Stress Corrosion Cracking of Tunnel Ventilation Fan Blades: A Case Study

Round 1

Reviewer 1 Report

Stress-Corrosion Cracking of Tunnel Ventilation Fan Blades

ID: metals-2007413

Although it is an extensive work on the failure analysis of Tunnel Ventilation Fan Blades. The authors made a successful attempt in finding the root cause of the problem and presented a reasonably good solution to avoid the issues in the future. However, the work sounds more like a technical report rather than a journal article. I strongly advise the authors to restructure the manuscript and present it as an article with more scientific reasons rather than generic statements.

1. Whole of section 2 (Methods) seems more like introduction. It should be re-written and arranged with the intro.
2. The structure of the manuscript doesn’t flow well as an article. I would classify this as a technical report/case study! This is my major critique.
3. The authors did a good job by estimating the surface roughness and providing all the parameters for the fatigue testing. However, the general scatter in fatigue is not taken into account or not considered during the analysis.
4. Please annotate Fig. 4. Also, revise the legend in Fig. 7 to reflect the data lines correctly.
5. The conclusions are too generic. “The combination of design, material, manufacturing process, corrosion protection and corrosive environment led to unexpected tensile stresses and therefore crack formation.” – The wear and tear of the material, corresponding stress concentration, and many such reasons could also lead to formation of crack initiation sites. Also, the wedging effect could be a potential site. The authors are advised to revise the conclusions and make them more scientific.

Author Response

Dear Reviewer

Thank You very much for Your encouraging comments.

General comment:

More scientific reasons rather than generic statements: We have eliminated generic statements which are not necessary for the conclusions of the study. Were possible we have written more specific statements.

 

We have the following comments on specific comments:

• Section "Methods" is shorted and merged with section "Results" to one main chapter.
• Case study: Yes it is clearly a case study which is now stated in the title
• Consideration of scatter in fatigue data: We were limited in number of specimens and a full statistical evaluation was out of the range of this study – nevertheless a conclusion could be drawn based on a limited number of specimens.
• Legend of Fig.4 rewritten, Legend of data in Fig 7 corrected.
• Conclusions rewritten

We are looking forward to the publication of our manuscript.

Reviewer 2 Report

In view of the failure of the tunnel ventilation fan bladest, the authors have formulated a detailed analysis plan. The research content is comprehensive and sufficient, and the results are accurate. I suggest that this manuscript can be published directly.

Author Response

Dear Reviewer

Thank You very much for Your encouraging comments. We have checked English writing and style as far as possible. We are looking forward to the publication of our manuscript.

Reviewer 3 Report

Dear authors, thank you for having submitted your paper to MDPI Metals.

First it is interesting to note that even with a very limited number of failed components, FEDRO carries out investigation to determine their cause in order to maintain a high safety level of their tunnels.

The determination of the cause is done by successive elimination of more or less possible causes and finally to retain the most likely cause. To make the analysis as convincing as possible, there are missing information or analysis. These should be included in the text if available or the authors must provide well-argued answers to the following comment or questions.

11.   About the material:

a.       the chemical composition is given but the Al series is not indicated

b.      the heat treatment is not specified: only in paragraph 3.4.4 it is understood that it was in the T5 condition

c.       the microstructure should be better described based e.g. on Figure 4a and 4c

d.      did the authors observe porosities inherent to the casting process ?

e.       about the estimation of K_IC from K_CV; it should be better to find value of KIC from the literature instead of using a relation which is not valid as the authors of reference 5 mentioned

22.  About the fractography:

The authors claimed for brittle fracture in the final fracture surface: a micrograph at a higher magnification would have been welcome

3.  About the corrosion test:

Corrosion tests have been performed at +23°C while it is written in the introduction that the operating temperature can be +40°C: do the author believe that there is no impact ?

 

4-         about the cause of the failure:

The authors concluded to SCC. It seems that going through the paper they rejected hydrogen embrittlement as a possible cause because they never mention HE as a possible cause. Can the authors explain?

Indeed, in presence of such localized corrosion within corrosion pits, on can also consider that the local pH decreases and hydrogen production may occur.

I am asking the author to confirm their point of view especially because the paper title contains “stress corrosion cracking” term.

5. The session 2 "methods" can be shortened. It contains too basic informations

 

Author Response

Dear Reviewer

Thank You very much for Your encouraging comments.

 

Point 11.a.        The Series No. of all material is included

Point 11.b.        The heat treatment of all material is included

Point 11.c.        The microstructure is now better commented in the text. An additional figure is added and the sequence of figures 4a to 4c rearranged.

 

Point 11.d.        Porosity: Yes we have observed porosity. The text is added with the reference to Fig. 2d and 4a. 

Point 11.e.        K_IC of casted aluminum has been added.

 

Point 22            Fractography – micrograph of final fracture surface has been added (Fig. 3c).

 

Point 3 corrosion tests @ 23°C instead of +40°C: The environmental conditions in a tunnel change with seasons and the weather; 40°C is an upper bound of the temperature which has to be expected in a tunnel. But also temperature down to -20°C are possible. Therefore a temperature of 23°C is an adequate choice. The effect of temperature on corrosion was not the focus of this study.

 

Point 4: rejection of hydrogen embrittlement? – No this is a misunderstanding: We clearly agree, that hydrogen embrittlement is an important mechanism of SCC in this case. The critical factor which was not understood in the first evaluation was the high stress level mandatory for initiating a brittle final failure – Without assuming a wedging effect there is no explanation for high stresses. More explanations added in the discussion part.

Point 5: "Methods" has been shortened and included in the main chapter.

We are looking forward to the publication of our manuscript.

Reviewer 4 Report

The authors must carefully edit the manuscript to be considered for publication as it is not organized and needs major revision. Some of my comments are as follows: -

Page 2, 2. Methods, from line 54 to line 88, this part has no relation to Methods section.

Page 2, line 89 and line 90, 2. Methods, the authors mentioned that “the on-site investigations were carried out without any problems and the failed parts were easily identified, han0dled and brought to the laboratory.”. What kind of investigations have you carried out?

Page 2, line 92 and line 95, 2. Methods, “Therefore, not only a mechanical but also an electrochemical characterization of the blade material was carried out: The chemical and mechanical properties were compared with the specified values to check whether the material of the defective parts met the specifications.”, what kind of mechanical and electrochemical characterizations were carried out? What instruments have you used to carry out the mechanical and electrochemical characterization?

Page 2 and page 3, the section “2. Methods” should mention the materials and instruments used in the study as well as the conditions at which the measurements were carried out, but unfortunately, this part does not report any methods or materials or even conditions. For that, this part must be extremely edited.

Page 3, line 120, 3. Results, “This fire rating has to comply with EN 12101-3.”, the authors must cite a reference for this sentence, it is not accepted to mention EN 12101-3 without citing its source.

Page 3, page 4 and page 5, 3. Results, how the authors have got the images of Figure 1 and Figure 2? The machine that was used to make photos of these figures as well as its operating conditions must be mentioned in the Methods section.

Page 5 and page 6, Table 1, how were the dimensions of the failure case listed in the second column measured? This should be mentioned in the Methods section.

Page 6, “The laboratory corrosion analysis of these samples show high magnitude of chloride deposits of approximately 50 - 200 μg/cm2. In the presence of chloride deposits >10 μg/cm2, corrosion attack of metallic materials can be expected in the presence of relative humidity >50%.”. How could you analyze corrosion? What instrument have you used?  How could you measure the thickness of the deposits of chloride and also the percentage of humidity?

Page 6, line200 to line 202, “Three samples of white corrosion products, one from the failed blade and two from the reference blades were taken for a corrosion chemistry analyses with X-ray microanalysis, using a scanning electron microscope (SEM/EDX).”, a full description such as brand, company, country, for the X-ray, SEM and EDX instruments should be mentioned in the Methods part.

Page 16, 3.7.1. Evaluation of the corrosion resistance, the used aluminum alloys should be described in the Methods section. Also, the potentiostat and electrochemical cell and its component should be described in the Methods part.

The references list has very less references and more references must be cited.

 

 

Author Response

Dear Reviewer

Thank You very much for Your comments.

Major revision:

We have restructured the paper by merging the section "methods" and "results". We believe that with this change the flow of the text is improved. The main reason is that the paper is basically a case study and various well established methods have been used to characterize the material; we have not developed a new method – all the standardized methods are now referenced.

We have the following answers to specific comments:

1. Page2 line 54 to 88: We agree that this is not necessary and the text has been deleted.
2. Page2 line 89 to 90: The text has been revised. It was more an inspection than an investigation.
3. Page 2 line 92 and line 95, 2. Methods: description of mechanical and electrochemical characterization is given
4. Page 2 & 3, section 2. Methods: included in new chapter ""
5. Reference for EN 12101-3 added
6. Page 3, 4 and 5 3.Results: Information on instrument for pictures Fig.1 and 2 added.
7. Page 5 and page 6: Information on methods of determining the dimensions added (legend of table 1)
8. Page 6: Information on corrosion analysis methods added
9. Page 6 line200 to 202: Missing information added
10. Information on potentiostat and electrochemical cell added
11. References have been updated

We are looking forward to the publication of our manuscript.

Round 2

Reviewer 1 Report

The authors have address most of the reviewer’s comments. My only comment now is on the figures. They are poorly presented as well as the resolution in poor. Once the figures are extensively improved, the manuscript can be accepted in that form.

Author Response

Dear Reviewer

Thank You for Your comment. We have reworked the figures and checked the resolution. 

Fig 1a: replaced.

Fig 1b: dimensions in black, consistant with all other dimensions in other figures.

Fig 3a & b: resolution improved. 

Fig 5a  & b: resoltion improved, dimensions in black.

Fi 1c: Dimensions in black.

Fig. 6: Reworked and simplified to two scematics.

Fig. 9: legend in larger letters.

I hope with these improvements the figures are better presented. 

Sincerely

S. Michel 

 

 

Reviewer 3 Report

Dear authors,

Thank you for some answers of my questions but I would have appreciated to see them in the text as it is usually done for helping the reviewer. Here, the revised paper I received is in a form that takes to much time to be reviewed. There are a lot of strikethrough words, graphs  sentences, too many !

I do not clearly in the text where are your answers and modified parts.

Please provide a simplified corrected paper showing where you have introduced the required modifications and the answers but underlined them with colors

Thank you 

 

Author Response

Dear reviewer

We send You a revised manuscript with the etxt marked related to Your comments, see attachement.

The answers to reviewers comments is added with refering to the lines in the manuscript were the corrections have been made, see below. Thanks for Your friendly review.

 

Answers to reviewer 3

 

Dear Reviewer 3

Thank You very much for Your encouraging comments.

 

Point 11.a.        The Series No. of all material is included, the material specifications follow European standards and the references are given for all materials involved. See line 189,209,446,456-461

Point 11.b.        The heat treatment of all material is included: See line 209, 459-461

Point 11.c.        The microstructure is now better commented in the text. An additional figure is added and the sequence of figures 4a to 4c rearranged: See line 244-261

Point 11.d.        Porosity: Yes we have observed porosity. The text is added with the reference to Fig. 2d and 4a. : See line 259-261

Point 11.e.        K_IC of casted aluminum has been added: See line 308-309 Ref [11]

Point 22            Fractography – micrograph of final fracture surface has be added (Fig. 3c): See line 238-239

Point 3 corrosion tests @ 23°C instead of +40°C: The environmental conditions in a tunnel change with seasons and the weather; 40°C is an upper bound of the temperature which has to be expected in a tunnel. But also temperature down to -20°C are possible. Therefore a temperature of 23°C is an adequate choice. The effect of temperature on corrosion was not the focus of this study.

Point 4: rejection of hydrogen embrittlement? – No this is a misunderstanding: We clearly agree, that hydrogen embrittlement is an important mechanism of SCC in this case. The critical factor which was not understood in the first evaluation was the high stress level mandatory for initiating a brittle final failure – Without assuming a wedging effect there is no explanation for high stresses. More explanations added in the discussion part. See line 383-388

Point 5: "Methods" has been shortened and included in the main chapter.

We are looking forward to the publication of our manuscript.

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors have revised the manuscript according to the suggested recommendations and the revised version may be accepted for publication.

Author Response

Thank You for Your positiv feedback. We are looking forward to the publication. 

Sincerely

S. Michel