Short Fiber-Reinforced Polymer Polyamide 6 Lugs and Selective Laser-Melted Ti-6Al-4V Bushing Contact Cohesive Zone Model Mode II Parameters’ Evaluation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors reported their interesting experiments and simulations on the effects of SML titanium alloy bushings with different surface roughness on the tensile bearing capacity of SFRP lugs. My comments are as follows,

1. The title should be shorter and focused.

2. The first sentence of the abstract has two “with”. It would be better to replace a long sentence with multiple short sentences.

3. In a uniaxial tensile test of lug with bushing, is the bushing-lug interface dominated by mode II cracks?

4. The formula should be numbered. The symbols in the main text and formulas should be completely consistent.

5. There is a significant difference in the deformation characteristics of the bushing-lug interfaces between the push out tests of bushing and the uniaxial tensile test of lug. Can a CZM model based on the former be used to predict the latter?

6. In Table 2, Fig. 13, Fig. 14, Fig. 17, Fig. 18, Table 4, lake of date for bushing with ribs.

7. The content of section 3.2 is weakly linked to the topic of the paper and is recommended to be deleted.

8. In Table 3, why the uniaxial tensile bearing capacity of the lug of the vibratory finishing bushing is lower than that of the lug without bushing?

9. The difference in the failure characteristics of the specimens is not clearly evident from Fig. 18.

Comments on the Quality of English Language

See comments for authors.

Author Response

The authors reported their interesting experiments and simulations on the effects of SML titanium alloy bushings with different surface roughness on the tensile bearing capacity of SFRP lugs. My comments are as follows,

1. The title should be shorter and focused.

We’ve changed the title to “SFRP PA6 lugs and SLM Ti-6Al-4V bushing contact CZM Mode II parameters evaluation” to make it shorter and focus on studied materials, their contact model and the lug and bushing objects, covered in the manuscript. We apologize for the use of abbreviations in the title; however, they are generally accepted, reflect the main points of the article, such as the materials investigated in the manuscript, significantly shorten the title and make it easier to understand, which is especially important when a reader is browsing through a series of titles and searching for an article in lists.

2. The first sentence of the abstract has two “with”. It would be better to replace a long sentence with multiple short sentences.

We thank you for the recommendation. We have completely rewritten the abstract on the advice of the reviewers.

3. In a uniaxial tensile test of lug with bushing, is the bushing-lug interface dominated by mode II cracks?

The Mode II refers in the considered case to the contact between the bushing and the plastic part. The importance of this mode is confirmed by the fact that the presence of ribs on the surface of the bushing, which increase the transmission of the tangential part of the contact stress, increases the bearing capacity of the lugs in the experiment. After failure of the contact between the bushing, the load is redistributed around the perimeter of the lug hole and failure occurs by stretching of the lug bridge at the side (in the area of maximum stress) or at the front (where the weld line is located), but this can no longer be attributed to the CZM model mods.

4. The formula should be numbered. The symbols in the main text and formulas should be completely consistent.

Thanks for the recommendation. We’ve correct it.

5. There is a significant difference in the deformation characteristics of the bushing-lug interfaces between the push out tests of bushing and the uniaxial tensile test of lug. Can a CZM model based on the former be used to predict the latter?

We have addressed this issue in detail and compared the approach we used with standard ASTM samples for layered composite materials:

The parameters of adhesive interaction in mode II, which determines the fracture caused by shear, corresponds the case of bushing extrusion along the lug axis. The characteristics of mode II fracture along the tangential direction correspond to shear loading, which is manifested when the bushing is extruded along the axis of the lug. This approach has the advantages compared to using specimens according to standard test method [66]: samples for identification of adhesion parameters are in conditions closer to the lug under study than standard [66] samples, proposed samples are much smaller in comparison with [66] samples which is especially important in SLM Ti-6Al-4V production case, testing samples can made in an existing mold.

Technological difficulties of manufacturing large samples with the use of SLM technology as well as the issues of the PA6 shrinkage make our approach a good choice for rapid analysis of adhesive model parameters for such common products as lugs with bushings.

6. In Table 2, Fig. 13, Fig. 14, Fig. 17, Fig. 18, Table 4, lake of date for bushing with ribs.

As for roughness measurements (Table 2, Fig. 13, Fig. 14) - the ribbed bushing has a raw material with a SLM roughness. As for numerical study of load carrying capacity (Fig. 17, Table 4) - we added a ribbed bushing.

7. The content of section 3.2 is weakly linked to the topic of the paper and is recommended to be deleted.

The section on injection molding research (now 3.3) is necessary to explain the location of the weld line and the orientation of the reinforcing fibers needed to calculate the stiffness and strength of the lugs in the anisotropic formulation. We have added the weld line calculation and the orientation tensor field of the reinforcing fibers to show more explicitly this relationship. We have also added a more detailed description of the problem statement and its solution. We hope that in this form this section can be published as part of the present manuscript.

8. In Table 3, why the uniaxial tensile bearing capacity of the lug of the vibratory finishing bushing is lower than that of the lug without bushing?

We have added a description of this point to the manuscript:

An lug without bushings has the same external contours and axis diameter. Therefore, the elimination of the 1 mm thick bushing results in an increase of the bridging width from 5 mm to 6 mm, that leads to a higher load-bearing capacity than that of lugs with vibratory finishing bushings with low adhesion.

9. The difference in the failure characteristics of the specimens is not clearly evident from Fig. 18.

We have completely updated the calculation of the strength criteria and the analysis of the results by supplementing the examples with S and M type lugs with bushings with different roughnesses. We hope it has become more clear in this way.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper investigates the load capacity of short fiber reinforced polymer lugs with Ti-6Al-4V selective laser melting (SLM) bushings. The research content is good, but it is difficult to read and understand. The following points need to be improved.

(1) Abbreviations such as SLM, PA6 (polyamide 6) should be clearly explained when first used.

(2) Figure 2：A clear explanation is required regarding the underlined part of “the contact interaction strength between the bushing and the lug material“.

(3) Figure 9: The analysis results for stress and strain in the figure are difficult to see.

(4) Line 302: Please correct “3.1.” to “3.2.”

　 Line 323: Please correct “3.2.” to “3.3.”

　 Line 335: Please correct “3.2.” to “3.4.”

   Line 344: Please correct “3.3.” to “3.5.”

(5) Please add the calculation method regarding stress and ”G” described in Figure 14.

(6) The explanation regarding Figure 15 is insufficient.

Author Response

This paper investigates the load capacity of short fiber reinforced polymer lugs with Ti-6Al-4V selective laser melting (SLM) bushings. The research content is good, but it is difficult to read and understand. The following points need to be improved.

(1) Abbreviations such as SLM, PA6 (polyamide 6) should be clearly explained when first used.

Thank you for the recommendation. We have explained abbreviations. We apologize for the use of abbreviations in the title; however, they are generally accepted, reflect the main points of the article, such as the materials investigated in the manuscript, significantly shorten the title and make it easier to understand, which is especially important when a reader is browsing through a series of titles and searching for an article in lists.

(2) Figure 2：A clear explanation is required regarding the underlined part of “the contact interaction strength between the bushing and the lug material“.

A strength limit of stresses was assumed for the second mode CZM model, but these are estimates for the current lug geometry and we have reformulated this location to exclude the issue of estimating contact strength.

(3) Figure 9: The analysis results for stress and strain in the figure are difficult to see.

Thank you for the recommendation. We’ve correct it.

(4) Line 302: Please correct “3.1.” to “3.2.”,  Line 323: Please correct “3.2.” to “3.3.”,  Line 335: Please correct “3.2.” to “3.4.”,    Line 344: Please correct “3.3.” to “3.5.”

Thank you for the recommendation. We’ve correct it.

(5) Please add the calculation method regarding stress and ”G” described in Figure 14.

We have added a full description of the calculation used to fit the model parameters in Figure 17 (old Fig. 14), including mesh, boundary conditions (Fig 15), hardware and convergence plot (Fig 16): “The parameters of the adhesion interaction model for mode II were fitted on the basis of studies on the extrusion of bushings along the axis. To achieve this, the finite element problem of extruding the bushing from the lug along its axis is solved.  … “

(6) The explanation regarding Figure 15 is insufficient.

We have added a complete description of the injection-molding model, including problem formulation, computational mesh (Figure 18), weld line calculation, and tensor of fiber orientation (Figure 19).

Reviewer 3 Report

Comments and Suggestions for Authors

The following comments are to be considered while preparing the revised manuscript:

Abstract is qualitative and sounds like a mere summary. It is suggested to add some important findings from the present work

Also, delete some generic statements like "Lugs are an important part of pin-joints" from the abstract

Introduction last paragraph: Did the authors use the text directly from a thesis. It is not clear why chapters are specified in the journal paper? No such summary of sections are required.

As a replacement for the last paragraph of the introduction section, it is suggested to add the novelty of the proposed work. How does this work fill in the existing knowledge gap?

Figure 1: Why only one sample results are presented? It is suggested to add three graphs and make an average comparison along with the values of standard deviation.

Please clarify the source for the data presented in Table 1.

Figures 6 to 8 can be merged into a single graph.

Figure 14: As specified earlier, it is suggested to present the average results for comparison

Author Response

The following comments are to be considered while preparing the revised manuscript:

Abstract is qualitative and sounds like a mere summary. It is suggested to add some important findings from the present work.

Thanks for the recommendation. We have completely rewritten the abstract, adding specific values of the found quantities.

Also, delete some generic statements like "Lugs are an important part of pin-joints" from the abstract

Thank you. We've been corrected.

Introduction last paragraph: Did the authors use the text directly from a thesis. It is not clear why chapters are specified in the journal paper? No such summary of sections are required.

We have removed this paragraph, taking into account the reviewer's recommendation.

As a replacement for the last paragraph of the introduction section, it is suggested to add the novelty of the proposed work. How does this work fill in the existing knowledge gap?

Thank you for the recommendation, we've added some sentences about novelty: “The novelty of the work is the use of axial bushing extrusion to determine the parameters of adhesive interaction between the plastic part and the embedded element. This makes it possible to significantly simplify the estimation of contact parameters in cases where the fabrication of large crack opening specimens is difficult due to process limitations, such as shrinkage or limitation of SLM capabilities.”

Figure 1: Why only one sample results are presented? It is suggested to add three graphs and make an average comparison along with the values of standard deviation.

We have updated the figure 1 by adding individual test results and choosing to display the three standard deviation instead of one.

Please clarify the source for the data presented in Table 1.

The technological parameters of the SLM were selected experimentally.

Figures 6 to 8 can be merged into a single graph.

We thank you for the recommendation. It still seems to us that these figures address different aspects of the production and testing process. We would ask that they be left in their original form as separate figures.

Figure 14: As specified earlier, it is suggested to present the average results for comparison

We added a comparison on this figure (now figure 16) and also did an analysis of this comparison in the text.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accept in present form

Reviewer 3 Report

Comments and Suggestions for Authors

Comments are addressed. Paper can be accepted for publication.