Experimental Study of Stepped-Lap Scarf Joint Repair for Spar Cap Damage of Wind Turbine Blade in Service

Round 1

Reviewer 1 Report

Objective (as stated by authors): Tensile strength and fatigue tests were conducted to investigate stepped-lap scarf joint repairment for spar cap damage of wind turbine blades in service. In this paper, a full experimental research was carried out on the spar cap test samples which were repaired using the stepped-lap scarf joint method. The large aspect ratio samples with multi configuration parameters of stepped-lap scarf joint unidirectional tape were selected for the tensile test. The control variable method was used to analyze the internal relation and variation rule of the multiple test results.

Results (as stated by authors): The influence of the spar cap’s stepped-lap scarf joint parameters on strength repairment and the failure damage mode were confirmed. The test results show that the fracture load of test specimen 13 can be increased to 1.65~1.96 times of the original value by using reinforcing layers. It is confirmed 14 that the shear delamination and the coupling of fiber fracture in the stepped-lap zone are the main 15 failure modes.

Using carbon nanotube buckypaper transducer, the quasi-static test and fatigue performance study of double stepped-lap stepped scarf repair samples with overlap reinforcing layers were carried out. Based on the resistance response of the transducers, the damage evolution and cumulative extended fracture process of the sample could be accurately captured which provided an important basis for using the carbon nanotube buckypaper transducer to detect the structural characteristics of the stepped-lap areas. Infrared thermal imaging technology was used to image the samples after fatigue damage failure, which furtherly verified that carbon nanotube buckypaper transducer can successfully monitor the damage evolution and running status of the spar cap after the repairment.

 

Specific Observations:
Language and writing style: good, the article can be read fluently and main ideas are clearly exposed. Nonetheless, Authors are encouraged to ask a native English speaker to read the article and highlight phrases that could be rewritten, with views to improve the overall readability.
State of the art: good, it reflects the most important methods for blade repair, although the theoretical background is somewhat limited.
Scientific soundness: very good, authors provided adequate images and descriptions to understand the topic.
Key references: excellent and well distributed across the paper.

Suggested corrections:
Page 2, line 55, correct "f" in "finite element analysis".
Page 2, line 76, rephrase, so as to avoid redundant "accuracy" term.

Page 3, add key reference(s) to the baseline stepped-lap scarf joint repairing method.
Page 3, line 122, correct brackets and parenthesis in equation 8.

Increase font size of figures 1, 3, 5 and 9. It is not possible to read the contents.
Decrease font size of tables 1 and 2, so as to properly fit titles and contents and double-check upper- and lower-case letters.

Conclusion section: the Authors did good work while summarizing the findings of the experiments. However, no paths for future research were highlighted. Authors are encouraged to present a summary of potential lines of study that can be derived from the findings of this work.

Final remarks:
This work is high-quality; it deals with an appealing subject and is well structured and documented. The experimental setup is comprehensive; however, it does not make a clear connection with available theoretical frameworks for failure/damage prediction, as the work is purely experimental (this has no critical impacts on the overall merit or expected impact of the work).

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Overall the manuscript presents an interesting analysis and some good experimental work. The main failing is that the thesis of the research is not clear and the conclusions need significant tidying up. The manuscript also requires a review of the terminology used throughout, as much of it is not correct or commonly used. The overall merit of the work is high, if an appropriate explanation of goals and potential benefits can be provided.

Abstract

“Repairment” is not a commonly used word, it should be replaced with “repair” throughout the manuscript.

1.65-1.96 times of the original value by using reinforcing layers

Damage polymerization?

Rapid accumulation propagation?

NB: Needs review, unclear what the overall goal of the paper is from the abstract. It seems like one aim of the paper is to determine how well the buckypaper works (from wording at least). Is this a goal of the work, or is the buckypaper simply one type of instrumentation used? From the introduction section this still remains unclear and is a serious lack of focus for the manuscript.

The title mentions the damage to a blade in service, nothing in the abstract refers to testing blades.

 

Introduction

L30 – Very general statement with no references to back it up

L32 – What failure modes and damage types are you referring to here?

L34 – “geometric concentrator” and “stiffness mutation” are not commonly used terms. These should be reviewed with careful attention to nomenclature used in this industry/scientific discipline so the correct information and context is communicated to the reader

L35 – What waves are you referring to here? Is this ply waviness? “Waves” is not a common way to describe this defect

L37 – While a deviation of 15° may reduce the strength significantly, this is much higher than would be expected in production. It would be more relevant to quote the strength reduction from fibre deviations typically seen in production (on the order of 1-3° perhaps?)

L39 – This is not always how blades fail, a further review of failure mechanisms of blades in service is required

L43 – What does “motherboard and patches configuration” mean?

L48 – “The maintenance of the blade bearing parts damage is basically using scarf method” - unclear what this sentence means

L51 – An explanation of the difference between stepped and tapered scarf repair types is needed here.

L58 – This section needs to be completely revised to provide a much clearer explanation of the different types of repair methods and the mechanics of how they transfer load. Possibly including figures/graphics to show the different types and to show a breakdown of the loading.

L68 – This sentence should be revised to provide a much clearer explanation to the reader. Refer to blade design standards to find the correct terminology (e.g. motherboard, interlayer, failure courses, etc.).

L76 – This sentence is too long and it is unclear what you are trying to communicate.

The introduction in general does not provide enough discussion of the background to composite spar cap repair or details of the repair method under investigation (and viable alternative methods). The reader should be able to understand the context of this research from the explanations provided in the introduction and should be provided with a list of the relevant sources to further their own understanding of the key topics and methodologies used in this work. The introduction as it is written requires significant revision to meet these requirements.

The level of English in the paper should be reviewed in detail. There are multiple locations throughout the manuscript where words are repeated, e.g. L89 and L76

 

Stepped-lap scarf joint repairing method

L112 – Figures explaining the single and double lap method should also be included.

L122 – What is the constant c used for?

 

Tensile strength experiments on the large aspect ratio test specimens with multiple stepped lap scarf joint unidirectional tape

L131 – Do you mean vacuum infusion? Not familiar with vacuum perfusion.

L146 – The differences between the three main groups (W1-W3) should be properly explained in the text. The variation in length, width, and other factors within the groups also needs to be adequately explained. Along with a more detailed explanation of the goals of the research in the introduction, this will make the following results and discussion section much easier to understand.

L156 – The title of this section should be reviewed for readability.  

 

The research on the quasi-static and fatigue test of stepped-lap scarf joint repaired monitoring using carbon nanotube buckypaper

L248 – Are the buckypaper transducers laid up between GFRP plies? Have you considered how their presence inside the laminate might impact the failure mode of the samples? Did you test any of the same types of samples without the buckypaper for a comparison?

L261 – Do you mean tension-compression fatigue instead of tensile-pressure fatigue?

L284 – By resistance do you mean material strength or stiffness? Or do you just mean the resistance reading of the transducer? What does transducer resistance signify, e.g. high strain? A clearer explanation of the buckypaper sensors and how they work is required. How were the readings captured from the buckypaper?

L290 – Figure 7 should be annotated to show where sensors 1, 2 and 3 are located.

L361 – These figures should be revised. The inset figures into each make the information difficult to process. Further comments/explanation should be included than is currently provided in the manuscript. Similarly for Figure 16.

L373 – Are these the results for 6000 fatigue cycles, or 6000 fatigue tests. Terminology throughout the manuscript should be reviewed.

L379 – The locations of transducers (e.g. L2-1, L2-2, etc.) should be given in the manuscript.

L385 – Use the word until not “till”.

 

Conclusions

L419 – The first sentence describes the goals of the paper, this should be included in the abstract and introduction sections

What is the control variable method???

L425 – Revise the use of a numbered list here. Not clear why there is an initial list level separating out the next lists?

L431 – This point needs revision, it appears to be contradictory (you say the stepped-lap mode has little influence on strength and then that the stepped-lap joint decreases the overall strength in the same sentence).

L435 – “Intensity decay degree” is not a common term

L438 – What is the reinforcement layer described here?

L452 – What size effect? Your conclusion points should be self-contained and not require referencing back to specific sections in the manuscript to understand them.

It is not clear what the actual thesis of the work presented here is. From the long list of conclusion bullet points, it doesn’t appear that there was a particular goal set out for the research. Some of the concluding remarks belong in the discussion section (e.g. using infrared thermal imaging technology) as they are not direct results from your work that have relevance to the wider research community.

This final comment:

“The 476 experimental results in this paper are transformed into a repair method of stepped-lap scarf joint for spar cap. The method has been successfully applied to repair the major structural damage of spar cap of blade in service. The repaired blade has been running for more than 3 years and has been running in good condition.”

Is not entirely suitable for the conclusion of the manuscript, since it is not the focus of the current work. The manuscript describes an assessment of the structural response of the repair method, but does not indicate how it could be used in an actual blade, hence this comment should be removed (or elaborated on in the main body of the text).

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript contains the elements of an interesting and useful study of wind turbine blade spar cap repair.  However, it is badly organized and written, and requires a thorough editing to improve the English and the layout before it could be considered for publication.

The problems start with the title which does not mention "wind turbine".  The actual application is not mentioned until line 43 and "wind turbine blades" should appear in the the keywords.  There are many terms used poorly: "repairment" for "repair" (in most cases); "motherobard" which occur only in computers; and "furtherly" are examples.  Many phrases are hard to understand, such as "For the normal and given by adhesive layer" (line 114) and "No matter single lap joint" (line 164) and many others.

The Introduction should be revised to be complete and consistent. Introductory material scattered throughout the manuscript, such as in lines 396-397, should be consolidated.  Some discussion of the context of the study is needed.  Is is actually possible to repair spar caps on blades that are to be rerturned to service? The methods of repair given in line 43 are not described and I would think that a simple diagram expalining the principles of the methods would be valuable. In line 83, the research on damaged spar caps is mentioned without describing the damage.  Were the damaged specimens taken from actual blades or were they artifically damaged? There seems to be a practical issue here.  I would have thought that inspection and removal of damaged sections of spar caps would be relatively easy near the hub but almost impossible near the tip.  If spar cap damage is concentrated to the hub region, this should be stated.  Otherwise more explanation is needed.

The equations are often poorly formatted in a font size different to the text. The headings for Table 2 are badly formatted and figure 12 lacks axis labels, and figures 4, 7, and 11 showing spar cap samples have no scales. The Sensor1, Sensor2, and Sensor3 in figure 9 are not explained.  The horizontal axes in figures 12, 14, and 16 are labeled "Circle period" when "Cycle number" is probably an imrpovement.

The referencing is poor with some author names truncated as in [33] and [34] and the author of [38] is L. Mishnaevsky.  The URL of [39] should be given.

The most important deficiency is the description of the carbon nanotube buckypaper transducers.  We are not even told what they measure, no information is given about their accuracy, or suitability for this purpose.  There is a reasonable literature on this measurement technology but there are no specific references in the manuscript.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors have made a substantial effort to improve the quality of the paper and to address the comments I made on the previous version.  I have only three remaining issues:

Point 12. My query regarding where on the blade, spar cap repair can be done has not been addressed.

Point 17. The new information on buckypaper sensors in new lines 103-123 is a good addition. I suspect that on line 106 “is shown” should be replaced by “acts”.  Also, there is still no information on the accuracy of the sensors.  It appears that they are used as qualitative indicators of material degradation and, possibly, strict error estimates are not possible. I have no problems if this is the case but it should be stated.

Finally, my comment “The horizontal axes in figures 12, 14, and 16 [now 15 – 17] are labeled "Circle period" when "Cycle number" is probably an improvement.”

These comments are minor and can be handled by the editor without sending it back to me.

Author Response

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Author Response File: Author Response.docx