Fatigue Life (Limit) Analysis Through Infrared Thermography on Flax/PLA Composites with Different Reinforcement Configurations

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article "Fatigue life (limit) analysis through infrared thermography on flax/PLA composites with different reinforcement configurations" presents an interesting experimental study on different reinforcement configurations in composites. The following comments and suggestions are provided to help the authors improve the quality of the manuscript.

1. The novelty of the study needs to be clarified in the abstract.

2. The first paragraph of the introduction is out of context and must be removed.

3. The introduction should be updated with more recent research.

4. In Section 2.3, for the tensile quasistatic and fatigue tests, did you follow any standards to define the test methodology? If so, they should be cited.

5. Why did you choose to use infrared thermography simultaneously with the fatigue test?

6. I believe that the volume fraction and degree of crimp are not results of the study. If you intend to analyze these parameters, the methodology should be described in Section 2.

7. There are several referencing errors related to figures and tables, which hinder the understanding of the results. The authors should review and correct them.

8. The conclusions should be updated to include the study’s limitations and opportunities for future research.

Author Response

First of all, we would like to thank the reviewers for their valuable and useful comments. All the comments for improvement have been well received and the manuscript has been modified accordingly.

 

Reviewer #1:

SPECIFIC QUESTIONS

The article "Fatigue life (limit) analysis through infrared thermography on flax/PLA composites with different reinforcement configurations" presents an interesting experimental study on different reinforcement configurations in composites. The following comments and suggestions are provided to help the authors improve the quality of the manuscript.

1. The novelty of the study needs to be clarified in the abstract.

The abstract was rewritten.

2. The first paragraph of the introduction is out of context and must be removed.

The first paragraph was removed.

3. The introduction should be updated with more recent research.

In the second paragraph, the part that does not contribute directly to the study was removed; furthermore, some references from recent years have been included.

4. In Section 2.3, for the tensile quasistatic and fatigue tests, did you follow any standards to define the test methodology? If so, they should be cited.

For this study, ASTM D3479 and ASTM D3039 standards were used as references for some general criteria; however, for test sample size, gauge length, strain rate for tensile testing, and others, studies developed in our facilities and previously published were used as references, with the purpose of being able to compare and validate the results obtained. It is for this reason that the above standards have not been included in the manuscript.

5. Why did you choose to use infrared thermography simultaneously with the fatigue test?

The purpose of this research was to evaluated whether the thermography can be used on flax/PLA composites in order to determine the fatigue limit. To this end, during cyclic loading in the sample, the surface temperature of the specimens should be measured using an infrared camera at the same time an extensometer was used to measure the deformation in the loading zone. Although thermography is useful, it has high variability in results.

6. I believe that the volume fraction and degree of crimp are not results of the study. If you intend to analyze these parameters, the methodology should be described in Section 2.

To determine the degree of crimp and the volume fraction, the test pieces manufactured for this study were analyzed using the micrography method. To do this, the samples were polished until a mirror surface; thereafter, the Olympus GX51 optical microscope was used to obtain the micrography images, and these were processed with the Olympus Stream Basic software to obtain the volume fraction and degree of crimp. A description was included at the end of section 2.2

7. There are several referencing errors related to figures and tables, which hinder the understanding of the results. The authors should review and correct them.

All errors were corrected.

8. The conclusions should be updated to include the study’s limitations and opportunities for future research.

The conclusion was rewritten.

 

Reviewer 2 Report

Comments and Suggestions for Authors

Abstract
Re-write the abstract to make it more clear.
Introduction
First paragraph is just instructions on how to write the introduction from MDPI. Delete it.
Introduction section is jumping all around. Authors need to present a concise introduction on the problem and show analysis of the data already reported/published. I don't agree with the introduction since it doesn't present any valuable insight into the problem statement and why authors conducted his research. Showing a small picture of flax fibers with orientation angle that defines load transfer will help reader digesting the core language of this study. Overall, there are several issues related to the English Language which must be corrected. 
Methodology
section 2.2 is not written correctly.
section 2.4: Why emissivity is assumed to be 0.9?
Results and discussion
section 3.1: Please indicate what is warp and weft directions for the readers. For volume fraction, what equation was used for image analysis? How many images were analyzed for volume fraction calculations? What is the outcome of this test or how is this useful?
Section 3.2: There are a lot of Error signs in this section. please fix. make the data in Fig. 3 as versus strain percentage and show same in table 2.
The error referencing is continuing throughout the manuscript. 
Did authors take 2D images from the thermographic camera during fatigue test? I believe that was the main purpose of this manuscript.
Conclusions: please indicate a broader picture of this study where it could be used or what could be done beyond this work to amplify the outcomes of this study.

 

Author Response

First of all, we would like to thank the reviewers for their valuable and useful comments. All the comments for improvement have been well received and the manuscript has been modified accordingly.

 

Reviewer #2:

1. Abstract - Re-write the abstract to make it more clear.

The abstract was rewritten.

 

2. Introduction - First paragraph is just instructions on how to write the introduction from MDPI. Delete it.

 This paragraph was removed.

 

3. Introduction section is jumping all around. Authors need to present a concise introduction on the problem and show analysis of the data already reported/published. I don't agree with the introduction since it doesn't present any valuable insight into the problem statement and why authors conducted his research. Showing a small picture of flax fibers with orientation angle that defines load transfer will help reader digesting the core language of this study. Overall, there are several issues related to the English Language which must be corrected.

 According to the existing literature, no study has been found that has determined the fatigue limit of Flax/PLA composites (in laminates similar to this study); hence, a preliminary analysis of the behavior of this type of composite under fatigue loading could not be performed. Therefore, the introduction focuses on the importance of these materials, methods used in similar composites, and how the fibers behave under cyclic loading.

For clarity, the introduction has been rewritten, and references from recent years have been added.

 

4. Methodology

section 2.2 is not written correctly.

Section 2.2 has been partially rewritten, and additional information has been added on how the volume fraction and degree of crimp of the fibers were measured.

section 2.4: Why emissivity is assumed to be 0.9?

According to previous studies (Charca et. al, 2024) during the calibration of the infrared thermal camera (Flir SC6000) the emissivity of 0.9 was adequate for this type of composite. This study was used as a reference to consider an emissivity of 0.9.

 

5. Results and discussion

section 3.1: Please indicate what is warp and weft directions for the readers. For volume fraction, what equation was used for image analysis? How many images were analyzed for volume fraction calculations? What is the outcome of this test or how is this useful?

Figure 2 shows the warp and weft direction. On the other hand, the description of the method used to determine the volume fraction and degree of crimp was added at the end of section 2.2.

Section 3.2: There are a lot of Error signs in this section. please fix. make the data in Fig. 3 as versus strain percentage and show same in table 2.

For the stress-strain diagram, representative data have been taken from each type of laminate, but they do not represent the average, which is why there may be a discrepancy in the values ​​of the figure with the table, especially in deformation, as seen in the table for the laminate [+45/-45]s its standard deviation is 0.0276, adding to 0.0847 gives a value of 0.1123, which is equal to the value shown in the figure. In the first line of section 3.2 it is clearly mentioned that “Figure 3 shows the representative stress-strain curves for the different studied laminate” not the average.

We decide to maintain the strain unit as mm/mm.

 

6. The error referencing is continuing throughout the manuscript.

Referencing was corrected in this version.

 

7. Did authors take 2D images from the thermographic camera during fatigue test? I believe that was the main purpose of this manuscript.

In section 2.4, describe how the infrared camera was set to measure the superficial temperatures, even do the image frame rate was specified and how the images were processed.

 

8. Conclusions: please indicate a broader picture of this study where it could be used or what could be done beyond this work to amplify the outcomes of this study.

The conclusion was rewritten

Reviewer 3 Report

Comments and Suggestions for Authors

The paper presents an investigation of PLA samples reinforced with flax fibers, employing an MTS system for mechanical testing that has been enhanced with infrared thermography. The study critically discusses the temperature measurements obtained during testing and addresses the limitations associated with the thermographic method. The topic is original and relevant to the field, particularly within the context of sustainable composite materials. However, additional references related to the reinforcement of resins and laminates would strengthen the background and contextual framing.

 

The material selection—flax and PLA—is appropriate, given their relevance in the development of bio-based composites. The study successfully explores the applicability of infrared thermography for gaining insights into fatigue behavior, a promising approach since energy dissipation during testing provides meaningful thermal signals. This makes the methodology both feasible and of interest to the composites research community.

 

Linguistically, the manuscript is well-written, though the formatting requires attention. Specifically, figures are not currently referenced in the text, which should be corrected to improve readability and coherence. Additionally, author affiliations should include the respective city and country for clarity.

 

Lines 31 to 39 (“The Introduction should briefly place…for further details on references”) appear to be remnants of a submission template and should be removed.

 

To better describe the test specimens, it would be beneficial to include a schematic or image that visually explains the laminate layups, such as \[0°/90°]s, \[90°]₄, and \[+45°/−45°]s configurations. Furthermore, the introduction would benefit from a comparative discussion of similar reinforcement approaches using resin matrices filled with graphene or carbon nanotubes.

 

Regarding figures, Figure 1 should be enlarged to clearly display the sample details. Also you may add some explanantion on the small image on the left. Additionally, the font size of axis labels and legends in Figures 5, 7, and 8 should be increased to improve readability.

 

The reference list is generally adequate, but a more detailed discussion on the materials—flax and PLA—and their applications in biocomposites would enrich the paper. The methodology is sound, and the conclusions are consistent with the presented data.

Author Response

First of all, we would like to thank the reviewers for their valuable and useful comments. All the comments for improvement have been well received and the manuscript has been modified accordingly.

 

Reviewer #3:

The paper presents an investigation of PLA samples reinforced with flax fibers, employing an MTS system for mechanical testing that has been enhanced with infrared thermography. The study critically discusses the temperature measurements obtained during testing and addresses the limitations associated with the thermographic method. The topic is original and relevant to the field, particularly within the context of sustainable composite materials. However, additional references related to the reinforcement of resins and laminates would strengthen the background and contextual framing.

Additional references from the last years were added, and extra information was added as well.

 

The material selection—flax and PLA—is appropriate, given their relevance in the development of bio-based composites. The study successfully explores the applicability of infrared thermography for gaining insights into fatigue behavior, a promising approach since energy dissipation during testing provides meaningful thermal signals. This makes the methodology both feasible and of interest to the composites research community.

Thank you.

 

Linguistically, the manuscript is well-written, though the formatting requires attention. Specifically, figures are not currently referenced in the text, which should be corrected to improve readability and coherence. Additionally, author affiliations should include the respective city and country for clarity.

Some improvement was done along the manuscript.

 

Lines 31 to 39 (“The Introduction should briefly place…for further details on references”) appear to be remnants of a submission template and should be removed.

The first paragraph of the introduction was removed, and some references were added.

 

To better describe the test specimens, it would be beneficial to include a schematic or image that visually explains the laminate layups, such as \[0°/90°]s, \[90°]₄, and \[+45°/−45°]s configurations. Furthermore, the introduction would benefit from a comparative discussion of similar reinforcement approaches using resin matrices filled with graphene or carbon nanotubes.

Figure 1 was added to show the fiber direction in each laminate and the respective nomenclature.

Although there are many studies on carbon fiber and glass fiber composites, their behavior cannot be compared to natural fiber composites. Natural fibers exhibit nonlinear behavior when subjected to tensile loads; this behavior gives natural fiber composites higher complexity, and for this reason, direct comparisons were made only when necessary.

 

Regarding figures, Figure 1 should be enlarged to clearly display the sample details. Also you may add some explanantion on the small image on the left. Additionally, the font size of axis labels and legends in Figures 5, 7, and 8 should be increased to improve readability.

The description of Figure 2 (Figure 1 in the previous version) was added.

Font size on Figures 5, 6 and 7 was increased.

 

 The reference list is generally adequate, but a more detailed discussion on the materials—flax and PLA—and their applications in biocomposites would enrich the paper. The methodology is sound, and the conclusions are consistent with the presented data.

Some references were added.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No further comments. Accept