1 Introduction

Temperature is one of the main factors affecting the mechanical properties of rocks (Wang 1995). Rock mass involved in projects such as nuclear waste disposal, geothermal energy generation, and underground development in large cities is generally under high temperature conditions. Engineers need to know the rock’s mechanical parameters for excavation of underground rocks, design of supports, and for stability analysis of the surrounding rock. Therefore, the strength of the rock and its deformation behavior at high temperatures need to be understood (Wai et al. 1982; Al-Shayea et al. 2000).

Since the 1970s, many researchers have investigated the effects of temperature on the mechanical properties of rocks (Yin et al. 2012a, b; Heuze 1983; Lau et al. 1995). Chen et al. (2012) measured the peak stress, peak strain, and elastic modulus of granite subjected to thermal treatment from 20 to 1,000 °C. They found that the peak stress and elastic modulus of heated granite decreased as the heating temperature increased, while the peak strain increased. Xu et al. (2008) studied the mechanical characteristics of granite under the action of temperatures ranging from room temperature to 1,200 °C and found that mechanical characteristics did not show obvious variations below 800 °C; strength decreased suddenly above 800 °C and bearing capacity was almost lost at 1,200 °C. Dwivedi et al. (2008) reviewed the thermo-mechanical properties of granites from India and other locations. Through a series of physical and mechanical tests on salt rock at different temperatures (20–240 °C), Liang et al. (2006) found that the ultrasonic velocity of the samples declined with rising temperature, while the uniaxial compressive strength and axial strain increased, whereas the tangent modulus had an opposite trend. Ferrero and Marini (2001) applied microscopic analysis to study crack densities in limestone and marble samples at temperatures up to 600 °C. They found a correlation between the increase in open porosity due to new fractures and the crack density for both rocks. Zhao et al. (2012) studied the thermal deformation and failure mode of large-size granite specimens at high temperatures and pressures, and obtained the behavior of the thermodynamic parameters of the specimens, such as Young’s modulus, for various temperatures. Other studies (Zhang and Mao 2009; Mao et al. 2009; Luo and Wang 2011; Wu et al. 2005) concluded that the strength of most rocks decreased with increasing temperatures and that the drop in strength depended on the rock type.

Although much knowledge has been gained through theoretical and experimental studies of granite, salt rock, and sandstone, few experimental studies on the mechanical properties of coal measures mudstone at real-time have been carried out because of limitations due to experimental conditions. In our study, we explored the mechanical properties of mudstone at high temperatures under uniaxial compression using the MTS810 electro-hydraulic servo system and the MTS652.02 high-temperature furnace with temperatures up to 800 °C.

2 Experimental Method and Procedure

The specimens were drill cores from the Zhangshuanglou mine in the Peixian region, Jiangsu Province, China. They comprised 50.3 % kaolinite, 25.6 % quartz, 4.5 % illite, 4.5 % green cone stone, and 12.4 % siderite along with small amounts of other minerals.

The rock specimens were 45 mm in height and 20 mm in diameter. The two ends of each specimen were polished with a grinder to ensure that the parallelism error between the two ends of the sample was less than 0.05 mm. Twenty-one specimens were divided evenly into seven groups. The sample groups were subjected to temperatures at 25, 100, 200, 400, 600, 700, and 800 °C.

The experiments were performed by an MTS810 electro-hydraulic servo system and an MTS653.02 high-temperature furnace, as illustrated in Fig. 1. The whole experimental process was automatically controlled by the Teststar II control system at the seven temperature levels listed above. The main menu provides the flexibility of choosing the distributing sensors, defining the control model, setting the boundary conditions, and automatically zeroing the sensing element. Moreover, the output signals could be selected and some parameters could be set if necessary. The system software included a graphical user interface, a data interface, a software function generator, and a program design and system tool.

Fig. 1
figure 1

The MTS810 hydraulic servo system and MTS653.02 high-temperature furnace

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The experimental procedure was as follows:

  1. 1.

    Activate the MTS810 electro-hydraulic servo test system by switching on the oil pumps of the experimental system.

  2. 2.

    Place the mudstone specimen into the indenter of the testing machine, adjust the position, and close the high-temperature furnace.

  3. 3.

    Heat up the rock specimen. The specimen was heated to the predetermined temperature at a rate of 2 °C/s. Uniform heating of the rock sample was ensured by maintaining the temperature constant for 20 min before commencing with the measurements.

  4. 4.

    Apply the load to the specimen. In a uniform temperature environment, the electro-hydraulic servo displacement control was used to apply the load, thereby increasing the displacement at a rate of 0.003 mm/s. This was continued until the specimen was destroyed.

We completed the experimental process using the Teststar II control program and obtained the mechanical parameters such as axial load, displacement, and stress and strain.

3 Deformation Characteristics at High Temperatures

The stress–strain curves of the mudstone samples at high temperature were determined by a uniaxial compression test (Fig. 2). The elastic modulus was calculated by fitting the experimental stress–strain data before the peak strength using an approximate straight line. The experimental and fitting results, including elastic modulus, peak strength, and peak strains of the rock specimens, are listed in Table 1.

Fig. 2
figure 2

Axial stress, \( \sigma \), strain, \( \varepsilon \), curves showing repeatability using three samples at each temperature

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Table 1 Experimental results showing the main mechanical characteristics of the mudstone specimens
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3.1 Stress–Strain Relation at High Temperature

The stress–strain curves indicate that (1) for temperatures below or equal to 400 °C, the features of the stress–strain curves are similar to those at 25 °C. To some degree, they experience the compression phase, the elastic deformation phase, and the destruction phase; and (2) above 600 °C, besides the compression stage and the deformation stage, the stress–strain curves also show the nonlinear stage, the strain-softening stage, and to some extent the residual strength stage.

Figure 2 shows that (1) between 25 and 400 °C, the residual stress is very small or even zero when the axial stress drops rapidly to zero. For temperatures of 600–800 °C, the specimen exhibits a residual stress after the peak value; and (2) when the temperature is over 600 °C, there are many drops in the stress–strain curve, especially at 800 °C. Chemical transformation in the mineral composition and changes in the mineral distribution with increasing temperature contribute to the variety of stress–strain curves for the different temperature ranges. From the microstructure view, when the temperature is below 400 °C, the mineral composition is stable and evenly distributed, and the mudstone samples break along their main weak structural planes. Once the temperature reaches 400–600 °C, the mineral composition changes because of the emergence of the feldspar group minerals and the dehydration of kaolinite. Consequently, the uniformity and pore orientation deteriorate and the rock ruptures along the partial weak structure planes as well as the main weak structural planes. Between 600 and 800 °C, with the dehydration effect and kaolinite transforming to illite, groups with parallel orientation, weak structural planes are formed, so that the whole stress–strain curve is more disordered than it is at 400 °C (Zhang 2012).

3.2 Variation of Elastic Modulus with Temperature

The variation of the elastic modulus E with temperature is shown in Fig. 3. When the temperature is below 400 °C, the elastic modulus of mudstone increases considerably as the temperature rises. At the temperature range of 25–400 °C, the elastic modulus of mudstone rises from 15.1 to 23.9 GPa, an increase of about 79 %. The gradient of the elastic modulus also varies, with the largest gradient occurring between 25 and 100 °C. However, when temperatures are above 400 °C, the elastic modulus of mudstone decreases as the temperature increases. This downward trend slows down for temperatures above 600 °C. At 400–800 °C, the elastic modulus falls from 23.9 to 5.8 GPa, a decrease of 76 %.

Fig. 3
figure 3

The elastic modulus, E, as a function of temperature, T

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The main factors affecting the variation of the elastic modulus with temperature include:

  1. 1.

    Thermal evaporation: when the temperature varies from 100 to 400 °C, the volatilization of water from the mineral surface and gaps creates more void space for compaction. Under axial compression, the micro-cracks become smaller, or even completely close. Consequently, the density of the specimens rises after the stress state reaches the elastic stage. Therefore, the combined action of thermal evaporation and axial compression increases the density of the rock and its ability to resist deformation.

  2. 2.

    Thermal cracking: the mineral composition of mudstone varies, and the thermal expansion coefficients vary depending on the temperature and the type of mineral grain. Different thermal expansion deformation in the different mineral grains causes the thermal stresses in the mineral structure to rise. When the temperature changes from 400 to 600 °C, new micro-cracks appear when the structure thermal stress exceeds the peak strength. The existence of the crack and the irreversible micro-crack extension lead to a change in the microstructure and an obvious break in the structure, causing a sharp decrease of the elastic modulus.

  3. 3.

    Thermal softening: when mudstone experiences high temperature (above 600 °C), the mineral grain cohesion is weakened and the mineral grains begin to slip under the effect of high temperature, resulting in a reduction in the elastic modulus.

4 Strength Characteristics under High Temperature

We used the high temperature stress–strain curves to obtain the peak stress \( \sigma_{\text{c}} \) and the peak strain \( \varepsilon_{\text{c}} \) of various specimens.

4.1 Variation of the Peak Stress

The peak stress \( \sigma_{\text{c}} \) as a function of temperature is shown in Fig. 4. The peak stress of mudstone has an approximately exponential relation with temperature below 400 °C. Between 25 and 400 °C, the peak stress of mudstone jumps from 90.6 to 252.2 MPa, increasing by 161.6 MPa. This is because the evaporation of the water and gas present in the mudstone reduces the lubricating effect of the relative sliding between the rock particles. Meanwhile, the original cracks close because of the thermal expansion of the internal mineral particles. This reduces the number of micro-cracks, improves the densification, and enhances the peak strength. Thus, high temperatures have a significant impact on the strength of mudstone, enhancing it when the temperature is below 400 °C. For temperatures above 400 °C, the peak stress \( \sigma_{\text{c}} \) decreases with temperature, following an approximately negative exponential function. In the range 400–600 °C, the peak stress varies rapidly, dropping from 252.2 to 69.9 MPa, a decrease of 182.3 MPa. This is because mudstone is a classical clay stone; when the temperature reaches 600 °C, some minerals begin to melt, decompose, and evaporate, producing new micro-cracks in the specimens. The primary and secondary cracks then extend, causing the strength of mudstone to fall sharply (Yin et al. 2012a, b). When the temperature exceeds 600 °C, the peak stress of mudstone decreases continuously but the rate slows down with increasing temperature. Further studies will be required to determine whether the observation of a sharp reduction in stress strength at 600 °C is an indication that this temperature is the critical temperature for the microstructure change of mudstones.

Fig. 4
figure 4

The variation in peak stress, \( \sigma_{\text{c}} \), as a function of temperature, T

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4.2 Variation of the Peak Strain

The variation in the peak strain of mudstone with temperature is shown in Fig. 5. Within the range of 25–100 °C, the peak strain varies only slightly with increasing temperature. At temperatures between 100 and 400 °C, the peak strain will follow a near-linear trend with rising temperature, increasing from 0.6456 × 10−2 to 1.0964 × 10−2, a change of about 70 %. This is because the loss of internal water and the expansion of particle volume in the mudstone increase the internal cracking and holes, leading to greater deformation in the compaction stage under stress at 100–400°. However, at 600 °C, the peak strain drops to 0.8214 × 10−2. When the temperature further increases to 700 and 800 °C, the peak strain reaches 1.2652 × 10−2 and 1.2692 × 10−2, respectively. This is because under high temperature (600–800 °C), the mineral composition varies significantly in certain conditions such as loosing of constituent water, recrystallization, and phase transitions, all of which lead to the fluctuation of the peak strains (Zhang 2012).

Fig. 5
figure 5

The variation in peak strain, \( \varepsilon_{\text{c}} \), as a function of temperature, T

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5 Conclusions

In this work, the mechanical properties of mudstone at high temperatures were tested using the MTS810 electro-hydraulic servo controlled system and the MTS652.02 high-temperature furnace. After analyzing the stress–strain curves, the deformation characteristics, and the strength of mudstone at high temperatures, we drew the following conclusions:

  1. 1.

    The stress–strain curves indicate four stages: a compaction stage, elastic deformation stage, a nonlinear deformation stage, and the destruction stage.

  2. 2.

    The elastic modulus and peak strength of mudstone first increase but then decrease with rising temperatures. From 25 to 400 °C, the elastic modulus jumps from 15.1 to 23.9 GPa and the peak strength grows from 90.6 to 252.2 MPa. When the temperature changes from 400 to 800 °C, the elastic modulus and peak strength of the mudstone decrease to 5.8 and 90.6 MPa, respectively.

  3. 3.

    The peak strain of mudstone increases with rising temperature until a reduction is noted around 600 °C. As the temperature reaches 800 °C, the peak strain soars rapidly from 0.6456 × 10−2 at 25 °C to 1.2692 × 10−2 at 800 °C, a change of about 97 %.

  4. 4.

    Mudstone is a typical clay stone. For temperatures below 400 °C, the deformation due to thermal expansion closes the micro-cracks. The shrinkage of the pores increases the density and improves the contact between the mineral grains. This increases the friction between the grains, and therefore, enhances the ability to resist deformation. At temperatures above 400 °C, the thermal stress and transformation in the mineral composition and microstructure create micro-cracks, which cause the deformation of the specimens and weaken their load-bearing capacity and their ability to resist deformation.