Acoustic Emission Characterization Analysis of Quasi-Static and Fatigue Compression Properties of Aluminum Foam

Round 1

Reviewer 1 Report

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Author Response

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Reviewer 2 Report

The present work entitled “Acoustic emission characterization analysis of quasi-static and fatigue compression properties of aluminium foam”, deals with the study of physical and mechanical properties and acoustic emission (AE) characteristics of aluminum foam under fatigue and quasi-static compression.

The topic meets the wider scope of the journal.

The structure of the manuscript is adequate. The keywords are appropriate and compatible to the topic of the manuscript.

In general, the work can be considered original, given that certain interesting findings are presented.

I adduce some comments, additions and recommendations that must be taken into consideration in order to improve the quality of the submitted manuscript.

 1. The last paragraph of the introduction must be further enriched in order to emphasize on the originality and the contribution of the work on the scientific topic.

2. More information must be provided on the sensor probe characteristics (like the resonant frequency). Furthermore, the authors must report the frequency band they filtered the AE signatures.

3. Figure 5 must be further enriched with the amplitudes of the AE signals by adding three corresponding figures (d, e, f) and provide the corresponding discussion on them with respect to the compression rate. It would be also interesting to study the AE rate.

4. Provided the fact that the manuscript is not submitted to a Journal focusing solely on mechanics’ technologies, the term AE energy count [line 185] must be briefly described or at least provide a relevant reference.

In the above frame I would suggest to add a figure in section 3.3 presenting the characteristics of single AE wave demonstrating for example the threshold, the peak amplitude, the duration, the ring down count, the energy etc.

5. Authors must examine the correctness of Eq. 1 [line 206]. See: ASTM, E. "Standard terminology for nondestructive examinations." West Conshohocken, Pennsylvania, USA: ASTM International (2006).

The described quantity corresponds to the absolute energy or the MARSE energy?

In the case of absolute energy the unit should be in aJ. Reference [45] is not adequate since it is in Chinese.

6. I would suggest for the Figure 15 the AE cumulative ringing/energy counts axis  (vertical) scale to be changed in logarithmic (if this provides further hidden details on low AE levels).

7. Optionally, in line 98 where the Authors present the AE technique in the study of deformation damage mechanisms in rocks, the following references can be added:

a) Mechanical and Acoustic Emission (AE) Characteristics of Rocks under Biaxial Confinements. Appl. Sci. 2021, 11, 769.

b) Notched marble plates under tension: Detecting pre-failure indicators and predicting entrance to the “critical stage”, Fatigue & Fracture of Engineering Materials & Structures, 41, 776-786 (2018).

Conclusively, this manuscript will be in publishable condition after the authors’ major revision according to my revision comments.

Author Response

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Reviewer 3 Report

The paper proposes an interesting approach to correlate the compressive fatigue behaviour of aluminium foams to the measurements of their acoustic emissions.

The subject is well-presented and described but some aspects need still to be clarified. The revised version of the manuscript should solve the following issues:

·         Figure 1 – To avoid any confusion, considering that the specimen is tested with compression loads, it should be better to draw the curve in the negative semi-plane or to express the legend of the y-axis with the absolute value of F.

·         Figure 2 – Increase the size of the aluminium foam specimen.

·         Line 122 – Report the value of the base material yield strength.

·         Line 148 – Given that there was no evident yield point between the elastic stage and the yield stage, how was the yielding detected? Was the conventional definition of 0.2% of permanent deformation used or was some other criterium utilized?

·         Equations (2) and (4) – Both equations define the energy absorption efficiency with different symbologies for this quantity. Moreover, the true stress appears for the first time in equation (2) but its definition is at Line 217. Reconsider this part of the paragraph in order to improve clarity.

·         Line 221 – The reported decrement of energy absorption is actually present only in the curve with the rate of 2 mm/min, the other two curves show a continuous increase until the maximum value; this difference should be specified. Also, it is also useful to conclude that the lower the deformation rate, the higher the energy absorption efficiency.

·         Figures 7 and 8 – Vertical lines in correspondence with the true stress values that delimit the four stages could be added to the figures to better identify the different regions.

·         Figure 15 – The change in the trend of accumulated energy passing from k=1.0 to k=1.1 is very relevant, how can this aspect be justified?

·         Conclusions – This paragraph should also give a brief outlook on the practical implications of the correlations between the AE and the fatigue behaviour that have been presented in the study.

 

·         Scientific literature about the compressive behaviour of aluminium foams properties is wide and other papers should be added to the references list, such as: https://doi.org/10.1016/j.compositesa.2020.105923; https://doi.org/10.1016/j.euromechsol.2021.104291; https://doi.org/10.1016/j.commatsci.2013.08.021

Author Response

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Round 2

Reviewer 1 Report

Review of  “Acoustic emission characterization analysis of quasi-static and fatigue compression properties of aluminium foam” by Qiong Song, Jian Shi *, Xu Chen

The quality of the paper has improved a lot, now it contains some explanation. There is one main concern: the usage of true stress and true strain since in the case of foams, the VOLUME IS NOT CONSERVED during plastic deformation. Therefore, most of the evaluation is not correct and eq. 3 cannot be used since it supposes that the volume is conserved during the deformation.

 

See:

[Comment#19] How was the true stress and the true strain calculated? Were the changes in the cross-sectional area measured? If yes, were two extensometers used for that? Why did the authors use true curves instead of engineering curves? (Usually, the engineering stress and strain are evaluated since the volume changes during plastic deformation, and the deformation is mostly localized; thus, it is hard to tell where to measure the changes in the cross-sectional area.)

[Response#19] The authors thank the reviewer for your carefully reading and suggestion. We collected the compression displacement data by compression testing machine or AE instrument. According to the volume invariance theorem, the area under the corresponding displacement can be calculated, so as to further calculate the true stress-strain curve.

 

In the case of foams, the VOLUME IS NOT CONSERVED during plastic deformation, since the size of the pores changes. Therefore, there are two possibilities. 1) use the engineering stress-engineering strain curves, and change your findings accordingly. 2) Using the video, an average width can be defined, so the changes in cross-sectional area can be estimated as the changes in the square of the width. However, the 2) one is not applicable if there is barreling (the pictures are cut, so it’s not visible).

 

 

[Comment#20] The densification stage is when the already collapsed cells are densified. According to Fig 4, at 20% the cells only start to collapse. In Fig 3 densification starts at ~12%; later in the text, it is written that it starts at 15-20% of true strain.

[Response#20] The authors thank the reviewer for your carefully reading and suggestion. Fig.3（as shown in Fig.4 in the updated manuscript）was obtained by referring to the relevant literature and based on the macroscopic analysis of the compression process based on the characteristics of acoustic emission parameters and the CCD deformation image of the specimen. Although the collapse of the cell hole at 20 % of the strain can be observed from the deformation image of the specimen, there is a sudden energy count at 12-15 % of the strain, and the energy absorption efficiency also shows a downward trend at about 15 % of the strain. Therefore, we define the compactness point of the specimen at 12 %, and then it will be in the densification stage.

According to Fig. 7. and 8., there is no change in the characteristics of AE. The decrease in the ring down count and energy count is continuous. The high energy signals may indicate fracture in the structure. Since the images of the deformed surface are shown only up to 20%, the densification may start at a higher strain. The downward trend starts at the beginning of the plateau stage. The engineering curve might show the starting of the densification more clearly.

[Comment#22] l.161 What do the authors mean by "discontinuous" crushed band?

[Response#22] The authors thank the reviewer for your carefully reading and suggestion. Due to the uneven thickness of the cell pore wall and random distribution of pore wall defects，the deformation occured in the direction of structural weakness and foam aluminum would have multiple local deformation bands during compression and the severely deformed position were not all on one deformation band, so this was expressed as a local ' discontinuous ' fracture zone.

Andreas Mortensen made similar Al foam with smaller pore size and they found uniform deformation. Is it not the same here? Instead of "discontinuous" crushed band, use multiple crush bands.

[Comment#28] l. 195 burst AE is not equivalent with (very) high energy signals

[Response#28] The authors thank the reviewer for your carefully reading and suggestion. We carefully examined the meaning of the article here to express is suddenly generated high energy signals, i.e. sudden-type acoustic emission signals. So we made the following change in line 460: burst -type AE signals—sudden-type AE signals.

In the literature, there is no such thing as sudden-type AE signal. Here, I think you mean simply high energy burst-type AE signal.

 

The English still should be improved: e.g. elastic phase (stage/regime), ideal energy absorption efficiency (it should be 100% if it’s ideal, shouldn’t it?), plateau stress stage, field of ocean, deformation damage, specimen, total counts (cumulative counts), ascending time (rise-time), accumulative count (cumulative), dense AE ring down count etc.

What are AE rate (l.109) and energy absorption rate?

l.125 [Comment#31] How is the yield strength defined? Why do the authors use yield strength and not proof stress?

[Response#31] When the curve did not show a clear yield point, but rather a gradual increase in stress with strain, we used the stress at a strain of 0.05 as the yield strength according to reference [51]. The reference is as follows：

[51]Cluff, D.R.A.;Esmaeili, S..Prediction of the effect of artificial aging heat treatment on the yield strength of an open-cell aluminum foam.[J].Journal of Materials Science,2008,Vol.43(3): 1121-1127

In [51] engineering stress and engineering strain were used in the definition, most probably the yield strength is lower than 6MPa.

l.239 the same holds for a piece of mild steel at small strains while it deforms totally elastically, thus, it is not due to local plastic deformation.

l.255, l. 268:  missing ref.; l.266  remove 31

l. 272 should be explained by dislocations motion…

[Comment#34] In Fig.10 6000 for "Ringing count" seems too high; the ring down count is defined for one event.

[Response#34] Considering that the signals was collected at a frequency of 10 Hz during cyclic loading, the ring down count had a higher value.

The ring down count is defined for ONE event. Does this mean that the events are overlapping?

[Comment#35] l. 282 What is the "extrusion deformation" of the cell edges? And the compression deformation? Shear stress will act on the cell struts.

[Response#35] The authors thank the reviewer for your carefully reading and suggestion. We carefully examined the expression here in the article and referred to the relevant literature. There is an error in the expression of the deformation of the cell edge. The deformation and failure mode of the cell edge is mainly shear deformation. We also modified the corresponding position in the article.

Not changed, see l. 306, 308, 314, 317

Take out the [J] from the references; the style of the newly added references are different; check [44][45][48]

Comments for author File: Comments.pdf

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Reviewer 2 Report

After careful consideration of the revised manuscript, it can be said that the authors have properly addressed the comments and suggestions and the manuscript is considerably improved.

The answers given by the authors are convincing and the issues raised were properly confronted.

Before publication the authors should consider an additional minor issue: In Fig.17 it should be clarified what the unit of the energy counts is.

Recapitulating the reviewer suggests that the manuscript can be published assuming that the above comment will be addressed.

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Reviewer 3 Report

The points raised with the review have been discussed and solved.

In my opinion, the paper can now be accepted for publication.

Author Response

The authors thank the reviewer for your carefully reading and suggestion. Thank you very much for reviewing our manuscript and giving us valuable suggestions to enhance its quality.

Round 3

Reviewer 1 Report

Acoustic emission characterization analysis of quasi-static and fatigue compression properties of aluminum foam
Qiong Song Jian Shi* Xu Chen

 

My suggestions regarding the evaluations were taken into account, and the manuscript has improved. However, there are still some parts that need to be corrected, and there are parts that were asked to be corrected but remained unchanged. E.g. "Take out the [J] from the references; the style of the newly added references are different; check [44][45][48]" – It is still not done. Or changing the phrase "extrusion" /" compression deformation" of pore wall

·         "The English still should be improved: e.g. elastic phase (stage/regime), ideal energy absorption efficiency (it should be 100% if it's ideal, shouldn't it?), plateau stress stage, field of ocean, deformation damage, specimen, total counts (cumulative counts), ascending time (rise-time), accumulative count (cumulative), dense AE ring down count etc."

The above-mentioned misused words were corrected, but there are more to correct, as it was indicated by the "etc". For example: compression ratio (rate), ocean (marine) and there are more. Please check your text carefully. 

Other examples are: using different tenses in the same sentence; not the appropriate tense was used;" total of 9 specimen"; "compression rates was small"; missing articles (again, these are just examples)

·         deformation was slow/fast?? Deformation isn't defined as time-dependent quantity.

·         l.188 "Then the ring down count values showed a decreasing trend in the yield and strengthened stages, which may indicate the initiation and propagation of microcracks in the material. "According to the literature (micro)crack initiation and propagation cause burst type of AE with high ring down count. Please add a reference here to support your statement. 

·         Add ref. to your statement in l. 190.

·         The authors state that increasing the compression rate will localize the deformation. Please prove this statement. As I wrote previously, this may be due to lower density of this/ these specimen(s), as the deformation curve of 50mm/min  compression suggests.

·         l. 220: the lack of high E burst AE is most probably due to some averaging effect

·         l. 244: The authors wrote," We also modified the strain value corresponding to the deformation image of the specimen according to the engineering stress and strain. As you consider, combined with the comprehensive analysis of the engineering stress-strain curve and the deformation image of the specimen, when the engineering strain is 25 %, the cells begin to collapse…"

This means at 12%, the cells only collapse.

·         l.252: ref

·         l.266 add the unit time used in this evaluation

·         l.273 slip of grain? The local deformation is not as high as to have "slip of grains". Add a reference that proves your statement.

·         acoustic emissivity?

·         I still don't understand that if the fatigue measurement was continued until 12% strain (in Fig. 13-14 we can see the surface of the samples at 12% strain), then why are these data missing from figs 12 and 17?  

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