Effects of High Temperature Treatments on Strength and Failure Behavior of Sandstone under Dynamic Impact Loads

Abstract

With the increasing demand for resource consumption, the mining depths gradually increase, resulting in increases of temperature at tens or even hundreds of degrees. High temperature could damage to interior structures and alter the mechanical properties of rock mass. Therefore, studying the effects of temperature on dynamic mechanical properties and failure behaviors are of great significance for deep resources exploitation. In this study, to study the effects of high temperature treatment on the strength and failure behavior of typical sandstones, specimens were prepared and heated to different degrees. The longitudinal wave velocity, volume, and density of specimens before and after high-temperature treatment were examined. Then, the Thomas Hopkinson (SHPB) test was conducted on specimens with different air pressures and the dynamic stress-strain curve, peak stress, peak strain, and other dynamic characteristics were obtained. The variations regulations and failure behavior of sandstone under the effects of high-temperature treatments and different impact loads were analyzed and discussed from the aspects of stress-strain, peak strength, and peak strain. It was observed that with the increase of heating temperature, the average density, average wave velocity, peak strength, and average elastic modulus of the sandstone specimens all showed a decreasing trend and the highest decreasing rated occurred at the temperature between 600 °C and 800 °C. The obtained results provided a certain theoretical basis for deep mine exploitation, especially for mines with high temperature.

1. Introduction

Rock is a kind of discontinuous medium, and there are a large number of primary structural fractures, mineral particles, and micro voids [1,2,3,4]. The mechanical properties of rock are affected by environmental factors such as high temperature, stress and groundwater [5,6,7,8,9,10]. With the increasing demand for energy and resource exploitation, the exploitation of deep underground resources is increasing [11,12,13]. Compared with the shallow strata, the deep strata have more geological conditions and are in the state of ‘three highs’ which are high temperature, high pressure, and high-water pressure [14,15,16]. The temperature in underground conditions gradually increases with the increase of depth, which can reach tens or even hundreds of degrees [17,18,19,20]. The high temperature and pressure of deep rock strata lead to a certain degree of internal damage, which further leads to the nonlinear deformation or zoning failure characteristics that significantly different from shallow strata conditions [21,22,23]. Therefore, it is of great engineering significance for deep resource exploitation to study dynamic mechanical properties of rocks under high temperature conditions.

The macro and static mechanical properties of different types of rocks after high temperature treatments were studied. David et al. [24] measured the longitudinal wave velocity and porosity of granite specimen after heating in temperature between 200 °C and 400 °C. They concluded that the porosity of granite slightly increased while the longitudinal wave velocity decreased continuously with the increase of temperature. The high temperature increased the porosity and decreased the compactness of the internal structure of the granite. With the increase of temperature, the cutting rate of the granite increased, and the energy consumption led the rock failure to decrease gradually. The uniaxial compressive strength and average modulus of the sandstone and coal after high-temperature treatment were examined [25]. It was found that the mechanical properties of sandstone deteriorated obviously after the temperature exceeded 400 °C [26]. In the aspect of dynamic tests on rock mediums, Chiddister and Malvern [27] used the Thomas Hopkinson (SHPB) test to study and analyze the dynamic mechanical properties of materials, which laid a foundation for the application of SHPB technologies. Price and Knill [28] carried out dynamic splitting experiments on green sandstone and granite, which showed that the dynamic tensile strength of two type of rocks was obviously higher than that of static state. Kumar [29] used the SHPB system to study the dynamic strength of rocks, which promoted the application of SHPB technique in the field of rock mechanics. Friedman M [30] carried out uniaxial compression tests on granite using the SHPB device, and concluded that the dynamic compressive strength of granite increases with the increase of the ballast rates. The aforementioned studies were mainly focused on the effects of temperature on macro and static rock or the dynamic mechanical properties of the rock specimens, but few studies have combined both factors and studied the effects of high temperature on dynamic damage and failure characteristics of rock under dynamic load conditions. As the mining depth goes deeper, studying the effects of the high temperature and pressure of deep rock strata on the dynamic mechanical properties of the rock is of great significance for deep resource exploitation.

In this study, to study the dynamic damage and failure mechanism of rock under high temperature, the longitudinal wave velocity, volume, and density of the specimens after high temperature treatment were measured. The stress-strain curve, peak stress, peak strain, and other dynamic mechanical parameters of the specimens after high temperature treatment were obtained by SHPB tests. The dynamic damage regulation and failure behavior of sandstone under different air impact loads and temperature treatments were analyzed and discussed.

2. Materials and Methods

2.1. Specimens Preparation

As the most common rock type in strata of coal mines, the dynamic damage and failure characteristics of sandstone directly affect the exploitation and utilization efficiency of coal resources. Therefore, the sandstone was selected for laboratory testing. The sandstone samples used were collected from a deep buried coal mine in China which has stable structure and are generally uniform in texture. The sandstone is cyan white and the mineralogical compositions are studied by the X-ray diffraction technique. The main components include quartz, albite, laumontite and clay. The specimens were drilled to a cylinder with an inner diameter of 50 mm which were then cut to a height of 100 mm. Both ends of the specimens were finely ground and polished using a face grinder, and the flatness errors did not exceed 0.1 mm. The polished specimens were measured with vernier calipers to ensure that the specimen size was consistent and met the requirements recommended by ISRM.

2.2. Testing Procedures

Before the test, each specimen was marked and then divided into groups which were then heated at the temperature gradient of 25 °C, 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C. Each group included three specimens and each group was heated at a certain temperature in electric heating furnace for 3 h. Then, the heating furnace was turned off and natural cooling was performed in furnace. After the heating treatment was completed, the specimens were taken out and stored in sealed bags to avoid influences of moisture in the air. The heating process and the specimens after high temperature treatment are shown in Figure 1. As can been seen in Figure 1, the appearance of the specimens changed from green to yellow under the action of high temperature from 25 °C to 500 °C.

To analyze the change of physical parameters of the specimens after high temperature treatment, the mass, volume, and wave velocity of the specimens treated at different temperatures were measured. A HS-YS301t non-metallic ultrasonic analyzer was used to test the longitudinal wave velocity of specimens (Figure 2). For the wave velocity examinations, Vaseline was applied between the sensor and specimens as a coupling agent. The prepared specimens were clamped at both ends by the longitudinal wave transducer of the acoustic emission detector. The position of the specimens was fixed to ensure the axes of the transducers and specimens were aligned. The damage caused by high temperature was analyzed and calculated by characteristics of the collected wave parameters.

After the wave velocity measurements, the SHPB tests were carried out on specimens using the SHPB loading device (Figure 3). The test device consists of four major parts which are the dynamic impact system, guide bar system, buffer system and data collection system. The SHPB test system with a 100-mm rod diameter was used to investigate the dynamic compression of sandstone specimens after heat treatment. The length of the incident rod and the transmission rod are 5.0 m and 3.0 m, respectively. The parameters of the SHPB test system are provided in

Table 1. Parameters of the SHPB experimental system.

Bar Diameter	Incident Bar Length	Transmission Bar Length	Poisson’s Ratio	Elastic Modulus	Longitudinal Wave Velocity
100 mm	5000 (mm)	3000 (mm)	0.28	210 (GPa)	4910 (m/s)

. To investigate the effects of air impact pressure on failure behavior of the sandstones, different impact pressure of 0.3 MPa, 0.4 MPa and 0.6 MPa were used. To reduce the friction effect between specimen and rod, butter was applied on the contact surface.

2.3. Statistical Analysis Methods

For fully elastic materials, the stress-strain relationship should satisfy the relationship described in Equation (1). In the equation, $\sigma$ is stress of rock specimen, E is the elastic modulus of rock specimen and $\epsilon$ is strain of rock specimen without considering damage. The damage variable D of rock specimens can be expressed by the elastic strain method. The stress-strain relationship of the damaged sandstone specimen satisfied Equation (2). E(1-D) is the elastic modulus of damaged rock specimen which is labeled as $\widehat{E}$. Then, Equation (2) can be converted to Equation (3).

$$\sigma =E{\epsilon }_{1\mathrm{c}}$$

(1)

$$\sigma =E\left(1-D\right){\epsilon }_{2\mathrm{c}}$$

(2)

$$D=1-\widehat{E}/E$$

(3)

The relationship between the longitudinal wave velocity $V_p$ and the elastic coefficient of the medium can be described by Equation (4). In the equation, $V_p$ is the longitudinal wave velocity after high temperature treatment and ρ is the density of the material. Combining Equations (3) and (4), Equation (5) can be obtained. In the equation, $V_{\mathrm{f}}$ is the average longitudinal wave velocity of the specimen before high temperature treatment.

$$V_{\mathrm{p}}=\sqrt{E/\rho }$$

(4)

$$D\approx 1-{\left\{V_{\mathrm{p}}/V_{\mathrm{f}}\right\}}^2$$

(5)

3. Results and Analysis

3.1. Physical and Damage Effects

After high temperature heating treatment, the physical parameters of specimens, including height, diameter, and mass of the specimens, were measured, with the results recorded in groups (

Table 2. Physical parameters of specimens after heat treatment.

Specimen ID	Mass (g)	Diameter (mm)	Height (mm)	Density (g/cm3)	Average Density (g/cm3)	Temperature (°C)
25-1 25-2 25-3	469.82 469.69 464.67	48.34 48.65 48.88	97.34 98.74 98.65	2.630 2.560 2.510	2.567	25
100-1 100-2 100-3	469.07 469.60 469.29	49.80 47.79 48.22	99.30 99.11 98.45	2.425 2.640 2.610	2.558	100
200-1 200-2 200-3	465.24 467.52 460.97	47.88 48.89 49.11	98.34 98.74 97.99	2.627 2.522 2.482	2.543	200
400-1 400-2 400-3	464.62 465.18 466.94	48.35 48.66 48.98	99.13 97.65 98.55	2.551 2.562 2.514	2.542	400
600-1 600-2 600-3	467.32 468.11 466.49	48.32 49.32 49.74	96.36 97.49 99.10	2.645 2.514 2.422	2.527	600
800-1 800-2 800-3	458.97 468.44 469.92	48.99 47.74 49.95	99.77 98.12 98.60	2.440 2.668 2.432	2.513	800
1000-1 1000-2 1000-3	468.02 469.57 469.59	50.11 49.55 49.07	97.82 98.43 98.60	2.425 2.473 2.517	2.472	1000

). The relationships between the average density of specimens with heating temperature are shown in Figure 4. The average densities of specimens decreased with the increases of heating temperature, and the mass of specimens was continuously lost under the action of high temperature. The density change of specimen was different in each temperature interval. The density decreased obviously after 25 °C and 200 °C, which may be due to the evaporation of water inside the specimen. After the heating temperature of 200 °C and 400 °C, the density of the specimen basically remained unchanged. At the heating temperature between 400 °C and 1000 °C, the average density showed an obvious decreasing trend again. The decreasing rate of density at the temperature of 800 °C and 1000 °C was obviously higher than that in the range of 400–800 °C. The average density of specimens after heating at 1000 °C was 96.2% of the average density at normal temperature. The maximum density loss after heat treatment reached around 4%, which indicated the significant influence of temperature on the density of the rock materials.

Based on Equation (5), the longitudinal wave velocity and average damage variations of specimens after high temperature treatment were obtained, with the results shown in

Table 3. Wave velocity, elastic modulus and damage variable parameters of the specimens.

Specimen ID	Wave Velocity (km/s)	Average (km/s)	Variation (%)	Elastic Modulus (GPa)	Average (GPa)	Damage Variation (%)
25-1 25-2 25-3	2.666 2.624 2.695	2.662	0	18.69 17.63 18.23	18.18	0
100-1 100-2 100-3	2.645 2.439 2.545	2.543	−4.5	18.50 14.96 15.69	16.38	8.7
200-1 200-2 200-3	2.439 2.563 2.433	2.478	−6.9	14.51 17.53 14.40	15.48	13.3
400-1 400-2 400-3	2.398 2.411 2.369	2.393	−10.1	13.94 15.35 14.65	14.65	19.2
600-1 600-2 600-3	1.898 2.242 2.252	2.131	−20.0	9.46 12.68 12.59	11.58	35.9
800-1 800-2 800-3	1.263 1.295 1.295	1.284	−51.8	4.07 4.30 4.22	4.20	76.7
1000-1 1000-2 1000-3	0.745 0.904 0.724	0.791	−70.3	1.35 2.02 1.32	1.56	91.2

. The variation trend of the average wave velocity and reduction rates are shown in Figure 5, which clearly reflected the relationship between wave velocity variation and temperature. The longitudinal wave velocity of the specimen showed a downward trend with the increase of heating temperature. At the heating temperature between 25 °C and 600 °C, the longitudinal wave velocity decreased slowly. When the temperature was higher than 600 °C, the longitudinal wave velocity decreasing rates were relatively higher. At the temperature between 800 °C and 1000 °C, the decreasing rate of longitudinal wave velocity was again slower than that between 600 °C and 800 °C, but the overall decreasing rate was still higher than that between 25 °C and 600 °C. At the heating temperature of 1000 °C, the average wave velocity of the specimens was only 29.7% of that at normal temperature, which indicated the significant influence of temperature on the physical properties of specimens.

The change trend of average elastic modulus and damage variation of the specimens with temperature are shown in Figure 6. In the heating range from 25 °C to 400 °C, the average elastic modulus and damage variation of specimens gradually decreased. At the heating temperature of 400 °C, the damage variation of the specimens was about 19.2% of that at normal temperature. In the heating range from 400 °C to 800 °C, the average elastic modulus of the specimens decreased rapidly while the damage variable value increased rapidly. When the heating temperature was 800 °C, the damage variation reached 76.7% of that at normal temperature. At the temperature between 800 °C and 1000 °C, the decline rate of average elastic modulus was slower than that between 600 °C and 800 °C. When the heating temperature was 1000 °C, the damage variation of the specimens reached 91.2%. Generally, under the heating temperature between 600 °C and 800 °C, the damage variation increased fastest, and the bearing capacity was the most unfavorable.

3.2. SHPB Testing Results

For SHPB tests, three different air pressure impacts were set for the specimens heated at various temperature conditions. The impact air pressure impacts were set to 0.3 MPa, 0.4 MPa and 0.6 MPa, respectively. The stress-strain curves of the specimens under different impact pressures after different temperature treatments are shown in Figure 7. Generally, as can be seen in all figures when the stress reached the maximum, the slope of the stress-strain curve was approximately a straight-line segment, which indicated the stress showed an approximate linear relationship with the strain. This stage could be regarded as the elastic stage. Once the maximum stress reached, the stress decreased sharply with the increase of strain. At this stage, the generation of new cracks and the continuous evolution of original cracks leaded to the failure of the specimens. In Figure 7a,b, it can be observed that under the air pressure impacts of 0.3 MPa and 0.4 MPa, the peak stress of the specimens at normal temperature was the highest among all temperatures. While under the air pressure impacts of 0.6 MPa, the peak stress of the specimens heated at 200 °C was the highest among all temperatures. On the whole, the trend of the peak stress of specimens with temperature showed a certain regularity. Under the impact of all different impact pressures, the peak stress of the specimens after the treatment of 1000 °C was the smallest. This was because after the specimens heated at 1000 °C, the internal cracks increase, resulting in great damage and destruction of the specimens. The internal damage of the specimens led to the rapid reduction of cohesion and significant deterioration of the mechanical properties of the specimens.

The variation curve of peak strain of specimens with temperature under different impact pressures is shown in Figure 8. As shown in Figure 8, at the impact pressure of 0.3 MPa, the peak strain of the specimens at high temperature from 25 °C to 600 °C decreased with increase of temperature. The peak strain of the specimens after high temperature treatment at 600 °C decreased by 30.7% compared with that at normal temperature. At the temperature range from 600 °C to 1000 °C, the peak strain of the specimens showed a slight increase at 800 °C, then decreased at 1000 °C. The variation trend of the peak strain at the air impact pressure of 0.4 MPa was almost the same as that at 0.3 MPa, except for a slight difference from 25 °C to 200 °C. From 25 °C to 400 °C, the peak strain of the specimens showed rapid decline trend at 200 °C then slight increases. The peak strain of specimens under the heating condition of 200 °C was 39.2% lower than that under normal temperature. Compared with air impact pressure of 0.3 MPa and 0.4 MPa, the peak strain of the specimens under air impact pressure of 0.6 MPa showed a completely different law with the increase of temperature. The peak strain of the specimens showed increases from 25 °C to 200 °C, then sharp decreases and was lowest at the temperature of 600 °C. Similar to the air impact pressure of 0.3 MPa and 0.4 Mpa, the peak strain of the specimens increased at 800 °C, then slightly decreased at 1000 °C. Under air impact pressure of 0.6 Mpa, the peak strain of the specimens showed the largest response at 200 °C and the smallest at 600 °C, with a difference of nearly 76.9%.

The curve of peak stress versus temperature under different impact pressures is shown in Figure 8. Under the three different impact pressure conditions, the variation trend of peak stress with temperature was basically consistent. The peak stress had a tendency to decrease with the increase of temperature. At the impact pressure of 0.6 MPa, the peak stress under normal temperature is 192.5 MPa, which decreased by 16.8% to 160.1 MPa under the temperature of 600 °C. At 800 °C, the peak stress was 145.5 Mpa with the decrease rate was 24.4%, while the peak stress was 61.1 MPa with the decrease rate of 68.3% at 1000 °C. At the impact pressure of 0.4 MPa, the peak stresses were 170.1 MPa at 25 °C and 119.7 MPa at 800 °C. The peak stress after 1000 °C was 40.7 MPa, and the reduction rate was 76.7%. At the impact pressure of 0.3 MPa, the peak stress of specimens was 150.0 MPa at normal temperature, which was 23.3 MPa at temperature of 1000 °C with reduction rate at 84.5%. The peak strength decreased with the increase of the impact pressure at the same temperature and decrease rates were generally higher at the temperature range from 600 °C to 1000 °C.

4. Discussions

Generally, under the coupling action of high temperature and impact load, the stress-strain curves of sandstone specimens were consistent, which could be divided into compaction stage, elastic stage, yield stage and failure stage. The average peak stress decreased with increase of temperature, and the decrease rates were the largest in temperature range from 600 °C to 1000 °C. This observation was consistent with the relatively higher decreasing rate of average density of specimens at the temperature of 800 °C and 1000 °C. The average density after heating at1000 °C was 96.2% of the average density at normal temperature, and the significant decrease in density may account for high decreases of the mechanical properties of the specimens. Additionally, in the heating range from 400 °C to 1000 °C, the average elastic modulus of the specimens decreased rapidly, and the damage variable value increased rapidly. At 800 °C, the damage variation reached nearly 76.7% of that at normal temperature and increased to 91.2% at 1000 °C. At the temperature of 800 °C and 1000 °C, the damage variation of the specimens increased fastest and the dynamic bearing capacity of the specimens was the most unfavorable.

The heating process may affect the physical properties of specimens through internal reactions, such as adsorption water of internal particles of minerals and separation of interlayer water, thermal expansion, structural dehydration, recrystallization, and phase transformation of clay minerals [31]. The above-mentioned internal reactions increased the internal microcracks and porosity of the specimens, which ultimately leaded to the decrease of longitudinal wave velocity. The values of mass loss of specimens increased with the increases of the treating temperatures, leading to the decrease of density of specimens. The continued deterioration of specimens indicated that with increase of temperature, the internal deterioration of specimens was intensified. When temperature was below 600 °C, the separation of adsorbed water and interlayer water as well as thermal expansion of clay minerals mainly occurred. The separation of water caused a loss of bearing structure in specimens, and the thermal expansion of clay minerals leaded to some microcracks. Both of the actions led to a reduction of strength as well as the elastic modulus. The continued deterioration of specimens indicated that with the increase of temperature, the internal deterioration of specimens was intensified. When temperature was below 600 °C, the separation of adsorbed water and interlayer water as well as thermal expansion of clay minerals mainly occurred. The separation of water caused a loss of bearing structure in specimens, and the thermal expansion of clay minerals led to some microcracks. Both of the actions led to a reduction of strength, but the reduction was not significant, and the deterioration tended to decrease slowly. When the temperature was above 600 °C, the interiors of specimens were mainly clay mineral thermal expansion, structural dehydration, recrystallization, and phase transformation. A large number of micro internal cracks were generated, leading to seriously deteriorated and decreased bearing capacities [32]. It was suggested that at the heating temperatures lower than 600 °C, separation interlayer water mainly occurred, and the reduction of strength was not significant. When temperature was higher than 600 °C, thermal expansion of clay mainly occurred, which generated micro internal cracks and led to the significant reduction of mechanical properties. Additionally, the average wave velocity of the specimens decreased significantly. With the increase of temperature, the internal thermal stress gradually increased, which led to the incoordination of thermal expansion of various mineral particles and microcracks between particles. The primary and new cracks further expanded and connected, which directly affected propagation velocity of acoustic wave and led to the continuous deterioration of the dynamic mechanical properties of specimens [33]. On the other hand, some components of the specimens itself may be degraded due to chemical reactions after high-temperature treatment, resulting in significant changes in the internal structure of the rock and a decrease in the acoustic wave velocity [34].

5. Conclusions

The influences of the high-temperature and pressure on the dynamic mechanical properties of the rock were examined through conducting the SHPB tests with different air pressures. The variations regulations and failure behavior of sandstone under the effects of high-temperature treatments and different impact loads were analyzed and discussed from the aspects of stress-strain, peak strength, and peak strain. The influence of temperature on physical properties of sandstones was evaluated through the variation of density and wave velocity measurement. The major finding of the studies can be summarized as follows:

• With the increase of heating temperature, the average density, average wave velocity, and average elastic modulus all showed a decreasing trend and the highest decreasing rates occurred at temperature between 600 °C and 800 °C.
• The temperature treatment and impact loads had obvious effects on dynamic mechanical properties and damage degree of the specimens. Under the action of high temperature and impact load, the average peak stress and modulus decreased with increase of temperature, and peak stress decreased fastest in the range from 600 °C to 1000 °C.
• It was suggested that when the temperature was below 600 °C, the separation of adsorbed water and interlayer water as well as thermal expansion of clay minerals mainly occurred. The microcracks generated led to the slight deterioration of the specimens. When the temperature was above 600 °C, a large number of micro internal cracks were generated, leading to seriously deteriorated and decreased bearing capacities.
• The obtained results provided a certain theoretical basis for deep mine exploitation, especially for the mines with high temperature.