Energy Dissipation and Damage Evolution of Water-Saturated Skarn Under Impact Loading

Abstract

Understanding the combined effects of water and dynamic disturbance on rock behavior is essential for deep underground engineering, where groundwater and blasting often coexist. Existing studies have mainly emphasized static weakening by water or the strength characteristics under impact, while the energy evolution process remains insufficiently addressed. To fill this gap, uniaxial impact compression tests were conducted on dry and water-saturated skarn specimens using a separated Split Hopkinson Pressure Bar system. The relationship between peak stress and impact pressure was analyzed, and the total input energy, releasable elastic strain energy, and dissipated energy were quantified to examine their evolution with strain. The results indicate that water saturation significantly reduces dynamic strength and modifies the damage process. During the compaction and elastic stages, dissipated energy is low but slightly higher in water-saturated specimens due to microcrack initiation. In the plastic stage, dry specimens exhibit faster energy dissipation, while water-saturated specimens show reduced capacity for crack propagation dissipation. Damage–strain curves follow an S-shaped pattern, with water-saturated specimens presenting higher damage growth rates in the plastic stage. These findings clarify the energy-based damage mechanisms of skarn under impact loading and provide theoretical support for evaluating stability in water-rich underground environments.

1. Introduction

With the gradual exploitation of shallow resources and the construction of deep transportation, railways, and other projects, underground projects such as tunnels and mining in China have gradually developed more. Due to the existence of groundwater, the deep rock mass is in water saturation, and the water will produce physical (softening) and chemical reaction (dissolution) reactions on the rock mass, resulting in changes in the mechanical properties of the rock [1,2]. The blasting load will attenuate in the rock mass, so different degrees of dynamic disturbance will occur at different distances from the blasting source so that the rock shows different dynamic mechanical properties. Therefore, it is of great theoretical significance and engineering value to study the dynamic characteristics of water-saturated rock.

The use of separated Hopkinson rods for the dynamic characterization of rock masses has been widely recognized by researchers in this field. Zhou et al. [3] used a separated Hopkinson pressure bar (SHPB) system to perform impact test tests on sandstone at different strain rates, analyzed the infrared thermal image evolution characteristics of sandstone at different strain rates, and discussed the relationship between the strain rate and energy density of the specimen. Zhu et al. [4] conducted an experimental study on the kinetic properties of sandstone under single and repeated impact loads using a large-diameter SHPB test apparatus. The damage process of the rock was analyzed from the perspective of the fine view crack expansion and energy absorption of the rock. Tan et al. [5] used a modified SHPB-based combined rock dynamic and static loading test system to perform a one-dimensional static load and cyclic impact loading test on a skarn rock sample taken from a 900 m depth below the Dongguashan copper mine. Yang et al. [6] conducted impact compression and impact splitting tests on sandstone and sand-like rocks using SHPB and RFPA2D numerical calculation software for an in-depth comparative analysis of the dynamic mechanical and damage characteristics of sandstone and sand-like rocks and compared the stress–strain curve evolution characteristics and mechanisms as well as the peak stresses of sandstone and sand-like rocks. Tang et al. [7] employed an experimental system for coupled static and dynamic loading based on an SHPB device to investigate the deformation characteristics, energy laws, and failure modes of skarn under the combined action of high static stress and frequent dynamic disturbances at medium to high strain rates, thereby further revealing the failure mechanism of deep rock masses subjected to dynamic effects such as excavation and blasting. Wang et al. [8] investigated the energy evolution and fractal characteristics of coal rocks at different driving air pressures (0.3–0.5 MPa) and specimen lengths (15–50 mm). Dynamic compression tests were conducted using a split Hopkinson compression bar system to clarify the variation in energy evolution parameters with strain rate for both approaches. Wen et al. [9] applied an SHPB test apparatus to perform dynamic compression tests on composite rock samples with different dip angles at different strain rates. The energy absorption rate of the specimens at 0°, 30°, and 90° shows a trend of increasing and then decreasing with increasing strain rate.

At present, a large number of achievements are concentrated on the study of the influence of water on the mechanical properties of rocks under static load. The results of rock statics showed that water has a strong deterioration effect on the mechanical properties of rocks. Burshtei [10] took quartz arenite and sandstone as research objects and found that water content has a great impact on their compressive strength. Xu et al. [11] conducted uniaxial compression tests on rocks in different states (dry, natural, and water-saturated) and found that the uniaxial compressive strength, elastic modulus, and Poisson’s ratio of rocks decrease with an increase in water content. Lajtai et al. [12] took granite as an experimental material and found that the effect of water would reduce its strength characteristics. Li et al. [13] carried out a shear creep test on sandstone in the Wuhan river-crossing tunnel under dry and water-saturated conditions and found that water can improve the creep strain and strain rate of sandstone but reduce its strength. Hawkins [14] took sandstone as the research object and analyzed the strength difference in sandstone under dry and water-bearing conditions. Zhu et al. [15] conducted a uniaxial compression test on argillite and found that water content would have a great impact on the compressive strength of argillite. Deng et al. [16] considered three states, dry, natural, and full water, and conducted a comparative systematic test and test analysis on the compressive and tensile strength characteristics, deformation and damage characteristics, and microstructure characteristics of red-bedded soft rocks and concluded that the softening characteristics of red-bedded soft rocks in the full water state are obvious, and the ductility characteristics at the time of damage are more obvious. The above researchers studied water-bearing rocks from the perspective of statics, but there is less research on the influence of water on the dynamic properties of rocks from the perspective of dynamics. Yuan et al. [17,18] studied the deterioration effect of the dry–wet cycle on the dynamic mechanical properties of coal mine sandstone using an SHPB device and analyzed the impact energy absorption of sandstone in different states. Lou [19] performed impact tests on granite in dry and water saturation states and found that the sample under water saturation has a higher elastic modulus and dynamic strength than that in the dry state. Wang et al. [20] carried out a uniaxial impact compression test on sandstone in the air-dried and water-saturated states and found that the dynamic strength of sandstone in the water-saturated state is similar to that in the air-dried state. Rubin et al. [21] compared the dynamic tensile strength of dry and water-saturated granite using an SHPB compression bar test and found that water-saturated granite is more difficult to break than that in the dry state. Jia et al. [22] analyzed the dynamic stress–strain patterns, mechanical parameter degradation, and microstructural characteristics of tuffs using separated Hopkinson compression rods for dynamic impact tests on tuffs with different water contents. The results showed that the average dynamic peak stress, dynamic strain, and elastic modulus were negatively correlated with the water content. Wang et al. [23] adopted methods that take into account density, unloading rate, preloaded axial stress, and impact air pressure; analyzed the deformation characteristics and damage failure modes of copper-bearing skarn; and established its constitutive model after loading, unloading, and hierarchical dynamic load disturbance. Lu et al. [24] conducted dynamic mechanical tests on deep granite specimens under different strain rates using a Split Hopkinson Pressure Bar (SHPB) device. The test results show that there is a consistent corresponding relationship between the damage degree of granite specimens and the dissipation energy per unit volume. Pi et al. [25] conducted conventional uniaxial compression tests and five-stage cyclic loading–unloading tests with two different schemes on skarn using a YAW-2000 microcomputer-controlled electro-hydraulic servo rigid testing machine. The research results show that the change in the elastic modulus of skarn is significantly affected by the stress path, with the curve showing a typical wavy shape, while the influence of the loading–unloading rate on it is not significant. Chu et al. [26] used sandstone as the test material to investigate the variation in the mechanical properties of rocks with different water contents and prepared dry, semi-water-saturated, and water-saturated rock specimens with saturation coefficients of 2.82%, 52.11%, and 100%, respectively, to investigate the dynamic properties under static load and eight different impact energies. It was found that the stress–strain curves of the semi-water-saturated and water-saturated rock specimens showed a significant decrease in the peak value with increasing water content under static load compared to the dry rock. Lou et al. [19] investigated the dynamic stress–strain relationship between dry and water-saturated granite specimens using an SHPB experiment. The average dynamic tensile strengths of dry and water-saturated granite specimens were obtained, indicating that the tensile strengths of water-saturated granite specimens and dry granite specimens have different relationships with loading rates. Wang et al. [20] used a modified SHPB test apparatus to perform impact compression tests on Kaiyang phosphate sandstone with an aspect ratio of 0.5 under natural air-drying and water-saturated conditions, and the results showed that the stress–strain relationship of water-saturated sandstone under impact loading was different from its static stress–strain relationship, and the dynamic strength of water-saturated sandstone under medium strain rate loading was similar to that of air-dried sandstone, which is opposite to the result of the strength reduction in water-saturated sandstone under static loading conditions. In Jin et al. [27], four specimens of red sandstone were prepared with four water contents, and impact tests with the same impact pressure were conducted using an SHPB test system. The ratio of dissipated energy to total strain energy at the beginning of the damage (unloading) phase increased and then decreased with increasing water content. Tan et al. [28] conducted separated Hopkinson compression bar experiments on chert, red sandstone, and barite in the dried, natural water-bearing, and free-absorbing states, and the dynamic compressive strength of chert and red sandstone in the free-absorbing state decreased significantly, while the dynamic compressive strength of barite did not change much. At present, studies on rock dynamic response mainly focus on the strength and deformation characteristics of water on rock, while there are fewer studies on the impact of water on the energy evolution process of rock under dynamic load.

Therefore, this study employs a Split Hopkinson Pressure Bar (SHPB) system to perform uniaxial impact compression tests on dry and water-saturated skarn specimens under different impact pressures. The dynamic strength and failure characteristics are examined, and the energy evolution during loading is quantified from stress–strain data. Based on the calculated input energy, releasable elastic energy, and dissipated energy, energy–strain evolution curves are established and divided into three characteristic stages. A damage variable is then defined using dissipated energy to analyze the damage–strain evolution, highlighting the influence of water saturation on dynamic damage behavior. These findings offer new insights into the energy-based mechanisms of water-saturated rocks under impact loading, with practical implications for underground construction in water-rich environments.

The remainder of this paper is structured as follows. Section 2 introduces the materials, specimen preparation procedures, and experimental methods. Section 3 presents the dynamic mechanical responses and failure characteristics of the specimens. Section 4 focuses on the analysis of energy evolution and damage behavior. Finally, Section 5 summarizes the key findings and discusses their engineering significance.

2. Test Method

2.1. Materials and Equipment

The test rock samples were collected from skarn in the mining pit of the M1 ore segment of an open-pit mine in Qinghai. The main components of the skarn include garnet, pyroxene, and andradite. Skarn blocks with good integrity and uniformity were selected. In accordance with the one-dimensional stress wave propagation characteristics of the SHPB (Split Hopkinson Pressure Bar) and the International Society for Rock Mechanics (ISRM) sample standards, the rock samples were processed into standard cylindrical specimens with dimensions of 50 mm × 50 mm (diameter × height) [29,30]. The two end faces of the specimens were polished, ensuring that the non-parallelism of the end faces was controlled within 0.05 mm and the axial deviation was less than ±0.25°.

After the specimens were processed, skarn specimens with small differences in wave velocity were selected for drying and water saturation treatments. The polished skarn specimens were placed in an oven at 110 °C for 24 h to dry. When the mass of the skarn specimens almost no longer decreased, it could be considered that the skarn had reached a dry state [31]. For the preparation of water-saturated skarn specimens, a water saturation device was used to perform vacuum water saturation treatment on the rock samples. The vacuum pressure of the water saturation device was set to 0.15 MPa, and the specimens were placed 2 cm below the water surface. The air extraction time was 6 h. After air extraction was completed, the water-saturated rock specimens were placed in water for 28 days of curing. During the curing process, it was ensured that the water surface was at least 2 cm higher than the upper surface of the rock. Tests were conducted on the static physical and mechanical properties of the skarn, and the results are shown in Table 1.

Table 1. Static physical and mechanical properties of rock.

Density/g·cm−3	Elastic Modulus/GPa	Uniaxial Compressive Strength/MPa	Tensile Strength/MPa	Poisson’s Ratio	P-Wave Velocity/m·s−1
3.83	102.55	117.02	9.48	0.22	4465

2.2. Test Equipment and Principle

For the dynamic uniaxial compression test of skarn, a Split Hopkinson Pressure Bar (SHPB) test device provided by the Rock Mechanics Laboratory of Kunming University of Science and Technology was adopted, and the test system is shown in Figure 1. The SHPB test device is mainly composed of a driving system, a pressure bar system, and a data acquisition system. In the pressure bar system, both the incident bar and the transmission bar have a diameter of 50 mm, with lengths of 200 cm and 150 cm, respectively. They are all made of high-strength alloy steel, with a density of 7.81 g/cm³, an elastic modulus of 210 GPa, and a wave velocity of 5410 m/s.

Figure 1. Schematic diagram of test system of SHPB.

During the test, the impact of the projectile on the incident bar generates an incident wave. When the incident wave reaches the contact surface between the incident bar and the specimen, part of the stress wave is reflected to form a reflected wave, while the other part penetrates the specimen and propagates toward the transmission bar to form a transmitted wave. A rubber sheet with a diameter of 20 mm was used as the waveform shaper, and finally, the obtained waveform diagram is shown in Figure 2. Strain gauges were attached to the middle positions of the incident bar and the transmission bar to measure the strain under impact loading.

Figure 2. Measured waveform.

In the SHPB tests, resistive strain gauges mounted on the incident and transmitted bars were connected to a Wheatstone bridge circuit for the real-time monitoring of stress waves. Signal amplification was performed using a FYLDE transducer amplifier with a bandwidth of 0–500 kHz and a gain of 100 applied to both channels, ensuring the coverage of the dominant stress wave frequencies. Signals outside this range were filtered to suppress noise and preserve physically relevant waveform features. A high-speed data acquisition system operating at MHz-level sampling rates recorded the incident, reflected, and transmitted strain signals. The conditioned signals were processed using one-dimensional stress wave theory to obtain the stress, strain, and strain rate responses of the specimen under impact loading.

Based on one-dimensional stress wave theory, and using the strain values measured by the strain gauges attached to the incident bar and the transmission bar, the stress, strain, and strain rate values of the specimen can be calculated via the “three-wave method” [32]:

$$\left\{\begin{array}{l}\sigma \left(t\right)=-{\displaystyle \frac{E_0{A}_0}{A_s}}\left[{\epsilon }_I\left(t\right)+{\epsilon }_R\left(t\right)+{\epsilon }_T\left(t\right)\right]\hfill \\ \dot{\epsilon }\left(t\right)=-{\displaystyle \frac{2C_0}{L_s}}\left[{\epsilon }_I\left(t\right)-{\epsilon }_R\left(t\right)-{\epsilon }_T\left(t\right)\right]\hfill \\ \epsilon \left(t\right)={\displaystyle \frac{2C_0}{L_s}}{\int }_0^t\left[{\epsilon }_I\left(t\right)-{\epsilon }_R\left(t\right)-{\epsilon }_T\left(t\right)\right]\mathrm{d}t\hfill \end{array}\right.$$

(1)

where $A_0$, $C_0$, and $E_0$ are the cross-sectional area, wave velocity, and elastic modulus of the elastic rod, respectively; $A_s$ and $L_s$ are the cross-sectional area and length of the sample, respectively.

To better investigate the dynamic properties and energy evolution law of water-saturated skarn, a preliminary SHPB compression test was conducted on skarn specimens before the formal SHPB compression test. This preliminary test aimed to determine the critical failure value of the specimens, and it was found that the critical impact air pressure was approximately 0.25 MPa. When the impact air pressure exceeded 0.25 MPa, macroscopic cracks appeared obviously; moreover, as the impact air pressure increased, the macroscopic cracks further propagated. Therefore, in this study, impact air pressures of 0.25, 0.30, 0.35, and 0.40 MPa were selected for the formal tests.

2.3. Stress Balance Verification

For the SHPB test, stress balance before the failure of the rock specimen is a prerequisite for effective test results [33]. It can be verified by comparing the dynamic stress at both ends of the rock sample during the loading process. As shown in Figure 3, the curve of the sum of the incident wave and the reflected wave almost coincides with the transmission wave curve, which indicates that the loads at both ends of the rock sample are equal before the peak. In this case, the axial inertia effect of the sample can be ignored, so it can be considered that the sample has reached the stress balance state, and the test results are effective. In the test, all the test results are strictly verified by stress balance, and the results that do not meet the requirements will be eliminated.

Figure 3. State of stress balance.

3. Analysis of SHPB Test Results

3.1. Analysis of Stress Wave Propagation Law

Figure 4 shows the incident wave, reflected wave, and transmitted wave of dry and water-saturated skarn under different impact pressures. It can be seen from the diagram that the impact pressure and water have a significant effect on the rock stress wave waveform. When the rock is dry or water-saturated, the amplitude values of the incident wave, reflected wave, and transmitted wave increase with the increase in impact pressure. Under the same impact pressure, that is, under the same incident wave condition, the reflected wave amplitude of water-saturated rock is higher than that under the dry condition, while the transmitted wave amplitude is lower than that under the dry condition. According to one-dimensional stress wave theory, the wave impedance of the water-saturated specimen is lower than that in the dry state, which is manifested by the increase in the reflected tensile wave and the decrease in the compressive wave. The stress wave transmitted through the specimen to the transmission rod is reduced, which is mainly manifested by the decrease in the amplitude of the reflected wave and the decrease in the amplitude of the transmitted wave.

Figure 4. Original waveform diagram.

3.2. Stress–Strain Curve Analysis

The dynamic compressive stress–strain curves of skarn specimens under dry and water-saturated conditions are shown in Figure 5. It can be seen that under the same impact load, the dynamic compression stress–strain curves of dry and water-saturated skarn specimens are basically the same, which can be roughly divided into four stages. The first stage is the crack compaction stage. At this stage, the curve is concave, which is mainly caused by the closure of microcracks inside the skarn specimen caused by external load. Under dynamic load, the duration of this stage is very short, so it is not obvious on the stress–strain curve. The second stage is the elastic stage. The curve at this stage is approximately a straight line, and the corresponding slope is the dynamic elastic modulus of the skarn specimen. The slope of the curve increases with the increase in the impact pressure, and the slope of the curve of the water-saturated specimen is smaller than that of the dry state. The third stage is the plastic deformation stage. At this stage, the curve is convex, and the slope of the curve gradually decreases. When the peak value of the curve is reached, the slope is 0. The fourth stage is the failure stage, and the slope of the curve is negative. In this stage, the dry state is different from the water-saturated state. In the dry state, the curve shows the phenomenon of “stress drop”, and the skarn specimen shows obvious brittle characteristics. The time curve of water-saturated skarn becomes slower, and the rock changes from brittleness to plasticity. This shows that the existence of water changes the mechanical properties of rock.

Figure 5. Dynamic compressive stress–strain curve of test piece.

In order to quantitatively study the influence of dry and water saturation conditions on the peak stress of skarn specimens, a histogram of the peak stress and impact load of skarn specimens was drawn and fitted, as shown in Figure 6.

Figure 6. Relationship between peak stress and impact load.

It can be seen from this figure that as the impact load increases, the dynamic compressive strength of the skarn specimen also increases. When the impact load is consistent, the peak stress in the dry state is greater than that in the water-saturated state. Through linear fitting, the correlation coefficients of drying and saturation were 0.99 and 0.93, respectively, which are at a high level.

3.3. Analysis of Specimen Failure Morphology

Dynamic impact tests using the SHPB device were conducted on twenty dry specimens and twenty water-saturated specimens. A representative subset of fragments was selected from the results for detailed analysis and presentation. The SHPB impact compression failure modes of dry and water-saturated specimens are shown in Figure 7. It can be seen from the diagram that almost all of the rock crushing is due to splitting failure along the radial loading direction, indicating that the test results are good. Under the action of low impact energy, the rock damage is small. With the increase in impact pressure, the degree of fragmentation of rock samples gradually increases. Due to the high strength, the dry rock sample has the least damage under the action of low impact energy.

Figure 7. Impact failure morphology of skarn specimens.

When the impact pressure is 0.25 MPa, the skarn specimens in the dry state are destroyed into two main blocks and some fragments, and the water-saturated specimens are destroyed into three blocks and some small blocks. When the impact pressure is 0.40 MPa, the skarn specimen loses its bearing capacity, but the failure block diameter of the specimen in the dry state is significantly larger than that of the water-saturated specimen. That is, under the same impact load, the damage degree of skarn specimens under water saturation is more severe than that in the dry state, indicating that the ability of water-saturated rock to resist crushing is lower than that of the dry state; that is, water has a deterioration effect on rock.

4. Analysis of Energy Evolution Characteristics

4.1. Evolution Characteristics of Energy with Strain

From the energy point of view, the deformation and failure of rock under stress consist of a process of energy input, elastic energy accumulation, energy dissipation, and energy release. The rock causes damage under the energy drive until macroscopic instability and failure. Energy dissipation is mainly used for the initiation and propagation of cracks, and energy release is the internal cause of the sudden failure of rock mass. For the actual engineering rock mass, the elastic strain energy stored in the rock volume before excavation is the main source of energy released from the final failure of surrounding rock. The failure process of rock is accompanied by the evolution of energy, so it is very necessary to analyze the failure of the specimen from the perspective of energy [34,35]. In addition, following the classical one-dimensional stress wave assumption in the SHPB methodology [36], the cross-sectional area of the specimen is assumed to remain constant during wave propagation, i.e., lateral dilations are neglected. This assumption is reasonable because lateral strains are much smaller than axial strains in the elastic stage and thus have little influence on stress transmission [36]. In the main stress space, the total input strain energy, releasable elastic strain energy, and dissipation energy of the skarn unit can be expressed as follows [37,38,39,40]:

$$\left\{\begin{array}{l}U={\displaystyle {\int }_0^{\epsilon }\sigma d\epsilon }\\ U_e={\displaystyle \frac{1}{2}}\sigma {\epsilon }_e\\ U_d=U-U_e\end{array}\right.$$

(2)

where $U$, $U_e$, and $U_d$ are the total strain energy, releasable elastic strain energy, and dissipation energy of the skarn unit, respectively; $\sigma$ is the dynamic compressive stress of skarn; $\epsilon$ is the total strain of skarn; and ${\epsilon }_e$ is the elastic strain of skarn.

The relationship between total strain energy per unit volume, releasable elastic strain energy, and dissipated energy with strain during dynamic impact under dry and water-saturated conditions is shown in Figure 8. The total strain energy and dissipation energy per unit volume of skarn increase with the increase in strain, and the growth rate first rises gradually, then tends to be stable. The released elastic strain energy first increases and then decreases with the increase in strain. The energy evolution of skarn with strain during impact can be divided into four stages. In stage I, the total strain energy of the skarn element is not significantly increased, which is due to the fact that the specimen is in the pore compaction stage, at which time the strain continues to increase, but the stress increases relatively slowly. The total strain energy and the released elastic strain energy of the skarn element in the second stage increase synchronously, and the dissipated energy is almost not accumulated. At this time, the skarn is in the stage of elastic deformation, with almost no plastic deformation. The total strain energy is stored in the skarn as elastic energy. In the third stage, the dissipated energy of the skarn unit begins to accumulate, which means that the specimen begins to enter the plastic stage, and the initial crack expands and is accompanied by the formation of new cracks, which gradually produces a large amount of plastic deformation. The starting point of this stage can be called the “crack initiation point” [41]. Stage IV is called the failure stage. In this stage, the skarn produces macroscopic damage, and its bearing capacity decreases. The stress in the skarn begins to drop, and the elastic strain energy cannot continue to accumulate and instead begins to be released. The released elastic strain energy is completely converted into dissipated energy, and the total strain energy tends to remain unchanged after a continuous increase. Therefore, similarly to the deformation stage of skarn, the energy evolution curve can also be divided into four stages: the pore compaction stage, elastic deformation stage, plastic stage, and rock failure stage [42,43,44].

Figure 8. Energy–strain evolution curve of skarn specimen.

From this figure, it is evident that during the compaction and elastic stages, the energy dissipation values of the specimen remain relatively low, with a gradual increase over time. This indicates that the internal structure of the rock remains largely intact during these phases, with no significant macroscopic failure observed. However, a comparative analysis between dry and saturated samples reveals notable differences: the saturated specimens exhibit a higher rate of energy dissipation growth during both the compaction and elastic stages, suggesting that under identical loading conditions, they require greater strain energy absorption. This disparity is primarily attributable to water–rock interactions that induce a series of physico-chemical effects; for instance, water molecules infiltrating mineral lattices or filling microcracks weaken the bonding forces between mineral grains, facilitating the initiation and propagation of microcracks in localized regions. During this process, part of the input energy is converted into the dissipation energy necessary for crack extension, resulting in significantly higher energy dissipation in saturated samples compared to dry ones at this stage.

In contrast, dry specimens, after complete compaction, exhibit fully closed pores and behave as nearly homogeneous, continuous media. Consequently, during the subsequent linear elastic deformation, the formation of new microcracks is minimal, and the stress–strain relationship remains predominantly linear. The energy is primarily stored as elastic strain energy, with a negligible proportion dissipated.

Upon entering the plastic stage, the internal structure of the rock undergoes pronounced damage, and the energy dissipation ratio increases rapidly. During this phase, dry samples demonstrate a higher energy dissipation ratio, with a growth rate markedly exceeding that of earlier stages, indicating the substantial consumption of input energy during plastic deformation and crack propagation. In the final failure stage, the energy dissipation rate in dry specimens surpasses that of saturated samples, reflecting the greater energy expenditure required for rapid crack extension, interconnection, and network failure. Conversely, saturated samples, after initial high energy dissipation, show decelerating growth in energy dissipation as cracks become interconnected, implying that the energy demand for crack propagation during ultimate failure is less pronounced than in dry specimens.

Overall, water–rock interactions not only alter early-stage energy dissipation characteristics but also influence the dominant mechanisms of rock failure at different stages. Saturated rocks are more prone to early crack initiation and energy consumption, whereas dry rocks exhibit higher energy dissipation rates during later plastic failure and ultimate instability.

4.2. Damage Characteristics Based on Energy Evolution

In the dynamic fragmentation process of rock, energy dissipation is the main factor driving the failure of rock materials. According to the laws of thermodynamics, the rock failure process is essentially a process of energy absorption, conversion, and release. Under the action of impact loading, the specimen exhibits irreversible energy dissipation. The failure process of rock is always accompanied by energy evolution; therefore, it is of great significance to analyze the failure mechanism of the specimen from the perspective of energy [45,46]. The relationship between various types of energy during the specimen compression process can be expressed as follows [47]:

$$\left\{\begin{array}{l}{E}_I(t)=A_0{C}_0{E}_0{\displaystyle {\int }_0^t{\epsilon }_{\mathrm{i}}^2}(t)dt\hfill \\ E_R(t)=A_0{C}_0{E}_0{\displaystyle {\int }_0^t{\epsilon }_{\mathrm{r}}^2}(t)dt\hfill \\ E_T(t)=A_0{C}_0{E}_0{\displaystyle {\int }_0^t{\epsilon }_{\mathrm{t}}^2}(t)dt\hfill \end{array}\right.$$

(3)

where $E_I\left(t\right)$, $E_R\left(t\right)$, and $E_T\left(t\right)$ are the incident energy, reflected energy, and transmitted energy of the skarn, respectively.

In the SHPB test, regardless of whether the specimens are in the dry or saturated state, the energy variation process shows a three-stage growth pattern, as illustrated in Figure 9a. Stage 1: Low-speed growth stage. With the increase in time, the energy of the incident wave, reflected wave, and transmitted wave gradually starts to increase at a low speed. Stage 2: High-speed growth stage. The energy of the incident wave, transmitted wave, and reflected wave increases linearly. Stage 3: Stable stage. After growing to a certain time point, the energy of the incident wave, reflected wave, and transmitted wave all tend to stabilize. In the stable stage, the incident energy, reflected energy, and transmitted energy of the specimens in the dry state are higher than those in the saturated state. During the test, the energy of the incident wave is greater than the sum of the energy of the reflected wave and the transmitted wave. The difference between the two (incident wave energy minus the sum of reflected and transmitted wave energy) represents the energy consumed for rock failure during the impact process, i.e., the energy absorbed by the specimen. Figure 9b shows the variation curve of the energy absorbed by the specimens. It can be observed that although the incident energy of the dry specimens is higher than that of the saturated specimens, the energy absorbed by the saturated specimens is greater. This further explains why the saturated specimens exhibit a higher degree of damage under the same impact pressure condition.

Figure 9. Energy evolution change in skarn specimen: (a) energy evolution curve; (b) absorbed energy of specimens.

5. Conclusions

Dynamic uniaxial compression tests were performed on dry and water-saturated skarn specimens using an SHPB system to investigate the effects of water saturation on strength, fragmentation, energy evolution, and damage behavior. The main conclusions are as follows:

(1) The degree of fragmentation intensifies with impact pressure. Under the same loading conditions, water-saturated specimens display finer fragment distributions and higher fractal dimensions, reflecting a more severe damage state than dry specimens. Water saturation thus accelerates the transition from brittle fracture to ductile failure and weakens the rock’s resistance to dynamic crushing.

(2) Energy variation during impact loading can be divided into three stages: initial slow growth, rapid increase, and final stabilization. Although dry specimens absorb less energy overall, their dissipation rate in the plastic stage remains higher. In contrast, water-saturated specimens absorb more energy at failure, which explains their more pronounced fragmentation and damage.

(3) The damage–strain relationship follows an S-shaped pattern of gradual accumulation, rapid growth, and stabilization. In the plastic stage, damage in water-saturated specimens grows faster than in dry specimens, confirming that water accelerates crack propagation and cumulative damage under dynamic loading.