The Influence of the Ratio of Circumference to Cross-Sectional Area of Tensile Bars on the Fatigue Life of Additive Manufactured AISI 316L Steel
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis manuscript presents an interesting work about the influence of the ratio of circumference over the cross-section area of tensile bars on the fatigue life of additive-manufactured AISI 316L steel. A minor revision is needed before the manuscript can be considered for publication. The main concerns are
1. Chapter numbers are confusing, for example, in section 2, there are 3.1, 2.3-2.6.
2. Lines from 337(page 14, 'The microstructure of an N4 specimen rod....') to 379 (page 15, '...... the manufacturing process.') appear to be irrelevant to the high cycle fatigue test. It is recommended to put these contents in Section 2.
Author Response
Dear reviewer, thank you for your comments and we have considered them in our paper:
Comments 1: Chapter numbers are confusing, for example, in section 2, there are 3.1, 2.3-2.6.
Response 1: According to your suggestion, we have fixed in manuscript.
Comments 2: Lines from 337(page 14, 'The microstructure of an N4 specimen rod....') to 379 (page 15, '...... the manufacturing process.') appear to be irrelevant to the high cycle fatigue test. It is recommended to put these contents in Section 2.
Response 1: According to your suggestion, we have fixed lines in manuscript.
Kind regards
Author
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Authors,
I think your paper sounds good to me. It means some corrections should be made. Please follow the paper.
Reviewer
Comments for author File: Comments.pdf
Minor editing of English language required.
Author Response
Dear reviewer, thank you for your comments and we have considered them in our paper.
Comments are given in file: peer-review-40488913.v1.pdf
Response: We have follow your suggestion and provide reply in text on your comments in attached pdf file.
However, we have particularly describe our response e.g.:
line 107-112:
Diameter of 12-rod specimen geometry was chosen to be at least 2mm in respect to maximum stress capacity. From experience maximum stress is reduced with small diameters of specimen (d < 1mm) therefore maximizing smallest diameter was needed for comparable results. Diameter of 2.01 mm for 12 rod geometry also allowed all specimens to be manufactured at the same time, reducing material properties between specimens to obtain as high as possible ratio of outer surface to cross-sectional are while keeping geometries within limitations of test and manufacturing equipment, 4 and 12 rod geometries were selected.
Line 398-403
This was performed in order to reduce recorded data during fatigue testing, as change in displacement is best visible at maximum loading force. Recorded displacement of each cycle allows to estimate number of cycles when first rod of specimen fails as stiffness of specimen will change with fatigue crack propagation through rod. Figure 22 shows change in displacement of actuator on N4 specimen that was tested at maximum stress level of 280 MPa,
And new Figure 22:
Figure 22: Measured displacements of actuator during fatigue test of specimen N4 tested at Maximum stress of 280 MPa
e.g. line 451 Figure 24:
e.g. 641- New sentence has been provided:
Different ratios of area and circumference have impact on results of fatigue life of tested AISI 316L stainless steel structure specimens manufactured with additive manufacturing as fatigue life of different structure specimen geometry were different at same stress level. Therefore, it is necessary to consider circumference length as parameter of fatigue life at such structures.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsDear Authors,
Major remarks:
1. Lines 40-42 – I propose to add references.
2. Figure 1 – I propose to add hatching (bars) at both cross-sections.
3. Table 1 – I propose to remove „[/]” (below Number of rods).
4. Section 3.1 – Figures with FE mesh should be provided for both examples. What were the finite element sizes in the areas in which stress concentrations were observed? Did you perform a verification of convergence of the solution?
5. Tables 2, 3, and Figures 4 and 5 – missing units.
6. Line 182 – Figure 8 is cited before Figure 7. It should be corrected.
7. Line 217 „is listed in Table 3.” – wrong reference.
8. Lines 243 „0.00025 mm/mm/s” and 0.002 mm/mm/s – not clear.
9. I did not find a reference to Figure 10.
10. Section 3.1 – what fit was used in the Cardan joint and what was the influence of such fit on the distribution of force and displacement in the tested sample?
11. Line 337 „The microstructure of an N4 specimen rod in parallel direction of layers and it is detailed in Figure 16.a)” – check grammar/style.
12. Line 347 „36.5 mm” – wrong unit.
13. Figure 20 – the description of the horizontal axis should be improved. The equations of trend lines should be added.
14. The process of enlargement of the images may be not clear to readers. I propose to revise such images or add some information in figure captions.
15. Figures 21-24 – missing captions (for a,b,c,d). The legend for colored arrows should be provided in captions
16. Line 418 „suggesting that significant plastic deformation occurred before failure.” – It looks like the overload of the residual cross-section after fatigue failure of the first rod. Please check and revise. How many cycles were counted after the failure of the first rod (from Figure 21b) to complete failure?
17. The whole paper should be carefully revised with respect to: a) a/an/the b) singular/plural (Figure/Figures).
18. What is the maximal stress? Is it the nominal stress or stress calculated with the use of the stress concentration factor?
19. Lines 522-541: It is difficult to evaluate in which cross-section failures occurred. In the description of the results of the fatigue tests, the additional image in the direction parallel to the sample axis should be given. If the failure occurred at the end of the rod it means that the radii at the end of the rods have a crucial influence on the fatigue life (this effect is not discussed in the paper). Such radii are different in samples N4 (R=2 mm) and N12 (R =1 mm) which may have a significant impact on stress concentration factors. Here are missing the analysis and calculation of such stress concentration factors in the cross-sections in which failure occurred.
Kind Regards
Comments on the Quality of English LanguageLine 337 „The microstructure of an N4 specimen rod in parallel direction of layers and it is detailed in Figure 16.a)” – check grammar/style.
The whole paper should be carefully revised with respect to: a) a/an/the b) singular/plural (Figure/Figures).
Author Response
Dear reviewer, thank you for your comments and we have considered them in our paper:
Major remarks:
Comment 1: Lines 40-42 – I propose to add references.
Response 1: We have fixed: Several AM technologies are employed for AISI 316L, including Powder Bed Fusion (PBF), Directed Energy Deposition (DED), Fused Deposition Modelling (FDM), and Binder Jetting (BJ) [15]. PBF and DED are notable for producing near full-density components with mechanical properties close to those of traditionally manufactured materials [15].
Comment 2: Figure 1 – I propose to add hatching (rods) at both cross-sections.
Response 2: We have change Figure 1:
Comment 3: Table 1 – I propose to remove „[/]” (below Number of rods).
Response 3: we have change in Table 1:
Comment 4: Section 3.1 – Figures with FE mesh should be provided for both examples. What were the finite element sizes in the areas in which stress concentrations were observed? Did you perform a verification of convergence of the solution?
Response 4: we have explained in chapter 3.1:
Size of mesh:
N4 and N12 finite element meshes are shown on Figure 4. Convergence of mesh was performed, resulting in N4 geometry meshed with 488083 C3D4 elements, and N12 geometry meshed with 720339 C3D4 elements, of approximate global size of 0.2, and approximate 79 number of elements per circle.
Added Figure 4.a) and b):
Comment 5: Tables 2, 3, and Figures 4 and 5 – missing units.
Response 5: We fixed in manuscript.
Comment 6: Line 182 – Figure 8 is cited before Figure 7. It should be corrected.
Response 6: We fixed in manuscript.
Comment 7: Line 217 „is listed in Table 3.” – wrong reference.
Response 7: We fixed reference in manuscript.
Comment 8: Lines 243 „0.00025 mm/mm/s” and 0.002 mm/mm/s – not clear.
Response 8: We have change to strain/s.
Comment 9: I did not find a reference to Figure 10.
Response 9: We have add Fig. 14.a) of specimen and mentioned in text.
However, comparing AM tested tensile specimens to commercial sheet material in annealed condition [43] shows that AM material has higher yield stress and elongation at break due to remelting and deposition which alter material properties, resulting in ductile fracture of tensile specimen as shown in Figure 14.
Comment 10: Section 3.1 – what fit was used in the Cardan joint and what was the influence of such fit on the distribution of force and displacement in the tested sample?
Response 10: We have explained in text:
Cardan joint with clearance fit was utilized between a specimen and the upper clamps of the MTS machine, as depicted in Figure 23. Cardan joint was used to achieve alignment of specimen symmetry line with load direction of MTS machine. This setup helped in mitigating any potential misalignment issues due to manufacturing process that could affect test results due to uneven stress distribution in tested specimen as it reduces bending of tested specimen.
Comment 11: Line 337 „The microstructure of an N4 specimen rod in parallel direction of layers and it is detailed in Figure 16.a)” – check grammar/style.
Response 11: We fixed in manuscript.
Comment 12: Line 347 „36.5 mm” – wrong unit.
Response 12: We fixed in manuscript.
Comment 13: Figure 20 – the description of the horizontal axis should be improved. The equations of trend lines should be added
Response 13: We have add equations in Figure 24
Comment 14: The process of enlargement of the images may be not clear to readers. I propose to revise such images or add some information in figure captions.
Response 14: We have change order of photos of fractured surfaces and more clearly explain by arrows and text main points.
Comment 15: Figures 21-24 – missing captions (for a,b,c,d). The legend for colored arrows should be provided in captions
Response 15: We have change order of photos of fractured surfaces and more clearly explain by arrows and text main points.
Comment 16: Line 418 „suggesting that significant plastic deformation occurred before failure.” – It looks like the overload of the residual cross-section after fatigue failure of the first rod. Please check and revise. How many cycles were counted after the failure of the first rod (from Figure 21b) to complete failure?
Response 16: After first rod failure, specimen shows different number of cycles to failure of second rod depends on stress level. Minor changes in text are follows:
In contrast, the originally circular shape is distorted in other rods, indicating that rods failed due to material reaching ultimate tensile strength due to higher stress values and not due to fatigue crack propagation through rod.
Comment 17: The whole paper should be carefully revised with respect to: a) a/an/the b) singular/plural (Figure/Figures).
Response 17: English was corrected by native speaker.
Comment 18: What is the maximal stress? Is it the nominal stress or stress calculated with the use of the stress concentration factor?
Response 18: Maximum fatigue stress, R=0.1 - fixed in manuscript
Comment 19: Lines 522-541: It is difficult to evaluate in which cross-section failures occurred. In the description of the results of the fatigue tests, the additional image in the direction parallel to the sample axis should be given. If the failure occurred at the end of the rod it means that the radii at the end of the rods have a crucial influence on the fatigue life (this effect is not discussed in the paper). Such radii are different in samples N4 (R=2 mm) and N12 (R =1 mm) which may have a significant impact on stress concentration factors. Here are missing the analysis and calculation of such stress concentration factors in the cross-sections in which failure occurred.
Response 19: We have add the photos of fractured specimens N4 and N12, where is obvious that failure occurred in middle rods of specimen, and root's radii R=2 mm or R=1 mm has no influence on stress concentration factor and fatigue crack initiation.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsDear Authors,
The submitted manuscript consists of basic numerical modelling following the finite element fundamentals and conventional fatigue testing. Two geometries were introduced as the benchmark, however, the motivation and the reason why the Authors selected them amongst others did not described in the paper. Thus, the others cannot repeat what you have done on their own demands. The language of the paper must be improved, typos were found several times.
The reviewer is against the paper due to the serious flaws as:
1- Lack of statement on the geometry selection
2- Basic numerical modeling and fatigue testing, more advanced aspects should have been addressed to have added value.
3- Application of the proposed geometry is vague.
Other comments:
- Paper should be reviewed carefully in terms of language. Many typos were found!
- The authors made a comprehensive literature review which is quite appreciated, then, I expected to find your statement regarding the main novelty of the your proposed methodology and how it will fill the gap in the state of the art. It is VAGUE!
- It was state that the specimen geometries were defined based on the numerical simulations, however, the Authors did not provide details on that. Imagine others want to repeat your methodology, they do not have enough data how to design the samples. This is very weak point of your work.
- Figure 1, what is the dimension units? Inch?
- The reason why the sample include 4 and 12 rods was not given in the paper.
- Section 3.1: “FEA analysis of geometries” in fact, FEA stands for finite element analysis. So, please correct the title.
- Section 3.1, opening paragraph, last sentence, what was the acceptable level of the stress concentration? Any calculation? Formulations?
- Fatigue test, what was the maximum load applied?
- What about fatigue test in compression? This would be an added value to your work, which I cannot find it in the current version.
- You presented some fractography, good, but they are not stand alone descriptive.
- Validation of the results should have been included in the paper. You proposed a new geometry of the samples, good, so, it is expected to have some comparable results to confirm your proposal.
- Do you believe that this proposed geometry will be applicable and useful in the market? If so, what will be the applications?
- More importantly, do you believe that this experimental outcome will be enough to feed numerical simulations? If so, how will be the contribution?
Best regards,
The Reviewer
Comments on the Quality of English LanguageShould be reviewed
Author Response
Dear reviewer, thank you for your comments and we have considered them in our paper:
Comment 1: The submitted manuscript consists of basic numerical modelling following the finite element fundamentals and conventional fatigue testing. Two geometries were introduced as the benchmark, however, the motivation and the reason why the Authors selected them amongst others did not described in the paper. Thus, the others cannot repeat what you have done on their own demands. The language of the paper must be improved, typos were found several times.
Response 1: We have described motivation additionally with text in manuscript as follows:
Many studies on fatigue properties of AM components focus on standard fatigue specimens, typically featuring circular or rectangular cross sections. This standard geometry maintains a constant ratio between outer surfaces and near-surface defects compared to internal defects. However, additive manufacturing enables far more complex shapes e.g. topologically optimized geometries that might not maintain a similar defect to load bearing area ratio. Consequently, this study aims to evaluate the impact of the ratio between the outer surface and cross-sectional area on the fatigue behavior of additive manufactured components from AISI 316L steel.
and
Diameter of 12-rod specimen geometry was chosen to be at least 2mm in respect to maximum stress capacity. From experience maximum stress is reduced with small diameters of specimen (d < 1mm) therefore maximizing smallest diameter was needed for comparable results. Diameter of 2.01 mm for 12 rod geometry also allowed all specimens to be manufactured at the same time, reducing material properties between specimens to obtain as high as possible ratio of outer surface to cross-sectional are while keeping geometries within limitations of test and manufacturing equipment, 4 and 12 rod geometries were selected. These specific dimensions resulted in distinct ratios of outer surface area to cross-sectional area: 0.44 for the 4-rod design and 0.25 for the 12-rod design, as shown in Table 1 and Figure 1.
The reviewer is against the paper due to the serious flaws as:
Comment 2: Lack of statement on the geometry selection.
Response 2: The purpose of the geometry was to evaluate the influence of different ratios of cross-sectional area to external surface on the durability of the samples. The geometry was chosen because it provides a uniaxial, as uniformly as possible, distributed stress in the tested rods, which allows comparison of different specimen geometries. A significant amount of time was spent determining these geometries through FEM analysis. Particular attention is paid to determining the correct root radius from the base of the specimen to the test rods to ensure minimum stress concentration and to ensure fatigue failure at the center of the specimen rod.
Comment 3: Basic numerical modeling and fatigue testing, more advanced aspects should have been addressed to have added value.
Response 3: The purpose of the numerical modeling was to establish a geometry with a low stress concentration to minimize its influence on fatigue testing. For illustration we have added a finite element mesh of both elements but the design of the pattern is not emphasized in the article as we have only tried to avoid stress concentration in the selected geometry as described in response 1.
Comment 4: Application of the proposed geometry is vague.
Response 4: As mentioned in the text of the manuscript, the main purpose of the geometry used is to keep the same cross-sectional size but with a different number of rods to increase the surface area of the specimen where fatigue cracks can form. It can be seen from the broken surface that both samples have the same geometry with the same fatigue crack propagation at the same stress amplitude level, which confirms the correctness of the choice of sample geometry. Of course, crack growth at a higher stress amplitude level was different from that at a lower stress amplitude level regardless of the difference in specimen geometry.
Other comments:
Comment 5: Paper should be reviewed carefully in terms of language. Many typos were found!
Response 5: Paper is corrected by native English speaker.
Comment 6: The authors made a comprehensive literature review which is quite appreciated, then, I expected to find your statement regarding the main novelty of the your proposed methodology and how it will fill the gap in the state of the art. It is VAGUE!
Response 6: The main innovation is the finding that samples with more rods and thus larger surfaces have a lower durability at the same voltage, which means that, for safety reasons, a smaller diameter must be considered when determining the durability of components with smaller sections of additively manufactured components, which is not the case with conventional components homogeneous materials.
Comment 7: It was state that the specimen geometries were defined based on the numerical simulations, however, the Authors did not provide details on that. Imagine others want to repeat your methodology, they do not have enough data how to design the samples. This is very weak point of your work.
Response 7: A drawing of the geometry used is attached. With numerical simulations, we only checked whether there are differences in the stress value on the middle plane of the sample, where the diameter of the bars is constant. After we confirmed this with numerical simulations, we made the samples. Since the fatigue result is important in the article, we have described the numerical simulations in a way that someone else can repeat them.
Comment 8: Figure 1, what is the dimension units? Inch?
Response 8: We have provided statement: All units are in mm.
Comment 9: The reason why the sample include 4 and 12 rods was not given in the paper.
Response 9: We described in Answer 2 and mentioned in the text and Table 1 that the ratio of the free surface area/surround line was 0.44 for N12 and 0.25 for N4.
Comment 10: Section 3.1: “FEA analysis of geometries” in fact, FEA stands for finite element analysis. So, please correct the title.
Response 10: We have corrected to 2.1 FEA analysis of geometries.
Comment 11: Section 3.1, opening paragraph, last sentence, what was the acceptable level of stress concentration? Any calculation? Formulations?
Response 11: The stress concentration can be determined from the Von Mises stress results provided, which compare the global maximum values to the mid-plane maximum values, resulting in a stress concentration of 1.01 for Specimen N12 and 1.004 for Specimen N4.
Comment 12: Fatigue test, what was the maximum load applied?
Response 12: Additionally, we provided formulas for calculating the maximum tensile load.
Comment 13: What about fatigue test in compression? This would be added value to your work, which I cannot find it in the current version.
Response 13: In our article, we only considered uniaxial tensile loading with a ratio of R=0.1 of two different specimens in a wide stress range from low to high cyclic fatigue. Testing took a long time. This result alone shows significant differences in fatigue behavior, which is worthy of publication. To avoid misalignment due to the AM process, we used a gimbal, which is only possible with a tensile load with positive R!
This is the reason why we did not consider compression because some buckling stress may occur and also fatigue crack initiation may vary from specimen to specimen!
Comment 14: You presented some fractography, good, but they are not stand alone descriptive.
Response 14: Thank you! In these photos and in the manuscript, we have added additional description with arrows and text.
Comment 15: Validation of the results should have been included in the paper. You proposed a new geometry of the samples, good, so, it is expected to have some comparable results to confirm your proposal.
Response 15: In the diagram (Figure 24) we have added a comparison with the AM results of the standard fatigue sample added in the manuscript.
Comment 16: Do you believe that this proposed geometry will be applicable and useful in the market? If so, what will be the applications?
Response 16: The proposed geometry was used to determine the effect of surface area on fatigue life. It is used as a basis for determining the effect of the outer surface on fatigue life. Its practical use is comparable to the use of tensile samples to assess material properties, which means that the selected geometry is the basis for durability assessment. The results show that the fatigue results for N12 can be used for a conservative approach, provided that the part under tension is not less than 2.01 mm in diameter. In the event that no cross-section has a diameter smaller than 3.48 mm, the results of the N4 samples may be used.
Comment 17: More importantly, do you believe that this experimental outcome will be enough to feed numerical simulations? If so, how will be the contribution?
Response 17: The article is part of an ongoing research work on the numerical evaluation of the impact of fatigue crack formation on fatigue life.
Kind regards,
Authors!
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Authors,
You have done a good job. I recommend the paper for publication in Metals.
Reviewer
Comments on the Quality of English LanguageMinor editing of English language required.
Author Response
Dear reviewer,
thank you very much for your recommendation,
Kind regards
Prof. Dr. Nenad Gubeljak
Reviewer 3 Report
Comments and Suggestions for AuthorsDear Authors,
minor remarks:
1.Line 164 „global size of 0.2” – If it is the finite element edge size then please add a unit.
2. Line 273 – typo “.”
3. The Figures 4, 13 and 14 should be given after citations in the main text.
4. Line 295 “extensimeter,” – typo.
5. Lines 353-354 “9,5”, “9,7” and 366-367 “0,3%. The density of the N4 specimen was measured at 7,8433 g/cm3, and density of the N12 specimen at 7,8667 g/cm3.” – decimal separator and “cm3” must be corrected.
6. Lines 494 and 709 “R” – italic.
Kind Regards,
Comments on the Quality of English LanguageLine 273 – typo “.”
Line 295 “extensimeter,” – typo.
Lines 353-354 “9,5”, “9,7” and 366-367 “0,3%. The density of the N4 specimen was measured at 7,8433 g/cm3, and density of the N12 specimen at 7,8667 g/cm3.” – decimal separator and “cm3” must be corrected.
Author Response
Dear reviewer, thank you for your comments and we have considered them in our paper:
Comments 1: Line 164 „global size of 0.2” – If it is the finite element edge size then please add a unit.
Response 1: According to your suggestion, we have added unit in manuscript.
Comment 2: Line 273 – typo “.”
Response 2: According to your suggestion, we have fixed typo in manuscript.
Comment 3: The Figures 4, 13 and 14 should be given after citations in the main text.
Response 3: According to your suggestion, we have moved Figures after citations in the main text.
Comment 4: Line 295 “extensimeter,” – typo.
Response 4: According to your suggestion, we have fixed typo.
Comment 5: Lines 353-354 “9,5”, “9,7” and 366-367 “0,3%. The density of the N4 specimen was measured at 7,8433 g/cm3, and density of the N12 specimen at 7,8667 g/cm3.” – decimal separator and “cm3” must be corrected.
Response 5: According to your suggestion, we have changed decimal separator and fixed units.
Comment 6: Lines 494 and 709 “R” – italic.
Response 6: According to your suggestion, we have changed “R” to italic.
In behalf of the authors
Prof. Dr. Nenad Gublejak
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsNo comments
Author Response
Dear reviwer,
thank you very much for your comments,
Kind regards
Prof. Dr. Nenad Gubeljak
