Granite samples were obtained from the Kafang tin mine in Honghe Hani and Yi Autonomous Prefecture, Yunnan Province, China. According to the International Society for Rock Mechanics (ISRM) standard (Zhou et al., 2012), granite was processed into a cylindrical sample with a diameter of 50 mm and an aspect ratio of 0.5. A KRX-17B box-type resistance furnace was used to treat granite samples at high temperatures. The heating rate was set at 2°C/min. After heating to the target temperature, the temperature was maintained constant for 2 h. After heating, the sample was cooled to room temperature (25°C) in a furnace chamber before removal. To prevent the reaction between the rock samples and water vapor in the air following the high-temperature treatment, the samples were stored in a Tester WGLL-230BE electric blast-drying oven. The granite specimen after high-temperature treatment is shown in Figure 1. Evidently, with an increase in the heating temperature, the surface color of the granite specimens gradually deepened. When the heating temperature exceeded 400°C, thermal cracks appeared on the surface of granite specimens.

Figure 2 shows the schematic of a 50 mm diameter SHPB test system used in this study. The system includes a dynamic loading part (nitrogen tank, gas gun, and spindle punch), rod part (incident bar and transmission bar), and data acquisition part (infrared speed measuring, single recording device, and data-processing device). The incident, transmission, and impact bars were all ⁴⁰Cr high-strength alloy steels. The density was 7.81 g/cm³. The lengths of the incident rod and transmission rod were 2 m. The longitudinal wave velocity was 5,100 m/s. The elastic modulus was 210 GPa. The strain gauge was pasted on 1/2 of the rod.

During the SHPB experiment, the sample was placed between the incident and transmission bars, and the bullet was discharged at a certain speed. A stress wave was thus formed in the incident bar after impacting the incident bar. The incident waves were then propagated forward in the incident bar. When transmitted to the interface between the incident rod and the sample, a part of the incident wave was reflected back to the incident rod as a reflected wave, owing to the wave impedance difference between the rod and the sample. The other part was transmitted through the sample into the transmission rod as a transmission wave. Based on the one-dimensional stress assumption and stress uniformity assumption (Hu et al., 2015), the following method was described by Wang (2005). By substituting the signal collected by the strain gauge in Eq. 1, the stress σ ( t ) , strain ε ( t ) , and strain rate ε ˙ ( t ) of the specimen can be obtained as follows: σ ( t ) = E A 2 A s [ ε i ( t ) + ε r ( t ) + ε t ( t ) ] ε ˙ ( t ) = C l s [ ε i ( t ) − ε r ( t ) − ε t ( t ) ] ε ( t ) = C l s ∫ 0 t [ ε i ( t ) − ε r ( t ) − ε t ( t ) ] d t } , (1) where ε i ( t ) , ε r ( t ) , and ε t ( t ) are the incident, reflected, and transmitted strains on the bar, respectively; A, E, and C represent the cross-sectional area, elastic modulus, and longitudinal wave velocity of the pressure rod, respectively.

In this experiment, the granite specimens were heated and naturally cooled at room temperature (25°C) and high temperatures at different temperature levels (200°C, 400°C, 600°C, and 800°C). The specimens were then subjected to the dynamic impact compression experiments at different impact velocities (8.5 m/s, 11.5 m/s, and 13.5 m/s). The standard circular hole screens with different diameters (0.3, 0.5, 1, 2.5, 5, 10, 15, 20, and 25 mm) were used to screen the granite fragments following the experiment. A total of three-independent experiments were conducted for each working condition. Statistically significant data were selected for statistical analysis.

To ensure the validity of the experimental data, the stress balance must be ensured at both ends of the rock sample before the dynamic impact compression test. Typical waveforms at both ends of the sample are shown in Figure 3A. Figure 3B shows that the overlapping waveforms of the incident and reflection stresses have a high degree of coincidence with those of the transmission stress, which can guarantee the stress balance state in the dynamic impact process and the validity of the test data results.

The dynamic stress–strain curves of the granite specimens affected by different temperatures at different impact rates are shown in Figure 4.

As shown in Figure 4, the dynamic stress–strain curves of granite can be roughly divided into three stages: elasticity, yield, and failure. The initial stage of the curve was approximately a straight line, indicating that granite had a strong linear elastic relationship during the initial stage of dynamic impact compression. The slope of the curve at this stage can be approximated as the initial elastic modulus of rock. The stress–strain curve at the same impact rate initially moved up slightly and then moved sharply to the lower right along with the position of the ascending curve of the temperature grade. The initial elastic modulus and peak stress of the rock first increased and then decreased. They reached the maximum at 200°C. The curve moved down significantly at 400°C than at normal temperature and 200°C, indicating that there is a threshold temperature for the deterioration of rock mechanical properties between 200 and 400°C. At 25–800°C, the curve gradually moved to the lower right. Thus, the continuous increase in rock failure strain indicates that the ductility of the rock was enhanced by high temperature, and it transitioned from brittleness to plasticity. With an increase in the strain rate at the same temperature level, the peak stress and elastic modulus of the rock have different degrees of buoyancy, which is an evident strain rate effect.

The development of fractures in rocks is the result of energy absorption (Xia et al., 2006; Zhao et al., 2019). Eq. 2 was used to calculate the incident, reflected, transmitted, and dissipated energies during the dynamic impact compression tests of granite, as presented in Table 1. W I = A C E ∫ 0 t ε i 2 ( t ) d ( t ) W R = A C E ∫ 0 t ε r 2 ( t ) d ( t ) W T = A C E ∫ 0 t ε t 2 ( t ) d ( t ) } , (2) where W I represents the incident energy; W R represents the reflected energy; and W T represents the transmitted energy; ε i ( t ) , ε r ( t ) , and ε t ( t ) are the incident, reflected, and transmitted strains on the bar, respectively; and A, E, and C represent the cross-sectional area, elastic modulus, and longitudinal wave velocity of the pressure rod, respectively.

The relationship among the incident, reflected, and transmitted energies under different working conditions is shown in Figure 5. At the same temperature, the energy levels of the incident, reflected, and transmitted energies increased with an increase in the impact rate. The growth rate was also positively correlated with the impact rate, exhibiting an evident strain rate effect. This conforms to the relevant laws of kinetic energy theorem (Shu et al., 2019). The influence of temperature was primarily reflected in the evolution of reflected and transmitted energies. With an increase in temperature, the transmission energy first increased and reached a maximum value at 200°C. When the temperature level exceeded 400°C, the transmitted energy gradually decreased with an increase in temperature; however, the effect on reflected energy was the opposite.

The influence of reflected or transmitted energy on granite specimens cannot be directly characterized by the influence of temperature on granite specimens during the crushing process. Xia et al. (2006) introduced the concept of crushing energy (Eq. 3) to characterize the energy dissipation during granite destruction. W S = W I − [ W R + W T ] , (3) where W S represents the dissipated energy, W I represents the incident energy, W R represents the reflected energy, and W T represents the transmitted energy.

Compared with other dissipated energies (such as heat energy), previous studies (Zhang and Jing, 2018; Shu et al., 2019) have considered dissipated energy as the main energy causing the propagation of crack and breakage of rock materials during the dynamic impact. In this case, the dissipated energy can be approximated as broken energy. The statistics of crushing energy under different working conditions are shown in Figure 6. Figure 6 shows that the dissipated energy has an evident strain rate effect and temperature effect on the failure process of the granite specimen. Dissipated energy increases with an increase in the strain rate and becomes a quadratic parabola with an increase in temperature. The overall evolution law of the crushing energy of granite during the dynamic impact was consistent compared to the dynamic compressive strength under different working conditions. This can also explain the strain rate and temperature effects on the dynamic compressive strength of granite from the perspective of energy dissipation.

The fracture morphology of the granite after high-temperature treatment is shown in Figure 7. The failure modes of the granite specimens exhibit obvious temperature and strain rate effects (Figure 7). With an increase in temperature and impact rate grades, both showed a trend of deepening crushing degree, increasing granular fragments, and gradually decreasing particles after crushing. Thus, it was concluded that the temperature and the impact rate influence the final failure mode of the granite specimen by affecting the energy dissipation during the impact. Under the same impact rate, the granite specimen gradually developed from splitting failure mode with a less fracture surface to compression failure mode, with an increase in temperature and strain rate grade.

The distribution of rock fragmentation reflects the overall effect on it under the combined influence of temperature and impact load (Xu and Liu, 2012). A standard circular hole screen was used to screen-crushed granite specimens. Statistical analysis was conducted to obtain the distribution of broken lumpiness in granite specimens at different impact rates and temperature levels. The method introduced by Xu and Liu (2012) was implemented to calculate the average fragmentation, dm, of the granite specimens using thefollowing equation (Figure 8): d m = ∑ ( r i d i ) / ∑ r i , (4) where d i is the average size of residual screen fragments (0.15, 0.4, 0.75, 1.75, 3.75, 7.5, 12.5, 17.5, 22.5, and 37.5 mm) and r i is the percentage of residual screen fragments in the total mass of fragments.

It can be seen from Figure 8 that the average fragmentation of granite ranges from 7.22 to 34.27 mm. At the same impact velocity, and an increasing temperature grade, the average fragmentation degree of granite increases gradually and reaches a maximum value between 200 and 400°C. When the heating temperature exceeded 400°C, the average fragmentation of granite began to decrease, reaching a minimum value at 800°C. This indicates that there exists a threshold temperature that affects the variation in the average fragmentation size of granite. In our experiments for the present study, the threshold temperature ranged between 200 and 400°C. Under the same heating temperature grade, the average fragmentation of granite showed a gradually increasing trend with an increase in the impact rate grade.

According to the fractal dimension, D b of the mass–frequency relationship of the screening test, and the calculation formula can be expressed as follows (Xu and Liu, 2012): lg ( M x 0 / M T ) = ( 3 − D b ) lg ( x 0 x m ) , (5) where M x 0 and M T are the cumulative mass of the material under the sieve and the total mass of fragments, respectively; x 0 and x m represent the size of the broken fragments and maximum size of the fragments, respectively.

The fractal dimension D b of granite fragmentary lumpiness can be obtained by linearly fitting M x 0 / M T = Y , x 0 / x m = X on the data points in the lgY–lgX-type logarithmic coordinate system. The logarithmic coordinate curve for the calculation of the dynamic impact fractal dimension of granite after high-temperature treatment is shown in Figure 9. The statistical results for the fractal dimension are listed in Table 2.

It can be observed from Table 2 that the fitting line has a good correlation. With an improvement in the impact rate grade, the correlation of the fitting increases continuously. In the dynamic compression test, the fractal dimension of the granite fluctuated between 2.0138 and 2.3640. The fractal dimension is not only related to the strain rate but also to the properties of the rock itself (Ji et al., 2020). Under the condition of a constant impact load, the change in fractal dimension with temperature is opposite to the peak stress change in the dynamic impact of granite. The smaller the peak stress of granite, the more severe the degree of breakage, and the larger the fractal dimension. The change in the fractal dimension indirectly reflects the influence of temperature and impact load coupling on the mechanical properties of granite (Xu and Liu, 2012).

To explore the quantitative relationship between the macro- and micro-characteristics of granite after high-temperature treatment, this study established the relationship between the fractal dimension theory and the associated characterization indexes in the energy dissipation theory. Figure 10 shows the relationship among average fragmentation, fractal dimension, and energy consumption density.

As shown in Figure 10, under the same temperature grade, the average fragmentation of granite decreases linearly with an increase in the energy consumption density. The fractal dimension increased linearly with energy consumption density. The smaller the mean fragmentation, the larger is the fractal dimension. This indicates that the crushing degree of granite was higher during the dynamic impact process. Rock failure is the process of the development, expansion, and coalescence of microfractures in the rock, which is the result of internal damage due to macroscopic failure. The development and propagation of microfractures require energy absorption. As energy consumption density increased, so did the energy absorbed by granite per unit volume during dynamic impact compression and the energy used for rock damage and fracture; this led to the development and expansion of microcracks in the rock. Consequently, the degree of breakage became more intense, the number of new cracks and fracture surfaces, output of small particle size fragments, and the fractal dimension increased, while the average fragmentation size decreased.

The failure process of the rock under a given impact load and temperature is caused by the damage to breakage (Yu et al., 2020). The calculation of the degree of rock damage in this process forms the basis for constructing a dynamic constitutive equation of rock materials under different working conditions. So far, some studies have discussed the measurement and calculation methods for the rock damage degree from the perspective of non-destructive testing and damage testing. Non-destructive testing is mainly carried out through CT scanning of the damaged rock specimens (Wang et al., 2018), measurement of longitudinal wave velocity (Zuo et al., 2017), and other tests in a manner such that the test objects are not damaged. In contrast, the damage testing method mainly involves destructive mechanical experiments on the tested specimen; this method characterizes the damage degree of rock materials using data obtained from experiments involving dynamic elastic modulus (Xu et al., 2020), dynamic compressive strength (Guo et al., 2017), energy dissipation (Zhao et al., 2019), breakage of the test specimen (Zhai, 2015), and development and expansion mode of cracks in the failure process (Li et al., 2020). Owing to the limitation of the experimental conditions, this study only considered the degree of damage from the perspective of the energy absorption value.

Based on the strain equivalence principle (He et al., 2018), Eq. 6 was used to characterize the temperature damage from the perspective of energy dissipation: D T ( W ) = W T − W 0 1 − W 0 , (6) where D T ( W ) represents the damage factor, W 0 represents the dissipated energy at 25°C, and W T represents the corresponding temperature dissipated energy.

It can be observed from Figure 11 that D T ( W ) of different temperature grades presents a quadratic non-linear correlation under the same impact velocity. When the heating temperature ranges from 25 to 200°C, D T ( W ) < 0 . This reflects the strengthening effect of temperature on the dynamic mechanical properties of granite. When the heating temperature level exceeds 200°C, D T ( W ) shows an increasing trend with an increase in the heating temperature level. The results indicate that temperature degrades the dynamic mechanical properties of granite over time, which is consistent with the effect of temperature on the dynamic compressive strength. However, with the increase in impact velocity in the test, the energy used for crushing the rock increased, and the coupled effect of impact velocity and temperature aggravated the internal damage of the rock.

X-ray diffraction (XRD) experiments were performed on granite at room temperature and high temperature to analyze the changes in the physical phase characteristics of granite before and after high-temperature treatment. The XRD patterns of granite at various temperatures are shown in Figure 12.

As shown in Figure 12, the mineral composition and primary components of granite tend to change at different temperature grades. The variation in the diffraction intensities of the main components is shown in Figure 13. As shown in Figure 13, the primary mineral components of granite at normal temperatures are mica, quartz, feldspar, and small amounts of silicate minerals. When the heating temperature was 200°C, the diffraction intensity of mica increased to 6,354 counts, which continued to decrease with an increase in the heating temperature. When the heating temperature was 800°C, the diffraction intensity was 432 counts. With an increase in the heating temperature, the diffraction intensity of feldspar first decreased and then increased. When the heating temperature was 400°C, the diffraction intensity reached a minimum of 2,958 counts. Overall, the diffraction intensity of quartz was correlated with the change in mineral compositions, and the minimum diffraction intensity was observed at 800°C.

When the heating temperature was 200°C, the main mineral components in granite at normal temperature were the same. However, the effect of temperature on the diffraction intensity varied significantly; that is, the diffraction intensity of mica was greater than that of the other main mineral components, which exhibited similar diffraction intensities. Meanwhile, the grain was not structurally damaged, and the rock compressive strength improved significantly. When the temperature rose to 400°C, the diffraction intensity of quartz increased due to the effect of high temperature on the main mineral composition and decomposition reaction of silicate minerals. Between 400 and 600°C, α-quartz becomes β-modification. Subsequently, the crystal expanded and led to the formation of small cracks inside the rock, thereby weakening the cementation force between grains and decreasing the compressive strength of granite; this result is consistent with the apparent morphology of the granite specimen after the high-temperature treatment (Figure 1). When the temperature reaches 800°C, feldspar changes from a crystalline state to an amorphous state (Jia et al., 2021). Thus, after the high-temperature treatment, the granite strength decreases, failure strain increases, and the diffraction intensity fluctuates significantly compared to that in the normal temperature state. A change in the crystal structures of feldspar and quartz deteriorated the mechanical properties of the granite specimens. Meanwhile, decreasing the diffraction intensity of mica promotes the brittle–plastic transformation of granite specimens to a certain extent, which is consistent with the results of macroscopic mechanical experiments.

A tungsten wire scanning electron microscope (SEM) was used to observe the fracture morphology of granite specimens after impact compression failure at 25–800°C, and study the influence of the impact rate and temperature on the fracture failure form of granite. The fracture morphologies are shown in Figure 14.

Figure 14 shows that under dynamic impact compression conditions, the fracture morphology of granite presents different failure forms under different working conditions (temperature grade and impact rate). At a constant temperature, intergranular fractures tend to be dominant when the impact rate is 11.5 m/s. With an increase in the impact rate, transgranular fractures increase to 200°C, as shown in Figure 14. When the temperature is 400°C, fine cracks appear on the rock. When the temperature rises to 600–800°C, plastic failure characteristics are observed (Liang et al., 2015; Tao et al., 2019).

A low shock rate, low incident energy, and the formation of a crack that extends along the crystal boundary lead to an intergranular fracture. Meanwhile, the impact velocity increases, leading to a rise in the incident energy and dynamic impact load time; in a short period of time, the internal atomic bonds in the crystal with low energy consumption cannot be damaged to produce a transgranular fracture. Although the internal mineral composition and granite microstructure are significantly correlated at high temperatures (such as between 25 and 400°C), the fracture morphology of the rock gives priority to brittle fracture, does not exhibit ductile fracture characteristics, and demonstrates a neat rock fracture morphology; meanwhile, the irregular fracture of quartz and feldspar led to a line shaped transgranular (conchoidal) fracture morphology, with a step cleavage plane fracture. During 600–800°C, the fracture morphology changed from a strip structure to a coarse block structure. The high temperature changed the rock’s internal structure, the sliding and separation phenomenon indicates the brittle-to-plastic transformation of granite, which was consistent with the macro mechanical characteristics.