Fiber Bragg Grating Bonding Characterization under Long-Period Cyclic Loading

Round 1

Reviewer 1 Report

This manuscript presented a study on the FBG sensor used in the smart landing gear system. It proved the ability of FBG sensors to withstand the operational life-cycle of landing gear and selected the most suitable fiber-coating and bonding agents for this application. Although it gave relative detailed information on the usage and has confirmed the ability of FBG in fatigue-like testing, it did not give some sound scientific theoretical and experimental analyses. For example, what was the reason in Figure 13? How did the mechanical and geometric parameters affect the result?

There are some errors to be modified:  

1.     There were no units in Fig. 2.

2.     The value of “Expected Maximum Strain” in Table 2 was wrong.

3.     Fig. 4 did not give clear photos to illustrate the dissolve of the coating.

Author Response

Rebuttal letter: photonics-2536727

Reviewer no. 1

[…] Although it gave relative detailed information on the usage and has confirmed the ability of FBG in fatigue-like testing, it did not give some sound scientific theoretical and experimental analyses. For example, what was the reason in Figure 13? How did the mechanical and geometric parameters affect the result?

Probably using the term “fatigue-like” in the paper title may have induced a misunderstanding. We have updated the title: “Fiber Bragg Grating bonding characterization under long-period cyclic loading”. The text was updated accordingly. The meaning of “Fatigue-like” as intended by the authors in the paper was introduced and described. A sentence describing the aim of Figure 13 has been introduced. The only parameter the authors focused on was the adhesion stability over the required number of cycles. The study of the variability of mechanical and geometric parameters will be the object of further investigations and are out of the scope of the current paper. 

1. There were no units in Fig. 2. 

Figure two is an excerpt from the specimen design drawing, the units were, as per the standard, in the drawing legend. We thank the reviewer for the indication. Now, we have provided the units (mm) in the caption. 

2. The value of “Expected Maximum Strain” in Table 2 was wrong. 

The value was calculated considering 77% of s_Y, not 75% as erroneously written on line 227 and corrected in this new version. That is, 295 MPa, which is 77% of 393 MPa, given by the material data sheet. The same data sheet informs that Young's modulus, E, is 71.7 GPa. Applying Hooke's law, dividing s by E, the calculated strain value turns out to be 4.1*10³, as shown in the updated Table 2.

3. Fig. 4 did not give clear photos to illustrate the dissolve of the coating. 

Figures 4.a and 4.b were to give an idea of how the polyimide coating was immune to the action of cyanoacrylate dissolving the acrylate coating. They have been removed to give greater prominence to Fig. 4.c where, now, the points where the coating detaches from the fiber are highlighted.

Reviewer 2 Report

Correspondence of the title to the content of the manuscript: The content of the text of the manuscript fully corresponds to the title.

Briefly review: The work is devoted to the development of technology for the use of strain sensors in aircraft structures. In particular, the problem of the durability of the operation of FBG sensors in conditions of high cyclic loads is investigated. To do this, the authors made samples in two versions of optical fibers – with polyimide and acrylate coatings, and also used two adhesives – epoxy resin and cyanoacrylate glue. As a result, it was shown that the use of FBG in conditions of multiple excessive fatigue tests for use in aviation technology is quite possible.

Actuality of investigation: In my opinion, this study has a fairly high technical relevance, since the results of the study directly affect the development of aviation sensor devices.

Science novelity: This work is mainly of a technological plan, it searches for optimal parameters of the components of fiber-optic sensors, which is associated with an experimental search.

Reliability of results: The literature review used on various aspects related to the study, the experimental methods and installations used, as well as the previous results of the work of this group strongly indicate the reliability of the results of the study.

Qualitity of figures: good.

Other remarks:

1. Line 177: It is necessary to explain in more detail which specific structural element called «foil» we are talking about. Do I understand correctly that foil is not a thin layer of metal, but another designation of the metal part of the specimen ?

2. Line 185: The designation λ_B confuses with the sample number B. I advise you to replace the index, which apparently denotes the central wavelength of the FBG (Bragg).

3. If possible, I would like to see data on the type of optical fibers and the method of the FBG formation (UV, femtosecond point-to-point, recording through the coating or with the covering of the coating after).

4. Figure 13: The symbols on the vertical axis are rounded to the first sign, it is necessary to show the second sign, otherwise the values are the same for different levels.

Author Response

Reviewer no. 2

1. Line 177: It is necessary to explain in more detail which specific structural element called «foil» we are talking about. Do I understand correctly that foil is not a thin layer of metal, but another designation of the metal part of the specimen? 

This is evidently an error. In the new version 'foil' is replaced with 'plate'.

2. Line 185: The designation λ_B confuses with the sample number B. I advise you to replace the index, which apparently denotes the central wavelength of the FBG (Bragg). 

In order not to cause confusion, λ_B is replaced with Central Wavelength (CWL), which was already introduced in the text.

3. If possible, I would like to see data on the type of optical fibers and the method of the FBG formation (UV, femtosecond point-to-point, recording through the coating or with the covering of the coating after). 

The gratings are apodized FBGs (reflectivity ≥ 75%) directly written, with the excimer laser, on 9/125 single-mode optical fiber, which is locally stripped and then re-coated. This sentence was included in the updated version of the text.

4. Figure 13: The symbols on the vertical axis are rounded to the first sign, it is necessary to show the second sign, otherwise the values are the same for different levels. 

We thank you for the suggestion. In the updated version, the new figure presents rounding to the first decimal place for vertical axis values. 

Round 2

Reviewer 1 Report

No comments or suggestins