Fiber Bragg Gratings Based Cyclic Strain Measuring of Weld Toes of Cruciform Joints
Round 1
Reviewer 1 Report
This work describes the measurement of the strain in the weld toes of cruciform joints using fiber Bragg grating technology. I find this paper very interesting, showing a powerful application of these devices into structural health monitoring. The text provides a good technical introduction, well described procedures, and very interesting results, that demonstrate the applicability of this technology for this purpose. Although there are many typos throughout the text, the content is clear and the figures are adequate to support the results. Therefore, I recommend its publication in Applied Sciences, after performing just some minor revisions in order to improve the manuscript.
1.- I strongly recommend revising the English language of the manuscript. Although the text is mostly clear, there are many typos.
2.- Authors should include the name of their country in the affiliation section.
3.- In the introduction, other optical techniques to monitor welding defects should be referenced in order to make the article more complete. I recommend to include “NDT & E International 39 (5), 356-360 (2006)”, among others.
4.- The content of section 2.3 has no references. I think that authors should include some references to fundamental papers on FBGs. In addition, they should also include at least some references about several works in which FBGs are used for structural health monitoring. I recommend to include these recent articles: “Sensors 18 (4), 1094 (2018)”, “JOSA B 36 (5), 1176-1182 (2019)” and “Sensors 18 (2), 491 (2018)”.
5.- Fig. 4 does not represent the FBG sensing principle, but its working principle. To illustrate how FBG can act as sensors, a different figure should be included showcasing how FBGs produce a wavelength shift in the presence of some external stimuli.
6.- At the end of section 2.3, authors should clarify what do they mean when they say that the optical fiber has “good flexibility”. When the fiber is stripped to produce an FBG, its flexibility is reduced, so they should comment on this effect.
7.- Is table 1 based on specifications or on real measured data?
8.- Why authors chose the Bragg wavelength values appearing in table 4? Is there any logic?
9.- In figure 7, the FBG T is used as a temperate reference, as it is mentioned in table 4. Authors should mention that in order to get rid of any stress of the structure, the FBG should be placed exactly in the middle of the structure section.
10.- How were the FBGs attached/glued to the structure?
11.- When authors mention that “In order to prevent the FBGs from breaking during the testing, secondary coating protection could be employed”, I recommend to include a couple of references, as “IEEE Sensors Journal 18 (21), 8739-8746 (2018)” and “Sensors 18 (11), 3963 (2018)”.
12.- Authors have space to make all the figures inside Fig. 8 bigger. In the current version details cannot be appreciated, so please, make this figure bigger.
13.- I also recommend increasing the brightness of Fig. 9.
14.- I recommend to include an inlay in Fig. 10, as it has been done for Fig. 11.
15. In Fig. 11, I recommend to include in (a) an arrow pointing to the region where (b) and (c) correspond, for clarification.
16.- I recommend to include the axis values on the inlays of figures 12, 13 and 15 for better understanding of the results.
17.- I recommend to include a zoomed inlay in figure 14, as it has been done in figs 12, 13 and 15, for better understanding and consistency.
Author Response
Dear Reviewer:
We are truly grateful to yours and other reviewers’ critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modifications on the original manuscript. All changes made to the text are in red color. In addition, we have consulted native English speakers for paper revision before the submission this time. We hope the new manuscript will meet your magazine’s standard. Below you will find our point-by-point responses to the reviewers’ comments/ questions:
1.- I strongly recommend revising the English language of the manuscript. Although the text is mostly clear, there are many typos.
Response: We have checked the manuscript and made modifications according to this point.
2.- Authors should include the name of their country in the affiliation section.
Response: We have made modifications according to this point.
3.- In the introduction, other optical techniques to monitor welding defects should be referenced in order to make the article more complete. I recommend to include “NDT & E International 39 (5), 356-360 (2006)”, among others.
Response: We have read the recommended article and referenced it in the manuscript (line 95-98).
4.- The content of section 2.3 has no references. I think that authors should include some references to fundamental papers on FBGs. In addition, they should also include at least some references about several works in which FBGs are used for structural health monitoring. I recommend to include these recent articles: “Sensors 18 (4), 1094 (2018)”, “JOSA B 36 (5), 1176-1182 (2019)” and “Sensors 18 (2), 491 (2018)”.
Response: Great suggestion! We have referenced some new articles in the manuscript (line 95-98, line 203-204).
5.- Fig. 4 does not represent the FBG sensing principle, but its working principle. To illustrate how FBG can act as sensors, a different figure should be included showcasing how FBGs produce a wavelength shift in the presence of some external stimuli.
Response: Here, we just want to introduce the basic working principle of FBG, so we only modified the figure caption (line 204-208). Concerning the FBG sensing principle, in our opinion, the rest text and equations in section 2.3 can represent it clearly.
6.- At the end of section 2.3, authors should clarify what do they mean when they say that the optical fiber has “good flexibility”. When the fiber is stripped to produce an FBG, its flexibility is reduced, so they should comment on this effect.
Response: In our work, the sensors were attached at the weld toe corner of the specimen. They were not on a flat surface but on a curved surface (shown as Fig.7). During the test, they were always in the bending condition and should not been damaged, so we need the sensors have good flexural deformation resistance, which was referred as “good flexibility” (line 228-230).
In order to avoid the ambiguous meaning of “good flexibility”, we have revised “good flexibility” to “good flexural deformation resistance” in the manuscript.
7.- Is table 1 based on specifications or on real measured data?
Response: The data of table 1 is from some references. We have referenced them in the manuscript (line 240).
8.- Why authors chose the Bragg wavelength values appearing in table 4? Is there any logic?
Response: Actually, we didn’t consider a lot of the choice of the Bragg wavelength. As long as the FBG sensors of different test channels have different wavelength, it is ok!
9.- In figure 7, the FBG T is used as a temperate reference, as it is mentioned in table 4. Authors should mention that in order to get rid of any stress of the structure, the FBG should be placed exactly in the middle of the structure section.
Response: It is a great suggestion! We have made corresponding modifications in the manuscript (line 276-279).
10.- How were the FBGs attached/glued to the structure?
Response: We have made corresponding modifications in the manuscript (line 291-292).
11.- When authors mention that “In order to prevent the FBGs from breaking during the testing, secondary coating protection could be employed”, I recommend to include a couple of references, as “IEEE Sensors Journal 18 (21), 8739-8746 (2018)” and “Sensors 18 (11), 3963 (2018)”.
Response: We have include some new references in the manuscript (line 204, line 294).
12.- Authors have space to make all the figures inside Fig. 8 bigger. In the current version details cannot be appreciated, so please, make this figure bigger.
Response: We have made some modifications for Fig. 8.
13.- I also recommend increasing the brightness of Fig. 9.
Response: We have made corresponding modifications.
14.- I recommend to include an inlay in Fig. 10, as it has been done for Fig. 11.
Response: We included an inlay in Fig. 11 just because it had 3 distinct stages. However, Fig. 10 didn’t present the same characteristics. In order to show the differences of the strain time history between point 0# and other points (point 1#, 2#, 3#), we think it is better not include any inlay in Fig.10.
15. In Fig. 11, I recommend to include in (a) an arrow pointing to the region where (b) and (c) correspond, for clarification.
Response: We have made corresponding modifications.
16.- I recommend to include the axis values on the inlays of figures 12, 13 and 15 for better understanding of the results.
Response: We have made corresponding modifications.
17.- I recommend to include a zoomed inlay in figure 14, as it has been done in figs 12, 13 and 15, for better understanding and consistency.
Response: We have made corresponding modifications.
It should be noted that the manuscript in the attachment is a combination of two files, one file has a modified trace, and the other file is a modified manuscript.
Submission Date
12 July 2019
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript approaches a topic based on fiber Bragg grating (FBG) technology for strain measuring of weld toes of cruciform joints, during cyclic fatigue tests in three different points. For researchers in the area of optical fiber sensing (OFS) technology and structural health monitoring (SHM) this study is not considered as a novelty. In this way, a lack of references related with OFS technology and SHM is missing in the introduction section of the manuscript. A state of art about FBG sensors for strain measurements should be described, and why are the FBG selected for performed this measurement instead of others sensing technologies. English language, namely in section 2 and 4, should also be improved. It is my opinion that the manuscript has some lacks on scientific information and, in this way, needs to be major revised prior to being accepted for publication.
In the following topics, are stand out the main lacks that should be improved/revised on the manuscript:
Line 127 and 129 – The font size of the figure 1 and 2, should be adapted.
Line 130 - The formulae font size should be adapted.
Line 154 - The legend of the figure 3 must begin with capital letter.
Line 170 and 179 – Should be addressed any references in the sentences.
The section 2.3 (FBGs strain sensor) should be totally rewritten/revised. Please, check some papers/books about optical sensing technology, namely FBG sensors, and reference them in this section. The sentence in line 225-226 not make any sense. The type of fiber (SMF, MMF, other…), and the recorded FBG system used should also be described.
Line 255 – In the figure 5 should only appear a) and b), and in the figure legend the description of each lyric.
Line 257 and 283 – Table 2 and 3 should be adjusted.
Line 271 to 273 - Should be addressed any references in the sentences.
Line 294 to 300 – This sentence should be totally rewritten. All the FBG sensors were inscribed in the same optical fiber? How many fibers were used? Was performed any strain sensing calibration? If yes, this data should be inserted in the manuscript. Which strain sensitivities were obtained for each FBG? Any change in the spectral response of the FBGs was observed before and after being attached to the surface. How was performed the fixation of the FBGs? Along the fixation, the fibers were pre-tensioned?
Line 307 to 308 – Describe the fiber Bragg demodulator and software used (Company, country…)
Line 315 – This figure should be removed/replaced. Do not give any additional information to the manuscript.
Line 317 – This figure should be enhanced.
Line 320 to 326 – This sentence should be transferred to section 3.2 and rewritten with the correct sensitivities for each FBG.
Line 341 – Figure 10 should be adjusted to the margins of the page. The arrow that indicate the strain rises should also be adapted. Please increase all label axis in all figures, accordingly with the font size text.
Line 349 – It is missing the number 105 on the x-axis, in the figure 10a).
Line 350 to 362 – The word “figure”, when written in the start of sentence should appear “Figure”. When it is written in the middle of sentence should appear “Fig.”. Where is “Fig. (b) or (c)”, should be “Fig. 11 (b) and 11 (c)”.
In all the manuscript: Between the number and the unit should be given one space. For example, instead of ε=3962µε, should be written ε= 3962 µε.
Line 361 to 362 – The following sentence should be rewritten/revised: “In Fig.(c), the maximum εmax=3962με, with which the specimen fracture unstably.”
Line 371 - Table 4 should be adjusted.
Line 379 to 380 - The following sentence should be rewritten/revised: “If the strain coordinate axis exaggerated, clear hysteresis loops of different phases can be seen, as shown in the bottom-right sub-figure in Fig.12”.
Line 386, 400, and 418 - Figure 12, 13, and 15 should be adjusted to the margins of the page. Axis labels should be inserted in the inset images.
Line 411 – The y- and x-label scales should be adapted. 75% of this figure does not add any additional information to the document.
Line 431 – Fig. 16 not Figure 16.
The section 6 should be completely revised/rewritten, where should be comment the added value of this study to SHM and materials science technology community.
Line 4, 484 and 485 – The author name´s should be rewritten accordingly with the Applied Science Journal template.
Regarding the references: an article submitted to a journal should be consistent with the contents that it proposed in recent times. However, no references to “Applied Sciences” papers are present. Authors should justify this point, or (it’s my suggestion) better study the bibliography of this journal and update the manuscript with appropriate works recently published.
Author Response
Dear Reviewer:
We are truly grateful to yours and other reviewers’ critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modifications on the original manuscript. In the introduction, some reference survey on optical fiber sensing (OFS) technology and structural health monitoring (SHM) was added. All changes made to the text are in red color. In addition, we have consulted native English speakers for paper revision before the submission this time. We hope the new manuscript will meet your magazine’s standard. Below you will find our point-by-point responses to the reviewers’ comments/ questions:
In the following topics, are stand out the main lacks that should be improved/revised on the manuscript:
1. Line 127 and 129 – The font size of the figure 1 and 2, should be adapted.
Response: We have made modifications according to this point (Line 130-132).
2. Line 130 - The formulae font size should be adapted.
Response: We have made modifications according to this point (Line 133-134).
3. Line 154 - The legend of the figure 3 must begin with capital letter.
Response: We have made modifications according to this point (Line 157).
4. Line 170 and 179 – Should be addressed any references in the sentences.
Response: We have referenced some new articles in the manuscript (line 182).
5. The section 2.3 (FBGs strain sensor) should be totally rewritten/revised. Please, check some papers/books about optical sensing technology, namely FBG sensors, and reference them in this section. The sentence in line 225-226 not make any sense. The type of fiber (SMF, MMF, other…), and the recorded FBG system used should also be described.
Response: We have revised section 2.3 and included some new reference (Line 203-222).
The type of fiber was described as line 279-280 and the recorded FBG system used was described as line 301-303.
6. Line 255 – In the figure 5 should only appear a) and b), and in the figure legend the description of each lyric.
Response: We have made modifications according to this point (Line 241).
7. Line 257 and 283 – Table 2 and 3 should be adjusted.
Response: Due to the editor’s adding line number in the original manuscript, the format of several tables had been somewhat changes. However, we have made adjustment according to this point(Line 244, Line 270).
8. Line 271 to 273 - Should be addressed any references in the sentences.
Response: We have addressed a reference in this version (line 260).
9. Line 294 to 300 – This sentence should be totally rewritten. All the FBG sensors were inscribed in the same optical fiber? How many fibers were used? Was performed any strain sensing calibration? If yes, this data should be inserted in the manuscript. Which strain sensitivities were obtained for each FBG? Any change in the spectral response of the FBGs was observed before and after being attached to the surface. How was performed the fixation of the FBGs? Along the fixation, the fibers were pre-tensioned?
Response: We have rewritten this part and want to present some details as following:
(1) In our work, we used 5 optical fibers and each fiber inscribed only one FBG. All the FBG have different wave length, shown as table 4 (Line 283).
(2) We didn’t perform special strain sensing calibration for the test in our work. The FBG sensors and the record system used in our work were used very commonly in our lab. We think they have sufficient accuracy and reliability by conventional calibration parameters and fixation method. In fact, the strain result of the testing point 0# in our work showed great consistency with the theoretical calculating value, which can demonstrate this viewpoint.
(3) With the same reason, we used same strain sensitivities for each FBG, which is k_ε=1.2 pm/με. We have described it in line 322-324
(4) The fibers were not pre-tensioned in our work.
10. Line 307 to 308 – Describe the fiber Bragg demodulator and software used (Company, country…)
Response: We have made modifications according to this point (line 301-303).
11. Line 315 – This figure should be removed/replaced. Do not give any additional information to the manuscript.
Response: Fig.8 is used here to present the setup of the test. We think it is necessary to describe the used device and their relations of our test (Line 311).
12. Line 317 – This figure should be enhanced.
Response: We have made some modifications (Line 312).
13. Line 320 to 326 – This sentence should be transferred to section 3.2 and rewritten with the correct sensitivities for each FBG.
Response: In our opinion, when the test ended, we only obtained wavelength signal. In the beginning of the analysis, we need transfer the wavelength signal to strain signal at first. So, we think it is better to keep the sentence at the original position (Line 320-324).
14. Line 341 – Figure 10 should be adjusted to the margins of the page. The arrow that indicate the strain rises should also be adapted. Please increase all label axis in all figures, accordingly with the font size text.
Response: We have made modifications according to this point (Line 337).
15. Line 349 – It is missing the number 105on the x-axis, in the figure 11a).
Response: Due to the editor’s adding line number in the original manuscript, the format here had been changes. However, we have made modification according to this point (Line 346).
16. Line 350 to 362 – The word “figure”, when written in the start of sentence should appear “Figure”. When it is written in the middle of sentence should appear “Fig.”. Where is “Fig. (b) or (c)”, should be “Fig. 11 (b) and 11 (c)”.
Response: We have checked whole the manuscript and made modifications according to this point.
17. In all the manuscript: Between the number and the unit should be given one space. For example, instead of ε=3962µε, should be written ε= 3962 µε.
Response: We have checked whole the manuscript and made modifications according to this point.
18. Line 361 to 362 – The following sentence should be rewritten/revised: “In Fig.(c), the maximum εmax=3962με, with which the specimen fracture unstably.”
Response: We have rewritten this sentence (Line 361-362).
19. Line 371 - Table 4 should be adjusted.
Response: Due to the editor’s adding line number in the original manuscript, the format of several tables had been somewhat changes. However, we have made adjustment according to this point (Line 371).
20. Line 379 to 380 - The following sentence should be rewritten/revised: “If the strain coordinate axis exaggerated, clear hysteresis loops of different phases can be seen, as shown in the bottom-right sub-figure in Fig.12”.
Response: We have rewritten this sentence (Line 379-381).
21. Line 386, 400, and 418 - Figure 12, 13, and 15 should be adjusted to the margins of the page. Axis labels should be inserted in the inset images.
Response: We have made modifications according to this point (Line 386, 403, and 423).
22. Line 411 – The y- and x-label scales should be adapted. 75% of this figure does not add any additional information to the document.
Response: We have made modifications according to this point (Line 415).
23. Line 431 – Fig. 16 not Figure 16.
Response: We have made modifications according to this point (Line 437).
24. The section 6 should be completely revised/rewritten, where should be comment the added value of this study to SHM and materials science technology community.
Response: We have made modifications according to this point (Line 476-482).
25. Line 4, 484 and 485 – The author name´s should be rewritten accordingly with the Applied Science Journal template.
Response: We have made modifications according to this point (Line 491-497).
26. Regarding the references: an article submitted to a journal should be consistent with the contents that it proposed in recent times. However, no references to “Applied Sciences” papers are present. Authors should justify this point, or (it’s my suggestion) better study the bibliography of this journal and update the manuscript with appropriate works recently published.
Response: We have made modifications according to this point (Line 570).
It should be noted that the manuscript in the attachment is a combination of two files, one file has a modified trace, and the other file is a modified manuscript.
Submission Date
12 July 2019
Author Response File: Author Response.pdf
Reviewer 3 Report
The experiment is very interesting and worth reporting. Nevertheless the author needs to only claim strain measurement in only one dimension. The FBGs orientation chosen in the experiment can only measure strains in one dimension (that causes the tensile or compressive strain on the fiber sensor), but ignores the orthogonal strain components. A more careful consideration should have been practiced during the experimental design or at least a clear mention of this in the manuscript for reader's consideration. Some figures quality need be improved.
Comments for author File: Comments.pdf
Author Response
Dear Editor and Reviewer:
We are truly grateful to yours and other reviewers’ critical comments and thoughtful suggestions. Based on these comments and suggestions, we have made careful modifications on the original manuscript. The dimensional problem of strain measured by FBG has been explained in line 274-276. In the manuscript, in addition to modifying the comments you mentioned, the manuscript was also changed according to the opinions of other reviewers. All changes made to the text are in red color. In addition, we have consulted native English speakers for paper revision before the submission this time. We hope the new manuscript will meet your magazine’s standard.
It should be noted that the manuscript in the attachment is a combination of two files, one file has a modified trace, and the other file is a modified manuscript.
Submission Date
12 July 2019
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear authors,
Thank you for changing the manuscript as required. However, it is my opinion that the manuscript needs to be minor revised prior to being accepted for publication.
· Moderate English changes is still required;
· In figure 5 is missing the (a) and (b) letters;
· What were the dimensions of the FBG sensors?
· In line 292-294, the sentence “In order to prevent the FBG from breaking during the testing, secondary coating protection was employed [38,42]” should be clarified. How was make this coating protection and which material was used? It was a thin film layer? Which thickness dimensions?
Author Response
Dear Reviewer:
We are truly grateful for your affirmation of the manuscript change. Your critical comments and thoughtful suggestions are very effective for the improvement of this manuscript. We hope the new manuscript will meet your standard. Thank you for reviewing the manuscript. Below you will find our point-by-point responses to the reviewers’ comments/ questions:
· Moderate English changes is still required;
Response: We have checked the manuscript again and made modifications according to this point.
· In figure 5 is missing the (a) and (b) letters;
Response: We have made modifications according to this point (Line 243).
· What were the dimensions of the FBG sensors?
Response: The dimension of the FBG is explained in line 224-228, we are not very sure whether it can answer your question.
· In line 292-294, the sentence “In order to prevent the FBG from breaking during the testing, secondary coating protection was employed [38,42]” should be clarified. How was make this coating protection and which material was used? It was a thin film layer? Which thickness dimensions?
Response: In our work, the original optical fiber needs to be stripped of the original coating before the grating is inscribed into its core. Then it is coated with a coating again, which we call it secondary coating. After the secondary coating, the FBG has the same thickness as the original optical fiber (shown as line 224-228). According to this, we have modified line 294-299.
Author Response File: Author Response.pdf
Reviewer 3 Report
The revised manuscript is improved. The figures' captions must comment on the insets.
Author Response
Dear Reviewer:
We are truly grateful for your affirmation of the manuscript change. Your critical comments and thoughtful suggestions are very effective for the improvement of this manuscript. We hope the new manuscript will meet your standard. Thank you for reviewing the manuscript. Below you will find our point-by-point responses to the reviewers’ comments/ questions:
The revised manuscript is improved. The figures' captions must comment on the insets.
Response: Thanks, reviewer. We checked some papers of this journal and found none figures with insets. So we have no idea about their formats and how to caption them. We try to make some modifications in this manuscript.
Author Response File: Author Response.pdf
