Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigates the effect of water aging on the static bending behavior and damage mechanisms of 3D-printed auxetic core sandwich structures using acoustic emission (AE) monitoring. The experimental work is comprehensive. Below are my comments to help improve the manuscript.

 

(1) In the Introduction, the scientific merit and novelty of the paper are not clear. The authors should explain clearly in the Introduction what is the novelty of this manuscript and what is the added value in this paper?

(2) The motivation of the work should be clearly clarified. It could be added in the abstract, introduction and conclusion sections.

(3) The threshold in testing was set to be 38dB. Is there any theoretical basis? Or from experiment?

(4) The AE setup parameters (PDT=50ms, HDT=100ms, HLT=200ms) are provided, but their justification is lacking. Were these values determined from preliminary tests or a pencil-lead break calibration?

(5) The manuscript states that the water absorption behavior corresponds to the Fickian model. For the S1C configuration in Fig. 4, the experimental data shows a deviation from the Fickian curve at the beginning. An explanation for this non-Fickian "sudden rise" should be provided.

(6) Where exactly were the two AE sensors placed on the sandwich specimen during the three-point bending test? How was the manual leakage level controlled in the context of this study?

(7) The author cites a lot of literature describing 3D-printed materials and methods for enhancing specific properties of composites. I don't think this is necessary. The focus of this research is on acoustic emission. The authors state that no studies have been conducted on the effect of humidity on this type of material. I think the study of AE parameters and water aging effects on other materials is beneficial as well. For example, real-time damage assessment of hydrous sandstone based on synergism of AE-CT techniques, underground sedimentary rock moisture permeation damage assessment based on AE mutual information.

(8) The data analysis method used in this paper is simple, mainly based on time domain parameter analysis of the k-means algorithm. It is recommended to add algorithm comparison to eliminate the errors caused by the algorithm.

(9) I think the scientific contribution of this paper should not only lie in the experiment. The potential of the subsequent AE applications needs to be further supplemented, including the potential application scenarios, common methods, limitations, etc.

(10) Is the punctuation at the end of line 118, 190, 224 wrong?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The acoustic emission (AE) method is widely used as an effective non-destructive technique for detecting damage in various engineering materials at an early stage. A bio-based sandwich with an auxetic core could be found in a variety of applications, such as automobile, aeronautic, and aerospace. Such materials have a complex structure, and detecting the early stages of their damage is too difficult. The authors have interesting research that deserves attention.

The manuscript may be published after some corrections.

 

The manuscript has the following comments:

1. The design of the literature needs correcting. See rules for authors: Sensors | Instructions for Authors
2. The authors indicate that the static tests were conducted according to ASTM C393 (line 140), which recommends using at least 5 samples in experiments. The authors write that 3 samples were tested (line 143). What is the level of confidence in the authors’ estimations under the given conditions?
3. In the article, the authors did not indicate which transducer was used to detect AE and its technical characteristics.
4. There are several studies by authors of this type of composite. For example, in the work Essassi K., et al. Investigation and identification of damage mechanisms of sandwich with auxetic core using acoustic emission. In book: Design and Modeling of Mechanical Systems - V, Proceedings of the 9th Conference on Design and Modeling of Mechanical Systems, CMSM'2021, December 20-22, 2021, Hammamet, Tunisia (pp.580-587). DOI:10.1007/978-3-031-14615-2_65, the authors using the acoustic emission technique under indentation test classified three different classes of AE events: core cracking, matrix cracking, and the fiber/matrix debonding.

In [25], the authors investigated the static behavior and failure mechanisms of such a composite using acoustic emission (without water immersion). Based on AE analysis and microphotography, three failure mechanisms were identified: core-skin debonding, skin cracking, and core cracking. In this study, based on the classification of AE signals, two mechanisms of composite failure were identified: cracking of the matrix and fiber/matrix debonding. It would be advisable for the authors to conduct a comparative analysis of the results of various studies.

 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Paper explores the effect of water absorption on mechanical properties and damage evolution in a bio-composite sandwich panel. Through well documented experimentation, it showed mechanical properties deterioration over time with water absorption following the Finks law. This is as expected. However, more revealing is the evolution of damage mechanism in the bio-composite under 3-point bending text. Using cumulative AE metrics and principal component analysis, it unpicks two dominate modes. Results and conclusion are relevant to both researchers in the field.

typographical comment
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lines 190, 224, 281: Typo Error! Bookmark not defined.
lines 191, 204: use lowercase "where"
lines 368-472: check the formatting of the references. Prefixed superscript number [1] needs to be deleted. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Thank you for submitting your manuscript “Acoustic emission analysis of moisture damage mechanisms in 3D printed auxetic core sandwiches” to Sensors. Your study presents a well-structured and comprehensive approach to understanding water-aging effects in bio-based auxetic sandwich composites, combining acoustic emission (AE) analysis with mechanical testing. The integration of AE data classification using k-means and PCA visualization is particularly commendable, as it provides clear insight into the damage evolution in these innovative 3D-printed bio-composites.

I have a few questions and comments that could help further strengthen your work:

1. You mention two damage mechanisms (matrix cracking and fiber/matrix debonding) based on amplitude ranges. Could you elaborate on how you validated these classifications? For example, was there any correlation with microscopic or post-failure surface observations to confirm AE-based interpretations? can you add this to the paper.

2. The absorption data are modeled using Fick’s law, which fits the experimental results well. However, since bio-based composites often show non-Fickian diffusion behavior at longer immersion times, have you considered fitting alternative models (e.g., dual-stage diffusion) to verify the robustness of your assumption?

3. You report a ~15% loss in flexural and shear stress at saturation. Did you observe any corresponding changes in failure mode or AE signal density that might indicate a transition from fiber-dominated to matrix-dominated failure with aging? maybe do a quantitative study

4. The use of unsupervised k-means clustering with the Davies–Bouldin index is a strong methodological choice. Could you discuss how sensitive your classification results were to the chosen input parameters (e.g., number of clusters, normalization method)? Would a supervised approach—trained on known damage events—potentially improve the resolution of your AE damage categorization? have you studied other models too.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thanks to the author for their hard work and responses to the comments. Most of the comments have been addressed satisfactorily. However, a few remain unresolved. Please read and respond carefully to comments 1, 7, and 9. The paper can be accepted after these issues are addressed.

Author Response

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Author Response File: Author Response.pdf