New Concept of Dual-Sinusoid Distributed Fiber-Optic Sensors Antiphase-Placed for the SHM of Smart Composite Structures for Offshore

Round 1

Reviewer 1 Report

The presents an interesting concept of using dual-sinusoidal placement of distributed fibre-optic sensors for structural health monitoring in offshore wind turbine blades. The work extends previous research to include bending and torsion, and the study involves finite element modelling to analyse the strain patterns and sensing capabilities of the proposed sensor placement under various loading conditions. Prior to acceptance, there are a few comments, both minor and major, provided below.

1.      It is highly recommended to explain various methods of structural health monitoring such as electrical impedance measurement by using CNT [a], GNP [b], thermography [c] etc. then point out the importance of the SHM with fibre optic sensor in composite structures.

[a] Structural health monitoring of adhesive joints under pure mode I loading using the electrical impedance measurement. Engineering Fracture Mechanics, 245, 2021 107585.

[b] Structural health monitoring of defective single lap adhesive joints using graphene nanoplatelets. Journal of Manufacturing Processes,2020, 55, 119-130.

[c] Delamination defect evaluation in CFRP composite patches by the use of active thermography. Journal of Nondestructive Evaluation, 2022, 41(3), 61.

2.      The production name of the epoxy adhesive is not indicated in the paper.

3.      The authors should provide more details on the experimental setup and the standard of adherent composite and joint preparation

4.      Why the total thickness of 0.320 mm selected for adhesive layer? How the authors address this factor in their results?

5.      Could the authors elaborate on the importance of these analyses and how they contribute to a more comprehensive understanding of the proposed sensor placement in real structures?

6.      The Authors mention the need for further optimization of key parameters through parametric studies. What specific parameters are being referred to, and how do they influence both mechanical behaviour and sensing functions?

7.      How the

8.      Use bullets in Conclusions to emphasise the main achievements of the paper

Author Response

Dear Reviewer 1

Allow me first to thank you very much for your very valuable comments that will sure enhance the quality of the manuscript. Hereafter are the responses to all your comments.

The presents an interesting concept of using dual-sinusoidal placement of distributed fibre-optic sensors for structural health monitoring in offshore wind turbine blades. The work extends previous research to include bending and torsion, and the study involves finite element modelling to analyse the strain patterns and sensing capabilities of the proposed sensor placement under various loading conditions. Prior to acceptance, there are a few comments, both minor and major, provided below.

1. It is highly recommended to explain various methods of structural health monitoring such as electrical impedance measurement by using CNT [a], GNP [b], thermography [c] etc. then point out the importance of the SHM with fibre optic sensor in composite structures.

[a] Structural health monitoring of adhesive joints under pure mode I loading using the electrical impedance measurement. Engineering Fracture Mechanics, 245, 2021 107585.

[b] Structural health monitoring of defective single lap adhesive joints using graphene nanoplatelets. Journal of Manufacturing Processes,2020, 55, 119-130.

[c] Delamination defect evaluation in CFRP composite patches by the use of active thermography. Journal of Nondestructive Evaluation, 2022, 41(3), 61.

Let me first point out that the optical fiber sensors are solely used as strain sensors and not by any means as damage sensors … By using optical fiber, the idea is strictly to monitor strain that must remain below any strain that leads to onset of damage ... and stop the use of the composite anytime an overstrain is detected … Which is different from the sensors that are listed in the articles you are suggesting. Neverthless, I will add 2 of them in the reference list.

2. The production name of the epoxy adhesive is not indicated in the paper.

The composite was manufactured by prepreg method using a matrix volume fraction of 34% and volume fraction of fibers of 66%. HexPly M79/34%/UD300/CHS type laminates were considered. 

3. The authors should provide more details on the experimental setup and the standard of adherent composite and joint preparation

The article is the follow-up of the previous work by the author and uses the analytical results that were published. This has been added to current article.

Raman, V.; Drissi-Habti, M.; Limje, P.; Khadour, A. Finer SHM-Coverage of Inter-Plies and Bondings in Smart Composite by Dual Sinusoidal Placed Distributed Optical Fiber Sensors. Sensors 2019, 19, 742. https://doi.org/10.3390/s19030742

4. Why the total thickness of 0.320 mm selected for adhesive layer? How the authors address this factor in their results?

We are focusing on the placement, only. We are not considering the effects of the thickness of the optical fiber this time. Our main and unique focus is to check whether using double-sinusoid we can monitor the strain correctly.

5. Could the authors elaborate on the importance of these analyses and how they contribute to a more comprehensive understanding of the proposed sensor placement in real structures?

This is for sure our prime target. But we are still far from this target that needs extensive work to come … That we are trying our best to solve out step by step

6. The Authors mention the need for further optimization of key parameters through parametric studies. What specific parameters are being referred to, and how do they influence both mechanical behaviour and sensing functions?

The influence of the type, properties and thickness of the glue on the mechanical and physical behavior of the optical sensor ... Sliding/friction at the interface optical sensor-composite plies almong others

7. How the
8. Use bullets in Conclusions to emphasise the main achievements of the paper

This has been done on the modified version

Author Response File: Author Response.pdf

Reviewer 2 Report

1. The authors are correct in that there is significant potential in the use of dual-sinusoidal placement of FOSs but their claim that its use has not been considered "under various loading conditions on structural mechanical behavior and sensing functionality" is not correct since a body of research already exists in the published literature on this. The authors are urged to conduct a more thorough literature review to gain from prior work and to cite the most important aspects.

2. The authors claim that "next-generation offshore wind turbine blades will exceed 100 meters" suggests that this has not yet been done. However there are blades already in production exceeding 100 m produced by companies such as Vestas, MingYang, Siemens etc. Their statement would have been correct in 2022 but not in 2023. This needs to be updated.

3 How was the appropriate level of mesh density determined? What were the criteria to assess accuracy? Details on this need to be provided since the application itself is based on extreme sensitivity and hence this is very important

4. Further detail on the interface between the adhesive and the CFRP need to be provided. Why are interface elements not considered to enable better transition?

5. While the analytical results are extremely interesting the lack of comparison with experiments or even prior results to show that the predictions are valid is concerning. Without this the validity of the entire study is at risk. Just providing results of FEA studies with no comparison is insufficient to (a) prove the viability of the dual-sinusoidal placement of FOSs, and (b) prove that results are correct and sensitive enough to be used as predictions of mechanical loading.

6. Without some level of comparison the paper is incomplete and cannot be considered for publication.

Minor editing is required. Some sentences appear incomplete whereas others are poorly structured.

Author Response

Dear Reviewer 2

Allow me first to thank you very much for your very valuable comments that will sure enhance the quality of the manuscript. Hereafter are the responses to all your comments.

1. The authors are correct in that there is significant potential in the use of dual-sinusoidal placement of FOSs but their claim that its use has not been considered "under various loading conditions on structural mechanical behavior and sensing functionality" is not correct since a body of research already exists in the published literature on this. The authors are urged to conduct a more thorough literature review to gain from prior work and to cite the most important aspects.

We added the 2 following articles to the references list

Mei Yang, Qidi Liu , Hamza Sayed Naqawe and Mable P. Fok, Movement Detection in Soft Robotic Gripper Using Sinusoidally Embedded Fiber Optic Sensor, Sensors 2020, 20, 1312; doi:10.3390/s20051312

Samir Mekid, Asad Muhammad Butt, Khurram Qureshi, Integrity assessment under various conditions of embedded fiber optics based multi-sensing materials, Optical Fiber Technology, http://dx.doi.org/10.1016/j.yofte.2017.05.010 1068-5200/2017 Elsevier Inc. All rights reserved.

2. The authors claim that "next-generation offshore wind turbine blades will exceed 100 meters" suggests that this has not yet been done. However there are blades already in production exceeding 100 m produced by companies such as Vestas, MingYang, Siemens etc. Their statement would have been correct in 2022 but not in 2023. This needs to be updated.

The largest wind-blade manufactured by General Electric is 107m and is placed in Cherbourg (France) … But it is not equipped with SHM advanced technologies are expected. Anyway, this point has been updated.

3 How was the appropriate level of mesh density determined? What were the criteria to assess accuracy? Details on this need to be provided since the application itself is based on extreme sensitivity and hence this is very important

The details about meshing are continuum shell elements SC8R for the two CFRP face panels; 3D stress solid elements C3D8R for all other parts. The total number of elements is 125376, in which 121856 elements are for C3D8R and 3520 for SC8R elements. This has been added to the article along with an illustration of meshing.

3. Further detail on the interface between the adhesive and the CFRP need to be provided. Why are interface elements not considered to enable better transition?

You are perfectly right and this will be the topic of further research. Our model and associated concept are exploratory work and should not be considered otherwise. The idea is to use this research to convince Wind-Blade maufacturers to start joint research with our team (This work is not funded). There are so many points that need to be worked-out ..., not at this stage unfortunately. For example, we did not consider at this step the effects of the interface. So is rheology effects (epoxy behavior), the effects of fiber-optic coating, fatigue behavior, micromechanics ... While the analytical results are extremely interesting the lack of comparison with experiments or even prior results to show that the predictions are valid is concerning. Without this the validity of the entire study is at risk. Just providing results of FEA studies with no comparison is insufficient to (a) prove the viability of the dual-sinusoidal placement of FOSs, and (b) prove that results are correct and sensitive enough to be used as predictions of mechanical loading.

The analytical results were added to the article. Figures and illustration were added.

4. Without some level of comparison the paper is incomplete and cannot be considered for publication.

Analytical results have been added and show that the results are coherent. As stated clearly in the article the prime objective is the proof of concept of double-antiphasic sinusoid placement … Next research will tackle additional issues …

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors accepted all requested comments.

Moderate editing of the English language is required.

Author Response

Dear Reviewer 3,

Than k you very much !

Best regards

Reviewer 4 Report

This paper is a continuation of the previous research of the author's team and is dedicated to the study of the dual-sinusoidal placement of distributed optical sensors (FOS) embedded within the adhesive joint between two composite laminates.

This work is still an introductory concept aimed at refining the key parameters highlighted in previous research, before starting an applied study that will consider both numerical and validation steps on real large smart composite structures.

Explain in more detail which key parameters were emphasized in previous research by the author's team.

To complement the explanation of the three main shortcomings of SHM through embedded distributed FOSs, as mentioned in Section 1, the FOSs are modeled as embedded within the adhesive joint between two CFRP laminates in a dual-sinusoidal pattern.

Author Response

Dear Reviewer4,

Our answers are embedded into your report ....

This paper is a continuation of the previous research of the author's team and is dedicated to the study of the dual-sinusoidal placement of distributed optical sensors (FOS) embedded within the adhesive joint between two composite laminates.

This work is still an introductory concept aimed at refining the key parameters highlighted in previous research, before starting an applied study that will consider both numerical and validation steps on real large smart composite structures.

Explain in more detail which key parameters were emphasized in previous research by the author's team.

The parameters that we raised in the previous article were linked in particular to the ability of optical fiber sensors placed in double sinusoids to provide monitoring of the total surface of a composite structure, without these same sensors not become mechanical defects likely to facilitate delamination. This has been cited in the text.

Best regards

To complement the explanation of the three main shortcomings of SHM through embedded distributed FOSs, as mentioned in Section 1, the FOSs are modeled as embedded within the adhesive joint between two CFRP laminates in a dual-sinusoidal pattern.

Round 2

Reviewer 1 Report

 Accept in present form

Author Response

Dear Reviewer 2

Allow me first and again to thank you very much for your very valuable comments that will sure enhance the quality of the manuscript. Hereafter are the responses to all your past comments and their updates, as well as more explanation about the objectives of current research, which is at very low TRL.

1. The authors are correct in that there is significant potential in the use of dual-sinusoidal placement of FOSs but their claim that its use has not been considered "under various loading conditions on structural mechanical behavior and sensing functionality" is not correct since a body of research already exists in the published literature on this. The authors are urged to conduct a more thorough literature review to gain from prior work and to cite the most important aspects.

We fully followed your comments. We added the 2 following articles to the references list. There are two other references that I found that were added too.

Mei Yang, Qidi Liu , Hamza Sayed Naqawe and Mable P. Fok, Movement Detection in Soft Robotic Gripper Using Sinusoidally Embedded Fiber Optic Sensor, Sensors 2020, 20, 1312; doi:10.3390/s20051312

Samir Mekid, Asad Muhammad Butt, Khurram Qureshi, Integrity assessment under various conditions of embedded fiber optics based multi-sensing materials, Optical Fiber Technology, http://dx.doi.org/10.1016/j.yofte.2017.05.010 1068-5200/2017 Elsevier Inc. All rights reserved.

Bahareh Gholamzadeh, and Hooman Nabovati, Fiber Optic Sensors, World Academy of Science, Engineering and Technology International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering Vol:2, No:6, 2008

Floris, I.; Adam, JM.; Calderón García, PA.; Sales Maicas, S. (2021). Fiber Optic Shape Sensors: A comprehensive review. Optics and Lasers in Engineering. 139:1-17. https://doi.org/10.1016/j.optlaseng.2020.106508

2. The authors claim that "next-generation offshore wind turbine blades will exceed 100 meters" suggests that this has not yet been done. However there are blades already in production exceeding 100 m produced by companies such as Vestas, MingYang, Siemens etc. Their statement would have been correct in 2022 but not in 2023. This needs to be updated.

The largest wind-blade manufactured by General Electric is 107m and is placed in Cherbourg (France) … But it is not equipped with SHM advanced technologies are expected (I am perfectly aware of it because I am the PI of this project). Anyway, this point has been updated. I added within the text of the revised version the names of the companies that you kindly mentioned.

3 How was the appropriate level of mesh density determined? What were the criteria to assess accuracy? Details on this need to be provided since the application itself is based on extreme sensitivity and hence this is very important. While the analytical results are extremely interesting the lack of comparison with experiments or even prior results to show that the predictions are valid is concerning. Without this the validity of the entire study is at risk. Just providing results of FEA studies with no comparison is insufficient to (a) prove the viability of the dual-sinusoidal placement of FOSs, and (b) prove that results are correct and sensitive enough to be used as predictions of mechanical loading.

3b. Further detail on the interface between the adhesive and the CFRP need to be provided. Why are interface elements not considered to enable better transition?

You are right. Nevertheless, for the time being, our focus is on sensor response and its capacity to monitor damage on the total surface considered, in a reliable way ... The aspects relating to the interfacial micro-mechanics aspects will be addressed in future articles.

3c. Without some level of comparison the paper is incomplete and cannot be considered for publication.

I think many explanations have been given to your comment. The modeling is introductory as mentioned throughout the article and experimental validations have been shown proof of promising results. As mentioned earlier, we started from scratch and we are trying to complexify the study of FOS placement, solely and step by step. Next step will be devoted to micromechanics at the interface FOS-Composite ply, the effects of the thickness of the bonding and its viscoelasticity, among other aspects

The assessment of the finite element discretization was performed by assuming that the adhesive layer with the embedded FOSs was the most relevant zone for strain concentration. The mesh discretization had a twofold purpose: to evaluate the variation of the strain field with the mesh density in the adhesive layer (number of elements) and to reduce the computational time. The mesh of the adhesive layer was built with varying density, higher density in the zones with higher curvatures of the FOSs. The minimum size of the element was set as the thickness of the adhesive (approximately 0.023 mm) between the FOS surface and the interface between the CFRP and the epoxy adhesive (see Figure 3). This resulted in a total number of 125376 elements. Additionally, assuming the minimum size of the finite element as half of the above, the maximum strain in the adhesive layer had a 3% difference. Therefore, considering the computational time, we have adopted the first mesh with a minimum element size of about 0.023 mm for the results presented in section 3."

The concept described in current article as applied science is new and let’s say it is at very low TRL (around 1-2). The objective upstream is to conduct exploratory deep research that can lead us to TRL 3-4, that can convince about the potential of offshore engineering to use it, raise the research issues (analytical and numerical) that have to be solved and the guidelines for future integrated research project that can lead us from the proof of concept (current article) to the pilot wind-blade that is reliable enough for industrial use in offshore industry. The pathway as you mentioned is very complex and tedious, as the challenges are numerous

There are so many points that need to be worked-out ..., not at this stage unfortunately. For example, we did not consider yet the effects of the interface. So is rheology effects (epoxy behavior), the effects of fiber-optic coating (acrylate or polyimide …), contact fatigue behavior, micromechanics ...

Analytical results have been added as you asked and show that the results are coherent. As stated clearly in the article the prime objective is the proof of concept of double-sinusoid anti-phasically placed as promising SHM concept …

More generally, I perfectly understand your ambition to get an article that is as complete as possible on both numerical and analytical modeling. We willingly share this ambition because we are, above all and after all, professors, very concerned about providing quality scientific work. However, we also know that to achieve this type of research, it would have to go through intermediate stages and provide a proof of concept, accompanied by experimental results that support the numerical predictions. It is in this regard that this research work was proposed and next step of this research will go deeper into the issues that are still ahead to be solved.

Best regards

Author Response File: Author Response.docx

Reviewer 2 Report

The authors fail to address the issues raised. In fact they state that the work is "to convince Wind-Blade maufacturers to start joint research with our team." This is an inappropriate reason for publication of a paper. Papers in high level journals need to address topics of scientific depth not to serve as advertisements to attract funding

Needs minor editing

Author Response

Dear Reviewer 2

Allow me first and again to thank you very much for your very valuable comments that will sure enhance the quality of the manuscript. Hereafter are the responses to all your past comments and their updates, as well as more explanation about the objectives of current research, which is at very low TRL.

1. The authors are correct in that there is significant potential in the use of dual-sinusoidal placement of FOSs but their claim that its use has not been considered "under various loading conditions on structural mechanical behavior and sensing functionality" is not correct since a body of research already exists in the published literature on this. The authors are urged to conduct a more thorough literature review to gain from prior work and to cite the most important aspects.

We fully followed your comments. We added the 2 following articles to the references list. There are two other references that I found that were added too.

Mei Yang, Qidi Liu , Hamza Sayed Naqawe and Mable P. Fok, Movement Detection in Soft Robotic Gripper Using Sinusoidally Embedded Fiber Optic Sensor, Sensors 2020, 20, 1312; doi:10.3390/s20051312

Samir Mekid, Asad Muhammad Butt, Khurram Qureshi, Integrity assessment under various conditions of embedded fiber optics based multi-sensing materials, Optical Fiber Technology, http://dx.doi.org/10.1016/j.yofte.2017.05.010 1068-5200/2017 Elsevier Inc. All rights reserved.

Bahareh Gholamzadeh, and Hooman Nabovati, Fiber Optic Sensors, World Academy of Science, Engineering and Technology International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering Vol:2, No:6, 2008

Floris, I.; Adam, JM.; Calderón García, PA.; Sales Maicas, S. (2021). Fiber Optic Shape Sensors: A comprehensive review. Optics and Lasers in Engineering. 139:1-17. https://doi.org/10.1016/j.optlaseng.2020.106508

2. The authors claim that "next-generation offshore wind turbine blades will exceed 100 meters" suggests that this has not yet been done. However there are blades already in production exceeding 100 m produced by companies such as Vestas, MingYang, Siemens etc. Their statement would have been correct in 2022 but not in 2023. This needs to be updated.

The largest wind-blade manufactured by General Electric is 107m and is placed in Cherbourg (France) … But it is not equipped with SHM advanced technologies are expected (I am perfectly aware of it because I am the PI of this project). Anyway, this point has been updated. I added within the text of the revised version the names of the companies that you kindly mentioned.

3 How was the appropriate level of mesh density determined? What were the criteria to assess accuracy? Details on this need to be provided since the application itself is based on extreme sensitivity and hence this is very important. While the analytical results are extremely interesting the lack of comparison with experiments or even prior results to show that the predictions are valid is concerning. Without this the validity of the entire study is at risk. Just providing results of FEA studies with no comparison is insufficient to (a) prove the viability of the dual-sinusoidal placement of FOSs, and (b) prove that results are correct and sensitive enough to be used as predictions of mechanical loading.

3b. Further detail on the interface between the adhesive and the CFRP need to be provided. Why are interface elements not considered to enable better transition?

3c. Without some level of comparison the paper is incomplete and cannot be considered for publication.

The assessment of the finite element discretization was performed by assuming that the adhesive layer with the embedded FOSs was the most relevant zone for strain concentration. The mesh discretization had a twofold purpose: to evaluate the variation of the strain field with the mesh density in the adhesive layer (number of elements) and to reduce the computational time. The mesh of the adhesive layer was built with varying density, higher density in the zones with higher curvatures of the FOSs. The minimum size of the element was set as the thickness of the adhesive (approximately 0.023 mm) between the FOS surface and the interface between the CFRP and the epoxy adhesive (see Figure 3). This resulted in a total number of 125376 elements. Additionally, assuming the minimum size of the finite element as half of the above, the maximum strain in the adhesive layer had a 3% difference. Therefore, considering the computational time, we have adopted the first mesh with a minimum element size of about 0.023 mm for the results presented in section 3."

The concept described in current article as applied science is new and let’s say it is at very low TRL (around 1-2). The objective upstream is to conduct exploratory deep research that can lead us to TRL 3-4, that can convince about the potential of offshore engineering to use it, raise the research issues (analytical and numerical) that have to be solved and the guidelines for future integrated research project that can lead us from the proof of concept (current article) to the pilot wind-blade that is reliable enough for industrial use in offshore industry. The pathway as you mentioned is very complex and tedious, as the challenges are numerous

There are so many points that need to be worked-out ..., not at this stage unfortunately. For example, we did not consider yet the effects of the interface. So is rheology effects (epoxy behavior), the effects of fiber-optic coating (acrylate or polyimide …), contact fatigue behavior, micromechanics ...

Analytical results have been added as you asked and show that the results are coherent. As stated clearly in the article the prime objective is the proof of concept of double-sinusoid anti-phasically placed as promising SHM concept …

More generally, I perfectly understand your ambition to get an article that is as complete as possible on both numerical and analytical modeling. We willingly share this ambition because we are, above all and after all, professors, very concerned about providing quality scientific work. However, we also know that to achieve this type of research, it would have to go through intermediate stages and provide a proof of concept, accompanied by experimental results that support the numerical predictions. It is in this regard that this research work was proposed and next step of this research will go deeper into the issues that are still ahead to be solved.

Author Response File: Author Response.pdf