Numerical and Experimental Characterisation of Polylactic Acid (PLA) Processed by Additive Manufacturing (AM): Bending and Tensile Tests

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper presents a detailed study of the mechanical properties of a 3D modeling product made of polylactic acid. The study includes both experimental and finite element analysis, and the results are recognized for their engineering value.

However, I have identified several issues that need to be addressed before publication in this journal.

Firstly, there are instances where subscripts are not properly formatted.

Secondly, there are duplicate reference numbers.

Thirdly, it is important to note that Poisson's ratio cannot exceed 0.5.

Fourthly, in Figure 20, the subscripts of Poisson's ratio are reversed and need to be corrected.

Finally, The test results should include the frequency and standard deviation of each characteristic value.

Author Response

The authors appreciate the Reviewers’ suggestions and recommendations to improve the manuscript.

This paper presents a detailed study of the mechanical properties of a 3D modeling product made of polylactic acid. The study includes both experimental and finite element analysis, and the results are recognized for their engineering value.

However, I have identified several issues that need to be addressed before publication in this journal.

(a)  Firstly, there are instances where subscripts are not properly formatted.

Response to query (a): The authors performed the necessary corrections (mainly in new sections 3.1. and 4.2.), following the Reviewer's suggestions.

(b) Secondly, there are duplicate reference numbers.

Response to query (b): The authors corrected the identified error. Those sections were renumbered correctly.

(c) Thirdly, it is important to note that Poisson's ratio cannot exceed 0.5.

Response to query (c): By the time the experimental data was processed, part of it was disregarded for the evaluation of the Poisson's ratio. The Reviewer's observation led the authors to rectify this point, leading to a result of 0.48, according to the new version of Figure 20.

Nevertheless, the authors would like to emphasize that depending on the scale on which the analysis is performed, these results can be analysed in the light of the response of the material, or the response of the structure. Consequently, it is worthwhile noting that the authors view these measurements in the light of the structure's response. Therefore, the result shown in Figure 20 for the orientation at 90° was determined by calculating the ratio of the strain in the direction of the filament (transverse extensional strain) to the strain in the direction normal to the filament (axial extensional strain). In structures obtained by AM with 90° orientation, which exhibit higher values of Poisson's ratio, the conclusion to be drawn is that they are more likely to suffer small increases in stiffness in the direction normal to the filament (direction of loading, i.e. x-direction; Fig. 2(b)) than those oriented at 0°. This conclusion may have to do with the larger number of interfaces between layers in the loading direction for structures at 90 degrees being significantly higher than those for structures with the filament aligned with the loading direction (0 degrees).

 

(d) Fourthly, in Figure 20, the subscripts of Poisson's ratio are reversed and need to be corrected.

Response to query (d): The authors verified that there was indeed an error in the equation used to calculate the Poisson ratio. The correction was performed in the new version of the manuscript (marked in green in section 4.2.).

 

(e) Finally, the test results should include the frequency and standard deviation of each characteristic value.

Response to query (e): Figures 13 (a)-(d) show two load-displacement curves per condition (0°/90°, 45/-45°, 0° and 90°). Given that an excellent reproducibility of the load-displacement curves was attained in the experimental work, the authors didn’t consider testing more specimens for the referred material configurations. As referred in section 4.2, the printing conditions were considered steady for the material production, demonstrating a meticulous preparation of the test specimens. It was also concluded that the filament orientations demonstrate a clear difference among material samples.

For the above referred reasons, the authors have concluded that it is not adequate to include the frequency and the standard deviation of each characteristic value in Table 5.

Reviewer 2 Report

Comments and Suggestions for Authors

1) Abstract Improvement: The abstract should be enhanced to encapsulate the numerical outcomes obtained from the study. Including specific numerical results will provide a more comprehensive overview of the findings and their significance.

2) Introduction Novelty Inclusion: In the introduction section, ensure the incorporation of the novelty of the research work. Clearly articulate the unique contributions and advancements brought about by the study in the field of additive manufacturing and the characterization of Polylactic Acid (PLA).

3) Incorporate Relative Density and Porosity Results: Integrate the relative density and porosity percentage results into the manuscript. This additional information will contribute to a more thorough understanding of the material properties and the implications for additive manufacturing processes.

4) Dynamic Mechanical Analysis (DMA) Results Inclusion:  Add the outcomes of Dynamic Mechanical Analysis (DMA) to substantiate the assertion that the specimens' tests confirm the additive manufacturing process's generation of anisotropic behavior in the fabricated structural elements. This will strengthen the analysis of structural behavior under dynamic loading conditions.

5) Numerical Results in Conclusions: Ensure that the conclusions section incorporates the numerical results obtained throughout the study. This will reinforce the key findings and their implications, providing a solid basis for the overall conclusions drawn from the research. Additionally, craft the conclusion section effectively to summarize the essential outcomes and their broader significance.

By addressing these points, the manuscript will be enriched with more comprehensive and impactful information, enhancing its overall quality and contribution to the field.

Comments on the Quality of English Language

The article requires minor English chanages.

Author Response

The authors appreciate the Reviewers’ suggestions and recommendations to improve the manuscript.

1) Abstract Improvement: The abstract should be enhanced to encapsulate the numerical outcomes obtained from the study. Including specific numerical results will provide a more comprehensive overview of the findings and their significance.

Response to query (1): The authors include the reviewer's suggestion: “The obtained numerical agreement indicated that an inverse method based only on the load-displacement curve can yield an accurate value for this material's elastic modulus.”

2) Introduction Novelty Inclusion: In the introduction section, ensure the incorporation of the novelty of the research work. Clearly articulate the unique contributions and advancements brought about by the study in the field of additive manufacturing and the characterization of Polylactic Acid (PLA).

Response to query (2): The authors follow the suggestions in the Introduction section: “Notably, this study goes beyond conventional approaches employed in previous studies by incorporating a combination of numerical modeling and experimental data, integrating non-contact full-field experimental techniques to evaluate the mechanical performance of 3D printed materials. By doing so, aiming to bridge the gap between theoretical predictions and practical results, thereby improving the understanding of the mechanical behavior of 3D printed components. Furthermore, this work introduces a unique focus on filament orientation and fill percentage, two crucial factors that influence the overall stiffness of 3D printed structures.”

 

3) Incorporate Relative Density and Porosity Results: Integrate the relative density and porosity percentage results into the manuscript. This additional information will contribute to a more thorough understanding of the material properties and the implications for additive manufacturing processes.

4) Dynamic Mechanical Analysis (DMA) Results Inclusion:  Add the outcomes of Dynamic Mechanical Analysis (DMA) to substantiate the assertion that the specimens' tests confirm the additive manufacturing process's generation of anisotropic behavior in the fabricated structural elements. This will strengthen the analysis of structural behavior under dynamic loading conditions.

Response to queries (3) and (4): This is indeed a very relevant point that is worth doing in the characterization of additive manufacturing materials. The authors totally agree with the Reviewer's observation. However, the samples used in the mechanical tests were not preserved subsequently to the performed mechanical analysis. This makes the accomplishment of the proposed evaluations/measurements unfeasible.

Finally, the authors would like to emphasize that the proposed analyses are currently being performed in a new study, regarding fracture of PLA structures (lattice) both under mode I and mode II loading.

5) Numerical Results in Conclusions: Ensure that the conclusions section incorporates the numerical results obtained throughout the study. This will reinforce the key findings and their implications, providing a solid basis for the overall conclusions drawn from the research. Additionally, craft the conclusion section effectively to summarize the essential outcomes and their broader significance.

Response to query (5): The authors followed the Reviewer's suggestion adding the following sentences “In addition, the elastic modulus was shown to increase with the infill percentage regardless of filament orientation. A clear linear response (P-δ curve) was obtained by properly reproducing the experimental elastic stiffness. By achieving a strict numerical agreement on the P-δ curves that emerged from the experimental testing, the inverse approach provided for the numerical determination of the material's modulus of elasticity.”

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper has been appropriately corrected in the areas noted and is worthy of publication in this journal at this time.

Reviewer 2 Report

Comments and Suggestions for Authors

The article is accepted without any further revisions.