the length of the rock specimen.

2.3. Experimental Scheme

In the impact compression experiments of heat-treated granite, three impact velocity levels were set: the first impact velocity (V₁ = 11.2 m/s), the second impact velocity (V₂ = 14.0 m/s), and the third impact velocity (V₃ = 16.8 m/s). The loading rates corresponding to the three impact velocities are 2264 GPa/s (${\stackrel{·}{\sigma }}_1$), 2834 GPa/s (${\stackrel{·}{\sigma }}_2$), and 3136 GPa/s (${\stackrel{·}{\sigma }}_3$), respectively. The loading rate is the slope of the approximately straight section of the rising section of the incident stress wave. At least five impact compression experiments were conducted on the heat-treated granite specimen at each temperature under each loading rate. Peak stress was selected as an evaluation parameter. The specimens with the highest and lowest peak stresses were removed, and the remaining three samples were used as valid data. Under the same impact velocity, the pressure value of nitrogen gas and the position of the striker bar inside the chamber remained unchanged to ensure that the peak intensity of the incident wave was equal. Vaseline was smeared on both ends of the granite specimen to eliminate the end friction effect.

3. Dynamic Properties of the Heat-Treated Granite under Impact Loading

3.1. Impact Stress–Strain Curve

Under three loading rates, the impact stress–strain curves of the granite specimens treated at different temperatures have the same characteristics before the peak stress point (as shown in Figure 4). Unlike the static compression stress–strain curve, which will undergo a compaction stage, the impact stress–strain curve is approximately a straight line at the initial loading stage. It can be considered that the rock is in the elastic deformation stage. Subsequently, as the stress gradually increases, the tangent slope of the impact stress–strain curve gradually decreases. This indicates that the micro-cracks inside the rock initiate and extend gradually, and the damage continues to increase until the peak stress point is reached. Under the first and second loading rates, the strain of the heat-treated granite specimen at different temperatures increases with the decrease in stress during the initial unloading stage. After reaching the maximum strain, the strain decreases gradually. The impact stress–strain curve shows a significant rebound, presenting a typical “П-type” stress–strain curve [26]. In addition, the strain distance between the maximum strain and peak strain in the impact stress–strain curve of the granite specimens treated at 600~1000 °C is significantly greater than that of the granite specimens treated at 20–500 °C. Under the third loading rate, the impact stress–strain curves of the granite specimens treated at 20–900 °C also show a “П-type” stress–strain curve. For the granite specimen treated at 1000 °C, the strain continues to increase as the stress decreases after the peak stress point, presenting a typical “І-type” stress–strain curve [26].

3.2. Average Strain Rate

Strain rate is an important measurement parameter for distinguishing static mechanics of rocks from dynamic mechanics. In this paper, the ratio of the strain corresponding to the peak stress to the time required to reach the peak stress is selected as the average strain rate of rock specimens under the impact compression loading [12]. The average strain rate of granite specimens varies with heat treatment temperature under three loading rates, as shown in Figure 5. The average strain rate of the granite specimen treated at the same temperature increases with the increase in loading rate. For example, the average strain rates of the room-temperature granite specimen under three loading rates are 36.31 s⁻¹, 49.70 s⁻¹, and 61.69 s⁻¹, respectively. The average strain rates of the granite specimen treated at 800 °C are 100.85 s⁻¹, 112.18 s⁻¹, and 128.31 s⁻¹, respectively. The average strain rates of the granite specimen treated at 1000 °C are 154.43 s⁻¹, 192.40 s⁻¹, and 295.90 s⁻¹, respectively.

Figure 6 shows the variation in the normalized average strain rate with heat treatment temperature under three loading rates. It can be seen that under the three loading rates, the average strain rate of the granite specimens treated at 20–500 °C shows a trend of “slow increasing (20~200 °C)—slow decreasing (200~400 °C)—slow increasing (400~500 °C)”. Taking the first loading rate as an example, the average strain rates of the granite specimens treated at 100 °C, 200 °C, 300 °C, 400 °C, and 500 °C are 1.12 times, 1.23 times, 1.19 times, 1.15 times, and 1.42 times that of the room-temperature granite specimen, respectively. Within the temperature range of 20–500 °C, the average strain rate fluctuates and the range is small. The average strain rate of the granite specimen treated at 600 °C increases significantly, at 2.20 times, 1.86 times, and 1.60 times that of the room-temperature granite specimen, respectively, under three loading rates. The average strain rate of granite specimen increases slowly under the same loading rate within the temperature range of 600~800 °C. The average strain rate of the granite specimens treated at 900 °C and 1000 °C increases sharply. Under three loading rates, the average strain rates of the granite specimen treated at 900 °C are 3.42 times, 3.05 times, and 3.22 times that of the room-temperature granite specimen, respectively. The average strain rates of the granite specimen treated at 1000 °C are 4.25 times, 3.87 times, and 4.80 times that of the room-temperature granite specimen, respectively.

3.3. Peak Stress

The variation in peak stress of granite specimens with heat treatment temperature under three loading rates is shown in Figure 7. It can be seen that the peak stress of the granite specimen treated at the same temperature also has a loading rate effect; that is, the peak stress increases with the increase in loading rate. The peak stresses of the room-temperature granite specimen under three loading rates are 200.35 MPa, 257.01 MPa, and 310.37 MPa, respectively. Even if the granite specimen is heat-treated at 1000 °C, its peak stress still has a loading rate effect. The peak stresses under three loading rates are 114.80 MPa, 126.42 MPa, and 137.45 MPa, respectively.

Under the same loading rate, the peak stress of the granite specimen varies with heat treatment temperature. The variation in normalized peak stress with heat treatment temperature is shown in Figure 8. Within the heat treatment temperature range of 20–200 °C, the peak stress of granite specimen shows a slow decreasing trend. Under three loading rates, the peak stresses of the granite specimen treated at 200 °C are 94.34%, 92.71%, and 92.37% of those of the room-temperature granite specimen, respectively. The peak stress of the granite specimen is restored to a certain extent within the heat treatment temperature range of 200–400 °C. Under three loading rates, the peak stresses of the granite specimen treated at 400 °C are restored to 98.57%, 97.09%, and 96.60% of those of the room-temperature granite specimen, respectively. Under three loading rates, the peak stresses of the granite specimen treated at 500 °C are 94.91%, 92.16%, and 91.66% of those of the room-temperature granite specimen, respectively. Overall, within the heat treatment temperature range of 20–500 °C, the peak stress of the granite specimen shows fluctuating changes with a relatively small amplitude under the same loading rate.

After heat treatment at 600 °C, the peak stress of granite specimens decreases significantly. Under three loading rates, the peak stresses are 86.36%, 84.02%, and 82.21% of those of the room-temperature granite specimen, respectively. Within the heat treatment temperature range of 600–800 °C, the peak stress of granite specimen shows a slow decreasing trend. Under three loading rates, the peak stresses of the granite specimen treated at 800 °C are 79.36%, 77.32%, and 72.57% of those of the room-temperature granite specimen, respectively. Under the same loading rate, the peak stress of the granite specimens treated at 900 °C and 1000 °C decreases rapidly. Under three loading rates, the peak stresses of the granite specimen treated at 900 °C are 71.51%, 66.44%, and 60.33% of those of the room-temperature granite specimen, respectively. The peak stresses of the granite specimen treated at 1000 °C are 57.30%, 49.19%, and 44.29% of those of the room-temperature granite specimen, respectively.

3.4. Peak Strain

The variation in peak strain (the strain corresponding to the peak stress) of granite specimen with heat treatment temperature under three loading rates is shown in Figure 9. Similarly, the peak strain of the heat-treated granite specimen at the same temperature also increases with the increase in loading rate. Under three loading rates, the peak strains of the room-temperature granite specimen are 0.0038, 0.0052, and 0.0064, respectively. The peak strain of the heat-treated granite specimen at high temperatures still has a loading rate effect. Taking the granite specimen treated at 1000 °C as an example, under three loading rates, the peak strains are 0.0185, 0.022, and 0.0253, respectively.

Under three loading rates, the peak strain of the granite specimens treated at 20–500 °C fluctuates and varies within the ranges of 0.0038–0.0054, 0.0052–0.0063, and 0.0066–0.0077, respectively. The variation in peak strain is relatively small under the same loading rate. The peak strain of the granite specimen treated at 600 °C increases significantly. Under three different loading rates, the peak strains are 2.18 times, 1.77 times, and 1.72 times that of the room-temperature granite specimen, respectively, and 1.54 times, 1.46 times, and 1.43 times that of the granite specimen treated at 500 °C, respectively, (as shown in Figure 10). Within the heat treatment temperature range of 600–800 °C, the peak strain of the granite specimen increases slowly. The peak strains of the granite specimen treated at 800 °C are 2.87 times, 2.31 times, and 2.14 times that of the room-temperature granite specimen, respectively. The peak strain of the granite specimens treated at 900 °C and 1000 °C increases sharply. Under three loading rates, the peak strains of the granite specimen treated at 900 °C are 3.68 times, 3.10 times, and 2.94 times that of the room-temperature granite specimen, respectively. The peak strains of the granite specimen treated at 1000 °C are 4.87 times, 4.23 times, and 3.95 times that of the room-temperature granite specimen, respectively.

3.5. Elastic Modulus

The loading section of the impact stress–strain curve of the granite specimen treated at different temperatures is not a straight line. The tangent slope of the two points corresponding to 20% and 80% of peak stress on the loading section of the impact stress–strain curve is selected as the elastic modulus [27]. The variation in elastic modulus of granite specimen with heat treatment temperature under three different loading rates is shown in Figure 11. Different from the average strain rate, peak stress, and peak strain, the elastic modulus of the granite specimen treated at the same temperature does not increase with the increase in loading rate. Under three loading rates, the elastic moduli of the room-temperature granite specimen are 55.12 GPa, 54.76 GPa, and 50.79 GPa. The elastic moduli of the heat-treated granite specimen at 500 °C are 39.47 GPa, 40.82 GPa, and 40.73 GPa. The elastic moduli of the granite specimen treated at 1000 °C are 7.71 GPa, 6.18 GPa, and 6.75 GPa. The elastic modulus of the granite specimen treated at the same temperature does not have a loading rate effect.

Figure 12 shows the variation in normalized elastic modulus with heat treatment temperature. Under the same loading rate, the elastic modulus of the granite specimen decreases slowly within the temperature range of 20–200 °C. Under three loading rates, the elastic moduli of the heat-treated granite specimen at 200 °C are 79.10%, 87.22%, and 87.18% of those of the room-temperature granite specimen. Within the temperature range of 200–400 °C, the elastic modulus of granite specimen increases slowly. Under three loading rates, the elastic moduli of the heat-treated granite specimen at 400 °C are 85.41%, 90.74%, and 98.92% of those of the room-temperature granite specimen. The elastic modulus of the granite specimen is restored to a certain extent. Starting from the heat treatment temperature of 400 °C, the elastic modulus of the granite specimen decreases gradually. Under three loading rates, the elastic moduli of the heat-treated granite specimen at 500 °C decrease to 71.61%, 74.54%, and 80.19% of those of the room-temperature granite, respectively, which are 90.53%, 85.47%, and 91.98% of those of the granite specimen treated at 200 °C, respectively. Overall, the variation in elastic modulus of granite specimens is not severe within the temperature range of 20–500 °C.

After heat treatment at 600 °C, the elastic modulus of granite specimen decreases significantly. Under three loading rates, the elastic moduli of granite specimens treated at 600 °C decrease to 41.11%, 43.37%, and 52.49% of those of the room-temperature granite specimen. Within the heat treatment temperature range of 600–1000 °C, the elastic modulus of the granite specimen decreases gradually, showing an approximately linear decreasing trend. Under three loading rates, the elastic moduli of the heat-treated granite specimen at 1000 °C are decreased to 13.99%, 11.29%, and 13.29% of those of the room-temperature granite specimen, respectively.

3.6. Maximum Strain

The maximum strain is the maximum value of strain in the impact stress–strain curve. The variation in maximum strain of granite specimen with heat treatment temperature under three loading rates is shown in Figure 13. Like the strain rate, peak stress, and peak strain, the maximum strain of granite specimens treated at the same temperature increases with the increase in loading rate. Under the same loading rate, the variation in maximum strain of granite specimen with heat treatment temperature is similar to the average strain rate and peak strain within the temperature range of 20–500 °C. It also shows a trend of “slow increasing (20~200 °C)—slow decreasing (200~400 °C)—slow increasing (400~500 °C)”. The maximum strain of the granite specimen treated at 600 °C increases significantly to 2.20 times, 2.02 times, and 1.65 times those of the room-temperature granite specimen, respectively, under three loading rates, as shown in Figure 14. Within the heat treatment temperature range of 600–800 °C, the maximum strain of granite specimen under the same loading rate increases slowly with the increase in heat treatment temperature. The maximum strain of the granite specimens treated at 900 °C and 1000 °C increases sharply. Under three loading rates, the maximum strains of the granite specimens treated at 900 °C are 3.59 times, 3.44 times, and 4.46 times those of normal temperature granite specimens, respectively. The maximum strains of the granite specimen treated at 1000 °C are 4.75 times, 4.74 times, and 8.26 times those of the room-temperature granite specimen, respectively.

4. Thermal Damage Characteristics

4.1. Analysis of XRD Test Results

Figure 15 shows the X-ray diffraction pattern of granite treated at different temperatures. It can be seen that the main mineral components of fine-grained granite are quartz, plagioclase, microcline, and biotite. Within the heat treatment temperature range set in this paper (20–1000 °C), the diffraction information for the four main minerals does not disappear and no new minerals are generated. There are no significant changes in the diffraction pattern and corresponding diffraction angles of the four main minerals. This indicates that after heat treatment at different temperatures, the mineral composition of granite is not changed. Although the diffraction intensity of granite minerals at the same diffraction angle varies, this does not mean that decomposition or chemical reactions of minerals have occurred. This is because the spectral peak intensity of X-ray diffraction may be affected by multiple factors, such as light source condition, detection sensitivity, and environmental temperature and humidity.

4.2. Analysis of Scanning Electron Microscopy Results

To study the influence of temperature on the internal microstructure of granite, the FESEM images of the granite treated at 20–1000 °C were obtained using the Tescan MIRA4 field emission scanning electron microscope. Figure 16 shows the FESEM images of the granite treated at different temperatures under the same magnification (Mag = 3500 X). From Figure 16a, it can be seen that there are also some original cracks in the room-temperature granite. The initial condensation environment or later tectonic movement may lead to the generation of original cracks inside granite. Compared with the room-temperature granite, the width of internal cracks in the heat-treated granites at 100 °C and 200 °C increases slowly, as shown in Figure 16b and Figure 16c, respectively. This is because the water in different states (including attached water and bound water) in the granite will evaporate and overflow within this temperature range [28].

According to the one-dimensional stress wave theory [25], the stress wave is reflected and transmitted between the incident bar, rock specimen, and transmission bar. If only one-time transmission and reflection are considered, the following relationships exist:

$$F={\displaystyle \frac{{\rho }_2{c}_2-{\rho }_1{c}_1}{{\rho }_2{c}_2+{\rho }_1{c}_1}}\hspace{1em}\hspace{1em}\hspace{1em}(-1<F<0)$$

(7)

$${\epsilon }_{\mathrm{R}}(t)=F{\epsilon }_{\mathrm{I}}(t)$$

(8)

$${\epsilon }_{\mathrm{T}}(t)=(1-F^2){\epsilon }_{\mathrm{I}}(t)$$

(9)

In the formula, ρ₁c₁ and ρ₂c₂ are the wave impedance of the elastic bar and rock specimen, respectively; F is the reflection coefficient.

The increase in the width of internal cracks in the granites treated at 100 °C and 200 °C leads to a decrease in the wave impedance of granite. According to Formula (7), the reflection coefficient F decreases. According to Formulas (8) and (9), the absolute value of the transmitted wave ε_T(t) decreases when the incident wave is the same. However, the absolute value of the reflected wave ε_R(t) increases. Moreover, according to the “two-wave method” theory, the peak stress is proportional to the transmitted wave and the average strain rate is proportional to the reflected wave. Thus, the peak stress and elastic modulus of the granite specimens treated at 100 °C and 200 °C are slightly smaller than those of the room-temperature granite specimen under the same loading rate. The average strain rate, peak strain, and maximum strain are slightly larger than those of the room-temperature granite specimen.

Compared with the granite treated at 200 °C, the width of internal cracks in the granites treated at 300 °C and 400 °C is not increased but rather decreased, as shown in Figure 16d,e. This is due to the reduction in the distance between mineral particles caused by thermal expansion produced during the heating process. The internal cracks in granite are partially closed. The contact relationship between mineral particles is improved [29,30]. It is called the fracture healing effect [31,32]. Under the fracture healing effect, the wave impedance of the granite specimen increases within the heat treatment temperature range of 200–400 °C. The reflection coefficient F increases, resulting in an increase in the absolute value of the transmitted wave ε_T(t) and a decrease in the absolute value of the reflected wave ε_R(t). Therefore, under the same loading rate, the peak stress and elastic modulus of the granite specimens treated at 300 °C and 400 °C are partially recovered. The average strain rate, peak strain, and maximum strain decrease gradually.

Compared with the granite treated at 400 °C, the width of internal cracks in the heat-treated granite at 500 °C increases again (as shown in Figure 16f). Granite is composed of minerals with different thermal expansion coefficients. The uneven expansion between minerals in a high-temperature environment will lead to the generation of thermal stress [33,34]. When the thermal stress exceeds the tensile or shear strength of the minerals, the width of intergranular cracks increases, resulting in the thermal cracking effect. Similarly, according to the one-dimensional stress wave theory and the “two-wave method” theory, the peak stress and elastic modulus of the granite treated at 500 °C decrease again, and the average strain rate, peak strain, and maximum strain increase again. However, on the whole, within the heat treatment temperature range of 20–500 °C, there is no obvious increase or decrease in the width of internal cracks in granite. Therefore, the dynamic mechanical parameters of granite specimens fluctuate, and the range is not significant.

Compared with the granite treated at 20–500 °C, the width of internal cracks in the granite treated at 600 °C increases significantly, as shown in Figure 16g. The increase in heat treatment temperature will inevitably lead to a more obvious thermal cracking effect. In addition, the phase transition of quartz (from the α phase to the β phase) will occur around 573 °C [35]. Although this transition is reversible (the β-quartz will return to the α-quartz form when cooled to room temperature), the lattice expansion, dislocations, and other lattice defects caused by the increase in quartz volume cannot be completely restored [36]. This will lead to the generation of intragranular or transgranular cracks within mineral crystals. Under the combined action of the thermal cracking effect and the phase transition of quartz, the wave impedance of the granite treated at 600 °C is significantly smaller than that of the granite treated at 20–500 °C. Therefore, under the same loading rate, the peak stress and elastic modulus of the granite treated at 600℃ decrease significantly. Conversely, the average strain rate, peak strain, and maximum strain increase significantly.

Under the thermal cracking effect, the width of internal cracks in granite increases gradually within the heat treatment temperature range of 600–800 °C, as shown in Figure 16g–i. Under the same loading rate, the peak stress and elastic modulus of the granite specimen decrease gradually, while the average strain rate, peak strain, and maximum strain increase gradually. The width of internal cracks in the granite treated at 900 °C increases significantly again, as shown in Figure 16j. In addition to the more obvious thermal cracking effect, quartz will transform from β-quartz to β-tridymite at around 870 °C [37]. Although this transition is also reversible, the lattice expansion, dislocations, and other lattice defects caused by the increase in quartz volume cannot be completely restored. Therefore, the wave impedance of the granite treated at 900 °C decreases significantly. This leads to a significant decrease in peak stress, while the average strain rate, peak strain, and maximum strain increase significantly. Compared with the granite treated at 900 °C, the width of internal cracks in the granite treated at 1000 °C increases sharply, as shown in Figure 16k. This is due to the non-linear positive correlation between the magnitude of thermal stress and heat treatment temperature. Under a more dramatic thermal cracking effect, the dynamic properties of the granite treated at 1000 °C deteriorated more rapidly.

5. Conclusions

In this paper, the improved SHPB was used to carry out impact compression tests on the granite specimens treated at 20–1000 °C under three loading rates. In addition, X-ray diffraction (XRD) and Tescan MIRA4 thermal field emission scanning electron microscopes (FESEM) were used to reveal the deterioration mechanism of dynamic properties of granite treated at different temperatures. The main conclusions are as follows:

(1) The shape of the impact stress–strain curve (“І type” or “П type”) is jointly affected by the loading rate and heat treatment temperature. The average strain rate, peak stress, peak strain, and maximum strain of granite treated at the same temperature all increase with the increase in loading rate. The elastic modulus of the granite specimen treated at the same temperature does not have a loading rate effect.

(2) Under the same loading rate, the average strain rate, peak strain, and maximum strain of granite specimen show a trend of “slowly increasing—slowly decreasing—slowly increasing” within the heat treatment temperature range of 20–500 °C. The peak stress and elastic modulus showed the opposite change trend. The average strain rate, peak strain, and maximum strain of the granite treated at 600 °C increase significantly, while the peak stress and elastic modulus decrease significantly. The dynamic properties of granite deteriorated slowly within the heat treatment temperature range of 600–800 °C. The average strain rate, peak strain, and maximum strain of granite treated at 900 °C and 1000 °C increased sharply, and the peak stress decreased sharply. Within the heat treatment temperature range of 600–1000 °C, the elastic modulus of granite specimen presents an approximately linear decreasing trend.

(3) The mineral composition of granite did not change within the heat treatment temperature range of 20–1000 °C. On the whole, there is no significant increase or decrease in the width of internal cracks of the granites treated at 20–500 °C. The width of internal cracks in the granite treated at 600 °C increases significantly. Within the heat treatment temperature range of 600–800 °C, the width of internal cracks of granite increases gradually. The width of internal cracks in the granites treated at 900 °C and 1000 °C increases sharply. The change in the dynamic properties of granite is determined by the internal microstructure of the granite treated at different temperatures.