Mechanical and Heterogeneous Properties of Coal and Rock Quantified and Mapped at the Microscale
Round 1
Reviewer 1 Report
The manuscript is well organized but requires minor revisions regarding some grammatical mistakes.
From the technical point of view, the authors have done an acceptable job in performing the tests and reporting the data. The only improvement that this reviewer expects to see is adding some comments regarding how the scattered results can be attributed to the micro-structure of the tested materials. Also, I recommend that the hardness and modulus values for the curves which show considerable pop-in (or micro-cracking) to be removed from the analysis since the calculated values might be inaccurate due to cracking.
Other than the aforementioned minor revisions, the study is interesting and merits publication with a minor revision.
Author Response
Response to Reviewer 1 Comments
Thank you very much for your nice comments and suggestion on our article. According to your suggestions, we have supplemented several data here and corrected several mistakes in your previous draft.
We have also written a point-by-point response letter to you as you can see below.
Point 1: The only improvement that this reviewer expects to see in adding some comments regarding how the scattered results can be attributed to the micro-structure of the tested materials.
Response 1: We have supplemented that “According to Figure 6 a and Figure 7, for shale-2, the slope of its load-displacement curve is much larger than others at the first 500 nm, so this position is a hard mineral particle. However, the sudden decrease of slope for these lines is due to the formation of cracks around it. These abnormal phenomena would cause the accurate measurements of mechanical parameters of coal and rock if they are not delated.”
Point 2: Also, I recommend that the hardness and modulus values for the curves which show considerable pop-in (or micro-cracking) to be removed from the analysis since the calculated values might be inaccurate due to crack.
Response 2: We have removed the abnormal phenomena would cause the accurate measurements of mechanical parameters.
Besides, we also asked the help from a scientist for whose English is mother tongue to correct our grammatical mistakes.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper is very well written and organized.Only minor revisions are necessary, concerning formatting or writing
errors.
A .pdf file with a series of suggested corrections is attached
Comments for author File: 
Comments.pdf
Author Response
Thank you very much for your nice comments and suggestion on our article. According to your suggestions, we have also corrected grammatical mistakes one-by-one.
Author Response File: Author Response.docx
Reviewer 3 Report
Authors and Editors of Applied Sciences,
I have no reservation to the technical content of the manuscript. The concept, method and results are clearly presented. The only drawback is the quality of English. The text is understandable, but, in my opinion, written below a publication standard. I made c. 150 corrections directly on the annotated manuscript that is an integral part of my review. Proof reading of the final version of the paper by a scientist for whose English is a mother tongue would be still recommended.
I hope that my review will help you to get the manuscript successfully published.
Comments for author File: Comments.pdf
Author Response
Response to Reviewer 3 Comments
Thank you very much for your nice comments and suggestion on our article. According to your suggestions, we have also corrected grammatical mistakes one-by-one.
Your review is very helpful for me. Besides, we also asked the help from a scientist for whose English is mother tongue to correct our grammatical mistakes.
Author Response File: Author Response.docx
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Review of the article: Experimental Investigation on the Rheological Characteristics of Coal and Rock at the Microscale. Manuscript ref. number: applsci-615498.
The manuscript is poorly written. Please have an editor revise the text. Since the surface roughness is important in nanoindentation measurements, it is important that the authors report the surface roughness in the manuscript. The authors have not mentioned the tip geometry in the manuscript, although it looks to be a Berkovich tip according to figure 9. Considering the data as reported in figures 5.b and 9, it seems that a spacing of 20 um between the adjacent indents is not sufficient and the data might be affected by the plastic field from the nearby indents. This will negatively affect the validity of the data. Why do the authors calculate the modulus and hardness from the loading in the third cycle? Usually, the first unloading is the most accurate for measurement. What is the drift rate during the test? The presented data in figure 10 reveals that the rate of deformation during creep and elastic aftereffect might be considerably affected by the drift and might not be the material behavior. In figure 8, displacement is plotted vs the hardness. What is the physical significance of this plot? I don’t understand how the authors split the response in figure 11 into transient and steady state deformations. Why there is no schematic representation of the viscoelastic models used in this study? What are the numerical values for model parameters based on fittings in section 4.2.2? Since a Berkovish tip is used in this study, the contact geometry changes during the hold time (due to variation of depth). Accordingly, the correspondence principal cannot be used for obtaining the viscoelastic response from an elastic solution. Since the authors did not present any background, the physical soundness of the model used for this study for a Berkovich tip is questionable.Reviewer 2 Report
The authors present a comprehensive experimental study about the elastic aftereffect and creep of coal and rock, focusing on the microscale properties.
Abstract:
Line 13: What do you mean with "simplify experimental procedures"? Nanoindentation is not an easy technique, that is available everywhere. Line 17: The fact that coal and rock are heterogeneous is quite obvious. Please specify this heterogeneity.
Introduction:
Line 31: Please specify why rheological phenomena occur more often in shallow coal resources. Line 43: What do you mean by extremely longer? Compared to what? A load-holding duration of 10 mins is not that long for an experiment. Line 52: Specify "other simple mechanical properties". Eventually, "simple" should be replaced by e.g. common. Line 58: What are some common materials beside the ones already mentioned? Line 59: Nanoindentation is not a simple technique. In particular, the sample preparation is time-consuming and should be performed carefully, as well is the evaluation of the obtained results. Especially for heterogeneous materials, nanoindentation is complex because it was intentionally developed for homogeneous materials.
Materials and methods
Line 88: Please specify intensity. Line 102: Please specify the polisher solutions and the size of the sand paper. Which surface roughness was obtained? What is the surface roughness requirement for nanoindentation? Also add a reference for that. Line 104: Which SEM device and which measurement specifications were used? Also specify the nanoindentation tests (which device, which indenter type). Line 113-114: Sentence completely unclear. Line 115: Arrows do not point to microcracks. Line 120: There is a distinction between reversible and irreversible recovery. Please mention it here. Is this considered in the following experimental campaign? Line 127: What are the typical experiments and what is the specific adoption here? Line 142: Add an explanation of how the hardness and elastic modulus were calculated. Line 137: Are the five nanoindentation tests representative and do you think that all material phases are appropriately covered?
Results:
Figure 5 (a). Typo (displacement) Line 157: Did you observe the nanoindentation tests under a SEM in order to get information about different material phases? Which phases do occur in the coal and rock samples? Line 160: What do you mean by "we think"? Can you specify the this? Line 170: How do you explain the scattering of the results? Line 172: Please specify "happen sometimes". Please explain the phenomenon in more detail.
Discussions:
194: Can the different indentations in one sample be assigned to specific material phases? Line 212: In Figure 9(b), it is hardly visible to see anything. Besides, the Figures do not show the deformations, they just show the indents. Also provide a length scale in Figure 9(a). Line 236: How did you determine the transition time? Line 254: The simplification of the models is unclear. Please add a derivation. The same holds for Equations (4) and (5).
