Assessment of Aircraft Coating Resistance to Lighting Strikes and Long-Term Environmental Impact

Round 1

Reviewer 1 Report

Overall, the manuscript is clearly written, and the goals are basically achieved. Meanwhile, I have some comments for the improvement of this manuscript.

 

Ling 19-20 How do you get the conclusion that aircraft metal coating requires less maintenance? It seems that this is a repetition of references [21 22]. How much financial and time costs can it save?

 

Line 103 In section 2. Materials and Methods, it seems that the methods mentioned are all existing methods. Is there any improvement in the application of these methods in your research? From Figure 1 to Figure 5, there are all pictures in the researchers' papers. Are you get authorized to use it?

 

Actually, your experiment and data are very valuable. I don't know if you can try to explain the following questions:

1.      Which combination of consumers and topcoats is most likely to cause light shock?

2.      Is there any quantitative analysis of coating samples differences after lightning damage? Such as these indicators, diameter- already existing, depth, area, weight change, component change, roughness, etc., are for your reference only.

 

Line 138 A full point is added before the reference.

 

Line 172 As stated by the authors,” The specific damage associated with an acoustic wave in material has not yet been studied in detail.”( Line 196). In the paper, you don't need to describe other people's research methods about acoustic waves too much. Would it be better to focus on your work?

 

Line 199-200 In the results section, we did not find the analysis of the propagation mode of acoustic waves in composite materials. The relevant analysis seems to be the existing findings in other researchers' papers.

 

Line 225 The title of section 2.2 is Coverage, but the specific content seems to be about Coating. Whether Section 2.2 and Section 2.3 can be combined?

 

Line 252-258 The format of this paragraph seems to be inconsistent. It needs to be adjusted according to the requirements of the journal.

 

Line 350-351 6 Collection device cannot be found in figure9. Is it missing?

Author Response

We would like to thank the reviewer for their time spent carefully reviewing the manuscript, and in their opinions regarding the science and presentation of the material. In what follows the referees’ comments are in red and the authors’ responses are in black.

 

Reviewer #1 (Remarks to the Author):

Ling 19-20 How do you get the conclusion that aircraft metal coating requires less maintenance? It seems that this is a repetition of references [21 22].How much financial and time costs can it save?

In paper states:

“The paper concludes that metallic aircraft coating samples cause less damage of aircraft coverage after a lightning strike…”

This conclusion follows from the fact that was found in the experiment, namely that non-metallic coatings tend to cause more damage to the aircraft coverage after a lightning strike. In this case, care should be taken with the terms Coating and Coverage.

Line 103 In section 2. Materials and Methods, it seems that the methods mentioned are all existing methods. Is there any improvement in the application of these methods in your research? From Figure 1 to Figure 5, there are all pictures in the researchers' papers. Are you get authorized to use it?

Following your recommendations and questions, as well as those of other reviewers, the Materials and Methods section has been redesigned. Studies by other researchers have been only briefly summarized to give the reader an understanding of the issues without more detailed descriptions.

Actually, your experiment and data are very valuable. I don't know if you can try to explain the following questions:

First thing, we very much appreciate your opinion about our experiment.

1. Which combination of consumers and topcoats is most likely to cause light shock?

Thank you for your question, but this was not the subject of the research and despite discussion with the authors, we are unable to answer this question.

2. Is there any quantitative analysis of coating samples differences after lightning damage? Such as these indicators, diameter-already existing, depth, area, weight change, component change, roughness, etc., are for your reference only.

Any results we collected we mentioned in the paper. Further tests, such as tests of strength, elasticity and more, with samples even after the salt chamber are future work. However, it has to be said that we have thought about doing measurements like those mentioned above, mainly strength and elasticity.

Line 138 A full point is added before the reference.

Thank you, corrected.

Line 172 As stated by the authors,” The specific damage associated with an acoustic wave in material has not yet been studied in detail.”( Line 196). In the paper, you don't need to describe other people's research methods about acoustic waves too much. Would it be better to focus on your work?

Thank you, as mentioned, this section has been changed and the content on acoustic force has only been mentioned as part of the force exerted when lightning strikes.

Line 199-200 In the results section, we did not find the analysis of the propagation mode of acoustic waves in composite materials. The relevant analysis seems to be the existing findings in other researchers' papers.

Thank you, describing section about acoustic waves was modified.

Line 225 The title of section 2.2 is Coverage, but the specific content seems to be about Coating. Whether Section 2.2 and Section 2.3 can be combined?

Thank you for the recommendation, the sections have been combined.

Line 252-258 The format of this paragraph seems to be inconsistent. It needs to be adjusted according to the requirements of the journal.

This part of the article, lines 252-258, (now 177-183), has been checked and shows no problems with the consistency of the text. If we have misunderstood a suggestion, please let us know.

Line 350-351 6 Collection device cannot be found in figure9. Is it missing?

Thank you for your attention and setting the scene. Indeed there Collection device is missing. The STN standard where the sources are located has been checked and their absence is also there. The picture has been edited so that everything fits.

Author Response File: Author Response.pdf

Reviewer 2 Report

This work presents a study that simulates the effect of lightning on metallic and non-metallic types of aircraft coverings where various types of paint coatings are applied.

Congratulations, the work presented shows some knowledge and demonstrates some commitment to writing the article, however, there are some concerns about your work that can be resolved by improving the work and your understanding. Please see the attachment.

Comments for author File: Comments.pdf

Author Response

We would like to thank the reviewer for their time spent carefully reviewing the manuscript, and in their opinions regarding the science and presentation of the material. In what follows the referees’ comments are in red and the authors’ responses are in black.

 

Reviewer #2 (Remarks to the Author):

1.Congratulations, the work presented shows some knowledge and demonstrates some commitment in writing the article, however, there are some concerns about your work that can be resolved by improving the work and your understanding, such as:

First of all, thank you so much.

2. In subchapter 2.1, General information, the authors can refer to some dimensions of the material used in the experiment. Thus, it is easier to “visualize” the layout used.

Thank you for the suggestion, the dimensions, thickness and shape of the samples has been added to section 2.3 Samples.

3. In subchapter 2.1.3 Materials, when the authors present the variables of the presented algorithms, they must also make reference to the respective units.

This section has been completely redesigned based on other reviewers and formulas describing various phenomena have been removed. Based on this, there is no need to add units.

4. - In subchapter 2.4, samples:

* In a scientific study, it is recommended to have at least 3 samples to eliminate possible errors or defects. From what I understand, the authors have 12 samples, but all of them with some difference between them. The ideal would be to have 3 copies of each of the 12 samples.

Thank you for the suggestion. In this case, we argue that each sample was hit by a larger number of discharges (10), which in our opinion is a sufficient number for investigation. In any case, thank you for the suggestion and we will keep an eye out for a bigger number of samples in the future.

* Authors can also add the dimensions and shape of the samples

Thank you, dimensions and shape were added.

- In figure 6 you should check that the dimensioning respects the design standards. I leave some suggestions:

Thank you, the image has been modified based on your recommendations.

- In subchapter 2.6, evaluation of results, the authors must identify the equipment used. For example, the authors state that they used a microscope to observe carefully and also confocal laser scanning microscopy, but do not identify the equipment used. Check and identify all equipment used.

Laser scanning confocal microscope and microscope used were added. Thank you.

- Authors should check when mentioning the standards used, they should correctly identify the standards. It is not enough to say that they followed the norm. They must identify the norm. For example, when they talk about the ISO 9227:2017 standard, it is correct, but when they talk about the STN standard (Slovak Technical Standard), the information is incomplete.

Thank you for the reminder, the standards have been described as correct.

- In the chapter on the results, when the authors say that they can be summarized in 3 points, by microscope and by confocal microscopy, they must have these equipment previously identified, or else, if they have not yet been identified, they must do so here. But the ideal will be in the previous chapter, because in this one you should present only the results.

Already done, thank you.

- In images that contain more than one, for example image 16 has two images, the authors must add (a) and (b). In the caption they should include (A) and (b) to be clearer. Example:

All pictures were properly labelled. Thank you.

- This information must be used in all existing images in the work.

Already satisfied.

- This work would be much richer and more complete if the authors presented experiences and comparative results of stress tests to evaluate these properties of the samples before and after the lightning. This is the only way to verify the efficiency of the coating in relation to the mechanical properties of the materials.

Your opinion agrees with our other practices. The measurement of mechanical properties will be the subject of further research.

In subchapter 3.5 the authors say that sample 8 suffered destructive damage. Did the authors discover the reason for what happened? Was it an experiment gone wrong? I reinforce that it was important to have 3 samples of each for these situations. The reason for this result was not clear.

Unfortunately we are unable to identify the problem associated with sample 8. There are sub-issues however research associated with this issue will be the subject of future research.

- In chapter 4, discussion, the authors state that “An interesting result is the appearance of hydrophobicity in non-metallic samples in the vicinity of lightning.” It would be interesting for the authors to comment on the reason for this appearance... If they cannot justify it, it is better not to mention it, or put it as a suggestion for future work. For example: Due to the appearance of hydrophobicity in non-metallic samples in the vicinity of lightning, it will be interesting to carry out future work in which the samples are exposed to water before using the salt chamber.

The reason for the hydrophobicity is unknown to us. We agree that further research in this area could yield interesting results and we are currently considering such a study. A sentence where this phenomena is mentioned has been adjusted mentioning future research.

- Conclusions should be improved: * The authors address the failure of sample 8 in the salt chamber, but do not mention the cause.

Sample 8, which showed destructive effects, will be the subject of further experiments. Conclusions was modified to mention this issue, but not to blame a salt chamber.

* They claim that the metallic samples were found to have suffered damage that would affect the mechanical properties of the material, but they did not perform tribological tests to support this claim. Since they claim that the mechanical properties were affected, they must justify the claim.

In the paper we refer to the fact that damage to the material has been observed along the surface and also to the depth of the material, which clearly compromises the internal integrity, but we do not specify the magnitude of this damage as this has not been the subject of research. The conclusion that non-metallic specimens damage the aircraft coating more than metallic specimens is related to all the observed damage, specially for the damage into the depth of the meterials, which is specific to non-metallic specimens. Anyway we thank you.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors!

The problems studied in the article are of great importance, as they are related to the safety of aircraft flights. However, both the structure of the article and its content have a large number of shortcomings, listed below.

1. The structure of the article. There is a lot of repetition in the text. In particular, subsections of part 2.7 repeat similar subsections of part 2.6, somewhat expanding their content. This only unreasonably increases the volume of the article and distracts the reader's attention from the main content of the sections.

The formulas given in Section 2 are not used anywhere else in the text. Are they needed? If necessary to explain the results obtained, such explanations should be given during the discussion.

2. Characteristics of the studied samples. Sections 2.2 - 2.4 contain a description of the investigated coatings. The authors do not provide any justification for the choice of these particular materials. Such justification could be an indication of the use of such coatings in some aircraft, or a significant difference in physical and mechanical properties, which, nevertheless, would allow them to be used in the production of aviation structures. Tables 1-3 show the names and a brief description of the materials, but there is no information about their electrically conductive, mechanical properties, thermal conductivity, and heat capacity. There is no indication of primer and top coat thicknesses. This does not allow a reasonable interpretation of the presented research results.

3. The given average results have low reliability. Line 367 indicates that such results were considered erroneous and were not taken into account. However, no simple statistical analysis was carried out. Perhaps these were not errors, but outliers. But discarding even one result with three studied samples is unacceptable.

4. In the final part of the article, there is no visual table showing the effect of the coating composition on various damages, degradation of properties after exposure to a salt chamber, and the prediction of the effect of an electric discharge on the fatigue and short-term strength of the material in the discharge zone.

5. Section Discussion begins with a trivial statement that does not carry any information

The conclusions should be specified taking into account the significant differences in the coatings used, the forecast of their reliability and safety for air transport.

The text also contains inaccuracies.

Line 123 indicates that E is the electric field strength and H is the air density. But these values are not included in the formula.

Lines 208, 209 say “….a lightning strike on the aircraft structure will cause its formation on the coating”. What exactly did the authors have in mind?

The very small font of the magnification scale against a dark background, presented in the photographs taken from the microscope, does not allow one to visually assess the size of the damage.

All photographs taken from microscopes require a detailed description of the various areas represented in them.

Figure 16 shows 3D images of "non-metallic" and "metallic" damage. Which of the investigated "non-metallic" and "metallic" samples are depicted?

A detailed explanation of the photo in Fig.20, 21 is necessary.

Author Response

We would like to thank the reviewer for their time spent carefully reviewing the manuscript, and in their opinions regarding the science and presentation of the material. In what follows the referees’ comments are in red and the authors’ responses are in black.

 

Reviewer #3 (Remarks to the Author):

1. The structure of the article. There is a lot of repetition in the text. In particular, subsections of part 2.7 repeat similar subsections of part 2.6, somewhat expanding their content. This only unreasonably increases the volume of the article and distracts the reader's attention from the main content of the sections.

Thank you for your suggestion. However, we argue that section 2.6 describes the procedures by which the samples were evaluated. Section 2.7 describes the impact, set-up and relevant information about the salt chamber. Thus, in this case, we do not think there is repetition. In any case, however, we thank you for your feedback

The formulas given in Section 2 are not used anywhere else in the text. Are they needed? If necessary to explain the results obtained, such explanations should be given during the discussion.

Thank you for the comment, section 2 has been completely redone and unused formulas removed.

Characteristics of the studied samples. Sections 2.2 - 2.4 contain a description of the investigated coatings. The authors do not provide any justification for the choice of these particular materials. Such justification could be an indication of the use of such coatings in some aircraft, or a significant difference in physical and mechanical properties, which, nevertheless, would allow them to be used in the production of aviation structures. Tables 1-3 show the names and a brief description of the materials, but there is no information about their electrically conductive, mechanical properties, thermal conductivity, and heat capacity. There is no indication of primer and top coat thicknesses. This does not allow a reasonable interpretation of the presented research results.

Thank you for your suggestion. In this case we were supplied with samples directly from the aircraft maintenance company. Due to the fact that both the sample material and the primer are used in real operation, we consider them to be relevant samples. As far as the mechanical properties are concerned, we have also not received detailed documentation on them. The dimensions and thickness of the samples have been added to the article. The basic properties of the coatings are summarized in Table 2 and are properly referenced. Any further and more detailed information on these coatings is available at the appropriate links. The reason for the selection of these samples, i.e. that they are from real aircraft operations, has been added to the article.

3. The given average results have low reliability. Line 367 indicates that such results were considered erroneous and were not taken into account. However, no simple statistical analysis was carried out. Perhaps these were not errors, but outliers. But discarding even one result with three studied samples is unacceptable.

Thank you for your feedback. In this case, the experiment contained 12 samples, each of which was struck 10 times with an electric discharge. This amounts to 120 strikes. 3 discharges deviated from the norm in the amount of voltages. It was not possible to determine which discharge belonged to which strike on the sample, but the 3 strikes that deviated from the norm were considered erroneous. More detailed research and more thorough equipment would be needed to investigate and observe individual electrical strikes more closely.

The individual strikes on the samples do not vary greatly in the form of the values in the tables, therefore, also for the purpose of a cleaner look, the values have been averaged. The purpose was to show the changes in material structure after an electric shock hit, especially between conductive and non-conductive coatings.

A brief description of this reason has been added to the article, thank you.

4. In the final part of the article, there is no visual table showing the effect of the coating composition on various damages, degradation of properties after exposure to a salt chamber, and the prediction of the effect of an electric discharge on the fatigue and short-term strength of the material in the discharge zone.

Thank you for your message. To our disappointment, the salt chamber results did not yield any significant impact worthy of more detailed examination and comparison in the form of a table. For this reason, the text mentions, "Closer examination under the microscope made it clear that the salt chamber, like the naked eye observation, did not cause any damage that would significantly affect the difference between lightning strike damage and subsequent exposure to external conditions."

The only difference is sample 8, which suffered fatal damage, but for which we have no explanation and further research is needed.

5. Section Discussion begins with a trivial statement that does not carry any information

The beginning of the Discussion section has been corrected, thank you.

The conclusions should be specified taking into account the significant differences in the coatings used, the forecast of their reliability and safety for air transport.

More results of the sample comparisons have been added to the Conclusion section. A conclusion has also been added that describes the dangers of such lightning strikes, which cause a small area of damage at first glance on the surface, but propagate towards the depth of the material.

Line 123 indicates that E is the electric field strength and H is the air density. But these values are not included in the formula.

Thank you for noticing. These values were described in the text but were not marked as a formula. Nevertheless, the whole section has been redone and there are no discrepancies.

Lines 208, 209 say “….a lightning strike on the aircraft structure will cause its formation on the coating”. What exactly did the authors have in mind?

Thank you, the text was indeed worded in the wrong way. The whole section has been edited and the discrepancy is no longer there.

The very small font of the magnification scale against a dark background, presented in the photographs taken from the microscope, does not allow one to visually assess the size of the damage.

All scales have been adjusted, thank you.

All photographs taken from microscopes require a detailed description of the various areas represented in them.

Agree. Description of measured characteristics was added.

All photographs taken from microscopes require a detailed description of the various areas represented in them.

All areas has been described. Thank you.

Figure 16 shows 3D images of "non-metallic" and "metallic" damage. Which of the investigated "non-metallic" and "metallic" samples are depicted?

Since the results of confocal microscopy were similar for all metallic and non-metallic samples, the purpose was to point out the main difference, which consists in the concentration of energy to one point in non-metallic samples. On the contrary, with metallic ones, there is a greater distribution of forces. In the article, we want to point out that such a difference occurs in all samples. Thank you for the feedback and therefore the description was changed from Comparison of metallic and non-metallic sample to General Comparison of metallic and non-metallic sample.

A detailed explanation of the photo in Fig.20, 21 is necessary.

The reason for the fatal damage to sample 8 is unknown to us and further research is needed, so we cannot interpret any more detailed results. In the case of figure 21 (now 19), there were no significant observations of deviations before and after a salt chamber, and therefore we consider a brief description to be sufficiently relevant.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

There are no comments on the revised paper. Please make sure figure 9 is correct. If the collection device is missing, you can clarify it in the title.  This will not confuse others when they want to repeat your experiment.

Author Response

We would like to thank the reviewer for their time spent carefully reviewing the manuscript, and in their opinions regarding the science and presentation of the material. In what follows the referees’ comments are in red and the authors’ responses are in black.

 

Reviewer #1 (Remarks to the Author):

There are no comments on the revised paper. Please make sure figure 9 is correct. If the collection device is missing, you can clarify it in the title.  This will not confuse others when they want to repeat your experiment.

Our sincere apologize. The figure 9 was modified, but was not replaced in the revised manuscript. Thank you, now it is all correct.

Reviewer 2 Report

With the changes made by the authors, the article has quality to be published. Congratulations.

Author Response

Based on the reviewer's comment, no changes needed to be made.

Reviewer 3 Report

The text of the article has been significantly improved, but there are still some comments.

1. In table 4, it is desirable to indicate for each of the 12 samples which of them are metallic and which are non-metallic. Such an explanation is also very desirable in the photographs of Fig. 10 - 14

2. The magnification scale on a dark background, presented in photographs taken with a microscope, is still very small and does not allow visual assessment of the size of the damage.

3. Callouts (Commented [P1-P12]) in Slovak are incomprehensible to the reviewer. There is no way to translate them well. Can they be presented in English?

4. There are two proposals in the Conclusions that require mandatory specification. Without such specification, the expressions are devoid of informativeness. These are the expressions:

"The combination of primers and top coats used also significantly influences the mode of damage to the specimen."

"Any damage caused by a lightning strike should be treated appropriately to avoid gradual degradation of the material strength and fatal consequences."

The article can be accepted after minor modifications.

Author Response

We would like to thank the reviewer for their time spent carefully reviewing the manuscript, and in their opinions regarding the science and presentation of the material. In what follows the referees’ comments are in red and the authors’ responses are in black.

 

Reviewer #3 (Remarks to the Author):

In table 4, it is desirable to indicate for each of the 12 samples which of them are metallic and which are non-metallic. Such an explanation is also very desirable in the photographs of Fig. 10 – 14

Thank you. The designation of non-metallic samples has been added to Table 4 using the * symbol and included in the text as an explanation. All figures are labeled with the associated number, which is described in the tables, by which the properties of the sample in the figure can be determined.

The magnification scale on a dark background, presented in photographs taken with a microscope, is still very small and does not allow visual assessment of the size of the damage.

Thank you, the scales have been adjusted to make them easy to read.

Callouts (Commented [P1-P12]) in Slovak are incomprehensible to the reviewer. There is no way to translate them well. Can they be presented in English?

Thank you for your suggestion. The article has been rechecked and there are no longer any descriptions in the Slovak language.

There are two proposals in the Conclusions that require mandatory specification. Without such specification, the expressions are devoid of informativeness. These are the expressions:

"The combination of primers and top coats used also significantly influences the mode of damage to the specimen."

The text has been edited to explain the concepts.

"Any damage caused by a lightning strike should be treated appropriately to avoid gradual degradation of the material strength and fatal consequences."

References have been added to a maintenance organization (Boeing) and also an aviation organization (EAA) and their specific articles regarding lightning damage to which we are referring. We also include a reference to a specific aviation accident associated with such damage.

Thank you for your revision.