Review Reports - Post-Forming of Carbon Fibre-Reinforced PEEK Thermoplastic Tubular Structures

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The experimental aspect of the work is weak, with a lack of thorough characterization of the material. This study seems more like a literature review. For instance, the influence of fiber concentration on this process should be addressed. It would be beneficial to include a Scanning Electron Microscope (SEM) image showing the dispersion of fibers in the matrix before and after thermal treatment. Did the mechanical response of the material change after this process? The concept of the study is intriguing, but the experimental development was lacking.”

Author Response

Comments 1:

Response 1:

SEM was not performed on the specimens used for this experiment. The main purpose of this paper is to present the newest technique to post-form CF/PEEK and the application of the CF/PA6 post-forming fibre angle prediction model onto CF/PEEK. The mechanical response of the tubes before and after post-forming will be discussed in detail in another publication to be made. This is mentioned at the end of the conclusion as future work. The authors apologise for the mistake in reference to CT. The keyword “CT analysis” was meant to be a reference to optical analysis using micro-Vu. Reference to CT in this paper has been deleted and a new keyword “optical analysis” has been added.

Reviewer 2 Report

Comments and Suggestions for Authors

In this study a new forming technique is developed for high-temperature reinforced thermoplastic tubes and a model to predict the change in fiber orientation is validated for a new material. Samples of 20 mm diameter straight tubes of [±60°]₄ CF/PEEK were fabricated and post-formed using induction heating and rotary-draw bending. Sections of the tubes were heated near the melting temperature of PEEK (340°C) to allow bending, and a flexible steel spring mandrel was used to prevent buckling and maintain ovality. Samples were bent at four angles, 45, 90, 135, and 180 degrees. Changes in fiber orientation at the bent sections were measured and compared to a model developed to predict fiber angles. Changes in fiber orientation of up to 15° were measured and the prediction model was mostly accurate for the [±60°]₄ CF/PEEK tubes at all four angles of bent. Removal of the steel spring mandrel out of the 180°-bent sample was a little challenging.

The paper is good, and the authors have experience developing a similar method of post-forming and a fiber orientation prediction model for tubes of a similar CF/PA6 material.

Comments and suggestions for authors:

Keyword “CT analysis”: CT usually stands for computed tomography which is not used in the study. If CT refers to something else, it is better to spell it out.

Please explain the “bending ratio”, why a bending ratio of 2 was used in the study, and whether it is dependent on the diameter of the tube or the size of bending die.

Suggest editing and re-writing the last paragraph of the Introduction in the past tense, since the work has been done already.

Page 3 Line 104: “The Coil has a total length span of 25 mm ….” The length and span of the coil should refer to two different things, the first is how long the coil is, and the second, span, is the length of the tube covered by the coiled part, similar to length A-B in Figure 2(a). Also, it seems the length of the coil should be greater than 25 mm since it has three turns of about 61 diameters plus the additional length to the heater.

Page 4 line 145: remove the word “per” and change to: “……. mandrel section corresponding to 22.5° of tube bending angle..”.

Figure 2: The tube is not shown on the mandrel in the heated section. Is the tube heated with the mandrel or is the mandrel heated in segments and then inserted in the tube? The text does not explain the process very well and the mandrel moving direction is counter intuitive because it is in the opposite direction of the bend. Please explain the process and the figure better and show the tube on the mandrel as in Figure 1.

I am not familiar with the policy of the journal, but usually the “f” in the word Figure is capitalized when it refers to a specific figure with a number such as Figure 2 or Figure 3..

Page 5, line 195: change to: “Emissivity of the CF/PEEK tube was obtained from dividing the temperature measured by ….”

Line 211: change to “normalized with respect to the initial fiber angle ..”

Lines 212-213: “… profiles of the tubes were relatively even as evident from …….” Something is missing after the word “from” …. Do you mean as evident from Figure 3? In that case what part of Figure 3 and what is the evidence? This paragraph needs editing to read better.

Page 6 line 227: Change to: “… while the rest of the tubes remained at lower temperature, fiber movements were rather constrained ….” The word “rather” would indicate that the constrain was not complete because the orientation of the fibers was changed by up to 15° in the bent section which is about 25% change in fiber direction.

Line 230: remove the word “unwanted”

Page 7, line 247: change the word “evening” to “evenly”.

Line 279: remove “the validity of”

The change in fiber orientation at the bent section merits more discussion because it is significant. For example, a decrease of 15° on the extrados will orient the fibers at ±75°, almost along the tube while an increase of 15° on the intrados will orient the fibers at ±45° boosting the strength in the hoop direction rather than the length direction in that region. How do these changes affect the properties and performance of the bent sections of the pipe? These issues might have been addressed in your previous paper, but they should also be addressed or referred to in this paper because the matrix is different. A future study may consist of testing the bent sections and compare the properties with straight sections based on materials and fiber orientation.

Line 269: Ovality was maintained by the spring mandrel. Was ovality measured? Does it change with the bend angle? Is there a loss in tube cross-sectional area with angle of bend? Considering these questions may shed more light on the behavior in the bend area.

Comments on the Quality of English Language

Good with minor edits as suggested above.

Author Response

Comments 1: Keyword “CT analysis”: CT usually stands for computed tomography which is not used in the study. If CT refers to something else, it is better to spell it out.

Response 1: The authors apologise for the mistake in reference to CT. The keyword “CT analysis” was meant to be a reference to optical analysis using micro-Vu. Reference to CT in this paper has been deleted and a new keyword “optical analysis” has been added.

Comments 2: Please explain the “bending ratio”, why a bending ratio of 2 was used in the study, and whether it is dependent on the diameter of the tube or the size of bending die.

Response 2: The significance of the bending ratio used is explained in line 132 as “For dimensions of the tubes used in this study, 2 is the smallest bending ratio for the tubes to be post-formed without failures as calculated from the forming limit model derived in [31].”

The definition of the bending ratio is listed in line 130 as “Bending ratio is defined as the ratio between the radius of the main bending die (the bend formed) and the diameter of the tube [25].”

Comments 3: Suggest editing and re-writing the last paragraph of the Introduction in the past tense, since the work has been done already.

Response 3: The final paragraph of the introduction from line 65 has been re-written as” In this study, CF/PEEK tube post-forming experiments were conducted to demonstrate the forming technique. Four sets of [±60°]₄ CF/PEEK tubes were post-formed to bend angles 45^°, 90^°, 135^°, and 180^° respectively using an induction heater incorporated bespoke RDB setup. A long wave infrared (LWIR) camera was used to record tube surface temperature profiles during post-forming and control heating. Optical measurements using a Micro-Vu Vertex automated precision measurement system were conducted to characterise changes in fibre angles of tube bending zones before and after post-forming. Fibre angles measured before forming were processed using the formerly derived fibre angle prediction model for CF/PA6 tubes. The predictions were evaluated against fibre angles measured after forming to validate the validity of the fibre angle prediction model in the post-forming of CF/PEEK tubes”

Comments 4: Page 3 Line 104: “The Coil has a total length span of 25 mm ….” The length and span of the coil should refer to two different things, the first is how long the coil is, and the second, span, is the length of the tube covered by the coiled part, similar to length A-B in Figure 2(a). Also, it seems the length of the coil should be greater than 25 mm since it has three turns of about 61 diameters plus the additional length to the heater.

Response 4: The sentence in line 105 has been changed to delete “length”.

Comments 5: Page 4 line 145: remove the word “per” and change to: “……. mandrel section corresponding to 22.5° of tube bending angle..”.

Response 5: The word “per” has been deleted in line 154.

Comments 6: Figure 2: The tube is not shown on the mandrel in the heated section. Is the tube heated with the mandrel or is the mandrel heated in segments and then inserted in the tube? The text does not explain the process very well and the mandrel moving direction is counter intuitive because it is in the opposite direction of the bend. Please explain the process and the Figure better and show the tube on the mandrel as in Figure 1.

Response 6: Figures 1 and 2 have been merged for a clearer illustration of the heating process.

Comments 7: I am not familiar with the policy of the journal, but usually the “f” in the word Figure is capitalized when it refers to a specific Figure with a number such as Figure 2 or Figure 3..

Response 7: Reference to the Figures has been capitalised.

Comments 8: Page 5, line 195: change to: “Emissivity of the CF/PEEK tube was obtained from dividing the temperature measured by ….”

Response 8: The sentence in line 205 has been changed to: “Emissivity of the CF/PEEK tube was derived from dividing the temperature measured by the LWIR camera by the temperature measured by the thermocouple.”

Comments 9: Line 211: change to “normalized with respect to the initial fiber angle ..”

Response 9: The sentence in line 24 has been changed to: “The post-formed fibre angle measurements and predictions were normalised with respect to the initial fibre angle measurements to show potential discrepancies.”

Comments 10: Lines 212-213: “… profiles of the tubes were relatively even as evident from …….” Something is missing after the word “from” …. Do you mean as evident from Figure 3? In that case what part of Figure 3 and what is the evidence? This paragraph needs editing to read better.

Response 10: The sentence in line 228 has been changed to: “During the forming process, temperature profiles of the tubes were relatively even as evident from a small temperature gradient of ± 1° recorded on the outer tube surface inside the coil by the LWIR camera.”

Comments 11: Page 6 line 227: Change to: “… while the rest of the tubes remained at lower temperature, fiber movements were rather constrained ….” The word “rather” would indicate that the constrain was not complete because the orientation of the fibers was changed by up to 15° in the bent section which is about 25% change in fiber direction.

Response 11: The sentence has been changed in line 243.

Comments 12: Line 230: remove the word “unwanted”

Response 12: The word has been removed from the sentence in line 246.

Comments 13: Page 7, line 247: change the word “evening” to “evenly”.

Response 13: The word has been changed in the sentence in line 260.

Comments 14: Line 279: remove “the validity of”

Response 14: The word has been removed in the sentence in line 289.

Comments 15: The change in fiber orientation at the bent section merits more discussion because it is significant. For example, a decrease of 15° on the extrados will orient the fibers at ±75°, almost along the tube while an increase of 15° on the intrados will orient the fibers at ±45° boosting the strength in the hoop direction rather than the length direction in that region. How do these changes affect the properties and performance of the bent sections of the pipe? These issues might have been addressed in your previous paper, but they should also be addressed or referred to in this paper because the matrix is different. A future study may consist of testing the bent sections and compare the properties with straight sections based on materials and fiber orientation.

Response 15: The main purpose of this paper is to present the newest technique to post-form CF/PEEK and the application of the CF/PA6 post-forming fibre angle prediction model onto CF/PEEK. The mechanical response of the tubes before and after post-forming will be discussed in detail in another publication to be made. This is mentioned at the end of the conclusion as future work.

Comments 16: Line 269: Ovality was maintained by the spring mandrel. Was ovality measured? Does it change with the bend angle? Is there a loss in tube cross-sectional area with angle of bend? Considering these questions may shed more light on the behavior in the bend area.

Response 16: The function of the mandrel to maintain ovality is mentioned in section 2.2.1in line 115 with reference to a published article. The measurement of tube ovality was done in the cited article.

Reviewer 3 Report

Comments and Suggestions for Authors

The work "Post-forming of carbon fibre reinforced PEEK thermoplastic tubular structures" is good. This paper presents a post-forming technique utilising both induction heating and rotary draw bending (RDB) for carbon fibre reinforced polyetherentherketone (CF/PEEK) tubular structures. Optical characterisation was performed to analyse tube fibre angle changes. Following comments should be addressed:

1. Figure 1b should also be properly labelled just like 1a.

2. What is the rotation speed while tube is being bent?

3. Please show bent tubes at all considered angles for better understanding.

4. What is the bent (curvature) radius for 180 degree bend?

5. The words "measurement procedures" should be added in the captions of sub headings 2.3.1 and 2.3.2.

6. Figures 4, 5 and 6 should be presented in one figure as a, b, c and d. It would be great if these graphs are shown in 1 column x 4 rows pattern and the ranges of x axes in four graphs are same i.e. from 0 to 180. In first one, data will be from 0 to 45; in second one, data will be from 0 to 90; in third one, data will be from 0 to 135; and in last one, data will from 0 to 180. In this way, the comparative spread would be easy to visualize and understand.

7. Please mention the range of percentage errors between measure and predicted fibre angles for each bent.

8. The outcome/results of 2.3.2 are missing.

9. Conclusions are too long. These must be shortened. Discussion should be presented in a separate section before conclusions.

10. Please add a section about the practical aspect of this research.

Author Response

Comments 1: Figure 1b should also be properly labelled just like 1a.

Response 1: Figure 1 has been relabelled in line 110.

Comments 2: What is the rotation speed while tube is being bent?

Response 2: The bending rate of approximately 3 °/s was mentioned under section 2.2.3 in line 174.

Comments 3: Please show bent tubes at all considered angles for better understanding.

Response 3: Examples of bent tubes are added as Figure 3 in line 221.

Comments 4: What is the bent (curvature) radius for 180 degree bend?

Response 4: All tubes have the same bending radius of 46mm as written under section 2.2.2 in line 130.

Comments 5: The words "measurement procedures" should be added in the captions of sub headings 2.3.1 and 2.3.2.

Response 5: The subheadings of 2.3.1 in line 181 and 2.3.2 in line 194 have been changed.

Comments 6: Figures 4, 5 and 6 should be presented in one Figure as a, b, c and d. It would be great if these graphs are shown in 1 column x 4 rows pattern and the ranges of x axes in four graphs are same i.e. from 0 to 180. In first one, data will be from 0 to 45; in second one, data will be from 0 to 90; in third one, data will be from 0 to 135; and in last one, data will from 0 to 180. In this way, the comparative spread would be easy to visualize and understand.

Response 6: The Figures have been merged into Figure 4.

Comments 7: Please mention the range of percentage errors between measure and predicted fibre angles for each bent.

Response 7: The errors are shown in the form of error bars in Figure 4.

Comments 8: The outcome/results of 2.3.2 are missing.

Response 8: Outcome of section 2.3.2 is discussed in section 4 in line 228.

Comments 9: Conclusions are too long. These must be shortened. Discussion should be presented in a separate section before conclusions.

Response 9: Some contents from the conclusion has been moved to discussions. Discussions are separated into section 4.

Comments 10: Please add a section about the practical aspect of this research.

Response 10: The practical aspect of this research and applications are mentioned in section 1 line 47 as “In this case, structures with complex geometries and bends such as spaceframes and vehicle chassis reinforcements can be manufactured using CF/PEEK tubes efficiently without joints at the bends.”

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

After corrections and adjustments made by the author, this work must be published.

Author Response

Comment 1: After corrections and adjustments made by the author, this work must be published.

Responses 1: Thank you for your time and support.

Reviewer 2 Report

Comments and Suggestions for Authors

Line 153 - change "...sections A and B in Figure 2" to "... Section A and B in Figure 1(a & b)".

Author Response

Comments 1: Line 153 - change "...sections A and B in Figure 2" to "... Section A and B in Figure 1(a & b)".

Responses 1: Line 153 has been changed.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper is well received, but it still needs improvement. The results section should be expanded to explain results in detail with scientific soundness. It would be great to separately discuss the outcome of procedures (mentioned in sections 2.3.1 and 2.3.2) in results as sub-sections 3.1 and 3.2, respectively. Currently, the number of lines written for sections 3 and 4 are only 40 lines from line 210 to line 250. Whereas, the number of lines in conclusions are 42 from line 255 to line 297. Therefore, it is strongly suggested to explain results in detail with scientific soundness and shorten the conclusion section.

Author Response

Comments 1: The paper is well received, but it still needs improvement. The results section should be expanded to explain results in detail with scientific soundness. It would be great to separately discuss the outcome of procedures (mentioned in sections 2.3.1 and 2.3.2) in results as sub-sections 3.1 and 3.2, respectively. Currently, the number of lines written for sections 3 and 4 are only 40 lines from line 210 to line 250. Whereas, the number of lines in conclusions are 42 from line 255 to line 297. Therefore, it is strongly suggested to explain results in detail with scientific soundness and shorten the conclusion section.

Responses 1: We thank the reviewer for your constructive feedback. The discussion section has been separated into two parts to address 2.3.1 and 2.3..2 as suggested. The discussion section (section 4) has also been extended to include more details in the analysis, as well as an extra table (Table 1) discussing the similarities in fibre angle change between tube intrados and extrados and how they reflect consistency in the forming process.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The paper is all good by this reviewer.