Experimental Study on Fatigue Performance of M24 Twisted-Shear High-Strength Bolt for Assembled Steel Structure

Round 1

Reviewer 1 Report

The manuscript "Fatigue problem of M24 twisted-shear high-strength bolt node in the new prefabricated steel structure" reports a MTS fatigue test equipment and self-designed T loading device, and a constant amplitude fatigue test which was done for 25 M24 twisted-shear high-strength bolts. The fatigue failure modes of M24 twisted-shear high-strength bolt connection were obtained. In addition, the constant amplitude fatigue S-N curve was obtained by data fitting, and meanwhile the constant amplitude fatigue design method of M24 twisted-shear high strength bolt based on the nominal allowable stress amplitude was established. The paper in indeed very interesting especially from a practical point of view. Also, the analyses are comprehensive. I suggest this paper for publication after doing following mandatory minor revisions.  

1.     The most important issue is the introduction on the fatigue. The introduction on the fatigue is rather poor. Authors need to provide more recently published papers on the topic. I strongly advise authors consult and include following articles in their revisions:

-        A new comprehensive model of damage for flexural subassemblies prone to fatigue. Computers & Structures, 256, 2021, 106639. doi: https://doi.org/10.1016/j.compstruc.2021.106639

-        Defect tolerant fatigue assessment of AM materials: Size effect and probabilistic prospects. International Journal of Fatigue, 160, 2022, 106884. doi: https://doi.org/10.1016/j.ijfatigue.2022.106884

-        Probabilistic fatigue modelling of metallic materials under notch and size effect using the weakest link theory. International Journal of Fatigue, 159, 2022, 106788. doi: https://doi.org/10.1016/j.ijfatigue.2022.106788

2.     The conclusion is very short. Only paragraph 2 is really from your work. Please re-write the conclusion.

3.     Image have very low-quality. Please replace them with higher quality ones.

4.     Please mention based on what standard you have performed your tests?

5.     Please mention the composition of the steel.

Author Response

Dear reviewer ：

    Thanks a lot for your  sincere suggetions,  I have revised our manuscript according to your request. 

    I hope our new manuscript can be recognized by you.

 Best wishes,

 Rong Xing

Author Response File: Author Response.docx

Reviewer 2 Report

The paper propose a new type of prefabricated steel structure, the connections between the elements being assured by a new twisted-shear high-strength bolts. The experimental work is well developed. The results are properly analyzed, The conclusions are limited. There are a lot of drafting errors. The figures are not clear and must be improved, The text figures  are not aligned. The authors didn"t respect the publishing rules. In the text and in the tables appear drafting errors of the formulas.The paper rmust be revised.

Comments for author File: Comments.pdf

Author Response

Dear reviewer ：
    Thanks a lot for your  sincere suggetions,  I have revised our manuscript according to your request. 
    I hope our new manuscript can be recognized by you.
 Best wishes,
 Rong Xing

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript deals with fatigue evaluation of a steel bolted structure. Although the general concept of the study is important, it lacks serious problems related to introduction, methodology, questions about the implementation of methodology, and presentation of results. Thus, I think it needs serious improvement until it reach a suitable form. Consequently, I cannot suggest it for possible publication. Here are some minor and major comments/concerns which the authors can consider for improving their manuscript. Hope it helps and they continue their nice work until achieving an acceptable manuscript quality.

#1: the writing and english seems fine, while there are some extra spaces, punctuation, and disorders in writing. Also, it is needed to pay attention to the word selection, writing style and improving grammar mistakes and any typos. Please check the manuscript again for that.

#2: Title: I suggest to edit the title in a way it better present the novelty, methodology, materials, etc. Mentioning words such as "problem" seems to be too general.

#3: Abstract: The abstract seems to be written in a descriptive manner. Moreover, it is important to pay attention that the beginning lines are more like an introduction explanation. The abstract should begin with the importance of the specific subject of study and continues to show how essential it is. Then, the goal of the paper, its methodology, and any classification in work should be outlined. Finally, some highlighted results should be stated, while it is better to be in informative way by mentioning some numbers. So, this part needs serious modification.

#4: The introduction is not well written, it lacks proper flow and scenario, and lacks recent researches. So, I suggest the authors to have a clear classification for the paragraphs of the introduction section. For example, the first paragraph can discuss the importance of the structure, its use in previous research works/studies, and importance of fatigue study or any failure observation in bolted connections in the literature. The second paragraph can discuss the theories or methodologies used in previous research studies to evaluate fatigue life of bolted, bolted connections, and multi bolted connections. Finally, you can go through you work and clearly mention your novelty in a separate paragraph. 

Please note that most recent researches are missed here

#5: I think that the introduction section does not need pictures. Remember it is not a chapter book or review paper. This is an original research manuscript.

#6: the methodology is not well described. it is suggested to clearly mention the flowchart or method of your study in a separate section. Also, it is needed to mention that why you did not apply low-cycle fatigue regime estimation theories such as coffin-manson or SWT? 

#7: the presentation of results are poor and needs to be improved.

Author Response

Dear reviewer ：
    Thanks a lot for your  sincere suggetions,  I have revised our manuscript according to your request. 
    I hope our new manuscript can be recognized by you.
 Best wishes,
 Rong Xing

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Thanks to the efforts of the authors. Unfortunately, although some suirtable modifications were made (e.g., the abstract and some minor ones in the text), major concerns dealing with introduction, novelty, methodology and validity are still exist. So, I cannot accept it with the present form. Hope the respected authors make major changes to the manuscript and publish this work which have an interesting soul as soon as possible.

Author Response

Dear reviewer ：

    Thanks a lot for your  sincere suggetions,  I have revised our manuscript according to your request. 

    I hope our new manuscript can be recognized by you.

 Best wishes,

 Rong Xing

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Thanks to the efforts, I still think it does not fit the standards to satisfy the methodology and novelty. still there are many problems in illustrations, and tables.

Author Response

Dear reviewer: 

Response to Reviewer 3 Comments

Point 1: Does the introduction provide sufficient background and include all relevant references?

Response 1:

In Part 0 Introduction

High-strength bolt connection is the main connection mode of prefabricated steel structure, and it is widely applied in practical engineering because of its advantages of convenient installation, being easy to ensure the construction quality and high construction efficiency. On-site bolt connection is fast and convenient, but the fatigue damage of the bolt nodes will occur under the action of long-term repeated load, so the fatigue performance of bolt nodes can’t be ignored. Since the German engineer W A J Albert proposed fatigue problem as early as 1829, many scholars have paid attention to it and studied it [1], and different researches on structure’s fatigue problem are gradually appearing, especially considering from kinds of factors such as structural details, structure materials etc.[2-12]

However, most fatigue researches are focused on high strength bolts of grid structure. Nowadays, there is few research about sudden fatigue failure of the fabricated steel structure residence. Therefore, on the basis of the previous research on the fatigue failure type and mechanism of the steel structure, the paper have had the constant amplitude fatigue tests on the M24 twisted-shear high strength bolts used in the fabricated steel structure residence. Fatigue test is a typical means to study fatigue problems. The purpose of the test is to determine the different loading systems of high strength bolts by controlling the stress ratio. By recording the load cycle times of bolt fatigue fracture, and many different data obtained from the repetition of high strength bolt fatigue test, the S-N curve and equation of high strength bolt fatigue can be fitted through the regression analysis. By analyzing the S-N curve and the obtained equation, the fatigue life of high strength bolt under different loading systems can be estimated.

Point 2:Are all the cited references relevant to the research?

Response 2:

In Part  Reference

Deleting  the following references

[2]   Amit S，John E, Allison，Christopher J, and Szczepanski et al. Small fatigue crack growth in metallic materials: A model and its application to engineering alloys. Acta Material ia. November 2007; Volume 55, Issue 19: pp. 6606—6616．

[3]  Zhang J，He X D，and Du S Y，et al. Analyses of the fatigue crack propagation process and stress ratio effects using the two parameter method．International Journal of Fatigue. October-December 2005; Volume 27, Issue l0—12: pp. 1314—1318．

[4]  Leguillon D，Quesada D，Putot C, and Martin E et al. Prediction of crack initiation at blunt notches and cavities—size effects. Engineering Fracture Mechanics. October 2004, Volume74, issue l 5: pp. 2420—2436．

Point 3: Is the research design appropriate?

Response 3: having a geat restruction, as shown in following content.

1 Experimental methods

1.1 Specimen design

1.1.1M24 torsional shear high strength bolt connection

1.1.2T-type connector

1.2 Mechanical property of bolt’s material

1.3 Testing program and procedure

1.3.1 Fatigue test equipment

1.3.2 Process and method of the test

2 Experimental Results

2.1 Fatigue failure

2.2 Fatigue S-N curve

3 The Design Method for Constant-Amplitude Fatigue based on the nominal stress

3.1 The Establishment of Formula

3.2 Application of The Formal

4 Estimating the fatigue life of M24 high-strength bolts based on Broken mechanics

4.1 Parameter determination

4.2 Estimate of fatigue life of M24 twisted shear high strength bolt

5 Results and Discussion

5.1 Influence of the bolts’ pretension on the fatigue strength

5.2 Influence of bolt diameter on fatigue strength

6 Conclusion

Point 4: Are the methods adequately described?

1. The key to the test is that the high strength bolts should be put the pretension force onin order to achieve the purpose of reliable connection.
2. There are three steps in the constant amplitude fatigue test.
3. Generally, the relationship between external loads and its corresponding fatigue life is described by the S-N curve.

4.Taking the stress amplitude as the parameter, the constant-amplitude power regression curve and fatigue S-N curve are presented in Figure 8 and Figure 9, respectively, where the logarithmic equation for the corresponding part of the oblique line of the S-N curve in Figure 9 can be developed as Eq.(4).

5.The nominal stress method takes the nominal stress of the parts as the parameter. Through a lot of fatigue tests, the fatigue S-N curves of different structural details are established, so as to design the fatigue strength or fatigue life of the structure.

6.According to the fatigue crack extension parameters determined above, the fatigue life of M24 high strength bolt is estimated basing on Paris formula.

7.In order to compare the constant amplitude fatigue performance of M24 high strength bolts under different conditions, the relationship between the stress range and fatigue life was applied. Taking the nominal stress amplitude as the parameter, the power regression curve and S-N curve were established in the same coordinate system, as shown in Figure 10 and Figure 11.

8.In order to compare the constant fatigue performance of M24 and M20 twisted shear high-strength bolts under full pretension, the double-log S-N curve is made in the same coordinate system according to the fatigue test data in Table 4 and in Table 10, as is shown in Figure 12, where the logarithmic equation for the corresponding part of the oblique line of the S-N curve can be developed as the third equation in Table 16.

Point 5: Are the results clearly presented?

1. It is drawn that all the fatigue failure in the test occurred at the root of the high strength bolt thread, and most of the bolts were located at the first circle of thread where the nut was engaged with the bolt.
2. It can be seen from the Figure 8and Figure 9 that the fatigue life of the specimens is relatively scattered at the same stress amplitude, but in total the fitting correlation coefficient of the bolt test data is very satisfied, the value of which is about 0.8. Moreover, the test data follow the trend that the lower stress range, the longer the fatigue life. Besides, by calculating the standard deviation of fatigue test data, we can acquire the S-N curve equation under 97.72% survival probability, as shown in (5).

3. According to the fatigue S-N curve fitting formula, the fatigue design parameter,              ,  It can be seen that when the fatigue life of the bolt                 

times, the normal amplitude fatigue limit                   , and when the fatigue life of the bolt        times the allowable stress amplitude                  .

4.As can be seen above, the following findings can be drawn: 

As can be seen above, the following findings can be drawn: 

• The Grade-10.9 M24 twisted-shear highstrength bolt has a higher fatigue life, while the Grade-8 M24 large hexagonal bolt has the lowest fatigue life;

(2)Through comparing the test results of the M24 twisted-shear high strength bolt that put on 100% of pretension and Grade-10.9, Grade-8.8 M24 large hexagonal bolt that only put on 20%~40% of the pretension torque value, it is drawn the pretension force of the bolt will have a greater impact on the fatigue strength. The more higher the value of the pretension force is , the more higher the fatigue strength of the bolt is

5.As can be seen above, the following findings can be drawn: 

• Under full pretension the fatigue test data of the M20 twisted-shear high strengthbolt is higher than the M24 bolt;
• Under the guaranteed probability of 97.72%, the stress amplitude of the M20 twisted-shear high strength bolt corresponding to 2 million fatigue life is about 278.0MPa, which is greater than the stress amplitude the value of which is 181.0MPa of the M24 bolt, it was indicatedthat under the same stress amplitudethe fatigue life of M20 twisted-shear high strength bolt has a higher fatigue life than one of M24 twisted-shear high strength bolt. 

(3)The fatigue strength will decrease when the bolt diameter increases. The reason that the influence trend the bolt diameter has on the fatigue strength may be that when the diameter of the bolt has increased, the concentration of the stress between the thread and the bolt will exacerbate; meanwhile some defects or impurities will occur during the process of heat treatment.

Point 6: Are the conclusions supported by the results?

(1)Fatigue failure of all M24 twisted-shear high strength bolts in the test occurred at the root of the high strength bolt thread, and among them most bolts damaged at the first circle of thread between the nut and the bolt, where there was a serious stress concentration, and the fatigue gap effect is significant.

(2) The constant amplitude fatigue S-N curve was obtained by test data fitting, and the corresponding the S-N curve equation of M24 twisted-shear high strength bolts with the stress range as the design parameter, and the fitting correlation coefficient of the test data was about 0.8. Under the guarantee rate of 97.72% the allowable stress amplitude of 5 million fatigue life is about 167.0 MPa; the one of 2 million fatigue life is about 181.0 MPa.

(3) The Design Method for Constant-Amplitude Fatigue based on the nominal stress was established, as is indicated that when the fatigue life of the bolt is          times, the normal amplitude fatigue limit is               , and when the fatigue life of the bolt is           times, the allowable stress amplitude is                  .

(4)It is basically feasible to estimate the fatigue life of high strength bolts by using Paris formula, which can

generally reflect the changing trend of fatigue life.

(5)M24 twisted-shear high-strength bolts with 100% pretension force were of better performance in the certain stress range. Under the 97.72% survival probability, the fatigue strength of M24 pretension force high-strength bolts was 1.86 times and 1.71 times of the values of 8.8-grade M24 and 10.9 M24 large hexagonal bolts respectively.

(6)By comparing the test results of M24 and M20 bolts under full pretension, it is known that when the diameter of the bolt decreases, the fatigue strength of the bolt will reduce.

Thanks a lot  for your suggestions.

Best wishes,

Rong Xing

Author Response File: Author Response.docx