Evaluation of Crashworthiness Using High-Speed Imaging, 3D Digital Image Correlation, and Finite Element Analysis
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis paper presents experimental and numerical studies of a 3rd generation steel crash box, representative of many automotive crash structures, subjected to crushing. Comparisons between the experiments and simulations are given along with detailed 3D DIC of the experiments. The paper is well-written and can be accepted after the following minor changes are considered:
1) The authors should present one or two focused images to present examples of how well (or poorly) the numerical model replicates the fracture patterns in the 3rd generation steel, perhaps within Fig. 17/18 or with an additional figure.
2) The annotations added to Fig. 16 (i.e., the x’s and dashed lines) are too large for the reader to judge the mentioned distinctions for themselves. A more muted presentation is needed.
3) Remove the grid lines (at least the minor ones) from all figures and only leave the tick marks, similar to the current form of Fig. 7.
4) The overall energy absorption is generally predicted well by the FEA but there are significant deviations between the predicted peaks/valleys in the force response, especially beyond 60 mm of displacement. The authors should discuss some sources of error in the main text.
Author Response
Thank you for taking time to review and help improve our manuscript. Please see the responses below and refer to the attached pdf for modified manuscript. Once again, thank you.
Kind regards
The authors
Response to Reviewer 1 (modifications marked with red in attachment)
[Comment 1]: The authors should present one or two focused images to present examples of how well (or poorly) the numerical model replicates the fracture patterns in the 3rd generation steel, perhaps within Fig. 17/18 or with an additional figure.
[Author response]: This is a good idea and was certainly missing previously. We have included the requested images but under the FEM section instead together with the simulated CIDR response to fit the structure of the paper.
[Comment 2]: The annotations added to Fig. 16 (i.e., the x’s and dashed lines) are too large for the reader to judge the mentioned distinctions for themselves. A more muted presentation is needed.
[Author response]: The image is more for descriptive purposes than quantitative measurement (thus only the TBF 1180 material is presented). It shows that the method works for identifying cracks and when these cracks appear. It is thus left unmodified.
[Comment 3]: Remove the grid lines (at least the minor ones) from all figures and only leave the tick marks, similar to the current form of Fig. 7.
[Author response]: The authors think the data easier to interpret and compare with grid lines.
[Comment 4]: The overall energy absorption is generally predicted well by the FEA but there are significant deviations between the predicted peaks/valleys in the force response, especially beyond 60 mm of displacement. The authors should discuss some sources of error in the main text.
[Author response]: Good suggestion. The main reason is likely due to the difficulty in modelling the exact boundary conditions from the experimental setup due to the pre-crushed configuration. The model uses a simplified contact-based spotweld formulation which might not necessarily describe reality that well. Fractured spotwelds change the load state significantly and to mimic the entire deformation process is very difficult, especially for pre-deformed crash boxes. The reviewer is however correct that this should be elaborated on in the manuscript. Added a section under “General discussion”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe author presented a well detailed and interesting work. Its is recommended for publication. I have only one question.
1. Why the curves at 1 and 10 showing low EF compared to strain rate 0.001/s PHS 1000.
Author Response
Thank you for taking time to review and help improve our manuscript. Please see the responses below and refer to the attached pdf for modified manuscript. Once again, thank you.
Kind regards
The authors
Response to Reviewer 2
[General comment]: The author presented a well detailed and interesting work. It is recommended for publication.
[Author response]: Thank you so much for your kind words and thank you very much for taking time to review our manuscript.
[Comment 1]: Why the curves at 1 and 10 showing low EF compared to strain rate 0.001/s PHS 1000.
[Author response]: This is an interesting question. It is clear that fracture location in the PHS 1000 material varied quite a lot, while it was more consistent for the TBF 1180 material. The reason for these results are not known and was not further investigated in this work. No strong conclusion can be made. Further investigation is needed.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThis manuscript is devoted to evaluating the strain-rate sensitivity on the hardening response and the fracture behaviour at various stress triaxialities for two modern grades of steel used for cold and hot stamping applications, respectively. The authors performed many experimental tests including a high-speed axial compression test using 3D-DIC to follow the crushing response, crack formation and subsequent propagation. The crash index decreasing rate method was used to evaluate the crashworthiness of the analyzed steel grades. The results obtained from axial compression tests of the crash boxes were used to validate the calibrated material and damage models by comparing a commercial FE analysis using LS-DYNA software.
The manuscript is well and logically written and I think it provides very valuable research. Although I do not have any negative comments about the quality of the manuscript, it needs some editorial corrections:
1) The abstract provided by the authors is too long (349 words). It should be a total of about 200 words maximum.
2) Line 161 - the authors mention that Vision Research Phantom v2512 was used to determine the strain fields on the analyzed specimens when analyzing different stress states. However, Tables A1 and A2, respectively, provide information that Vision Research Phantom v1610/v2512 were used.
3) As the manuscript is relatively long, I suggest the authors remove some unnecessary figures, e.g. Fig. 3 is just an example without any quantitative information, only one of the figures can be used instead of Fig. 6 and Fig. 7, etc.
Author Response
Thank you for taking time to review and help improve our manuscript. Please see the responses below and refer to the attached pdf for modified manuscript. Once again, thank you.
Kind regards
The authors
Response to Reviewer 3 (modifications marked with blue in attached revision)
[General comment]: The manuscript is well and logically written and I think it provides very valuable research.
[Author response]: Thank you so much for the kind words and thank you so much for taking time to review and improve on our manuscript.
[Comment 1]: The abstract provided by the authors is too long (349 words). It should be a total of about 200 words maximum.
[Author response]: Good suggestion. Shortened it down to 260 words instead. Below this count, the authors do not think it would be descriptive enough.
[Comment 2]: Line 161 - the authors mention that Vision Research Phantom v2512 was used to determine the strain fields on the analyzed specimens when analyzing different stress states. However, Tables A1 and A2, respectively, provide information that Vision Research Phantom v1610/v2512 were used.
[Author response]: v1610 and v2512 were both used depending on availability in the lab. Similar settings were used for both though, and the results should be indistinguishable between the two for the resolutions and sampling rates used. Removed the specific model from text and appendix and just included vendor and sensor.
[Comment 3]: As the manuscript is relatively long, I suggest the authors remove some unnecessary figures, e.g. Fig. 3 is just an example without any quantitative information, only one of the figures can be used instead of Fig. 6 and Fig. 7, etc.
[Author response]: Valid point. Fig. 6 and Fig.7 combined into one figure instead. Although Fig. 3 does not contribute with any quantitative information, it does however contribute with qualitative information such as strain field distribution in the different specimens. Decided to keep it but added more information about its purpose. It was also not referenced in text which as also been corrected.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThis manuscript is clearly written and the results are well presented. However, the following modification could be improved the manuscript. The authors are kindly requested to follow these remarks:
Authors should add more related literature about DIC in introduction , gap of the research and also novelty was not clearly mentioned
Material supplier company name should be mentioned in methodology part
Line 140 (also line 151) authors mentioned 100 1/s strain is high that is not high please remove medium and high strain rate from this sentence, also please mention why they choose this range of strain rate. in real car crash it is possible to reach strain rate of 1000 1/s.
Authors should add more details about 3 DIC analysis. How did they coat sparkles on sample surface, size of sparkles. Also about analysis parameters such subsets size and step size
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Author Response
Thank you for taking time to review and help improve our manuscript. Please see the responses below and refer to the attached pdf for modified manuscript. Once again, thank you.
Kind regards
The authors
Response to Reviewer 4 (modifications marked with brown in attached revision)
[General comment]: The manuscript is well and logically written and I think it provides very valuable research.
[Author response]: Thank you so much for the kind words and thank you so much for taking time to review and improve on our manuscript.
[Comment 1]: Authors should add more related literature about DIC in introduction, gap of the research and also novelty was not clearly mentioned.
[Author response]: The authors use 3D DIC solely as a tool to evaluate crashworthiness, and a commercial software is used. This method is well established, and the goal for the authors is not to develop the method any further or contribute any novelty into the field of digital image correlation. Thus, the references to iDIC that covers the established research in the area should be enough. Added references about speckle size, however, since this is important.
[Comment 2]: Material supplier company name should be mentioned in methodology part.
[Author response]: Good point. Added to materials and acknowledgement.
[Comment 3]: Line 140 (also line 151) authors mentioned 100 1/s strain is high that is not high please remove medium and high strain rate from this sentence, also please mention why they choose this range of strain rate. in real car crash it is possible to reach strain rate of 1000 1/s.
[Author response]: Corrected. Maximum strain rate limited by machine and dynamic effects and thus these strain rates were chosen. It is true that the strain rates in a real crash can be significantly higher, but calibrating for these strain rates demands other experimental setups such as a Hopkinson bar designed for tensile pull. This could be interesting for future research.
[Comment 4]: Authors should add more details about 3D DIC analysis. How did they coat sparkles on sample surface, size of sparkles. Also about analysis parameters such subsets size and step size
[Author response]: Added images with speckle size from correlation into appendix. The other analysis parameters arealready available there.
Author Response File: Author Response.pdf
