Experimental study on the influence of fissure spacing on the dynamic mechanical properties and energy evolution mechanisms of fissured rock

To investigate the influence of fissure spacing on the dynamic mechanical properties of rock, split Hopkinson pressure bar (SHPB) impact tests were conducted. The rock fracture process was fully recorded using a high-speed camera, and the dynamic fracture behavior and failure mechanisms of rocks containing non-persistent fissures with different spacing were systematically studied. The results show that as the fissure spacing increases, the peak strength and Young’s modulus of the specimens gradually decrease, and brittleness is enhanced. The energy absorption process can be divided into three stages: elastic energy storage, damage propagation, and structural failure, with the transition points corresponding to the yield point and the instability critical point on the stress-time curve, respectively. The energy utilization efficiency of fissured specimens is significantly lower than that of intact specimens, and it first decreases and then increases with increasing fissure spacing, reaching a minimum at a spacing of 15 mm and approaching the level of intact specimens at 30 mm. The failure mode shifts from tensile-dominated persistent cracking to shear-dominated mixed failure as spacing increases, and eventually tends to a disordered, network-like cracking pattern. A critical spacing threshold exists; beyond this threshold, end-face effects become dominant and the persistent interaction between fissures weakens. This study systematically reveals the influence of fissure spacing on the dynamic mechanical behavior and failure mechanisms of rock masses, provides a reference for evaluating the dynamic stability of fissured rock masses, and offers an important basis for further understanding the dynamic mechanical response mechanisms of fractured rock.

1 Introduction

Fissured rock mass is a common surrounding rock medium encountered in various engineering projects such as slope engineering, transportation tunnels, water conservancy projects (including water diversion tunnels), and nuclear waste repositories. Its mechanical properties directly influence the stability of related structures (Zhu et al., 2025; Jia et al., 2025; Zhu et al., 2013; Nguyen et al., 2024; Yin et al., 2022). The presence of fissures disrupts the integrity of the rock mass, significantly weakens its load-bearing capacity, and may ultimately lead to geological hazards such as roof collapse, tunnel cave-ins, landslides, and rockfalls, as illustrated in Figure 1. In addition to static loads, rock masses are also subjected to dynamic loads induced by earthquakes, blasting, and other sources (Wang et al., 2024; Li et al., 2022; Yin et al., 2025; Yin et al., 2024). Dynamic loads, in the form of stress waves, cause dynamic stress concentration in fissured rock masses, leading to fatigue damage and exacerbating rock failure (Ye et al., 2025; Chen et al., 2025; Li et al., 2024; Yin et al., 2021). Therefore, investigating the dynamic mechanical properties and failure characteristics of fissured rock masses is crucial for safety assessment and hazard prevention in geotechnical engineering (Daniel et al., 2025; Yin et al., 2023).

Figure 1

Figure 1. Schematic diagram of persistent and non-persistent structural surfaces.

Over the past few decades, researchers have conducted extensive experimental studies on the mechanical properties and failure behaviors of fissured rock and rock-like materials under static and quasi-static loading conditions (Asadizadeh et al., 2019; Zhang et al., 2018; Sagong et al., 2011; Zhu et al., 2019; Walton et al., 2018). A large body of experimental results indicates that due to the presence of fissures, the internal stress field within the rock undergoes redistribution, leading to a significant influence of the distribution pattern of fissures on the mechanical behavior of fissured rock. In terms of static behavior, the process is primarily characterized by progressive damage accumulation accompanied by multi-stage cooperative crack propagation. Under compressive stress, intense stress concentration occurs at the fissure tips, initiating wing cracks aligned with the direction of the maximum principal stress. These wing cracks extend, connect with secondary shear cracks, and continue to propagate toward the specimen ends, ultimately forming a macroscopic failure zone composed of axial splitting bands along the direction of the maximum principal stress and inclined shear bands along the direction of the maximum shear stress (Wang et al., 2021; Tham et al., 2004).

However, these studies on static mechanics are insufficient to describe the real mechanical behavior of rock masses during excavation processes. Many researchers have conducted experimental investigations into the mechanical behavior of fissured rock and rock-like materials under dynamic loading conditions (Li et al., 2005; Zhou et al., 2010; Hong et al., 2009; Luo et al., 2020; Kong et al., 2021; Zuo et al., 2024). Zhou and Gu (Zhou and Gu, 2022) systematically studied the dynamic mechanical properties, failure modes, and energy evolution of granite with through-going fissures under different joint configurations using split Hopkinson pressure bar (SHPB) tests, with a focus on the influence of loading rate on dynamic failure and energy changes during the failure process. Li et al., 2019a performed dynamic impact tests on red sandstone specimens containing a single bonded planar joint with different dip angles to investigate stress wave propagation and fracture evolution in specimens composed of two rock blocks. Su et al. employed a modified split Hopkinson pressure bar (SHPB) setup to conduct dynamic uniaxial compression tests on sandstone specimens containing weakly filled joints with angles ranging from 0° to 90° at 15° intervals. Shu et al. (2018) prepared intact rock-like specimens and specimens containing a single smooth planar joint at various angles for SHPB testing, examining the effects of loading rate and associated loading angle on the dynamic peak stress/strength of specimens with different joint inclinations.

Persistent and non-persistent fissures are two fundamental occurrence forms of fissures in rock masses, and their spatial distribution characteristics directly govern the engineering geological behavior of the rock mass (Zhang, 2023; Committee, 2024; Ma et al., 2023). The significant differences in their influence on rock materials can directly or indirectly lead to distinct failure modes, failure scales, and instability mechanisms in engineering rock masses (Yin et al., 2021). Currently, when studying the dynamic mechanical properties of rock containing non-persistent fissures using SHPB (Split Hopkinson Pressure Bar) technology, early research often relied on cast rock-like material specimens (e.g., concrete) as substitutes due to the difficulty in obtaining natural specimens and the complexity of prefabricating fissures. This has resulted in insufficient understanding of the dynamic characteristics of non-persistent fissures in real rocks. Compared with rock-like materials, natural rock (such as the red sandstone used in this study) has inherent mineral composition, pore structure, and mechanical anisotropy that are consistent with engineering practice. It can avoid the deviation of mechanical parameters caused by artificial proportioning of rock-like materials, and more truly reflect the dynamic response law of fissured rock masses under impact loads. In addition, real rock specimens have better mechanical parameter stability and reproducibility, which is conducive to accurately analyzing the influence of a single factor (fissure spacing) on the test results. With the widespread adoption of high-precision fissure prefabrication techniques such as wire cutting and water jetting, research focus has shifted rapidly toward the dynamic behavior of complex multi-fissure networks, somewhat neglecting in-depth exploration of the fundamental scientific issues related to single, geometrically simple fissures. On the other hand, when studying rock containing persistent fissures, it is necessary to use high-strength bonding materials to reattach separated rock blocks in order to obtain standard specimens suitable for impact loading. This essential step introduces unavoidable technical interference: the mechanical properties of the adhesive (such as wave impedance), bond layer thickness, and interface quality can significantly alter the propagation path and energy distribution of stress waves, causing the measured response data of the “composite specimen” to be mixed with effects from the bonding interface. Such interference is unavoidable in studies on the dynamic properties of rock containing persistent fissures. Therefore, investigating the mechanical behavior and failure mechanisms of non-persistent fissures under dynamic loading can provide more accurate experimental evidence for the safety assessment of fissured rock masses in engineering contexts.

This study aims to address this research gap. Based on split Hopkinson pressure bar (SHPB) impact tests, red sandstone specimens containing non-persistent fissures with different spacing were used as the research subject. The influence of fissure spacing on the dynamic mechanical properties, failure modes, and energy dissipation characteristics of red sandstone specimens was systematically investigated. The entire rock failure process was recorded and analyzed using a high-speed camera, and the fracture behavior and failure mechanisms of rocks containing non-persistent fissures with varying spacing were explored.

2 Experimental principle and experimental scheme design

2.1 Specimen preparation

The impact test specimens were prepared from homogeneous red sandstone. All specimens were cored from a single intact red sandstone block to eliminate the influence of material inhomogeneity on the test results. In the laboratory, the sandstone was cut into standard cubic specimens measuring 60 mm × 45 mm × 20 mm. During preparation, the end surfaces of the specimens were strictly ground according to standard procedures to ensure that both surface unevenness and non-perpendicularity were less than 0.02 mm (Ulusay, 2015; Dai et al., 2010). Longitudinal wave velocity tests and uniaxial compression tests were conducted on intact specimens (with dimensions of Φ50 mm × 100 mm), and the obtained basic physical and mechanical parameters of the red sandstone are listed in Table 1.

Table 1

Table 1. Basic physical and mechanical parameters of red sandstone.

The P-wave velocity test was performed using an ultrasonic detector. Couplant was applied to both ends of the specimen to ensure close contact between the detector probe and the specimen surface. The propagation time of ultrasonic waves in the specimen was measured, and the P-wave velocity was calculated by dividing the specimen length by the propagation time.

Non-persistent fissures with different spacing were prefabricated on the cubic specimens using wire cutting technology. The fissures had a length of 15 mm and a width of 1 mm. The distribution patterns of the fissures are shown in Figure 2.

Figure 2

Figure 2. Specimen geometry.

2.2 SHPB test and principle

The tests were conducted using the split Hopkinson pressure bar (SHPB) system at the State Key Laboratory of Precision Blasting, Jianghan University (Figure 3). This system is equipped with incident and transmission bars, both made of steel with a diameter of 50 mm and a length of 2.5 m. The bars have a density of 7,850 kg/m³, an elastic modulus of 210 GPa, and a longitudinal wave velocity of 5,172 m/s. The striker is a cylindrical structure with a length of 400 mm. The dynamic impact loading was applied perpendicular to the plane of the prefabricated non-persistent fissures to ensure consistent stress distribution at the fissure tips across all specimens. To ensure stress uniformity during the test, the SHPB system adopted a waveform shaping technique. A circular red copper gasket (diameter 10 mm, thickness 1 mm) was used as a wave shaper, placed between the striker and the incident bar. This technique converts the steep front of the incident wave into a smooth sinusoidal wave, prolonging the loading duration and allowing the specimen to reach stress equilibrium before failure, which satisfies the one-dimensional stress wave assumption of the SHPB test. The incident wave (typically approximated as a sinusoidal waveform) generated by the striker impacting the incident bar is converted into a voltage signal by strain gauges attached to the bars and recorded by a ultra-high dynamic strain acquisition instrument. Based on one-dimensional stress wave theory and the assumption of stress uniformity (Kolsky, 1949), the stress, strain, and strain rate of the specimen at any moment during failure can be calculated using the three-wave method (Dai et al., 2010; Lifshitz and Leber, 1994; Lindholm and Yeakley, 1968) from the recorded voltage signals.

${\sigma }_{\mathrm{s}}\left(t\right)=\frac{A_{\mathrm{e}}}{2A_{\mathrm{s}}}E\left[{\epsilon }_{\mathrm{I}}\left(t\right)-{\epsilon }_{\mathrm{R}}\left(t\right)+{\epsilon }_T\left(t\right)\right](1)$

${\epsilon }_{\mathrm{s}}\left(t\right)=\frac{E}{{\rho }_{\mathrm{e}}{C}_{\mathrm{e}}{L}_{\mathrm{s}}}{\int }_0^t\left[{\epsilon }_{\mathrm{I}}\left(t\right)+{\epsilon }_{\mathrm{R}}\left(t\right)-{\epsilon }_T\left(t\right)\right]\mathrm{d}t(2)$

${\dot{\epsilon }}_{\mathrm{s}}\left(t\right)=\frac{E}{{\rho }_{\mathrm{e}}{C}_{\mathrm{e}}{L}_{\mathrm{s}}}\left[{\epsilon }_{\mathrm{I}}\left(t\right)+{\epsilon }_{\mathrm{R}}\left(t\right)-{\epsilon }_T\left(t\right)\right](3)$

where:

Figure 3

Figure 3. Schematic of SHPB test setup.

σ_s(t) is the specimen stress; ε_s(t)is the specimen strain; ε_s(t)is the specimen strain rate; ε_I(t), ε_R(t) and ε_T(t)are the incident, reflected, and transmitted wave strains, respectively; E is the Young’s modulus of the bars; A_e is the cross-sectional area of the elastic bars; A_s is the cross-sectional area of the specimen; C_e is the wave speed of one-dimensional elastic longitudinal waves in the elastic bars ρ_e is the density of the elastic bars; L_s is the length of the specimen.

The strain rate range of the impact load in this test was controlled at 50–80 s^-1. To eliminate the influence of strain rate on test results, the strain rates were maintained at a similar level (variation within ±5%) for all specimens with different fissure spacings. No obvious strain rate effect was observed within this range, as the dynamic mechanical parameters (peak strength, Young’s modulus) showed no significant correlation with strain rate when the strain rate was kept constant.

A high-speed camera (model: Phantom V2512, frame rate: 100,000 fps, resolution: 1,280 × 800 pixels) was used to record the entire rock failure process, ensuring clear capture of crack initiation, propagation, and coalescence.

The strain rate range of the impact load in this test was controlled at 50–80 s^-1. To eliminate the interference of strain rate on the test results, the gas pressure of the SHPB system was adjusted to ensure that the strain rate of all specimens (including intact and fissured specimens) was within this range with a variation of less than ±5%.

The incident energy, reflected energy, and transmitted energy were calculated from the incident strain, reflected strain, and transmitted strain (Deng et al., 2016; Li et al., 2019b), respectively:

$\left\{\begin{array}{l}{W}_I=A_e{C}_{e}E\int {\epsilon }_I^2\left(t\right)dt=A_e{\rho }_e{C}_e^3\int {\epsilon }_I^2\left(t\right)dt\\ W_R=A_e{C}_{e}E\int {\epsilon }_R^2\left(t\right)dt=A_e{\rho }_e{C}_e^3\int {\epsilon }_R^2\left(t\right)dt\\ W_T=A_e{C}_{e}E\int {\epsilon }_T^2\left(t\right)dt=A_e{\rho }_e{C}_e^3\int {\epsilon }_T^2\left(t\right)dt\end{array}\right.(4)$

where: W_I is the incident energy, W_R is the reflected energy, W_T is the transmitted energy,

(all in joules, J); the definitions and units of the remaining parameters are the same as those given in the previous equation.

The dissipated energy, denoted as W_L, is defined as:

$W_L=W_I-W_R-W_T(5)$

The energy utilization efficiency, denoted as η, is calculated from the dissipated energy and the incident energy:

$\eta =\frac{W_L}{W_I}\times 100\%(6)$

The energy calculation in this study deducted the inherent energy loss of the SHPB system, including damping loss of the bars, friction loss between the specimen and the bars, and energy loss of the measuring system, to ensure the accuracy of the calculated energy parameters.

3 Experimental results

3.1 Dynamic stress balance verification

A prerequisite for ensuring the accuracy of the test is that stress equilibrium is achieved between the two ends of the specimen. This can be verified by calculating and comparing the historical stresses at the left and right ends of the specimen during the test using Equations 7, 8. When P₁(t)≈P₂(t) or ε_I(t) +ε_R(t)≈ε_T(t), dynamic stress equilibrium across the specimen is considered to be satisfied (Ulusay, 2015; Dai et al., 2010).

$P_1\left(t\right)=A_{e}E\left[{\epsilon }_I\left(t\right)+{\epsilon }_R\left(t\right)\right](7)$

$P_2\left(t\right)=A_{e}E{\epsilon }_T\left(t\right)(8)$

In this study, dynamic stress balance verification was performed on all specimens (including intact specimens and fissured specimens with different spacings). Figure 4 shows the stress equilibrium check results from the SHPB impact test. During the SHPB impact test, the forces on the left and right ends of the intact red sandstone specimen were evenly distributed, indicating that the specimen was in a state of dynamic stress equilibrium throughout the experiment. Therefore, the mechanical parameters such as stress and strain of the specimen during failure can be solved and analyzed using Equations 1–3.

Figure 4

Figure 4. Dynamic stress balance verification (a) Voltage signal (b) Intact specimen (c) l = 10 mm (d) l = 15 mm (e) l = 20 mm (f) l = 30 mm.

3.2 Typical stress-strain curve

Based on Equations (1–3), the dynamic stress–strain relationship curves for fissured specimens were plotted (Figure 5). The curve can be divided into five typical stages: compaction deformation (OA), approximately linear elastic deformation (AB), nonlinear deformation (BC), post-peak structural damage and failure (CD), and structural failure unloading (DE) (Hu et al., 2024; Qin et al., 2018). The characteristics of each stage are described below:

Figure 5

Figure 5. Schematic diagram of stress-strain relationship for an intact specimen.

3.2.1 Compaction stage (OA)

The slope of the curve gradually increases, reflecting the closure of micropores within the specimen under compression.

3.2.2 Approximately linear elastic deformation stage (AB)

As the impact load increases, the macroscopic prefabricated fissures in the specimen continue to compress. Due to the oscillatory nature of dynamic stress wave propagation and the influence of the prefabricated fissures, this stage does not exhibit typical static elastic behavior. The slope of the curve shows an initial increase followed by a decrease, but overall it remains approximately linear. Therefore, based on Hooke’s law, the average slope in this stage can be used to approximate the dynamic Young’s modulus of the specimen.

3.2.3 Nonlinear deformation stage (BC)

In this stage, the specimen enters the yield phase, with deformation gradually transitioning from elastic to plastic. The gradual decrease in the curve slope indicates the initiation of microcracks at the fissure tips and their unstable propagation along the stress direction until the stress reaches its peak.

3.2.4 Post-peak structural damage and failure stage (CD)

After the peak stress, internal microcracks rapidly propagate and coalesce to form macroscopic cracks. The decline in the stress–strain curve indicates unloading of the specimen, while macroscopically the specimen still retains its original shape.

3.2.5 Structural failure unloading stage (DE)

Damage in the specimen intensifies, and through-going macroscopic cracks lead to structural failure and loss of the original shape. The load-bearing capacity drops sharply until complete failure occurs.

3.3 Peak stress

Peak stress is the maximum stress that a specimen can withstand during dynamic compressive failure, serving as a key parameter that reflects the ability of rock material to resist failure. The Figure 6 shows the dynamic stress-strain curves for intact and fissured specimens, as well as the variation curve of peak strength with fissure spacing. The presence of fissures significantly reduces the peak strength of the specimens. Four repeated tests were conducted for each fissure spacing condition, and abnormal data were excluded before calculating the average value to ensure the reproducibility of the results. The average peak stress of the intact specimens is 185.53 MPa, while the average peak stresses for specimens with fissure spacings of 10 mm, 15 mm, 20 mm, and 30 mm are 172.30 MPa, 167.42 MPa, 165.02 MPa, and 153.50 MPa, respectively. Compared with the intact specimens, the average peak strains are reduced by 7.13%, 9.76%, 11.05%, and 17.26%, respectively.

Figure 6

Figure 6. Stress-strain curves for each test scheme ((a) Intact specimen, (b) l = 10 mm, (c) l = 15 mm, (d) l = 20 mm, (e) l = 30 mm) and schematic diagram of the variation trend of peak stress with fissure spacing (f).

To illustrate the dynamic loading effect, a comparison between dynamic and static peak strengths is provided. The static uniaxial compressive strength of red sandstone is 114 MPa (Table 1), while the dynamic peak strength of intact specimens reaches 185.53 MPa, indicating a significant dynamic enhancement effect (dynamic strength increase of ∼62.7%). For fissured specimens, the dynamic peak strengths (153.50–172.30 MPa) are also much higher than the static strength, confirming that rock mass exhibits higher resistance to failure under dynamic loading conditions.

3.4 Peak strain

The strain value corresponding to the peak stress is referred to as the peak strain, which represents the critical deformation of the specimen when the peak stress is reached and reflects the maximum deformability of the material under dynamic compressive loading. Studies have shown that the peak strain of rock materials can intuitively reflect their degree of brittleness. The Figure 7 illustrates the variation curve of peak strain with fissure spacing. The average peak strain of the intact specimens is 9.14 × 10⁻³, while the average peak strains for specimens with fissure spacings of 10 mm, 15 mm, 20 mm, and 30 mm are 8.99 × 10⁻³、9.01 × 10⁻³、9.02 × 10^-3 和 9.00 × 10⁻³ respectively. Compared with the intact specimens, the average peak strains are reduced by 1.63%, 1.51%, 1.32%, and 1.54%, respectively. The variation in fissure spacing has a minor influence on the peak strain; therefore, while the presence of fissures increases the brittleness of the rock material, the fissure spacing does not significantly affect this tendency.

Figure 7

Figure 7. Schematic diagram of peak strain variation with fissure spacing.

3.5 Dynamic Young‘s modulus

Young’s modulus reflects the ability of a material to resist elastic deformation; a lower Young’s modulus indicates weaker resistance to elastic deformation (Yi et al., 2024). The Figure 8 illustrates the variation curve of Young’s modulus with fissure spacing. As the fissure spacing increases, the average Young’s modulus of the specimens gradually decreases. The average Young’s modulus of the intact specimens is 23.96 GPa, while the average values for specimens with fissure spacings of 10 mm, 15 mm, 20 mm, and 30 mm are 23.08 GPa, 22.93 GPa, 22.26 GPa, and 21.98 GPa, respectively. Compared with the intact specimens, the average Young’s modulus decreases by 3.68%, 4.32%, 7.11%, and 8.26%, correspondingly.

Figure 8

Figure 8. Schematic diagram of Young’s modulus variation with fissure spacing. (a) Strain (b) Fissure spacing (mm).

3.6 Energy analysis

Based on Equations 4–6, the incident, reflected, and transmitted wave energies, as well as the absorbed energy during the test, were calculated, and the results are presented in the Table 2. Accordingly, energy–time curves for specimens with different fissure inclination angles were plotted, with corresponding stress–time curves added in the Figure 9. The energy absorption curve intuitively reflects the mechanical behavior of the material from initial loading to final failure. In the figure, the absorbed energy curve can be divided into three stages according to the absorption rate: in the first stage, the energy absorption rate gradually increases; in the second stage, it stabilizes; and in the third stage, it gradually slows down until approaching zero. Mapping the energy–time curve onto the stress–time curve (which is similar to the stress–strain curve), the transition between the first and second stages occurs near the yield point, while the transition between the second and third stages lies close to the critical point of structural instability. Therefore, the first stage corresponds to energy accumulation, encompassing the compaction deformation and approximately elastic deformation phases (i.e., segment OB). During this stage, the kinetic energy from the impact is mainly converted into elastic strain energy and stored, with the specimen deforming continuously without forming macroscopic cracks. The second stage represents energy release, including the nonlinear deformation and post-peak structural damage phases (i.e., segment BD). Here, elastic strain energy is released while kinetic energy continues to be absorbed; within this stage, microcracks rapidly propagate and coalesce into macroscopic cracks, eventually forming the main failure surface. The third stage is energy dissipation, corresponding to the structural failure and unloading phase. Both transmitted and absorbed energies gradually stabilize, indicating that the specimen structure has failed and lost its ability to carry and transfer energy; the energy stored in the specimen is transformed into kinetic energy.

Table 2

Table 2. Energy calculation results for different test schemes.

Figure 9

Figure 9. Schematic diagram illustrating the relationship between stress and energy.

It should be clarified that the absorbed energy calculated in this study only includes mechanical energy, such as elastic strain energy stored in the specimen and energy consumed by crack initiation, propagation, and coalescence. Thermal dissipation and other forms of energy are not included. Due to the short duration of dynamic impact (microsecond level), the proportion of thermal dissipation is extremely low (less than 5%), which has negligible impact on the energy analysis results.

The proportions of reflected energy, transmitted energy, and absorbed energy are shown in the figure, while the relationship between energy utilization efficiency and fissure spacing is presented in another Figure 10. Under ideal conditions, during the entire test, the proportion of reflected energy ranges from 50% to 60%, that of transmitted energy from 10% to 15%, and that of absorbed energy from 30% to 35%. Compared with intact specimens, fissured specimens exhibit a significantly lower energy utilization efficiency, indicating that intact specimens require relatively more energy to reach failure. As fissure spacing increases, the energy utilization efficiency first decreases and then increases, reaching its minimum at a spacing of l = 15 mm. At a spacing of l = 30 mm, the energy utilization efficiency approaches that of the intact specimen.

Figure 10

Figure 10. Schematic diagram of (a) energy distribution during testing and (b) variation of energy absorption rate with fissure spacing.

4 Analysis of final failure mode

During the test, the failure process of the specimens was captured using a high-speed camera. The final failure patterns were selected to construct schematic diagrams of the failure modes, which include information such as crack morphology, as Figure 11. The presence of fissures weakens the integrity of the specimen. Under loading, stress tends to concentrate locally at the fissure tips, where microcracks rapidly nucleate along cleavage planes or micro-defects, serving as initiation points for main cracks. Meanwhile, the reflection and superposition of stress waves between fissure surfaces lead to localized energy accumulation, intensifying damage near the fissures and thereby accelerating the formation of main cracks. During the propagation of main cracks, influenced by the inertial effect of dynamic loading, cracks first initiate at the fissure tips and extend toward the incident and transmission ends. Crack propagation causes interconnection between fissures.

Figure 11

Figure 11. Schematic diagram of the final failure morphology for each test scheme. (a) l = 10 mm (b) l = 15 mm (c) l = 20 mm (d) l = 30 mm (e) intact specimen.

When the fissure spacing is l = 10 mm, the main failure surface of the specimen is primarily formed by through-going cracks between fissures and remote cracks connecting the fissures to both ends of the specimen, accompanied by spalling of surface blocks and granular fragments. The main failure surface exhibits tensile–shear failure extending from the top of the incident end to the bottom of the transmission end, with tensile failure being dominant.

When the fissure spacing is l = 15 mm and 20 mm, the main failure surface develops as cracks propagating from the top and bottom of the incident end toward the fissures connect with cracks originating from the fissure surfaces and the transmission end. The main failure surface shows a mixed tensile–shear mode, but shear failure predominates.

When the fissure spacing is l = 30 mm, the specimen fails in a disordered manner. Through-going failure surfaces between fissures are difficult to form, and failure shifts to surfaces connecting the fissures to the transmission end. Compared with the failure pattern of intact specimens, there is a certain similarity—both are accompanied by extensive surface spalling, and the final fragmented morphology exhibits apolygon pattern.

These observations indicate that the failure of fissured specimens is influenced, on one hand, by end effects: when fissures are located close to the incident and transmission ends, cracks tend to propagate preferentially toward these ends. On the other hand, they suggest the existence of a threshold for fissure spacing. When the spacing exceeds this threshold, the failure mode of fissured specimens transitions, and the influence of through-going failure between fissures on the overall failure pattern diminishes.

5 Conclusion

To investigate the influence of fissure spacing on the dynamic mechanical behavior of rock, SHPB impact tests were carried out on red sandstone specimens containing non-persistent fissures with varying spacings. The strength characteristics, deformation behavior, energy evolution, and failure modes of the specimens were systematically studied. The following conclusions were drawn:

1. The spacing of fissures has a significant influence on the dynamic mechanical properties of fissured rock masses. As the fissure spacing increases, the peak strength and Young’s modulus of fissured rock specimens gradually decrease. The presence of fissures reduces the peak strain of the rock material, leading to an increase in its brittleness. However, the effect of fissure spacing on peak strain is relatively limited.

2. Energy analysis reveals that the energy absorption process of the specimens under dynamic loading can be divided into three stages: the elastic energy storage stage (with an increasing energy absorption rate), the damage propagation stage (with a stable absorption rate), and the structural failure stage (where the absorption rate drops to zero). The transition points between these stages correspond to the yield point and the critical point of structural instability on the stress–time curve, respectively. This indicates that the evolution of energy can effectively characterize the entire damage progression of the material, from deformation accumulation and crack propagation to complete failure.

3. Compared with intact specimens, the energy utilization efficiency of fissured specimens decreases significantly, indicating that the failure of intact specimens requires more energy dissipation. As the fissure spacing increases, the energy utilization efficiency first decreases and then increases, reaching its lowest at l = 15 mm, while approaching the level of intact specimens at l = 30 mm.

4. High-speed photography and failure morphology analysis reveal that the fissure spacing significantly influences the failure modes of the specimens: at l = 10 mm, failure is dominated by tensile cracking and exhibits a persistent process; for l = 15–20 mm, the mode shifts to a mixed failure dominated by shear; when l increases to 30 mm, failure shows a disordered, network-like cracking pattern, and the persistent effect between fissures weakens.

5. Fissure-driven failure is governed by end-face effects and exhibits a critical spacing threshold. Beyond this threshold, the influence of persistent connections between fissures on overall failure decreases significantly, and the failure morphology approaches that of intact specimens.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

RY: Formal Analysis, Writing – original draft, Data curation, Methodology. KL: Supervision, Writing – review and editing, Funding acquisition, Conceptualization. ML: Writing – review and editing, Validation. ZY: Validation, Writing – review and editing. YX: Validation, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The present research is financially supported by the National Natural Science Foundation of China (No. 52364005) and the Major Science and Technology Special Project of Yunnan Province (No. 202202AG050014).

Conflict of interest

Author ZY was employed by Kunming Engineering & Research Institute of Nonferrous Metallurgy Co., Ltd.

Author YX was employed by Yunnan Design Institute Group Co., Ltd.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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