Experimental Study on Mechanical Properties, Energy Dissipation Characteristics and Acoustic Emission Parameters of Compression Failure of Sandstone Specimens Containing En Echelon Flaws

Round  1

Reviewer 1 Report

The authors presented experimental results of samples containing en echelon flaws under uniaxial compression.  Stress-strain curves, energies and acoustic emission events were recorded and analysed. Although correlation between drops of stress in the stress-strain curves and final failure is well known, addition of energy and acoustic emission analyses makes it worth to be recorded in the literature. So I recommend for publish, after some modifications are made as listed in the following. In addition some small suggestions have been marked in the attached pdf file.

 P2, Line 18-20  “To avoid the use of expensive large cutting equipment, pre-existing fissures of rock-like material can be easily fabricated by inserting paper or thin steel discs to simulate open or closed fissures. This statement does not make sense. I guess the authors want to state that “To avoid the use of expensive large cutting equipment to make real rock samples and it is easy to fabricate pre-existing fissures in rock-like materials by inserting paper or thin steel discs to simulate open or closed fissures, many scholars ….”

Here from the corresponding order of the words, inserting paper would simulate open fissure and inserting thin steel disc would simulate closed fissure. Is this correct?  If not, make the order correct.

P3 Line 5. Should  “multicrack brittle rock”  be “multicrack in brittle rock”?

 Change   “and seven types of coalescence were identified based …” to “and identified seven types of coalescence based …” to keep the same subject of the sentence.

P4 Line 8-10:  It would be better to change “The sandstone specimens containing two parallel fissures had different inclination angles, and the flaw 9 inclination angles ( ) were 30°, 45° and 60°.”  to  “The parallel fissures in sandstone specimens have different inclination angles of 30°, 45° and 60°.”

Also “The sample number reflects the "sample type-flaw inclination angle"” could be changed into “The samples were numbered as  “B-flaw inclination angle”” Then the sentence is continued as “B represents ….”. Since only B type samples were used in the study, there is no need to use general words “sample type”.

P6, Line 24-26: “After  comparing and analyzing the stress and strain values of stress drop points 1 and 2, the initial crack  initiation stress is approximately 90~98% of the peak strength, and the initial crack initiation strain  is approximately 93% of the peak strain.” The English grammar is not right here. The subject of the sentence is “initial crack initiation stress”, which cannot compare and analyse things. So this should be changed to “After  comparing and analyzing the stress and strain values of stress drop points 1 and 2, we found that the initial crack  initiation stress is approximately 90~98% of the peak strength, and the initial crack initiation strain  is approximately 93% of the peak strain.”

P15 Line 18-20:  Replacing “Second, macrocrack initiation occurred by the intermittent nucleation, propagation and coalescence of microcracks that generated crackling noises” with “Macrocrack initiation by the intermittent nucleation, propagation and coalescence of microcracks generated crackling noises, which can be detected by acoustic instruments.” would be better.

P16 Line 18-20: This is a scientific paper. It would be better to delete the last statement “If any abnormality is found, managers and technicians should be notified by SMS in time to achieve the purpose of monitoring and warning about the collapse and instability of an engineering rock mass.” in the section 4.

General suggestion: Since the specimens in this paper are those that contain en echelon flaws, I would suggest to change phrases “specimens containing en echelon flaws”  into “the specimens” after a few have been used at early of the paper.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer：

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. We hope that all these corrections and revisions would be satisfactory. Thanks a lot, again.

Author Response File: Author Response.docx

Reviewer 2 Report

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In the manuscript “Experimental Study on Mechanical properties, Energy Dissipation Characteristics and Acoustic Emission Parameters of Compression Failure of Sandstone Specimens Containing En Echelon flaws”, uniaxial compression tests on three sandstone samples with different” with different inclination angles of two parallel fissures were investigated, using three different measurement devices

The problem is described clearly, however, the following crucial issues are related to the manuscript at hand:

The motivation for the present publication is to develop a monitoring system for rock bodies in reality for determining potential collapse, as stated in the last paragraph of the discussion (L.13 to L.20). However it is highly questionable whether such a small sample size (~mm) under “perfect” laboratory conditions yields practicallymeaningful results. In reality, the dimensions of en echelon flaw range from millimeters to several meters, and certainly they are accompanied by many other discontinuities in the rock mass. Thus, the emphasized relation of the experimental study to reality is doubted.

Why are uniaxial compression tests chosen? Rock mass is usually pre-stressed and the behavior is influenced by in-situ stresses yielding a triaxial stress state.

The study is based on a rather small data set (3 specimens with each different parameters, so no information about the statistical spread is provided). For a statistical analysis the underlying data set is not sufficient. Thus, a more comprehensive study including the investigation ofmultiple samples with equal parameters for analyzing the statistical spread is inevitable.

The reported experimental results are stress-strain curves, which are no direct outcomes from the experiments.It must be discussed in detail, how stress-strain curves are derived from the measured data. The problem is the following: The discussion of the experiments by means of stress-strain curves tacitly assumes a homogeneous deformation along the specimen. However, as soon as cracking occurs (as in the present case), deformation is no longer uniform and certainly, the stress-strain curve is different in every material point (in uncracked elastic parts elastic unloading occurs). Thus, for energy considerationsin the specimens, it is not sufficient to consider only a single stress-strain curve.

The knowledgegain ofusing three different measurement devices should be highlighted in detail. What are the pros and cons of each measurement device?

The following specific comments are constituted:

p.1 L.8: what are “yield” pillars and where are they relevant?

Fig. 1: Which open-pit mine is shown? Which type of rock mass is prevailing? Was the photo taken by the author?

p.3 L.30: “… of rock mass structures.” This sentence does not justify the motivation for the publication since almost “intact” rock specimens are considered. A comment is required about the transfer from the experimental data (almost intact rock) to site investigations (rock mass with many discontinuities also apart from echelon flaws).

p.3 L.37: A general outline should be provided.

p.4 L.2: “Sandstone is a common porous material” → sandstone is a granular frictional-cohesive material

p.4 L.3: “We use ...” A consistent tense should be used in the manuscript.

p.4 L.3: Where does the sandstone come from? Was it bought from a stonecutter?

p.4 L.10: What do the parentheses mean “( )” after inclination angle?

p.5 L.3: “To ensure the homogeneity … “ Homogeneity of sandstone will never be ensured. It can be tested only, whether the mechanical properties are within a narrow statistical spread. This sentence should be reformulated.

p.5 L.5: The amount of three samples is too small for discussing variability of mechanical properties. Furthermore, not only the mean value should be provided but also the variation.

p.5 L.6: What are “deformation coefficients”? This expression is unclear.

p.5. L22: “The distance between two points of a sample was measured in millimeters”… Why and how?

p.6. L1: I am not sure if really “butter” was used  at the ends of the specimens?

p.6. L.8: In this line, one of the crucial information is missing: How can you really measure a unique stress-strain curve from the experiment? This point must be revised in any case. A single stress-strain curve has no meaning in case of non-uniform deformation of the specimen. The description of the transition from the recorded experimental data (load vs. displacement from the testing machine??) to the data shown in this paper is missing.

p.6. L.17-18: “If the loading ...” This sentence sounds very sloppy and must be revised properly.

p.6. L19: An interpretation for the fact that in the diagrams exactly 2 spikes are observed is missing.

Figure 4: Why does the recorded load not go to zero upon failure? An explanation must be provided.

Table 1: How is the Young’s modulus determined? Is it a secant modulus or a tangent modulus? The definition must be added.

Figure 5: Too little information about this figure is given. How is it recorded? What do the colors mean? Are vertical cracks categorized as tensile cracks and inclined ones as shear cracks? It is not clear for the reader. 

p.8 L.20: What is the theory based on energy dissipation? There is no unique theory, it rather stems from thermodynamics.

Eq. (3): Why is the elastic energy calculated like that? It would mean that the crack closure effect in the beginning dissipates energy which is certainly not the case because it is a reversible process. A more detailed explanation and derivation about the determination of the elastic energy must be provided.

p.9 L.16: “began to fail at the peak strength” A specimen either fails or not. Failure is abrupt. Furthermore “upward stepping again” sounds sloppy. The sentence must be revised.

Figure 6: It Is doubted whether energy is really dissipated in the pre-peak regime, so before the first peak. In the opinion of the reviewer there are no irreversible processes in the pre-peak regime (only crack closure and elastic loading). The split into dissipated energy and elastic energy should be overthought.

p.10 L.12: “...the dynamic phenomena caused by postpeak instability ...” What do the authors precisely mean with this statement? It must be explained more detailed.

p.11 L3: Abbreviations like “AE” must be introduced.

Eq. (5): How is R determined?

p.13 L23: “To avoid human error” sounds very sloppy and must be revised.

Eq. (8): What is o(1/n)? If it should indicate a higher order term, the notation O(n^-1) is more convenient.

Figure 9: What does the exponent r’ has for a physical meaning? Why is the energy related to r’ in Figure 9?

Author Response

Dear Reviewer：

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. We hope that all these corrections and revisions would be satisfactory. Thanks a lot, again.

Author Response File: Author Response.docx

Round  2

Reviewer 2 Report

The diagrams of the answer to the review regarding Point 2, where three individual individual curves are shown in the response letter should be added to the manuscript for completeness.

Regarding the answer to Point 3 "The reported experimental results are stress-strain curves, which are no direct outcomes from the experiments. It must be discussed in detail, how stress-strain curves are derived from the measured data. The problem is the following: The discussion of the experiments by means of stress-strain curves tacitly assumes a homogeneous deformation along the specimen. However, as soon as cracking occurs (as in the present case), deformation is no longer uniform and certainly, the stress-strain curve is different in every material point (in uncracked elastic parts elastic unloading occurs). Thus, for energy considerations in the specimens, it is not sufficient to consider only a single stress-strain curve."

The response is not satisfying. For the reviewer it is not clear, why the experimental load-displacement curves have to be transformed to stress-strain diagrams. The stress-strain diagram will be different in every sample point. The reviewer suggests to consider only the real measured load-displacement curves in the manuscript.

Response 21 should be explained in more detail: At which point is the tangent modulus determined? 

Author Response

Dear Reviewer：

Thank you for your serious and constructive comments on our manuscript. According to your suggestion, the manuscript has been revised as a letter to editor. We hope that all these corrections and revisions would be satisfactory. Thanks a lot, again.

Author Response File: Author Response.docx

Round  3

Reviewer 2 Report

The manuscript is ready for publication.