Experimental Study on Failure Characteristics and Energy Evolution Law of Coal–Rock Combination Body Under Different Quasi-Static Loading Rates

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors, the manuscript Experimental Study on Deformation, Failure, and Energy Evolution Characteristics of Coal-Rock Combination Body under Different Quasi-static Loading Rates, Manuscript ID: eng-3900231, has some limitations that must be addressed suitably.

Please refer to the list below of the most crucial issues:

1. The Abstract section does not emphasise the main novelty of the paper. Further, the main advantages are not revealed from the results mentioned.
2. Considering the Introduction section, each of the cited items must be referred to separately, especially since the Authors do not present any critical review.
3. Usually, the motivation for the work must be derived from the lack of knowledge, which is properly addressed by the quality literature review. The current form does not provide a suitable critical performance, which is negligible.
4. The sample preparation process, mentioned in section 2.1, must be presented with a diagram. It is not obvious which actions follow after another.
5. Similar to the previous comment, the flow chart of the main line of the experiment must be proposed in section 2.3, considering the full test scheme description.
6. With the words: ‘During the consolidation stage, compaction and closure of primary microcracks, pores, and coal-rock interfaces within the composite cause axial stress to increase slowly…’ How can it be explained?
7. ‘During the post-peak failure stage, axial stress exhibits two distinct rapid drops.’ Where does it derive from?
8. Please make some justification or discussion on the following: ‘The elastic modulus follows a similar pattern, while the peak axial strain shows an initial decrease followed by an increase.’
9. It is rather obvious: ‘The axial stress increases approximately linearly with time, during which three obvious stress drops occur, …’ but the Authors must provide some explanation on why?
10. How do the following respond to the advantage of the results received: ‘However, due to the small fracture scale, the AE amplitude and absolute energy remain at low levels, so the cumulative energy does not increase significantly.’
11. Equations (1)-(4) should be referenced to the primary sources that, from my understanding, are not newly proposed by the Authors.
12. ‘Thus, the macroscopic mechanical properties show increased compressive strength, increased elastic modulus, low energy dissipation degree, and high energy accumulation degree.’ How can this sentence be supported by previous studies, if exist?
13. What does the following indicate? ‘As a result, the total number of fracture events increases instead, and the macroscopic mechanical properties exhibit decreased compressive strength, decreased elastic modulus, and the emergence of impact failure effects.’
14. Any future proposals in the Conclusions, if the Authors do not mention The Outlook

Additionally, as a minor correction:

15. ‘…rate of 0.11 mm/min; The pre-peak…’ probably the dot should be used except for the semicolon?

From the above, the reviewed manuscript must be improved appropriately before any further processing of the Eng journal, if allowed by the handling Editor.

Author Response

Comments 1: The Abstract section does not emphasise the main novelty of the paper. Further, the main advantages are not revealed from the results mentioned.

Response 1: Thank you for pointing this out. We have made further revisions and improvements to the abstract section to better highlight the innovations of the paper. The specific revisions are as follows:

Abstract

The advancing speed of the coal mining face has a significant impact on the mining-induced stress and energy accumulation of the surrounding rock. To explain the influence mechanism from a mesoscopic perspective, this study conducted an uniaxial compression test on coal-rock combination body under different quasi-static loading rates, and analyzed their mechanical properties, failure characteristics, acoustic emission characteristics and energy evolution characteristics. The main findings are as follows: The uniaxial compressive strength and elastic modulus of the coal-rock combination body show a variation law of first increasing and then decreasing with the increase of loading rate, while the degree of impact failure significantly increases gradually as the loading rate rises. With the increase of loading rate, there is a tendency that the AE parameters concentrate from the first two stages to the latter two stages. The post-peak residual elastic energy density of the coal-rock combination body increases gradually with the increase of loading rate. The formation of the advancing speed effect of mining-induced stress concentration and elastic energy accumulation in coal-rock masses is caused by the "competitive" interaction between fracture propagation and coal matrix damage when the coal component in the coal-rock combination is deformed under stress.

Comments 2: Considering the Introduction section, each of the cited items must be referred to separately, especially since the Authors do not present any critical review.

Response 2: Thank you for pointing this out. We have added the critical review in the Introduction section. The specific revisions are as follows:

Among the current research results, the loading rates used in tests on the loading rate effect of coal-rock combination body fall outside the scope of quasi-static mechanics. However, during coal seam mining, the advancing process of the working face is basically a stable process, and most of it belongs to the scope of quasi-static mechanics. Within the range of quasi-static loading rates, in-depth research on the failure modes and energy evolution laws of coal-rock combinations under different loading rate conditions is still insufficient. Therefore, this study took the coal-rock combination body as the research object, conducts uniaxial compression tests on the coal-rock combination body under different quasi-static loading rates, analyzed its strength, deformation and failure characteristics, the variation law of acoustic emission characteristic parameters, as well as the accumulation, dissipation and release processes of strain energy, and revealed the formation mechanism of the advancing speed effect on the mining-induced stress concentration and elastic energy accumulation of coal-rock mass.

Comments 3: Usually, the motivation for the work must be derived from the lack of knowledge, which is properly addressed by the quality literature review. The current form does not provide a suitable critical performance, which is negligible.

Response 3: Thank you for pointing this out. We have added the critical review in the Introduction section.

Comments 4: The sample preparation process, mentioned in section 2.1, must be presented with a diagram. It is not obvious which actions follow after another.

Response 4: Thank you for pointing this out. We have added a clear diagram of the sample preparation process in section 2.1.

Comments 5: Similar to the previous comment, the flow chart of the main line of the experiment must be proposed in section 2.3, considering the full test scheme description.

Response 5: Thank you for pointing this out. We agree with this comment. We have added the main line of the experiment in section 2.3.

Comments 6: With the words: ‘During the consolidation stage, compaction and closure of primary microcracks, pores, and coal-rock interfaces within the composite cause axial stress to increase slowly…’ How can it be explained?

Response 6: Thank you for pointing this out. As typical sedimentary rocks, sandstone and coal naturally contain internal pores (such as intergranular pores and cement pores) and primary microcracks (such as bedding fractures and particle contact fractures formed during sedimentation). These defects directly determine their unique mechanical behavior in the "compaction stage" under uniaxial compression. The compaction stage is essentially a process in which external pressure drives the gradual elimination of internal microdefects; during this process, the axial strain increases gradually while the axial stress shows no significant growth.

Comments 7: ‘During the post-peak failure stage, axial stress exhibits two distinct rapid drops.’ Where does it derive from?

Response 7: Thank you for pointing this out. The basis for this conclusion is derived from Figure 3: the first obvious stress drop is caused by the penetration of macroscopic fractures in the coal sample, while the second obvious stress drop results from the complete bearing failure of the coal-rock composite.

Comments 8: Please make some justification or discussion on the following: ‘The elastic modulus follows a similar pattern, while the peak axial strain shows an initial decrease followed by an increase.’

Response 8: Thank you for pointing this out. We have conducted further analysis and discussion on this argument in the Discussion section.

Comments 9: It is rather obvious: ‘The axial stress increases approximately linearly with time, during which three obvious stress drops occur, …’ but the Authors must provide some explanation on why?

Response 9: Thank you for pointing this out. In figure 6, during the loading process from 8000 to 9500 s, the specimen is in the unstable crack propagation stage. The primary micropores and fractures in the sandstone component and coal component gradually expand and interconnect, leading to a decrease in the load-bearing capacity of the coal-rock composite. Consequently, the phenomenon of multiple stress drops is exhibited.

Comments 10: How do the following respond to the advantage of the results received: ‘However, due to the small fracture scale, the AE amplitude and absolute energy remain at low levels, so the cumulative energy does not increase significantly.’

Response 10: Thank you for pointing this out. We have conducted further analysis and discussion on this argument in the Discussion section.

Comments 11: Equations (1)-(4) should be referenced to the primary sources that, from my understanding, are not newly proposed by the Authors.

Response 11: Thank you for pointing this out. We have added the corresponding references to these formula.

Comments 12: ‘Thus, the macroscopic mechanical properties show increased compressive strength, increased elastic modulus, low energy dissipation degree, and high energy accumulation degree.’ How can this sentence be supported by previous studies, if exist?

Response 12: Thank you for pointing this out. We have added the corresponding references to these formula in the Discussion section.

Comments 13: What does the following indicate? ‘As a result, the total number of fracture events increases instead, and the macroscopic mechanical properties exhibit decreased compressive strength, decreased elastic modulus, and the emergence of impact failure effects.’

Response 13: Thank you for pointing this out. We have added the corresponding references to these formula in the Discussion section.

Comments 14: Any future proposals in the Conclusions, if the Authors do not mention The Outlook

Response 14: Thank you for pointing this out. We have elaborated on the prospects for future research based on the research foundation of this paper. The specific revisions are as follows:

6. Outlook

All the research in this paper on the mechanical properties, failure modes, acoustic emission characteristics, and energy evolution characteristics of coal-rock composites is based on uniaxial compression tests. However, in the actual environment, coal-rock masses are in a triaxial stress state, and the stress magnitude in each direction changes with the mining of the working face. Therefore, in future research, triaxial compression tests on coal-rock composites under mining-induced stress paths will be conducted to analyze the mechanical properties and energy evolution characteristics of coal-rock composites under different loading and unloading rate conditions.

Comments 15: ‘…rate of 0.11 mm/min; The pre-peak…’ probably the dot should be used except for the semicolon?

Response 15: Thank you for pointing this out. We have made comprehensive revisions to the relevant punctuation marks.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This article examines deformation, failure, and energy evolution in coal-rock samples under varying quasi-static loading rates. The study demonstrates that changes in loading rate directly affect the strength, elastic modulus, and susceptibility of the system to impact failure, and is accompanied by consistent changes in acoustic emission signals and energy redistribution. It was found that with increasing loading rate, strength characteristics initially increase and then decrease, while residual elastic energy after failure steadily increases, increasing the likelihood of sudden dynamic effects. These results are important for understanding the mechanism of energy accumulation and release in rock masses during underground coal mining. The practical value of this study lies in the possibility of predicting and mitigating the risk of rock bursts and other dynamic accidents, which is critical for ensuring the safety and sustainability of deep mining operations.
However, the article requires further revision. 1. The review covers the influence of loading/advancement rate and AE, but in places is enumerative in nature without critical synthesis: it does not explain the exact gaps in knowledge regarding coal-rock composites in quasi-statics, how your measurement scheme addresses these gaps, and how it differs from studies on single coal/rock.
2. The objective is stated as "revealed mechanism…", but no explicit hypotheses are set: (H1) nonlinear dependence of UCS/E on velocity; (H2) shift in the concentration stage of AE parameters; (H3) increase in post-peak elastic energy with increasing impact. Explicitly formulated hypotheses will allow for coordination of subsequent sections, the specification of statistical criteria, and verification of "data self-consistency" in the Discussion and Conclusions.
3. Materials – origin and representativeness of samples (lines 200–250). The mine and layer are described, but the petrography/petrographic groups of coals, ash/moisture content, porosity, and strength variability of coal and sandstone are not disclosed. These parameters directly influence fracturing, AE, and moduli.
4. Sampling plan and statistics (rows 330–360). Table 1 shows only four samples and physical parameters, but the results analyze four loading conditions. It is unclear how many replicates are per condition and how inter-sample variability is accounted for. For UCS/E/εpeak and energies, error bar plots, ANOVA/Kruskal-Wallis tests, post-hoc comparisons, and effect sizes are required.
5. Results 3.1 – Fracture morphology and the role of sandstone (rows 430–520). The sandstone is stated to be "almost free of visible damage," with failure concentrated in the coal. This is important for the model, but high-resolution photos of faces/ends, crack tracing, and interface interaction features are missing.
6. Results 3.2 — Selection of AE metrics (lines 540–680). The analysis is limited to "ring count, cumulative energy, amplitude, absolute energy." Since the differences between regimes are subtle, including the b-value, RA/AF, and the proportion of high-energy events could improve diagnostics. Consider introducing integral staging indicators (e.g., the proportion of events >90 dB before peak) and normalization by loading time/energy to avoid artifacts from different test durations.
7. Results 3.3 — Energy formulas and assumptions (lines 760–860). The energy balance is written for an adiabatic process without heat transfer. This is a strong simplification for coal with crack/weld friction. Describe the sensitivity of the results to this assumption, estimate the contribution of heat release (at least an order-of-magnitude estimate), and justify the use of the "elastic" expression in the presence of noticeable nonlinearity before the peak. This is important for the correct extraction of U1 and U2.
8. Results 3.3 — residual elastic energy and impact (lines 940–1020). A monotonic dependence of U1, post-peak on velocity (30.6 → 63.2 kJ/m³) is shown. However, the impact is described qualitatively ("not obvious/obvious/extremely obvious"). Introduce a quantitative indicator of impact (unloading front velocity, kinetic energy of fragment ejection, acoustic pulse amplitude >X dB after the peak) and calculate the correlation coefficient with U1, post-peak to prove the claimed relationship.
9. Lines 1080–1140. The increase in "impact" at high speeds may be due not only to the "competition" between cracks and the matrix, but also to weld interface effects/triboemission. Discuss alternatives: the influence of adhesive thickness, carbon inhomogeneities, localized heating in cracks, and grain size scaling effects.
10. Most of the conclusions are consistent with Sections 3.1–3.3. However, the claim of "uncovering the mechanism" appears stronger than the data suggest without microstructural visualization and quantitative impact metrics. We recommend softening the wording to "a mechanism consistent with observations is proposed" and explicitly noting the limitations: small sample size, lack of statistical tests, and the assumption of adiabaticity.

Author Response

Comments 1: The review covers the influence of loading/advancement rate and AE, but in places is enumerative in nature without critical synthesis: it does not explain the exact gaps in knowledge regarding coal-rock composites in quasi-statics, how your measurement scheme addresses these gaps, and how it differs from studies on single coal/rock.

Response 1: Thank you for pointing this out. We have revised and improved the literature review section. We further clarify the specific knowledge gaps existing in the research field of coal-rock combinations under quasi-static conditions, and explain how the test scheme of this study fills these gaps. The specific revisions are as follows:

Among the current research results, the loading rates used in tests on the loading rate effect of coal-rock combination body fall outside the scope of quasi-static mechanics. However, during coal seam mining, the advancing process of the working face is basically a stable process, and most of it belongs to the scope of quasi-static mechanics. Within the range of quasi-static loading rates, in-depth research on the failure modes and energy evolution laws of coal-rock combinations under different loading rate conditions is still insufficient. Therefore, this study took the coal-rock combination body as the research object, conducts uniaxial compression tests on the coal-rock combination body under different quasi-static loading rates, analyzed its strength, deformation and failure characteristics, the variation law of acoustic emission characteristic parameters, as well as the accumulation, dissipation and release processes of strain energy, and revealed the formation mechanism of the advancing speed effect on the mining-induced stress concentration and elastic energy accumulation of coal-rock mass.

Comments 2: The objective is stated as "revealed mechanism…", but no explicit hypotheses are set: (H1) nonlinear dependence of UCS/E on velocity; (H2) shift in the concentration stage of AE parameters; (H3) increase in post-peak elastic energy with increasing impact. Explicitly formulated hypotheses will allow for coordination of subsequent sections, the specification of statistical criteria, and verification of "data self-consistency" in the Discussion and Conclusions.

Response 2: Thank you for pointing this out. We have explicitly put forward the research hypotheses in the corresponding section.

Comments 3: Materials—origin and representativeness of samples (lines 200–250). The mine and layer are described, but the petrography/petrographic groups of coals, ash/moisture content, porosity, and strength variability of coal and sandstone are not disclosed. These parameters directly influence fracturing, AE, and moduli.

Response 3: Thank you for pointing this out. We have supplemented the relevant parameters of the coal samples and rock samples in Section 2.1. The specific revisions are as follows:

Through testing, the coal sample has well-developed internal fractures, with an ash content of 17.9%, a moisture content of 1.8%, and an uniaxial compressive strength of approximately 16 MPa, showing weak rock burst tendency. The rock sample is relatively dense, with well-developed bedding, an uniaxial compressive strength of about 50 MPa, and exhibits strong rock burst tendency.

Comments 4: Sampling plan and statistics (rows 330–360). Table 1 shows only four samples and physical parameters, but the results analyze four loading conditions. It is unclear how many replicates are per condition and how inter-sample variability is accounted for. For UCS/E/εpeak and energies, error bar plots, ANOVA/Kruskal-Wallis tests, post-hoc comparisons, and effect sizes are required.

Comments 5: Results 3.1 – Fracture morphology and the role of sandstone (rows 430–520). The sandstone is stated to be "almost free of visible damage," with failure concentrated in the coal. This is important for the model, but high-resolution photos of faces/ends, crack tracing, and interface interaction features are missing.

Comments 6: Results 3.2 — Selection of AE metrics (lines 540–680). The analysis is limited to "ring count, cumulative energy, amplitude, absolute energy." Since the differences between regimes are subtle, including the b-value, RA/AF, and the proportion of high-energy events could improve diagnostics. Consider introducing integral staging indicators (e.g., the proportion of events >90 dB before peak) and normalization by loading time/energy to avoid artifacts from different test durations.

Comments 7: Results 3.3 — Energy formulas and assumptions (lines 760–860). The energy balance is written for an adiabatic process without heat transfer. This is a strong simplification for coal with crack/weld friction. Describe the sensitivity of the results to this assumption, estimate the contribution of heat release (at least an order-of-magnitude estimate), and justify the use of the "elastic" expression in the presence of noticeable nonlinearity before the peak. This is important for the correct extraction of U1 and U2.

Comments 8: Results 3.3 — residual elastic energy and impact (lines 940–1020). A monotonic dependence of U1, post-peak on velocity (30.6 → 63.2 kJ/m³) is shown. However, the impact is described qualitatively ("not obvious/obvious/extremely obvious"). Introduce a quantitative indicator of impact (unloading front velocity, kinetic energy of fragment ejection, acoustic pulse amplitude >X dB after the peak) and calculate the correlation coefficient with U1, post-peak to prove the claimed relationship.

Comments 9: Lines 1080–1140. The increase in "impact" at high speeds may be due not only to the "competition" between cracks and the matrix, but also to weld interface effects/triboemission. Discuss alternatives: the influence of adhesive thickness, carbon inhomogeneities, localized heating in cracks, and grain size scaling effects.

Comments 10: Most of the conclusions are consistent with Sections 3.1–3.3. However, the claim of "uncovering the mechanism" appears stronger than the data suggest without microstructural visualization and quantitative impact metrics. We recommend softening the wording to "a mechanism consistent with observations is proposed" and explicitly noting the limitations: small sample size, lack of statistical tests, and the assumption of adiabaticity.

Response to 4-10: Thank you for pointing these out. We fully recognize that the research data in this paper is insufficient to support this mechanism, lacking micro-visualization evidence and impact-related quantitative indicators. Therefore, we have weakened the relevant expressions in the discussion section and clearly stated the limitations of this study. We would like to express our gratitude again to the reviewers for their valuable suggestions, which also provide guidance for our subsequent research. In our future studies, we will focus on the following aspects: Further increasing the number of samples in the experiments and analyzing the dispersion of the data. Introducing scanning electron microscopy (SEM) for microscopic observation to track the development process of cracks, obtain high-resolution images of the sample end faces, and clarify the interaction characteristics of the interfaces. Using high-speed cameras and other tools to monitor the ejection process of debris at the moment of sample failure, calculating the ejection kinetic energy of the debris, establishing quantitative analysis indicators for impact, and determining the correlation coefficient between these indicators and the post-peak residual elastic energy.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study takes the coal-rock combination body as the research object, conducts uniaxial compression tests on the coal-rock combination body under different quasi-static loading rates, analyzes its strength, deformation and failure characteristics, the variation law of acoustic emission characteristic parameters, as well as the accumulation, dissipation and release processes of strain energy, and reveals the formation mechanism of the effect of advancing speed on mining-induced stress concentration and elastic energy accumulation of coal-rock mass. However, there are the following issues that need to be revised:

1. Both the coal samples and rock samples used in the tests of this paper are taken from the 23908 working face of Zhangshuanglou Coal Mine. What is the reason for this selection? It is suggested to add a detailed explanation in the corresponding section.
2. The expression in the abstract needs to be revised again. The current format of the abstract fails to clearly present the research questions, methods, results and effects, making it inconvenient for readers to extract key information.
3. The article mainly focuses on the research of the failure process and energy evolution characteristics of the coal-rock combination body under different loading rates. The title needs to be refined, as the current title lacks a clear focus and is overly lengthy.
4. The title of Section 3.2 of the article is "Damage Evolution Characteristics", but the section mainly analyzes the evolution process of acoustic emission signals, and the "damage evolution characteristics" are not clearly reflected. It is suggested that the authors revise and improve the content of this part; in addition, the expression needs to be more concise.
5. Is the discussion and analysis of Figure 12 based on the previous research? If yes, the basis should be provided; if not, the corresponding references should be added. In addition, if there are analyses in other parts of the article that do not belong to the research results of this paper, the corresponding references should also be added.
6. The language of the article needs to be further improved. For example, there are spelling errors in many parts of the text, which need to be corrected.
7. The introduction section requires a more comprehensive literature review. Most of the literature review in this paper consists of research conclusions by domestic scholars, and relevant research conclusions by foreign scholars should be added.
8. What are the main differences between the research conclusions of this paper and those of other similar literatures, and what are the main innovations? These need to be further emphasized in the conclusion section.

Based on the above comments, this paper has good innovation, relatively substantial research content and rigorous logic, but there are still some shortcomings. A major revision is recommended.

Author Response

Comments 1: Both the coal samples and rock samples used in the tests of this paper are taken from the 23908 working face of Zhangshuanglou Coal Mine. What is the reason for this selection? It is suggested to add a detailed explanation in the corresponding section.

Response 1: Thank you for pointing this out. The results of the rock burst tendency assessment indicate that the coal seam in this working face has a weak rock burst tendency, while the fine sandstone immediate roof has a strong rock burst tendency. Conducting the relevant research in this paper based on samples collected from this working face is of strong pertinence and guiding significance.

Comments 2:  The expression in the abstract needs to be revised again. The current format of the abstract fails to clearly present the research questions, methods, results and effects, making it inconvenient for readers to extract key information.

Response 2: Thank you for pointing this out. We have revised and improved the abstract section to emphasise the main novelty of the paper.

Comments 3: The article mainly focuses on the research of the failure process and energy evolution characteristics of the coal-rock combination body under different loading rates. The title needs to be refined, as the current title lacks a clear focus and is overly lengthy.

Response 3: Thank you for pointing this out. We have revised and improved the title of the paper.

Comments 4: The title of Section 3.2 of the article is "Damage Evolution Characteristics", but the section mainly analyzes the evolution process of acoustic emission signals, and the "damage evolution characteristics" are not clearly reflected. It is suggested that the authors revise and improve the content of this part; in addition, the expression needs to be more concise.

Response 4: Thank you for pointing this out. We have revised and improved Section 3.2 to make its expression more concise and its content more aligned with the title.

Comments 5: Is the discussion and analysis of Figure 12 based on the previous research? If yes, the basis should be provided; if not, the corresponding references should be added. In addition, if there are analyses in other parts of the article that do not belong to the research results of this paper, the corresponding references should also be added.

Response 5: Thank you for pointing this out. The discussion and analysis of Figure 12 is based on other research results. We have added the corresponding references in this section.

Comments 6: The language of the article needs to be further improved. For example, there are spelling errors in many parts of the text, which need to be corrected.

Response 6: Thank you for pointing this out. We have improved the language of the paper and corrected the spelling errors.

Comments 7: The introduction section requires a more comprehensive literature review. Most of the literature review in this paper consists of research conclusions by domestic scholars, and relevant research conclusions by foreign scholars should be added.

Response 7: Thank you for pointing this out. We have added the relevant research conclusions by foreign scholars in the literature review and presented the critical review.

Comments 8: What are the main differences between the research conclusions of this paper and those of other similar literatures, and what are the main innovations? These need to be further emphasized in the conclusion section.

Response 8: Thank you for pointing this out. We have revised and refined the content of relevant sections such as the abstract and conclusions to further highlight the innovations of the paper.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

  Dear Author, find the attached comments for your paper improvement.   

Comments for author File: Comments.pdf

Author Response

Comments 1: In the introduction section the literature is not enough, mentioning your research objective.

Response 1: Thank you for pointing this out. We have conducted a further literature review and presented the critical review.

Comments 2: Compare your result with some international research, and how your result is novel from research published in the same fields.

Response 2: Thank you for pointing this out. Among the current research results, the loading rates used in tests on the loading rate effect of coal-rock combination body fall outside the scope of quasi-static mechanics. However, during coal seam mining, the advancing process of the working face is basically a stable process, and most of it belongs to the scope of quasi-static mechanics. Within the range of quasi-static loading rates, in-depth research on the failure modes and energy evolution laws of coal-rock combinations under different loading rate conditions is still insufficient. this study took the coal-rock combination body as the research object, conducts uniaxial compression tests on the coal-rock combination body under different quasi-static loading rates, analyzed its strength, deformation and failure characteristics, the variation law of acoustic emission characteristic parameters, as well as the accumulation, dissipation and release processes of strain energy, and revealed the formation mechanism of the advancing speed effect on the mining-induced stress concentration and elastic energy accumulation of coal-rock mass.

Comments 3: In the discussion section compare your discussion with international research published in the same fields.

Response 3: Thank you for pointing this out. We have compared our discussion with international research published in the same fields. The specific revisions are as follows:

Taking the coal-rock combinations made from on-site collected coal samples and rock samples as the research object, this paper explores their mechanical behaviors under different quasi-static loading rates. For different loading rates, this paper analyzes and compares the evolution laws of mechanical parameters, failure modes, and energy accumulation and release processes of the coal-rock combinations. Similar studies have been conducted separately in previous research. The relationship between rock mechanical behaviors and loading rates is one of the most important research directions in the field of laboratory rock mechanics. In terms of rock strength, some scholars have proposed that there is a positive correlation between rock strength and loading rate [33]. However, it should be noted that the research objects of these studies are mostly high-strength rocks, while the coal-rock combinations used in this paper are of low-strength type. In the study of strength characteristics, the test results show that the uniaxial strength of coal-rock combinations presents a trend of "first increasing and then decreasing" with the change of loading rate, and there is a threshold range of loading rate during this process. This result is highly consistent with the research conclusions of Li et al. [16, 34].

Comments 4: The discussion section is not enough. Please add more information.

Response 4: Thank you for pointing this out. We have added more information in the discussion section. The specific revisions are as follows:

Taking the coal-rock combinations made from on-site collected coal samples and rock samples as the research object, this paper explores their mechanical behaviors under different quasi-static loading rates. For different loading rates, this paper analyzes and compares the evolution laws of mechanical parameters, failure modes, and energy accumulation and release processes of the coal-rock combinations. Similar studies have been conducted separately in previous research. The relationship between rock mechanical behaviors and loading rates is one of the most important research directions in the field of laboratory rock mechanics. In terms of rock strength, some scholars have proposed that there is a positive correlation between rock strength and loading rate [33]. However, it should be noted that the research objects of these studies are mostly high-strength rocks, while the coal-rock combinations used in this paper are of low-strength type. In the study of strength characteristics, the test results show that the uniaxial strength of coal-rock combinations presents a trend of "first increasing and then decreasing" with the change of loading rate, and there is a threshold range of loading rate during this process. This result is highly consistent with the research conclusions of Li et al. [16, 34].

Comments 5: In the conclusion section where are the key findings, the implication and significance of your research, limitation and future research direction. Please ensure that the abstract and conclusion section do not repeat the same information.

Response 5: Thank you for pointing this out. We have added the key findings, the implication and significance of your research. We added the limitation and future research direction in the Outlook Section. The specific revisions are as follows:

5. Conclusions

The uniaxial compressive strength and elastic modulus of the coal-rock combination body show a variation law of first increasing and then decreasing with the increase of loading rate, while the degree of impact failure significantly increases gradually as the loading rate rises. The post-peak residual elastic energy density of the coal-rock combination body increases gradually with the increase of loading rate, which indicates that the post-peak residual elastic energy density of the coal-rock combination body has a positive correlation with its impact failure effect.

The formation of the advancing speed effect of mining-induced stress concentration and elastic energy accumulation in coal-rock masses is caused by the "competitive" interaction between fracture propagation and coal matrix damage when the coal component in the coal-rock combination is deformed under stress.

During low-speed advancing, the loading rate is relatively low, providing sufficient time for the development and propagation of fractures. Consequently, the macroscopic mechanical properties are characterized by low compressive strength, small elastic modulus, high energy dissipation, low energy accumulation, minimal energy release during failure, and a weak impact failure effect.

In the case of high-speed advancing, the higher loading rate leads to insufficient development and propagation of fractures. The coal matrix, acting as a framework, exerts a significant load-bearing effect. As a result, the macroscopic mechanical properties exhibit increased compressive strength, larger elastic modulus, low energy dissipation, high energy accumulation, substantial energy release during failure, and a strong impact failure effect.

6. Outlook

All the research in this paper on the mechanical properties, failure modes, acoustic emission characteristics, and energy evolution characteristics of coal-rock composites is based on uniaxial compression tests. However, in the actual environment, coal-rock masses are in a triaxial stress state, and the stress magnitude in each direction changes with the mining of the working face. Therefore, in future research, triaxial compression tests on coal-rock composites under mining-induced stress paths will be conducted to analyze the mechanical properties and energy evolution characteristics of coal-rock composites under different loading and unloading rate conditions.

Comments 6: Check all the figures and tables carefully.

Response 6: Thank you for pointing this out. We have checked all the figures and tables carefully and revised the existing issues.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Paper is now suitable to be recommended for publication.

Author Response

Dear Reviewer, Hello! First of all, on behalf of all the authors, I would like to extend our most sincere gratitude and highest respect to you for taking precious time out of your busy schedule to review our manuscript! Your review comments are professional, detailed and highly targeted. They not only accurately point out the deficiencies in the manuscript, but also put forward many constructive suggestions for improvement, providing a crucial guiding direction for us to further improve the quality of the manuscript. During the review process, your in-depth understanding of the research content, rigorous control of academic details, and keen insight into cutting-edge issues in the field have made us deeply admire, and also enabled us to have a more comprehensive and in-depth understanding of the research itself.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors significantly expanded the justification and description of the samples; added photographs of the final fracture morphology; systematized the description of AE stages and plotted a residual elastic energy graph. However, the statistical section, extended AE indicators, sensitivity of energy calculations to heat removal, quantitative metrics for "impactfulness," and alternative mechanisms remained poorly implemented.

What still needs to be improved:

Introduce explicit hypotheses (H1–H3). A short subsection in the introduction: H1 — nonlinear dependence of UCS/E on velocity; H2 — shift in the concentration of AE parameters to late stages; H3 — increase in U₁, post-peak with "impactfulness." Then, a connection to the statistical plan. (Currently missing.)

Statistics and repeatability.
Specify n for each velocity;
ANOVA/Kruskal-Wallis + post-hoc and effect sizes.

AE diagnostics.
Calculate the b-value, RA/AF (at least for key stages);
Introduce integral fractions (e.g., fraction of events >90 dB before peak) normalized by time/work;

Compare by speed.

Determine a quantitative indicator (unloading rate; emission energy by video/fragment mass; fraction of AE >X dB after peak);
Correlate this metric with U₁, post-peak (r, p, confidence interval). (Not currently available.)

Energetics: sensitivity to heat transfer.
Estimate the contribution of heat from friction/triboemission/viscous damping (order of magnitude);
How does this affect the decomposition of U = U₁ + U₂ and the estimate of U₁, post-peak;
Brief justification of "elastic" formulas for extreme nonlinearity before the peak. (Not currently considered.)

Morphology and interface.
Comment on adhesive thickness/rigidity and its possible influence on fracture kinematics. (Currently partially considered.)

Restate conclusions as "proposed mechanism consistent with observations," rather than definitively established.

Author Response

Dear Reviewer, Hello! First of all, on behalf of all the authors, I would like to extend our most sincere gratitude and highest respect to you for taking precious time out of your busy schedule to review our manuscript! Your review comments are professional, detailed and highly targeted. They not only accurately point out the deficiencies in the manuscript, but also put forward many constructive suggestions for improvement, providing a crucial guiding direction for us to further improve the quality of the manuscript. During the review process, your in-depth understanding of the research content, rigorous control of academic details, and keen insight into cutting-edge issues in the field have made us deeply admire, and also enabled us to have a more comprehensive and in-depth understanding of the research itself.

Comments 1: Introduce explicit hypotheses (H1–H3). A short subsection in the introduction: H1 — nonlinear dependence of UCS/E on velocity;

H2 — shift in the concentration of AE parameters to late stages;

H3 — increase in U₁, post-peak with "impactfulness." Then, a connection to the statistical plan. (Currently missing.)

Response 1: Considering the Reviewer’s suggestion, we have added the explicit hypotheses in the introduction. The corresponding changes have been marked in red and bold in the original text:

In this paper, we introduce 3 hypotheses: 1) The ratio of uniaxial compressive strength to elastic modulus varies nonlinearly with the loading rate. 2) The concentration interval of acoustic emission parameters shifts toward the later stage of the stress-strain curve. 3) The elastic strain energy in the post-peak stage increases with the enhancement of the impact property of specimen failure.

Comments 2: Statistics and repeatability.
Specify n for each velocity;
ANOVA/Kruskal-Wallis + post-hoc and effect sizes.

Response 2: Thank you for your professional insight! Due to the insufficient sample size, we cannot fully implement this experiment currently. However, we have discussed this limitation in the Outlook and proposed it as a future research direction. Your suggestion has greatly helped improve the study’s rigor.

Comments 3: AE diagnostics.
Calculate the b-value, RA/AF (at least for key stages);
Introduce integral fractions (e.g., fraction of events >90 dB before peak) normalized by time/work;

Compare by speed.

Response 3: Thank you for pointing this out. We introduced the RA/AF value to conduct a further analysis of the characteristics of acoustic emission. The corresponding changes have been marked in red and bold in the original text:

The data distribution of RA and AF values during the deformation and failure process of coal-rock combinations under different loading rates is shown in Figure 10. It can be seen that the failure mode of coal-rock combinations is dominated by tensile failure. When the loading rate is 0.01 mm/min, the proportions of tensile cracks and shear cracks are 91.01% and 8.99% respectively; when the loading rate is 0.03 mm/min, the proportions are 92% and 8% respectively; when the loading rate is 0.11 mm/min, the proportions are 91.4% and 8.6% respectively; and when the loading rate is 0.16 mm/min, the proportions are 90.5% and 9.5% respectively. The loading rate has little effect on the failure type of coal-rock combinations. In terms of the number of failure events, when the loading rate is 0.01 mm/min, the number of tensile failures is 42,415, the number of shear failures is 4,189, and the total number is 46,604; when the loading rate is 0.03 mm/min, the number of tensile failures is 36,744, the number of shear failures is 3,189, and the total number is 39,942; when the loading rate is 0.11 mm/min, the number of tensile failures is 21,159, the number of shear failures is 1,984, and the total number is 23,143; when the loading rate is 0.16 mm/min, the number of tensile failures is 25,640, the number of shear failures is 2,692, and the total number is 28,332.

(a)	(b)
(c)	(d)

Figure 10. Distribution of AE RA and AF values at different loading rates: (a) 0.01 mm/min; (b) 0.03 mm/min; (c) 0.11 mm/min; (d) 0.16 mm/min.

In the previous text, from an overall perspective, the distribution characteristics of different types of failure events after the final failure of the coal-rock combination were analyzed. To further compare and analyze the evolution process of tensile fractures and shear fractures in the coal-rock combination over time under different loading rate conditions, the growth curves of the number of tensile fractures and shear fractures over time under each loading rate condition were obtained through data statistics, as shown in Figure 11.

(a)	(b)
(c)	(d)

Figure 11. Evolution of the number of tensile and shear cracks under different loading rates: (a) 0.01 mm/min; (b) 0.03 mm/min; (c) 0.11 mm/min; (d) 0.16 mm/min.

It can be seen that before the coal-rock combination reaches the peak load, tensile failure mainly occurs inside it, with the cumulative number of tensile fracture events increasing slowly and the cumulative number of shear fracture events showing no significant growth. After reaching the peak load, the specimen undergoes mixed tension-shear failure, dominated by tensile failure, where the cumulative number of tensile fracture events increases rapidly and the cumulative number of shear fracture events increases slightly. To quantitatively analyze the damage evolution process of coal-rock combinations under different loading rate conditions, the damage degree of coal-rock combinations is defined here as the ratio of the cumulative number of fracture events at different loading progress to the total number of fracture events when the specimen is completely damaged:

 

(1)

 

Where X_σi is the cumulative number of fracture events when loaded to a certain stress level; X_σF is the total number of fracture events when the specimen is completely damaged.

When the loading rate is 0.01 mm/min, the number of tensile fracture events at the peak load reaches 20197, and the maximum number of shear fracture events reaches 1878; thus, X_σi is 22075, and the damage degree D at this time is 47.4%. When the loading rate is 0.03 mm/min, the number of tensile fracture events at the peak load reaches 3582, and the maximum number of shear fracture events reaches 331; accordingly, X_σi is 3913, with the damage degree D being 20.3%. At a loading rate of 0.11 mm/min, the number of tensile fracture events at the peak load is 2674, and the maximum number of shear fracture events is 145, resulting in an X_σi of 2819 and a damage degree D of 12.2%. When the loading rate is 0.16 mm/min, the number of tensile fracture events at the peak load reaches 5328, and the maximum number of shear fracture events reaches 423, making X_σi 5,751 and the damage degree D 9.8%. It can be seen that the damage degree of the coal-rock combination at the peak load decreases with the increase of the loading rate.

Comments 4: Determine a quantitative indicator (unloading rate; emission energy by video/fragment mass; fraction of AE >X dB after peak);
Correlate this metric with U₁, post-peak (r, p, confidence interval). (Not currently available.)

Response 4: Thank you for pointing this out. We have re-written the corresponding part according to the Reviewer’s suggestion.

Comments 5: Energetics: sensitivity to heat transfer.
Estimate the contribution of heat from friction/triboemission/viscous damping (order of magnitude);
How does this affect the decomposition of U = U₁ + U₂ and the estimate of U₁, post-peak;
Brief justification of "elastic" formulas for extreme nonlinearity before the peak. (Not currently considered.)

Response 5: Thank you for pointing this out. We have re-written the corresponding part in discussion according to the Reviewer’s suggestion.

Comments 6: Morphology and interface.
Comment on adhesive thickness/rigidity and its possible influence on fracture kinematics. (Currently partially considered.)

Response 6: Thank you for pointing this out. We have re-written the corresponding part in discussion according to the Reviewer’s suggestion.

Comments 7: Restate conclusions as "proposed mechanism consistent with observations," rather than definitively established.

Response 7: Considering the Reviewer’s suggestion, We have re-written the corresponding part in conclusion.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Publish.

Author Response

Dear Reviewer, Hello! First of all, on behalf of all the authors, I would like to extend our most sincere gratitude and highest respect to you for taking precious time out of your busy schedule to review our manuscript! Your review comments are professional, detailed and highly targeted. They not only accurately point out the deficiencies in the manuscript, but also put forward many constructive suggestions for improvement, providing a crucial guiding direction for us to further improve the quality of the manuscript. During the review process, your in-depth understanding of the research content, rigorous control of academic details, and keen insight into cutting-edge issues in the field have made us deeply admire, and also enabled us to have a more comprehensive and in-depth understanding of the research itself.