Experimental Analysis of Residual Stresses in CFRPs through Hole-Drilling Method: The Role of Stacking Sequence, Thickness, and Defects
and Thomas Niendorf 1
Round 1
Reviewer 1 Report
The HDM method studied and validated with reference to specific CFRP sample is well structured and presented. Results seem robust both in terms of numerical and experimental approaches. The paper is well structured in terms of contents and relative presentation to the reader. Some minor improvements are expected in terms of figure formatting. English is good ; somewhere could be slightly improved. I suggest a general rereading.
Author Response
Thank you for submitting your paper. The work done here draws attention to a significant subject residual stress in CFRP. I have found the paper to be interesting. However, several issues need to be addressed properly before the paper is being considered for publication. My comments including major and minor concerns are given below:
- Please consider reviewing the abstract and highlight the novelty, major findings, and conclusions. I suggest reorganizing the abstract, highlighting the novelties introduced. The abstract should contain answers to the following questions:
- What problem was studied and why is it important?
- What methods were used?
- What conclusions can be drawn from the results? (Please provide specific results and not generic ones).
- The abstract must be improved. It should be expanded. Please use numbers or % terms to clearly shows us the results in your experimental work.
Response: The authors thank the reviewer for his valuable comment. In light of the remarks (#1-5), the abstract has been revised.
- Please consider reporting on studies related to your work from mdpi journals.
Response: In light of the reviewer’s remark, now, studies from mdpi journals are reported within the Introduction (l. 133 ff.).
- The introduction must be expanded, please consider improving the introduction, provide more in-depth critical review about past studies similar to your work, mention what they did and what were their main findings then highlight how does your current study brings new difference to the field.
Response: The authors thank the reviewer for his valuable criticism. The introduction has been revised and, now, important studies being similar to this work are considered.
- The title should be improved, first of all it is too long, it does not indicate that there is FE work done. Perhaps consider the following: The effect of Stacking Sequence, Thickness and Defects on the developed residual stresses in CFRP: Experimental and finite analysis” or some other title more relating to the work done in this paper.
Response: In light of the reviewer’s suggestion, the title has been changed to “Experimental analysis of residual stresses in CFRP through hole drilling method: The role of stacking sequence, thickness and defects”. In particular, the measurement tool has been considered. However, the authors would like to refrain from adding the term “finite analysis” in the title. It may provide the misleading information to the readers that we simulated the manufacturing process using finite element method.
- Please stop using the word reliable, before making such statement it should be first it should be proved that it is reliable.
Response: the authors thank the reviewer for her/his comment. The usage of the word “reliable” has been checked carefully in the manuscript.
- In the abstract page 1 “By use of optical microscopy (OM)” font size/format is different?
Response: Thanks for carefully reading the manuscript. This error has been corrected.
- Authors forgot to number lines in the manuscript pages…. This makes it hard to indicate comments, but I will try as possible.
Response: The authors apologize for this. Now, line numbers have been added.
- Hole drilling method change to 2. Materials and methods.
Response: In light of the reviewer’s remark, the title of chapter 2 has been changed.
- Now I am a bit confused as there is drilling in this work in order to facilitate the measurement of residual stresses which is not mentioned in the title. Please reflect upon that as well in the title.
Response: The authors fully agree with the reviewer’s criticism and, thus, the title has been revised. Please see also response #8 above.
- Please check equation 1, the strains are in the right order?
Response: Indeed, the strains mentioned in equation 1 are in the right order. Please see Figure 2(a), providing the information for the directions of the strain and stress components.
- From figure 4 it appears that this work was done before on hybrid metal/composites so what is new here? If you have already managed to perform the work on multilaterals then what is the point of doing this work on CFRP alone? I don’t see any novelty after seeing figure 4.
Response: The authors thank the reviewer for her/his valuable comment. The objective of this work is to investigate the residual stresses in CFRP, considering the role of defects, stacking sequence, and thickness. To the best of the authors’ knowledge, no comprehensive study was performed before detailing the relations between calibration coefficients and resulting residual stresses, in particular, taking into account the aforementioned factors. In light of the reviewer’s concerns, the aim, focus and novelty of this work have been outlined in the Introduction part (Pages 2-3). In addition, in order to avoid to present some misleading information mentioned in the question, the review on hybrid metal/composites has been removed from the manuscript.
- Also the meshing strategy, it is not clear to me why it was adopted, the element aspect ratio is high, would this have any effect on the results? If not then what’s the point of having such a large variation in mesh density.
Response: The region in direct vicinity of the drilling hole is the key part of the simulation. Thus, local fine meshes close to the drilled hole were used to improve the accuracy of the drilling simulation. In turn, the reason for using coarse meshes far away from the hole is to reduce the computation time. For clarity, the classifications have been added to manuscript in lines 217-225.
- “The relaxed strains follow from the displacement data. The drilling process is simulated by removing material (elements) with the “Remove” function, using a Python script.” Did the authors write their own subroutine? Or used ones available from Abaqus? Please elaborate further for this part.
Response: The authors thank the reviewer for her/his comment. The subroutines were written by the authors themselves. This has been noted within the manuscript in line 245.
- We need more details about the FE study? Mesh convergence, drilling properties, coefficient of friction, boundary conditions and so on. Please do not refer for this from your past study as the stacking does have an effect on the results and also the residual stresses.
Response: The authors thank the reviewer for her/his comment. Please note, however, in the present work a real drilling simulation was not performed and, thus, the drilling properties and coefficient of friction are not required. The (residual) stresses are prescribed as the boundary conditions on the hole walls for every actual total hole depth, being equivalent to the released stresses within an increment during the drilling process. Furthermore, the “remove” function in ABAQUS was utilized. This allows to delete all elements from a part representing the procedure experimentally implemented in the incremental drilling process. This approach is well known and widely accepted in the hole drilling method community. For clarity, further information, i.e. on FE and boundary conditions applied, has been added in lines 241-247.
- Experiment details change to materials and methods and combine it with section 2. So 2 and 3 should be combined and renamed Materials and methods, then use subsections.
Response: In light of the reviewer’s criticism, the section structure has been revised.
- “Small drilling steps of 20 µm” was this done in FE or experimental?
Response: In the experiment, a constant increment size of 20 µm was employed in the entire hole drilling procedure. Such increment size promotes a smooth strain profile as a function of drilling depth, in particular across the interface between two adjacent layers. The experimentally determined strains were approximated with a polynomial of 6th order for residual stress evaluation. The strains at a given depth, for which the calibration coefficients were calculated, were chosen for evaluating the residual stresses.
For the calculation of the calibration coefficients, a hole drilled in 25 increments was considered: 10 increments of 20 µm thickness and from then onwards 15 increments of 40 µm thickness. The calculation time for these 25 increments was about 7 days in total, using a dual processor Intel Xean x 5660. Each processor had 24 GB RAM and 6 cores running at 2.8 GHz. In order to avoid high computational cost, the step size of 20 µm was not chosen for the whole simulation. This information has been added in lines 227-237.
- “300 000 rpm (3 bar)” these two units are not the same? Bar and rpm are different units, please explain.
Response: The authors fully agree with the reviewer’s comment and, thus, 3 bar has been removed. In a previous study, Nau and Scholtes [1] used high-speed drilling (300 000 rpm) for their investigations, ensuring high quality of the drilled hole and the accuracy of the residual stress results.
[1]: A. Nau, B. Scholtes, Evaluation of the High-Speed Drilling Technique for the Incremental Hole-Drilling Method, 53, pages 531–542 (2013).
- “It can be directly seen that the thickness of each ply is not exactly the same. The cross-sectional microstructures of the other samples are not shown for brevity.” This is trivial finding, nothing new here, due to manufacturing quality and process used, a minor variation in layers thickness is common. The question here whether this have high impact on the results or not?
- Validation of residual stress measurement some parts of this section can be moved to materials and method section.
Response: For the validation of the residual stresses, the comparison between the measurement and the numerical results must be drawn. Thus, the authors would like to keep those parts in the results section.
- The results are merely described and is limited to comparing the experimental observation and describing results. The authors are encouraged to include a more detailed results and discussion section and critically discuss the observations from this investigation with existing literature.
Response: In light of this valuable comment, the manuscript has been revised. More information has been added, see lines 604-627.
- Conclusion can be expanded or perhaps consider using bullet points (1-2 bullet points) from each of the subsections
Response: In light of the reviewer’s remark, more information has been added to the conclusion.
Reviewer 2 Report
Thank you for submitting your paper. The work done here draws attention to a significant subject residual stresses in CFRP. I have found the paper to be interesting. However, several issues need to be addressed properly before the paper is being considered for publication. My comments including major and minor concerns are given below:
- Please consider reviewing the abstract and highlight the novelty, major findings, and conclusions. I suggest reorganizing the abstract, highlighting the novelties introduced. The abstract should contain answers to the following questions:
- What problem was studied and why is it important?
- What methods were used?
- What conclusions can be drawn from the results? (Please provide specific results and not generic ones).
- The abstract must be improved. It should be expanded. Please use numbers or % terms to clearly shows us the results in your experimental work.
- Please consider reporting on studies related to your work from mdpi journals.
- The introduction must be expanded, please consider improving the introduction, provide more in-depth critical review about past studies similar to your work, mention what they did and what were their main findings then highlight how does your current study brings new difference to the field.
- The title should be improved, first of all it is too long, it does not indicate that there is FE work done. Perhaps consider the following: The effect of Stacking Sequence, Thickness and Defects on the developed residual stresses in CFRP: Experimental and finite analysis” or some other title more relating to the work done in this paper.
- Please stop using the word reliable, before making such statement it should be first it should be proved that it is reliable.
- In the abstract page 1 “By use of optical microscopy (OM)” font size/format is different?
- Authors forgot to number lines in the manuscript pages…. This makes it hard to indicate comments, but I will try as possible.
- Hole drilling method change to 2. Materials and methods.
- Now I am a bit confused as there is drilling in this work in order to facilitate the measurement of residual stresses which is not mentioned in the title. Please reflect upon that as well in the title.
- Please check equation 1, the strains are in the right order?
- From figure 4 it appears that this work was done before on hybrid metal/composites so what is new here? If you have already managed to perform the work on multilaterals then what is the point of doing this work on CFRP alone? I don’t see any novelty after seeing figure 4.
- Also the meshing strategy, it is not clear to me why it was adopted, the element aspect ratio is high, would this have any effect on the results? If not then what’s the point of having such a large variation in mesh density.
- “The relaxed strains follow from the displacement data. The drilling process is simulated by removing material (elements) with the “Remove” function, using a Python script.” Did the authors write their own subroutine? Or used ones available from Abaqus? Please elaborate further for this part.
- We need more details about the FE study? Mesh convergence, drilling properties, coefficient of friction, boundary conditions and so on. Please do not refer for this from your past study as the stacking does have an effect on the results and also the residual stresses.
- Experiment details change to materials and methods and combine it with section 2. So 2 and 3 should be combined and renamed Materials and methods, then use subsections.
- “Small drilling steps of 20 µm” was this done in FE or experimental?
- “300 000 rpm (3 bar)” these two units are not the same? Bar and rpm are different units, please explain.
- “It can be directly seen that the thickness of each ply is not exactly the same. The cross-sectional microstructures of the other samples are not shown for brevity.” This is trivial finding, nothing new here, due to manufacturing quality and process used, a minor variation in layers thickness is common. The question here whether this have high impact on the results or not?
- Validation of residual stress measurement some parts of this section can be moved to materials and method section.
- The results are merely described and is limited to comparing the experimental observation and describing results. The authors are encouraged to include a more detailed results and discussion section and critically discuss the observations from this investigation with existing literature.
- Conclusion can be expanded or perhaps consider using bullet points (1-2 bullet points) from each of the subsections
Author Response
Thank you for submitting your paper. The work done here draws attention to a significant subject residual stress in CFRP. I have found the paper to be interesting. However, several issues need to be addressed properly before the paper is being considered for publication. My comments including major and minor concerns are given below:
- Please consider reviewing the abstract and highlight the novelty, major findings, and conclusions. I suggest reorganizing the abstract, highlighting the novelties introduced. The abstract should contain answers to the following questions:
- What problem was studied and why is it important?
- What methods were used?
- What conclusions can be drawn from the results? (Please provide specific results and not generic ones).
- The abstract must be improved. It should be expanded. Please use numbers or % terms to clearly shows us the results in your experimental work.
Response: The authors thank the reviewer for his valuable comment. In light of the remarks (#1-5), the abstract has been revised.
- Please consider reporting on studies related to your work from mdpi journals.
Response: In light of the reviewer’s remark, now, studies from mdpi journals are reported within the Introduction (l. 133 ff.).
- The introduction must be expanded, please consider improving the introduction, provide more in-depth critical review about past studies similar to your work, mention what they did and what were their main findings then highlight how does your current study brings new difference to the field.
Response: The authors thank the reviewer for his valuable criticism. The introduction has been revised and, now, important studies being similar to this work are considered.
- The title should be improved, first of all it is too long, it does not indicate that there is FE work done. Perhaps consider the following: The effect of Stacking Sequence, Thickness and Defects on the developed residual stresses in CFRP: Experimental and finite analysis” or some other title more relating to the work done in this paper.
Response: In light of the reviewer’s suggestion, the title has been changed to “Experimental analysis of residual stresses in CFRP through hole drilling method: The role of stacking sequence, thickness and defects”. In particular, the measurement tool has been considered. However, the authors would like to refrain from adding the term “finite analysis” in the title. It may provide the misleading information to the readers that we simulated the manufacturing process using finite element method.
- Please stop using the word reliable, before making such statement it should be first it should be proved that it is reliable.
Response: the authors thank the reviewer for her/his comment. The usage of the word “reliable” has been checked carefully in the manuscript.
- In the abstract page 1 “By use of optical microscopy (OM)” font size/format is different?
Response: Thanks for carefully reading the manuscript. This error has been corrected.
- Authors forgot to number lines in the manuscript pages…. This makes it hard to indicate comments, but I will try as possible.
Response: The authors apologize for this. Now, line numbers have been added.
- Hole drilling method change to 2. Materials and methods.
Response: In light of the reviewer’s remark, the title of chapter 2 has been changed.
- Now I am a bit confused as there is drilling in this work in order to facilitate the measurement of residual stresses which is not mentioned in the title. Please reflect upon that as well in the title.
Response: The authors fully agree with the reviewer’s criticism and, thus, the title has been revised. Please see also response #8 above.
- Please check equation 1, the strains are in the right order?
Response: Indeed, the strains mentioned in equation 1 are in the right order. Please see Figure 2(a), providing the information for the directions of the strain and stress components.
- From figure 4 it appears that this work was done before on hybrid metal/composites so what is new here? If you have already managed to perform the work on multilaterals then what is the point of doing this work on CFRP alone? I don’t see any novelty after seeing figure 4.
Response: The authors thank the reviewer for her/his valuable comment. The objective of this work is to investigate the residual stresses in CFRP, considering the role of defects, stacking sequence, and thickness. To the best of the authors’ knowledge, no comprehensive study was performed before detailing the relations between calibration coefficients and resulting residual stresses, in particular, taking into account the aforementioned factors. In light of the reviewer’s concerns, the aim, focus and novelty of this work have been outlined in the Introduction part (Pages 2-3). In addition, in order to avoid to present some misleading information mentioned in the question, the review on hybrid metal/composites has been removed from the manuscript.
- Also the meshing strategy, it is not clear to me why it was adopted, the element aspect ratio is high, would this have any effect on the results? If not then what’s the point of having such a large variation in mesh density.
Response: The region in direct vicinity of the drilling hole is the key part of the simulation. Thus, local fine meshes close to the drilled hole were used to improve the accuracy of the drilling simulation. In turn, the reason for using coarse meshes far away from the hole is to reduce the computation time. For clarity, the classifications have been added to manuscript in lines 217-225.
- “The relaxed strains follow from the displacement data. The drilling process is simulated by removing material (elements) with the “Remove” function, using a Python script.” Did the authors write their own subroutine? Or used ones available from Abaqus? Please elaborate further for this part.
Response: The authors thank the reviewer for her/his comment. The subroutines were written by the authors themselves. This has been noted within the manuscript in line 245.
- We need more details about the FE study? Mesh convergence, drilling properties, coefficient of friction, boundary conditions and so on. Please do not refer for this from your past study as the stacking does have an effect on the results and also the residual stresses.
Response: The authors thank the reviewer for her/his comment. Please note, however, in the present work a real drilling simulation was not performed and, thus, the drilling properties and coefficient of friction are not required. The (residual) stresses are prescribed as the boundary conditions on the hole walls for every actual total hole depth, being equivalent to the released stresses within an increment during the drilling process. Furthermore, the “remove” function in ABAQUS was utilized. This allows to delete all elements from a part representing the procedure experimentally implemented in the incremental drilling process. This approach is well known and widely accepted in the hole drilling method community. For clarity, further information, i.e. on FE and boundary conditions applied, has been added in lines 241-247.
- Experiment details change to materials and methods and combine it with section 2. So 2 and 3 should be combined and renamed Materials and methods, then use subsections.
Response: In light of the reviewer’s criticism, the section structure has been revised.
- “Small drilling steps of 20 µm” was this done in FE or experimental?
Response: In the experiment, a constant increment size of 20 µm was employed in the entire hole drilling procedure. Such increment size promotes a smooth strain profile as a function of drilling depth, in particular across the interface between two adjacent layers. The experimentally determined strains were approximated with a polynomial of 6th order for residual stress evaluation. The strains at a given depth, for which the calibration coefficients were calculated, were chosen for evaluating the residual stresses.
For the calculation of the calibration coefficients, a hole drilled in 25 increments was considered: 10 increments of 20 µm thickness and from then onwards 15 increments of 40 µm thickness. The calculation time for these 25 increments was about 7 days in total, using a dual processor Intel Xean x 5660. Each processor had 24 GB RAM and 6 cores running at 2.8 GHz. In order to avoid high computational cost, the step size of 20 µm was not chosen for the whole simulation. This information has been added in lines 227-237.
- “300 000 rpm (3 bar)” these two units are not the same? Bar and rpm are different units, please explain.
Response: The authors fully agree with the reviewer’s comment and, thus, 3 bar has been removed. In a previous study, Nau and Scholtes [1] used high-speed drilling (300 000 rpm) for their investigations, ensuring high quality of the drilled hole and the accuracy of the residual stress results.
[1]: A. Nau, B. Scholtes, Evaluation of the High-Speed Drilling Technique for the Incremental Hole-Drilling Method, 53, pages 531–542 (2013).
- “It can be directly seen that the thickness of each ply is not exactly the same. The cross-sectional microstructures of the other samples are not shown for brevity.” This is trivial finding, nothing new here, due to manufacturing quality and process used, a minor variation in layers thickness is common. The question here whether this have high impact on the results or not?
- Validation of residual stress measurement some parts of this section can be moved to materials and method section.
Response: For the validation of the residual stresses, the comparison between the measurement and the numerical results must be drawn. Thus, the authors would like to keep those parts in the results section.
- The results are merely described and is limited to comparing the experimental observation and describing results. The authors are encouraged to include a more detailed results and discussion section and critically discuss the observations from this investigation with existing literature.
Response: In light of this valuable comment, the manuscript has been revised. More information has been added, see lines 604-627.
- Conclusion can be expanded or perhaps consider using bullet points (1-2 bullet points) from each of the subsections
Response: In light of the reviewer’s remark, more information has been added to the conclusion.
Reviewer 3 Report
Please use the same font size in abstract and overall of the paper
The abstract is too verbally it requires some brief quantitative results
Please highlight the industrial use at component level not only sector level.
Please highlight better the authors contribution in introduction
Overall the introduction is very limited and not details well the state of art
“As shown in Figure 13, for validation..” what you said about this figure do not correspond as there do not show any distance of hole…
A section of discussion is required
The FEM validation used in figure 14 is very far from what authors claim !
The conclusions are too verbally-please use quantitative interpretation such to link with your results
Author Response
1: Please use the same font size in abstract and overall of the paper
Response: In light of the reviewer’s remark, front sizes have been carefully checked in the manuscript.
2: The abstract is too verbally it requires some brief quantitative results
Response: The authors thank the reviewer for her/his comment. The abstract has been thoroughly revised. Please see also the response to questions #1-5 of reviewer #1.
3: Please highlight the industrial use at component level not only sector level.
Response: In light of the reviewer’s comment, the industrial use is now highlighted in lines 150-153.
4: Please highlight better the authors contribution in introduction
Response: In line with the reviewer’s comment, the Introduction part has been revised to highlight the authors contribution in more detail.
5: Overall the introduction is very limited and not details well the state of art
Response: In order to address the useful comment of the reviewer, the introduction has been carefully revised.
6: “As shown in Figure 13, for validation..” what you said about this figure do not correspond as there do not show any distance of hole…
Response: The authors thank the reviewer for her/his comment. For clarity, more details have been added in lines 604-617.
7: A section of discussion is required
Response: In order to increase the scientific depth of this work, some discussions have been added. Please see lines 604-627.
8: The FEM validation used in figure 14 is very far from what authors claim!
Response: The authors thank the reviewer for her/his comment. More descriptions have been added in lines 604-617.
9: The conclusions are too verbally-please use quantitative interpretation such to link with your results
Response: In line with the reviewer’s comment, the conclusion has been revised.
Round 2
Reviewer 2 Report
All questions answered and paper can be accepted
Reviewer 3 Report
.
