Evaluating the Effectiveness of Aluminum Coatings on Patch-Repaired Composites Using Electro-Thermal Analysis
Round 1
Reviewer 1 Report (New Reviewer)
Comments and Suggestions for AuthorsThe simulation and experimental result of this study is strong, well-structured, and provides valuable insights for the development of Patch-Repaired Composites field. I have only two question for the author.
1.The author mentioned that scaling this approach for broader application will pose challenges due to significant weight increases. Does author have any thought to overcome this issue?
2. These findings pave the way for the development of more effective repair techniques and lightning protection strategies in composite material applications. Does author has any future direction for further study?
Other than these two comments, I think this article is ready for publication.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report (Previous Reviewer 3)
Comments and Suggestions for AuthorsPlease find my comments in the attached file.
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report (New Reviewer)
Comments and Suggestions for AuthorsPart A: COMMENTS ON GENERAL ASPECTS AND RECOMMENDATION
Using a commercial software for a finite element electro-thermal analysis, Authors have carried out a modelling study on the effects of aluminium-foil coatings for repair patches suitable for parts of aircraft structures based on epoxy - CF composites, which have been damaged by lightning strikes. The main novelty of the work is in the use of different size and shapes of repair patches to determine the effect on the damaged area in different layers. The submitted report has many deficiencies, which would have to be overcome by completely rewriting the text and provide additional information before it can be considered for publication. The main shortcomings can be summarised as follows:
1) The terminology and statements are often vague and misleading. For instance, the word composite is used as if there was there way only one type, corresponding to the specific one that they have used in the analysis. And even when the authors, later in the text, state that the composite is a CF type and eventually mention that the matrix is an epoxy type, the nature of the latter has never been revealed. An example of vagueness is to be found in the statement of objectives, which are also very verbose (Lines 83 – 96).
2) The usefulness of the data produced is often overstated and the claims are not substantiated by experimental data. There also lack of clarity and precision in the wording, as exemplified in the statement made in Lines 41-42. Although the work is on repair patches for lightening induced damage, there is a mention, let alone elucidation, of the repairing methods used in practice to justify the assigned properties to the materials and the layering strategy of the patches examined.
The analysis used in the work is interesting and the obtained data are potentially useful for the applications mentioned in the report, which could be used in a resubmission of the manuscript. To this end Authors are advised to take into account the comments in Part B of this evaluation report and bear in mind that the readers of Applied Science would not have the specialistic knowledge of FE modelling and/or materials used. It would be useful, therefore, if the following suggestion are explained and/or clarified:
a) The assumptions made for response of each layer to the applied electric field and whether this is affected by the adjacent layers exhibiting different properties.
b) The role of the aluminium-foil coatings on the patch as a way of spreading out the electrical field from the point of the lightning strike, thereby reducing the current density perceived by the material.
c) Produce quantitative data for the variation of damage through the entire thickness, possibly as plots, to determine the minimum number of layers required for the patches.
Part B. COMMENTS ON SPECIFIC SECTIONS OF THE REPORT
TITLE: - 1) The term “aluminium coating” is vague. It could be replaced with “aluminium-foil coating”. 2) There is no indication of lightning induced damage.
ABSTRACT: - 1) There is no indication of the type of applications addressed by the work. 2) The word “optimized” in Line 9 is an overstatement.
1.INTRODUCTION: - 1) Authors provide an extensive account of problems arising from lightning induced damage in aircrafts but say nothing about methods used for repairing the damage. 2) There is no indication of the type of epoxy resin used as matrix for the CF composite under examination. 3) The information provided about the pyrolysis data in the literature (ref 14) is meaningless without identifying the type of resin. Note that there is an enormous difference between the conventional DGEBA and the high-performance types, such as TGDDM likely to be used for parts of aircrafts. 4) There are many vague statements throughout the report, e.g. Lines 42- 44, 85 – 86, 94- 96.
2.ELECTRO-THERMAL……., Section 2.1.3 – There is an invalid generalization that all composites are anisotropic in Line 117 (repeated later in Line 204 in Section 2.2.2). Section 2.2.1 – Regarding the Headings and Data in Tables 1 and 2 (ref 13, 20). 1) it should be stated that these are unidirectional composites, and an indication should be given of the resin type and CF content used. 2) There is inconsistency in denotations used for direction in the two tables. 3) How were these properties measured? Note the quoted accuracy to 2 decimal places for values at high temperatures, < 3000! 4) While the decrease in conductivity at 300 and 360 C in all directions is credible, the constant density up to 360 C is not. The Tg of the resin matrix (TGDDM) is likely to be ~200 C. 5) The statement in Lines 225 – 226 is not credible; Authors should consult the cited paper on TGA data and preferably reproduce in the text as it is valuable for the discussion of the results. 6) The terminology on the on the volatilization of products of the thermal decomposition is inconsistent and sometime wrong, e.g., “sublimation”, which denotes a volatilization of matter from a solid without undergoing decomposition. This is certainly not the case here. Section 2.2.3 – The Y axis in Figure 2 does not give any indication of the actual values for i, the current. Section 2.3 – (1) statement in Line 274 is not correct because Epoxy resins are cross-linked macromolecules which do not melt but undergo a large reduction in modulus above Tg, which is likely to be around 200 C for the matrix of the composite examined. However, the choice of 250 C as a reference temperature to denote onset of damage is very good with respect to both decomposition of the resin and the large increase in thermal expansion for mechanical damage. This should be emphasized. 2) The caption for figure 4 should include the term “experimental”. At the same time Authors should clarify the meaning of the images (e). Section 2.4 – (1) The meaning of legends (subfigure a,b,c,d) is not clear. (2) The captions for Figs. 5 and 6 need to be more accurate. Ablation for the type of epoxy resin used in the composite examined starts at temperatures much higher than 250 C. The colours for temperatures above 250 C are not clear. Maybe using much darker colours would be better and use a white background to enhance the visibility of the damaged area. (3) Images for layers at much greater distance from surface should also be shown. It is important to identify the layer at which the temperature falls below 250 C. A plot of temperature gradient at the centre of the damaged zone versus distance from top surface would be a useful parameter to characterize the damage resistance of the material.
3. ANALYSIS ………Section 3.1 – (1) The description and analysis of the repair patches is not at all clear. A sketch is required to show the cross-section of the damaged structure, with a clear identification of the damaged area, with the upper cross-section identified as the repair patch. At the same time, there must be a clear designation of the physical characteristics assigned to the composite structure underneath the patch. (2) The caption of Figure 7 does not accurately describe the nature of the images shown. Section 3.1.2 There are statements that are not substantiated by the data. For instance, in Lines 354 – 360 it is stated that there is not a perfect seamless fit resulting in minute gaps. How was this deduced? In Line 369, what is the meaning of (4.91 ~ 6.55 cm 2). The size of the damaged area must be clarified in terms of temperature. Concordantly a plot of “damaged area” versus distance from surface would be useful as design parameter in relation to patch thickness considerations. Section 3.2 - In Lines 386 – 392 are statements that are redundant for this section or even the entire paper. Section 3.2.1 – (1) Statements in Lines 398 – 401 are not clear. What do the following mean? mitigating breaks, misaligned fibres, junction. (2) Statements in Lines 406 – 409 should have been placed earlier, maybe after Table 3. (3) There is confusion regarding the statement regarding 20 cm coating when earlier the width was given a 30 cm value (Figs 9 and 10). Section 3.2.2 – There is the problem regarding patches smaller than the damaged zone in so far as the zone area and the physical characteristics have not been described. Section 3.2.3 – (1) There is a meaningless statement in lines 444 - 445, while that in Lines 448 – 451 is not substantiated by the data.
(X) Maybe the total number of images could be reduced and show simply how the geometry of the electrical field at the surface determines the shape of the damaged zone. More important is the area and how it decreases through the thickness. It would also be useful if the comparison between the pristine and repaired structure is presented as side-by-side images. The TGA trace which has been used for the ablation data (loss of weight) should be shown, as these would facilitate the interpretation of the results of the simulation.
4. CONCLUSIONS – (1) The first statement as an objective of the work should appear in the Introduction. (2) Statements in Lines 458 – 462 are vague and not substantiated. For instance, (a) What is “fibre misalignment” within the context of the simulation? (b) What data are there to determine the “optimized” repair structure? (3) Statement in Line 465 is redundant. (4) It is difficult to follow the explanation in point 1 without specifics. Also, the use of the term “dielectric breakdown” may not be appropriate as there has been no evidence. It is unlikely that this would occur, say at 250 C under an electrical spike (i.e., stress of short duration). (5) in relation to point 2, (a) what is the basis for the deduction in Lines 472 – 473, (b) what do the Authors mean by “broader applications”? Give examples! (5) In point 3 there is an association of the data from the work with change in weight with the data from the work without any evidence or rational basis without an elaboration. This is why the TGA curve would be necessary. (6) The statements in Lines 485 – 491 are speculative.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report (New Reviewer)
Comments and Suggestions for AuthorsRevised Manuscript
GENERAL COMMENTS
Authors have made some changes, mainly as descriptive additions, but have ignored some comments and not properly addressed some others. At the same time, they have not rectified the vagueness of some statements and terminology. For instance, a) in Lines 244 – 246 they sate that carbon fibres “sublime” at temperature >3000 C, even though in my previous report they were given a definition of the term and acknowledged in their answer. Graphitic carbon does not sublime but undergoes oxidation to CO2. b) In line 287 they state that the epoxy resin melts at .250 C even though they were told that the epoxy resin matrix consisted of a cross-linked network that cannot melt but undergoes a large reduction in modulus. Again, this was acknowledged in the answers but not corrected in the text. c) In lines 464 and 466 there are discussions about the increase in weight brought about by the Al coatings. In aircraft structures this is an infinitesimal fraction of the overall weight. d) In their answers they state that …repairs restore the “structural strength” but cannot recover the” original conductivity”. This is baseless without theoretical elaboration and possibly experimental evidence.
COMMENTS REQUIRING CORRECTIVE ACTIONS
TITLE:- In previous report it was suggested to change the term “aluminium coating” with “aluminium-foil coating”, because it is seems to be impossible to produce shaped Al zones on repair patches with techniques available for aluminium coatings, namely vacuum deposition nor powder spraying. It is possible, on the other hand, to cut shapes from thin Al foils and stick them on CF composite patches to suit the specific requirements for the repair job considered. How is done in industrial practice?
1.INTRODUCTION: - Authors say nothing about methods used in practice for repairing damages to aircraft structures. Authors must reassure the reader that the fibre arrangements they have used in the model is realistic.
2.ELECTRO-THERMAL……., 1) Section 2.2.1 – Regarding the Headings and Data in Tables 1 and 2 (ref 13, 20). It should be stated that these are unidirectional composites, and an indication should be given of the resin type and CF content used. 2) There is inconsistency in denotations used for direction in the two tables, i.e for directions in Table 1 are x, y and z, while in Table 2 these become longitudinal and transverse. 3) How were these properties measured? Note the quoted accuracy to 2 decimal places for values at high temperatures, < 3000 C! It is not sufficient, or even appropriate, to say that these are reliable because they have been taken from peer reviewed papers. Identified errors or inaccuracies should not be allowed to propagate without questioning or rectification. 3) The TGA trace by Ogasawara, which has been used for the ablation data (loss of weight) should be shown, as these would facilitate the interpretation of the results of the simulation This would also acts a finger print for the degradation characteristics of the polymer matrix to complement the data in Table 1 and 2, which are vital to the degradation induced by lightning strikes. ) Statement in Line 287 is not correct because Epoxy resins are cross-linked macromolecules which do not melt but undergo a large reduction in modulus above Tg, which is likely to be around 200 C for the matrix of the composite examined. 3) Section 2.4 – A plot of temperature gradient at the centre of the damaged zone versus distance from top surface would be a useful parameter to fully characterize the damage resistance of the material and make comparisons for different applied voltage or current.
3. ANALYSIS ………Section 3.1 – The description and analysis of the repair patches is not at all clear. A sketch is required to show the cross-section of the damaged structure, with a clear identification of the damaged area, with the upper cross-section identified as the repair patch. At the same time, there must be a clear designation of the physical characteristics assigned to the composite structure underneath the patch. Section 3.2.1 – There is confusion regarding the statement regarding 20 cm coating when earlier the width was given a 30 cm value (Figs 9 and 10).
4. CONCLUSIONS: - The majority of my comments have been ignored without justification. Authors should re-examine them and provide a proper answer to the points raised
Author Response
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Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report (New Reviewer)
Comments and Suggestions for Authors2nd Revised Version
GENERAL COMMENTS
Authors should know that have accepted the invitation to review this manuscript because of my experience in an industrial research laboratory on the electrical properties of polymers for high voltage insulation, which included tests for impulse breakdown voltage to assess the resistance to lightning strikes. I have also published many research papers based on work in academic research laboratories on epoxy resins and various forms of composites, ranging from particulate types to continuous fibres, both GF and CF types. I have also co-authored a few papers on FEA examination to simulate the stress distribution (mechanical) in the fibre/matrix interfacial regions, where I have provided the required data obtained from experimental evaluations, as well as assessing the level of accuracy of the FEA predictions. I was, therefore, expecting a similar approach for the work submitted for publication in Applied Science, where most readers are likely to be interested on the extent to which theoretical modelling by FEA can predict the behaviour of the material used for the structural component in practical situations, as well as obtaining a basic understanding of the principles used in the modelling work. In the light of this objective, I made extensive comments on the first submission to stimulate revisions that would elevate the quality of the paper to the possible highest standards and reach out a wide readership. I was disappointed that the authors appeared to have made hardly any efforts to meet this challenge. I lowered my expectations in the comments on the revised manuscript to a level that would make the paper just acceptable for publication in Applied Science. Again, I was disappointed about the lack of evident efforts to address my comments and to make the required changes in the text. Apart from making some corrections in the terminology used to describe the behaviour of the epoxy matrix, the only significant improvement is in the consistency of the terminology used in Table 1 and 2, which was ignored in my first report. Additions to the text were primarily of a descriptive nature, which increased further the highly verbose presentation without addressing the unsubstantiated claims about the value of the data obtained in practical situations. This must be viewed within the context that Authors have “refused” to include in the Introduction some information regarding the way repairs are carried out in practice, as a way of reassuring readers that they understand the problems addressed in their theoretical analysis. In this respect, Authors have not even stated explicitly, and in simple terms, that the model proposed assumes that the epoxy CF composite for the repair patches have the same characteristics as those used for the body structure of aircrafts.
COMMENTS ON SPECIFIC ASPECTS
Examples of lack of evidence:
1. In Lines 245 – 248 it is claimed that above 250 C the composite experience “ablation damage”. Authors have refused twice to include the published TGA trace for the epoxy resin to support this deduction. In my experience on epoxy resins used for these types of composites substantial weight losses are not experienced until a temperature in the region of 450 C is reached.
2. In Lines 297 – 299 it is stated that the damage area values predicted by analysis the is very close to those found in the literature (only 4 % discrepancy). However, they have not provided the related references and have not stated whether the composite materials, and the conditions used to induce the damage, were the same. In other words, “compare like with like”.
3. In Lines 475 – 490 (Conclusions section) there are a series of statements that at best can be described as “imaginative fiction”, as for instance, the attribution of sudden changes is attributed to “fibre misalignment” at the junction between patches and the laminates in the composite structure. This is unjustifiable because “fibre misalignment” as a parameter does not enter in the FEA model.
Author answers
In some cases, the changes indicated in the answers do not address the points raised in the comments. Examples: (1) Point c) about Lines 464 – 466 in the GENERAL COMMENTS, Authors still the weight of the Al coatings on the repair patches an important parameter to consider in aircraft structures, stating that this is aligned with my comments- which is not true. (2) Point d) about Lines 364 – 371 +, regarding the vagueness of statement that “repair patches cannot restore the original conductivity because of the fundamental nature of composite repairs” is baseless without elaboration, and it may not be true. The complementary statement that this is not the case the mechanical strength is also baseless. What is “effective load transfer mechanism”? and how is it achieved? The question is whether it is at all necessary to make such statements, as they are not evidenced by the results or by authoritative sources from the relevant practitioners. (3) The answers provided for section 3. ANALYSIS…… do not address in anyway the points raised.
At times, I have had the impression that Author regards Peer Review as a “box ticking” formality, evidenced by the reluctance to provide the available information in the literature.
Recommendations
The present revised version still does not address the issues raised in the comments The text is too verbose and contains far too many vague statements and unsupported claims that are more a distraction than assistance for the reader.
1. The reader of papers dealing with applied science topics needs be reassured that the data used for the analysis are reliable. An examination of the data in Tables 1 and 2 show that the values have an accuracy to something like 0.1%. In scientific reports these data are normally quoted as averages and the standard deviation. Observing that values are given for temperature up to around 3000 C, the natural question arises regarding the reliability, which is likely to be determined by the way they were obtained. It is not too much to ask to consult the original papers to obtain the information and add a few notes (say) just below the tables.
2. The suggestion to include the TGA trace to facilitate the interpretation of the data still applies, because it would make it easier to understand the results than trying to make sense of the longwinded explanations in the text. These data are as important as those in Tables 1,2 and3.
3. The many figures about the damaged areas are interesting to look at, but are very difficult understand simply through an examination of the colours. In line with my previous suggestions, it would be much easier if plots were provided for the “Temperature and Area of the damaged area” versus “Distance from the surface” with a marker is used to identify each layer of the repair patch.
4. A clarification is necessary for the way the analysis of the repaired patch differs from that used for the structure, particularly in relation to the physical property data that have been used for the damaged area of the structure.
As a final comment I would like to add that normally only two revisions are allowed but, due to the importance that I attach to the publication of this work in Applied Science, I am prepared to examine yet another Revision if this is accepted by the Editor. In any case I will not be able to recommend the paper for publication unless the points raised are “properly” addressed.
Author Response
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Author Response File: Author Response.pdf
Round 4
Reviewer 3 Report (New Reviewer)
Comments and Suggestions for AuthorsAuthors have not addressed the main concerned raised in my reports on the manuscript. The 4 points outlined in my last report have also been ignored.
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
Comments and Suggestions for AuthorsThis paper aims to assess the effectiveness of patch-repaired on composites damaged by lightning. While the topic has acceptable methodologies and results, the manuscript suffers from critical issues related to novelty and a lack of sufficient understanding of the literature, which together leads me to a conclusion to not recommend this work for publication.
Some comments to address:
1. The main contribution claimed in this paper does not offer sufficient novelty. The application of metallic coatings for lightning protection is a well-explored area and the findings presented here largely reiterate existing knowledge. There is little distinction between the outcomes of this study and the conclusions drawn in previously published work on lightning strike protection. A rethinking of the papers goals is recommended.
2. Several references cited in the manuscript do not align with the points being made in the text. And there is a notable lack of discussion on alternative methods for repairing and protecting composite materials from lightning strikes; such as the use of metallic meshes (or expanded metal foils), conductive nanomaterials, or hybrid composite systems. To strengthen the paper, the authors need to engage with recent literature, correct referencing inaccuracies, and offer a more robust analysis of their results in comparison to established solutions.
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsPage 2, Line 66: please insert "Carbon Fiber Reinforced Polymer" before its acronym CFRP.
Page 3, Line 111: please define x and sigma in Equation 2.
Page 3, Line 116: please define delta in Equation 3. Additionally, please provide at least a brief explanation to Equation 3.
Page 3, Line 118: Missing "T" in "The".
Page 3, Line 126: please define Pec in Equation 4.
Page 4, line 137: add space between condition and equation
Page 4, line 145: specify undefined terms in Eq.7
Page 5-6, Tables 1,2,3: please state the exact reference for this data with more accuracy with respect to the current version in the text ([13, 18-20])
Figure 2, Figure 4: please add reference to cited figure in the caption, if cited.
General note: in your simulations, have you considered the impact of the different coefficient of thermal expansions of the aluminum coating and composite, in terms of induced stresses on the composite?
Comments on the Quality of English LanguageEnglish is overall fine, but some grammar mistakes could be improved
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsPlease see my comments to the attached file.
Comments for author File: Comments.pdf
Please see my comments to the attached file above.
Author Response
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Author Response File: Author Response.pdf
