The Influence of Periodic Temperature on Salt Rock Acoustic Emission, Strength, and Deformation Characteristics

Abstract

During the long-term operation of salt cavern gas storage, multiple injections and extractions of gas will cause periodic temperature changes in the storage, resulting in thermal fatigue damage to the surrounding rock of the salt cavern and seriously affecting the stability of the storage. This article takes the salt rock samples after thermal fatigue treatment as the research object, adopts a uniaxial compression test, and combines DIC and Acoustic Emission (AE) technology to study the influence of different temperatures and cycle times on the mechanical properties of salt rock. The results indicate that as the number of cycles and upper limit temperature increase, thermal stress induces continuous propagation of microcracks, leading to continuous accumulation of structural damage, enhanced radial deformation, and intensified local displacement concentration, causing salt rock to enter the failure stage earlier. The initial stress for expansion and the volume expansion at the time of failure both show a decreasing trend. After 40 cycles, the compressive strength and elastic modulus decreased by 23.8% and 27.4%, respectively, and the crack failure mode gradually shifted from tension-dominated to tension-shear composite. At the same time, salt rock exhibits typical “elastic-plastic creep” behavior under uniaxial compression, but the uneven expansion and thermal fatigue effects caused by periodic temperature changes suppress plastic slip, resulting in an overall decrease in peak strain energy. The proportion of elastic strain energy increases from 21% to 38%, and the deformation process shows a trend of enhanced elastic dominant characteristics. The changes in the physical and mechanical properties of salt rock under periodic temperature effects revealed by this study can provide an important theoretical basis for the long-term safe operation of underground salt cavern storage facilities.

1. Introduction

During the long-term geological evolution, saturated multi-source brines undergo processes such as concentration, crystallization, sealing, and compaction to ultimately form salt rocks [1]. Salt rock, due to its low permeability, plasticity, and self-healing properties [2,3], has become an ideal medium for underground strategic energy storage, particularly suitable for constructing large-scale underground gas storage facilities [4,5] to meet the requirements of long-term stability and sealing of storage facilities [6]. During the long-term operation of salt cavern gas storage, multiple gas injections and extractions will cause periodic changes in the temperature of the salt rock inside the chamber [7,8]. Unlike constant high-temperature conditions, periodic temperature changes cause salt rock to repeatedly undergo thermal expansion and contraction, resulting in continuous adjustment of internal structural stress, which in turn affects its mechanical properties and seriously threatens the stability of salt cavern gas storage.

A large number of studies have found that the compressive strength of salt rock is lower than that of general limestone, sandstone, dolomite, etc., and the deformation modulus is 1–2 orders of magnitude lower, belonging to the category of soft rock [9,10,11]. In the stage division of stress-strain curves, salt rock shares common characteristics with other rocks and can be divided into four stages: compaction, elastic, inelastic, and failure [12,13]. However, compared with common rocks, salt rock, which is mainly composed of NaCl and has a polycrystalline structure, still exhibits distinct and unique mechanical properties [14,15]. Figure 1a–f shows the stress-strain curves prior to the peak under uniaxial compression for rocks, including the linear elastic type, elastic-plastic type, plastic-elastic type, plastic-elastic-plastic type I, plastic-elastic-plastic type II, and elastic-plastic-creep type. At low stress levels, the strain recovery ability is limited. As the load is continuously applied, the stress-strain curve rises gently until failure [11,12,16], exhibiting typical “elastic-plastic-creep” characteristics [12,17,18], as shown in Figure 1f. Especially in the storage and release cycle of salt cavern gas storage, the plastic strengthening stage of salt rock is inevitable [1,12], which has a decisive impact on the long-term stability of the gas storage. Given the unique “elastic-plastic-creep” stress–strain behavior of salt rock, its polycrystalline structure tends to evolve under the influence of cyclic temperature variations, thereby leading to corresponding changes in its mechanical properties.

Temperature changes have a significant impact on the mechanical properties of salt rocks [11]. Numerous studies have shown that the mechanical properties of salt rock exhibit different trends of change in different real-time high-temperature ranges. When the temperature is below 120 °C, the compressive strength and elastic modulus of salt rock gradually decrease with increasing temperature [19,20,21]. At 120–200 °C, the compressive strength and elastic modulus of salt rock increase with temperature, especially at 170 °C, where the stress-strain characteristics of salt rock undergo a sudden change. This temperature is considered the threshold for the sudden change in the mechanical properties of salt rock [20]. As the temperature exceeds 500 °C, the internal structure of salt rock undergoes significant changes, leading to a significant decrease in peak intensity [20,22]. In addition, the changes in the mechanical properties of salt rock after cooling under high temperature are consistent. As the temperature increases, the mechanical properties of salt rock gradually deteriorate, manifested by a decrease in elastic modulus and peak stress, as well as an increase in peak strain [11,23,24]. Through analysis of acoustic emission energy and counting, it was found that microcracks in salt rock are mainly concentrated during the process of temperature rise. Further analysis of the acoustic emission spectrum characteristics indicates that the cracks formed under low temperature conditions are mainly large-sized intergranular cracks, while under high temperature conditions, small-sized intragranular cracks are mainly generated [25,26]. On this basis, experts and scholars further studied the relationship between the elastic modulus of salt rock and temperature and derived a salt rock damage constitutive model based on temperature stress coupling conditions [27,28]. At present, research on the effect of temperature on the mechanical properties of salt rocks mainly focuses on the deformation laws and strength characteristics of salt rocks after real-time constant temperature or single heating and cooling. However, during the long-term operation of salt cavern gas storage, the internal temperature undergoes periodic fluctuations, and the resulting thermal fatigue damage in the salt rock cannot be ignored [7,24,29]. Previous studies on rocks such as granite, marble, and sandstone have shown that periodic temperature changes have cumulative destructive effects on the macroscopic mechanical properties and microstructure of rocks [30,31]. The cyclic temperature effect can promote the propagation of internal cracks and the increase in grain boundary spacing in rocks such as marble and sandstone [31], resulting in a gradual decrease in strength, wave velocity, and deformation ability [30,32,33], while the volume and permeability show an increasing trend [34,35]. In contrast, there are few reports on the correlation between periodic temperature changes and the mechanical properties of salt rocks. With the advancement of salt cavern gas storage construction, it is urgent to conduct in-depth research on the mechanical properties of salt rock under periodic temperature effects.

This article takes salt rock samples after thermal fatigue treatment as the research object and conducts physical and mechanical tests on the samples to study the influence of different upper limit temperatures and cycle times on the physical and mechanical properties of salt rock. Combined with DIC and acoustic emission technology, qualitative and quantitative analysis is conducted on the evolution process and expansion behavior of salt rock damage, revealing the mechanical behavior of salt rock under thermal fatigue damage. The research results contribute to a deeper understanding of the impact of temperature changes on the mechanical properties of salt rock during the storage and release cycle of salt cavern gas storage and provide a theoretical basis and technical reference for ensuring the long-term stability and sealing of salt cavern gas storage projects.

2. Experimental Materials and Methods

2.1. Specimen Preparation

The salt rock used in this experiment appears reddish-brown and semi-transparent in appearance, mainly composed of NaCl, with a natural density of about 2.16 g/cm³. Due to the easy dissolution of salt rock in contact with water, dry sawing and grinding methods were used to process the specimens, resulting in a standard cylindrical specimen with a height of 100 mm and a diameter of 50 mm. The basic physical and mechanical properties of the salt rock were determined in accordance with the International Standard for Rock Mechanics Testing (ISO 22282, 2012 [36]). The testing procedures and results are shown in Figure 2. The wave velocity and hardness of salt rock are mainly determined by RSM-SY6 (C) wave velocity meter and Leeb hardness tester (Produced in Beijing, China), which are 4.3–4.4 km/s and 256–269 HL, respectively (Figure 2a). According to the stress-strain curve of salt rock, the compressive strength is 37.35 MPa and the elastic modulus is 2.08 GPa (Figure 2b).

2.2. Experiment Scheme and System

Due to current limitations in experimental techniques, it is not yet possible to ensure that the heating-cooling process of the rock sample occurs simultaneously with the loading process, making it difficult to conduct truly coupled temperature-stress tests. Therefore, in order to achieve the research objectives, a simplified method was adopted, which first conducted temperature cycling tests on salt rock specimens, followed by physical property tests [37] and uniaxial compression tests. A total of 9 sets of experiments were designed for this experiment (

Table 1. Temperature cycle setting scheme.

Number	Tmax/°C	Tmin/°C	Number of Cycles	Temperature Cycle Diagram
T1	Normal temperature		0
T2	60	40	5
T3	60	40	10
T4	60	40	20
T5	60	40	40
T6	50	40	10
T7	60	40	10
T8	70	40	10
T9	80	40	10

), each using 3 parallel specimens. If there is significant variability in the test results, abnormal data should be removed and additional tests should be conducted to ensure data reliability. The specific experimental plan is as follows.

First, temperature cycling tests were conducted on salt rock specimens under different numbers of temperature cycles and various upper limit temperatures (Figure 3).

(1) Considering that salt cavern gas storage facilities are typically buried at depths of 600–800 m, and based on a geothermal gradient of approximately 3 °C per 100 m with a surface temperature of about 20 °C [15,20], as well as referring to actual temperature variations during the storage–withdrawal cycles of salt caverns [7,38], the upper and lower temperature limits were finally set as T_max = 60 °C and T_min = 40 °C, respectively. In low-frequency and high-amplitude operation mode, salt cavern gas storage facilities usually undergo 1–2 injection production cycles per year, and their designed service life is generally 40–50 years [39,40]. Based on this, this study set four types of cycle times, namely 5, 10, 20, and 40, representing the initial, middle, and long-term service stages of the storage operation, respectively, to ensure that the experimental plan has good representativeness and engineering relevance (Table 1).

(2) Considering the extreme changes in salt cavern temperature under different storage conditions, different upper limit temperatures T_max are set at 50 °C, 60 °C, 70 °C, and 80 °C, respectively. The lower limit temperature and cycle times are the same, set at 40 °C and 10 times, respectively (Table 1).

In addition, each temperature cycle is set to 4 h, with a heating and cooling rate of 3–5 °C/min. After heating or cooling to the target temperature, the temperature is maintained for 2 h. Then, the salt rock samples that have completed the temperature cycle are taken out of the heating test chamber and subjected to physical property testing using a TES-135A colorimeter (Produced in Beijing, China), RSM-SY6 (C) wave velocity meter, and Leeb hardness tester [41]. Finally, mechanical property tests were conducted on the specimens, during which acoustic emission (AE) sensors were installed and full-field deformation was monitored using DIC. The test procedures and equipment are shown in Figure 3. The Acoustic Emission test was conducted using a DS5 multi-channel monitoring system (Produced in Beijing, China), with a threshold set at 10 dB. During the test, six acoustic emission sensors were evenly arranged and fixed at positions 15 mm from both the top and bottom end faces of the specimen to collect acoustic emission signals. To compare and analyze the changes in the physical and mechanical properties of salt rock specimens before and after temperature cycling tests, corresponding tests were conducted on specimens that had not undergone temperature cycling tests.

The color measurement of salt rock was primarily conducted using the TES-135A color analyzer to extract color parameters, with the CIE-Lab color space employed for color representation. This coordinate system quantifies a color using three primary values: L*, a*, and b*. Specifically, L* represents the dimensionless lightness, ranging from 0 to 100, where 0 corresponds to black and 100 to white; a* denotes the green-red component, ranging from −100 (pure green) to 100 (pure red); and b* indicates the yellow-blue component, ranging from −100 (pure blue) to 100 (pure yellow).

3. Results

3.1. Changes in Physical Properties

Considering that salt cavern gas storage may experience temperature cycles of different amplitudes and frequencies during service, the temperature range mainly affects the strength of thermal fatigue, while the number of cycles controls its cumulative evolution process, thereby affecting the physical properties of salt rock to varying degrees. Figure 4 and Figure 5, respectively, show the experimental results of the influence of different temperature cycles and temperature ranges on the physical properties of salt rock.

To compare and analyze the effect of temperature cycling on the physical properties of salt rock, salt rock samples that have undergone 10, 20, and 40 thermal cycles, respectively, within the same temperature range (40–60 °C) were selected for analysis. As shown in Figure 4a, the variation of salt rock density, wave velocity, and hardness with the number of temperature cycles under periodic temperature changes. The main component of the salt rock used in this experiment is NaCl, which contains a small amount of impurity minerals such as gypsum, calcite, and dolomite. Under the temperature cycling of 40–60 °C, a large amount of free water in the salt rock sample evaporates, and gypsum partially dehydrates to form hemihydrate gypsum [15,42], resulting in a decreasing trend in salt rock density. And as the number of cycles increases, the water loss in salt rock gradually decreases, and the density decreases and eventually stabilizes. In addition, as the number of temperature cycles increases, the overall hardness of salt rock shows a decreasing trend, while the wave velocity briefly increases at the beginning of the cycle and then also shows a decreasing trend. This is because in the initial stage of temperature cycling (0–3 times), the natural cracks inside the rock sample partially close under the action of thermal expansion, but the bonding force between grain boundaries decreases, and local new cracks emerge. This leads to a short-term increase in wave velocity, while the hardness, which reflects local resistance to deformation, begins to decrease. Entering the midterm stage of temperature cycling (3–8 times), thermal stress accumulates and crack propagation dominates [43]. The wave velocity and hardness of salt rock samples show a decreasing trend. In the later stage of temperature cycling (8–20 times), the development of internal fractures in the rock sample tends to stabilize, and at this time, the changes in salt rock wave velocity and hardness tend to stabilize.

The chromaticity changes of salt rock under different temperature cycles are shown in Figure 4b. The parameter L*, which characterizes the brightness of salt rock, gradually decreases with the increase in temperature cycles, while the color difference ΔE shows a gradually increasing trend. In contrast, the values of parameters a* and b*, which reflect the changes in the red-green and yellow-blue chromaticity of salt rock, fluctuate overall with the increase in temperature cycles, without showing a significant upward or downward trend. This is because the salt rock used in the experiment is mainly NaCl, which has stable chemical properties and is not prone to redox reactions within the temperature cycling range of 40–60 °C. In addition, salt rocks are formed in terrestrial or shallow marine sedimentary environments [40], where a large amount of iron elements have been oxidized to Fe³⁺, further reducing the impact of temperature changes on their chromaticity. In summary, under periodic temperature changes, the physical properties of salt rocks undergo a dynamic adjustment process and eventually tend towards a new stable state.

As the upper limit temperature of the thermal cycle increases, the physical properties of salt rock exhibit a certain degree of temperature sensitivity. As shown in Figure 5, the decrease in salt rock density (Δρ), wave velocity (ΔV_p), hardness (ΔH_L), and color difference (ΔE) all gradually increase with the increase in the upper limit temperature, but their trends are not linear. Especially at low temperatures (50–60 °C), the changes in various parameters are significant, while at high temperatures (70–80 °C), the changes tend to stabilize. This indicates that as the upper limit temperature increases, the internal structure of the salt rock gradually weakens, but in the higher temperature stage, the structural damage tends to saturate, and further heating stabilizes its impact.

3.2. Stress-Strain Curve

Figure 6a–i shows the stress-strain curves and energy evolution characteristics of salt rock specimens under different temperature cycling cycles and upper temperature limits, respectively. From the figure, it can be seen that under uniaxial compression conditions, the axial stress-strain curve of the salt rock specimen exhibits a typical “elastic-plastic-creep” mechanical behavior [12,17], and its deformation process can be roughly divided into four stages, each corresponding to specific energy evolution characteristics:

① In the compaction stage (Stage I), under the action of axial stress, the original defects (microcracks and micropores, etc.) inside the salt rock quickly close, and the mineral particles inside the salt rock become more compact. The axial stress-strain curve shows a concave shape.

② In the elastic deformation stage (stage II), salt rock transitions from the compaction stage to the deformation stage dominated by elastic interaction between grains under continuous axial stress. The axial strain increases approximately linearly with the increase in axial stress, and the amount of axial deformation is relatively small. This stage mainly involves energy accumulation. Due to the low initial stress, the strain energy curves change smoothly, and the total strain energy absorbed by the sample is mainly elastic strain energy. The growth rate of the dissipated strain energy curve is lower than that of the elastic strain energy curve.

③ In the plastic deformation stage (stage III), when the axial stress exceeds the yield point, the stress-strain curve of salt rock shows an upward convex shape until it reaches the ultimate compressive strength. During this process, dislocations slide along the grain boundaries, exacerbating the micro rupture of pores and cracks, promoting stable propagation of micro cracks, and leading to a significant increase in axial deformation. In terms of energy evolution, this stage is mainly characterized by energy dissipation. In the initial stage, the energy absorbed by the sample is still mainly stored in the form of elastic strain energy, with less energy consumed for compaction and microcrack propagation. In the later stage, as the axial strain continues to increase, although the elastic strain energy is still increasing, the growth rate decreases, while the growth rate of dissipated strain energy gradually accelerates. This indicates that during the plastic deformation stage of salt rock, the energy dissipation process plays a dominant role and becomes a key factor affecting its mechanical behavior.

④ In the stage of failure (Stage IV), when the axial stress exceeds the compressive strength, the salt rock specimen undergoes deformation and failure, and the axial stress-strain curve rapidly drops. A large number of microcracks generated inside the salt rock specimen eventually penetrate to form a macroscopic fracture surface, and the bearing capacity rapidly decreases. At this stage, the elastic strain energy is rapidly released, the dissipated strain energy increases rapidly, and most of the input total strain energy is converted into dissipated strain energy. In addition, due to the presence of residual stress, although the structural integrity of the specimen is compromised, the total strain energy continues to rise.

As shown in Figure 7, the changes in compressive strength and elastic modulus of salt rock under different temperature cycle times and different upper limit temperature conditions are presented. From Figure 7a, it can be seen that the compressive strength and elastic modulus of the salt rock sample without periodic temperature action are 37.35 MPa and 2.1 GPa, respectively. After 10, 20, and 40 temperature cycles, the compressive strength of the salt rock decreased by 8.2%, 16.8%, and 23.8%, respectively, and the elastic modulus decreased by 20.4%, 29.7%, and 27.4%, respectively. It can be seen that the strength parameters of salt rock decrease with the increase in temperature cycles, and the rate of decrease gradually slows down. This is because as the number of temperature cycles increases, the damage caused by thermal stress to the internal structure of the specimen gradually accumulates, and microcracks initiate and gradually propagate inside the specimen, leading to a deterioration of the mechanical properties of the specimen. When the upper limit temperature remains constant and the number of temperature cycles increases, the number of microcracks generated inside the specimen gradually reaches the limit of this upper limit temperature, and the rate of decrease in compressive strength also slows down accordingly.

From Figure 7b it can be seen that the compressive strength and elastic modulus of the salt rock sample without periodic temperature action are 36.8 MPa and 1.7 GPa, respectively. When the number of cycles is 10 and the upper limit temperatures are 60, 70, and 80 °C, the compressive strength of the salt rock decreases by 6.9%, 11.9%, and 22.9%, and the elastic modulus decreases by 4.8%, 5.6%, and 6.5%, respectively. It can be seen that the strength parameters of salt rock decrease with the increase in the upper limit temperature, and the rate of decrease gradually accelerates. This is because different mineral particles have different coefficients of thermal expansion, which causes each mineral particle to be constrained by adjacent particles when deformed by periodic temperature effects, resulting in thermal stress [20]. Thermal stress often concentrates at the boundaries between mineral particles, causing stress concentration. When local stress exceeds the strength limit, microcracks are generated. As the upper limit temperature increases, the number and size of microcracks increase, eventually forming a widely distributed network of cracks, which gradually accelerates the deterioration of salt rock strength.

As shown in Figure 8, the peak strain energy (total strain energy U, elastic strain energy U_e, and dissipated strain energy U_d) of salt rock varies with the number of temperature cycles and the upper limit temperature. From the graph, it can be seen that as the number of cycles and upper limit temperature increase, the total strain energy, elastic strain energy, and dissipated strain energy at the peak point of the salt rock sample all decrease significantly. This indicates that salt rock absorbs more thermal energy under repeated thermal stress, leading to a large number of internal cracks, continuous accumulation of damage, and reduced structural integrity, making it more prone to failure under axial stress. Therefore, the energy required for the sample to reach peak strength is significantly reduced.

However, it is worth noting that although all types of strain energy show a decreasing trend, the proportion of elastic strain energy in the total strain energy shows an increasing trend. This is because, unlike other rock masses, salt rock itself has a polycrystalline structure. After repeated temperature cycles, the internal structure of salt rock deteriorates significantly, and grain structure damage and microcrack accumulation inhibit the full development of plastic slip [15]. The strain process that should have entered the energy dissipation stage is “compressed”, and its overall deformation tends to be more elastically dominated. This can also be confirmed by the strain values corresponding to the peak strength of the stress-strain curve in Figure 6. Therefore, the increase in the proportion of elastic strain energy essentially reflects the degradation trend of the plastic properties of salt rock under periodic temperature changes.

3.3. Acoustic Emission Characteristics

As shown in Figure 9, the evolution characteristics of ringing count (fracture activity) and total energy (damage scale) over time during the uniaxial compression test of salt rock under different temperature cycling times are presented [43]. From the figure, it can be seen that the process of sound emission ringing count and total energy change is divided into three stages:

① Active period: In the early stage of loading, the salt rock transitions from the compaction stage to the elastic stage. Due to the sudden loading of the salt rock (initial response triggered by fast loading), the initial cracks and pores inside the salt rock undergo instantaneous collision-type compression closure, resulting in impact failure. Therefore, during this stage, the sound emission signal is dense, and the ringing count and total energy continue to develop in a low-amplitude form.

② Calm period: After entering the elastic deformation stage, the acoustic emission ringing count and total energy begin to decrease, resulting in a sustained period of low signal, the “calm period”. This is because after the similar impact effect ends, the salt rock is in the stage of elastic deformation, and the instantaneous accumulated elastic deformation will have a certain rebound recovery space due to the rapid generation of cracks and voids. During this stage, it mainly undergoes the release and recovery of elastic strain, and no new crack propagation has yet occurred. As the load continues to increase and enters the plastic deformation stage, microcracks begin to propagate, and acoustic emission signals reappear and remain at a relatively low stable level.

③ Destruction period: As the volume of salt rock expands and microcracks develop unstably, the acoustic emission ringing count and total energy rapidly increase when the peak strength of salt rock is about to be reached. When the peak strength is reached, the main cracks that cause salt rock failure have already formed. Subsequently, stress rapidly decreases and is insufficient to generate new cracks. At this point, the acoustic emission signal is mainly caused by the relative displacement and friction between the main cracks.

As shown in Figure 9, with the increase in the number of thermal cycles, both the ringing counts and total energy of salt rock during uniaxial compression failure exhibit a significant upward trend. After 20 thermal fatigue cycles, the ringing counts and total energy increased by 63.4% and 92.3%, respectively. This indicates that thermal fatigue enhances the activity of the fracturing process and enlarges the failure scale of salt rock.

Through extensive analysis of acoustic emission data, it was found that dividing the ringing count in the acoustic emission parameters by the duration is recorded as the average frequency AF, and dividing the rise time by the amplitude is defined as RA. The ratio of the two can be used to determine the type and proportion of cracks in the specimen after failure. Process the acoustic emission characteristic data of the salt rock sample and plot it as an AF-RA scatter density map, as shown in Figure 10. The slope of the boundary between tensile and shear cracks in the AF-RA scatter plot is AF_max/RA_max [43,44], where the scatter located on the left side of the boundary is defined as tensile cracks, and the scatter located on the lower right side of the boundary indicates the occurrence of shear cracks (Figure 10a).

From Figure 10, it can be seen that the scatter points are distributed in a strip shape, closely following the RA axis and AF axis, and the distance from the scatter points to the density center is negatively correlated with the scatter density. From the density center to the outside, the scatter density decreases step by step. As the number of temperature cycles increases (Figure 10b–f), the position of the center of the scatter density changes significantly. In the absence of periodic temperature effects (Figure 10b), the number of data points for salt rock tensile failure is higher than that for shear failure [27], with tensile failure accounting for as high as 91%, indicating that the main form of sample failure is tensile failure. After 20 cycles of temperature action (Figure 10e), the proportion of salt rock tensile failure decreased to 86%, and the proportion of shear failure increased to 14%. Although tensile failure still dominated, the failure mode began to shift towards a combination of tensile and shear failure.

3.4. Compression Damage Characteristics Based on DIC

DIC stipulates that axial displacement is negative when it is vertically downward, and radial displacement is positive when it is horizontally rightward. As shown in Figure 11, the cloud map of the full field displacement of the salt rock surface after 40 temperature cycles is presented. For the convenience of analysis, three typical loading stages were selected as observation comparison points: elastic stage (①), plastic stage (②), and failure stage (③).

The left side of Figure 11 shows the radial displacement cloud map of the observed surface of the salt rock. In the elastic stage (①), the radial deformation of the salt rock is mainly concentrated in the lower half, and a significant local deformation state appears on the right side of the sample. As the load continues to be applied and enters the plastic stage (②), the radial displacement difference of the salt rock further intensifies, and the sample shows obvious expansion characteristics within a range of about 25–50 mm from the bottom. When entering the destruction stage (③), the displacement difference between the blue-green area on the left side and the red-yellow area on the right side of the salt rock significantly increases, forming a clear radial displacement mutation zone, exhibiting a strong Poisson effect. The deformation characteristics are consistent with the analysis results of AF-RA crack types and their proportions in Figure 10, indicating that the failure of salt rock is mainly caused by the tensile effect caused by the expansion on both sides.

The central image in Figure 11 shows the axial displacement cloud map of the observed surface of the salt rock. In the elastic stage (①), the displacement in the axial direction of the salt rock is negative, exhibiting a transfer-type compression state. After entering the plastic stage (②), the axial deformation of salt rock shows a clear gradient distribution characteristic, and the deformation of the upper part of the sample is significantly greater than that of the lower part, which is related to the loading end being located at the upper end of the sample. When entering the failure stage (③), the trend of axial displacement change remains unchanged, and the deformation of the upper part of the specimen continues to increase, without changing its evolution law. By comparing the numerical increments of axial and radial deformation of salt rock after entering the plastic stage, it was found that the final failure of the specimen was mainly caused by radial deformation.

On the right side of Figure 11 is the total displacement cloud map of the salt rock observation surface. In the elastic stage, the trend of total displacement variation is similar to axial deformation, indicating that the overall deformation of salt rock in this stage is mainly axial compression. After entering the plastic stage, although the total displacement and axial displacement cloud maps still show similar gradient distribution characteristics, there are significant differences between the two, mainly due to the gradual increase in radial deformation. According to the distribution characteristics of total displacement, the sample can be divided into three regions: A, B, and C. Region A is mainly red and has the highest total displacement. The B area is mainly yellow-green with a total displacement range of 0.6~0.9 mm, while the C area is mainly blue with a total displacement of about 0.2~0.6 mm. As the sample transitions from the plastic stage to the failure stage, the values in the red and yellow-green areas increase significantly, and a local concentration of total displacement appears in the lower left corner, showing a strain accumulation trend in the early stage of failure.

By comparing the stress-strain curves of salt rock under different temperature cycles (Figure 12a), it can be seen that with the increase in temperature cycles, the axial stress-strain curve shifts to the lower left, indicating that the axial deformation capacity of salt rock gradually weakens and the compressive strength also gradually decreases. To further analyze this trend, samples under the same strain conditions during the plastic stage were selected to compare the displacement cloud maps of the salt rock surface under different temperature cycling times.

From Figure 12b, it can be seen that during the plastic stage under the same strain conditions, as the number of temperature cycles increases, the axial deformation of the salt rock is basically similar, but there is a clear displacement discontinuity zone in its radial deformation, resulting in deformation accumulation in local areas of the total displacement cloud map. This is because the temperature is constantly alternating states, and the salt rock will repeatedly expand and contract accordingly. Different areas inside the sample will produce uneven expansion rates, which will lead to the formation of local residual stresses. In addition, during the continuous temperature cycling process, thermal fatigue can promote the early development of cracks and damage within the polycrystalline structure of salt rock, and local damage limits the slip behavior of dislocations, resulting in a damage accumulation effect far greater than the plastic strengthening effect [15], ultimately leading to the specimen entering the failure stage earlier, manifested as a decrease in peak stress and peak strain. This can also be confirmed by the variation of peak strain energy (total strain energy U, elastic strain energy U_e, and dissipated strain energy U_d) of salt rock with the number of temperature cycles in Figure 8.

4. Discussion

When salt rock undergoes periodic temperature changes, thermal stress induces the continuous propagation of microcracks, leading to the accumulation of structural damage, increased radial deformation, and concentrated local displacement, causing it to enter the failure stage earlier, and the type of crack failure gradually shifts from tension-dominated to tension-shear composite. As a polycrystalline rock, salt rock exhibits heterogeneity and discontinuity between its crystal grains and typically demonstrates “elastic-plastic-creep” behavior under uniaxial compression. When the stress reaches a certain threshold, microcracks continuously initiate, expand, and penetrate, ultimately leading to a significant volume expansion phenomenon (Figure 13 and Figure 14). To further reveal the influence of periodic temperature changes on the thermal expansion characteristics of salt rock during uniaxial compression, an analysis is conducted on the initial expansion stress and the volumetric expansion capacity changes of salt rock under different cycle times and upper temperature limits.

To obtain the radial deformation of the salt rock specimens, the virtual extensometer method was employed by leveraging the full-field displacement and strain measurement capabilities of DIC technology. This approach effectively overcomes the limitations of traditional strain gauges in measuring non-uniform deformation and allows for the calculation of volumetric strain in salt rock. In this study, three radial virtual extensometers were placed within the selected analysis area on the specimen surface at vertical positions of 25 mm, 50 mm, and 75 mm from bottom to top along the horizontal direction, as shown in Figure 13f. The strain curve in this article follows the regulations of rock mechanics tests; compressive strain is considered positive, and tensile strain is considered negative.

The radial strain of salt rock obtained based on DIC technology under periodic temperature changes is shown in Figure 13 and Figure 14. It is evident from the figure that the radial strain measured by the virtual extensometers is greatest at the midsection of the specimen. This is primarily due to the end effects experienced by the upper and lower ends of the specimen during mechanical testing. Additionally, the mechanical testing system used in this study employs active loading at the top and passive loading at the bottom, resulting in radial deformation being concentrated mainly in the specimen’s midsection, followed by the lower and upper ends. Therefore, to ensure the representativeness of the strain measurement, the radial strain at the specimen’s midsection was selected for calculating volumetric strain.

The strain of the salt rock mass obtained based on DIC technology under periodic temperature changes is shown in Figure 13 and Figure 14. From the figure, it can be seen that all samples exhibit significant volume expansion characteristics [20]. At the beginning of loading, the sample is in the compression and elastic deformation stage, and the internal pores gradually close. At this time, the radial strain is small, and the volumetric strain is mainly dominated by the axial strain. The sample volume is in the rapid shrinkage stage. After entering the initial stage of plastic deformation, as the radial strain gradually increases, the stress-strain curve enters a nonlinear stage, and the sample volume enters a short-term stable stage; that is, the volume is relatively stable. With the continuous development of plastic deformation, when the loading stress reaches the expansion starting stress, the stress-strain curve begins to reverse, and the volume of salt rock begins to expand, exhibiting expansion phenomenon. In the later stage of plasticity, the slope of the stress-strain curve decreases, and the specimen enters the accelerated expansion stage, eventually developing to failure.

Under periodic temperature action, the initial stress of salt rock expansion and the change in volume expansion capacity at failure are shown in Figure 15. From the graph, it can be seen that as the number of temperature cycles and the upper limit temperature increase, the initial stress of salt rock expansion and the volume expansion capacity at failure both show a decreasing trend. The expansion boundary of salt rock under uniaxial compression is usually closely related to its elastic ultimate strength [20]. The thermal expansion and contraction effects caused by multiple temperature cycles form a complex thermal stress field inside the sample, which promotes the loosening and sliding of bonds between grains in the polycrystalline structure of salt rock, induces the continuous initiation and propagation of microcracks, and weakens the overall integrity of the structure, resulting in a significant decrease in the initial stress for expansion. In addition, under the influence of periodic temperature, the total strain energy of salt rock decreases throughout the compression process, and the sample enters the plastic stage earlier. As the intergranular slip intensifies and exceeds the mutual constraint range, energy is gradually released [20], ultimately exhibiting the thermal expansion characteristics of “premature initiation-weakened failure”.

5. Conclusions

This article focuses on the study of the physical and mechanical properties of salt rock under periodic temperature changes and combines DIC and acoustic emission technology to qualitatively and quantitatively analyze the damage evolution process of salt rock. The influence of different upper limit temperatures and cycle times on the mechanical properties of salt rock is studied. The main conclusions are as follows:

(1) In the early stage of temperature cycling, the thermal expansion effect causes some cracks to close, and the wave velocity briefly increases. As the number of cycles increases, crack propagation gradually becomes dominant, and free water inside the salt rock evaporates continuously. Impurity minerals such as gypsum undergo dehydration, resulting in an overall decrease in density, wave velocity, and hardness, and ultimately stabilizing. In terms of chromaticity, as the number of cycles and the upper limit temperature increase, the brightness of the salt rock gradually decreases, the color difference continues to increase, and eventually it tends to a new stable state.

(2) Salt rock exhibits typical “elastic-plastic-creep” characteristics under uniaxial compression. Periodic temperature changes induce microcrack propagation in salt rock, leading to accumulated structural damage, resulting in a 23.8% decrease in compressive strength and a 27.4% decrease in elastic modulus after 40 thermal cycles. As the number of cycles and upper limit temperature increase, the peak strain energy overall shows a decreasing trend, but the proportion of elastic strain energy increases from 21% to 38%. During the deformation process of salt rock, plastic slip is limited, showing an increasing trend of elastic-dominant characteristics.

(3) The acoustic emission response during uniaxial compression of salt rock is divided into an active period, a calm period, and a failure period, corresponding to the initial crack closure, elastic strain accumulation, and unstable propagation of microcracks, respectively. Under periodic temperature action, the crack types of salt rock change during compression. When not subjected to thermal cycling, tensile cracks accounted for 91%. After 20 temperature cycles, the proportion of tensile cracks decreased to 86%, and the proportion of shear cracks increased to 14%. The failure mode of salt rock gradually shifted from tension-dominated to tension-shear composite.

(4) The strain of salt rock gradually shifts from axial compression to radial expansion control during uniaxial compression, exhibiting a typical Poisson effect. As the number of cycles and upper limit temperature increase, the uneven expansion and thermal fatigue effects caused by thermal cycling accelerate the propagation of microcracks and suppress plastic slip, resulting in a decreasing trend in both the initial stress of expansion and the volume expansion capacity at failure, exhibiting the thermal expansion characteristics of “premature initiation-weakened failure”. The sample structure fails prematurely, the bearing capacity decreases, and the peak stress and strain decay synchronously.