Functionally-Graded Metallic Syntactic Foams Produced via Particle Pre-Compaction

Round 1

Reviewer 1 Report

The manuscript concerns an analysis of  novel functionally-graded metallic syntactic foam. Quasi-static compression tests were performed to determine the mechanical foam properties. The results of paper showed particle pre-compaction as efficient tool to tailor the density and mechanical properties. The paper is interesting and can be published by explaining below remarks:

1)      What does mean “grey value” in Fig. 3? Is there any unit? It is suggested to better describe this.

2)      What is “quasi-elastic gradient” in units (MPa)? How physical meaning this magnitude possesses? Is it like/close to Young’s modulus? It should be explained how to refer this parameter.

Author Response

We want to thank all reviewers for their valuable time and inputs into our manuscript. Their remarks have indeed helped us to further improve our paper.

 *** Reviewer I

The manuscript concerns an analysis of novel functionally-graded metallic syntactic foam. Quasi-static compression tests were performed to determine the mechanical foam properties. The results of paper showed particle pre-compaction as efficient tool to tailor the density and mechanical properties. The paper is interesting and can be published by explaining below remarks:

1) What does mean “grey value” in Fig. 3? Is there any unit? It is suggested to better describe this.

Our reply: The following Section has been extended

To better analyse the density gradients, these projections were further processed. To this end, the average grey level of each horizontal line in these images was determined and plotted against the vertical direction (i.e. the sample height). These grey level values are dimensionless and defined between 0 (black) and 255 (white). [...]

2)      What is “quasi-elastic gradient” in units (MPa)? How physical meaning this magnitude possesses? Is it like/close to Young’s modulus? It should be explained how to refer this parameter.

Our reply: the following bullet point has been extended for clarification

The quasi-elastic gradient E corresponds to the maximum slope of the stress-strain data at low strains (ε<0.05). Following ISO13314, this value is selected on behalf of Young’s modulus. Young’s modulus is difficult to measure for metallic foams as it is sensitive to settling effects and a direct attachment of strain gauges to the sample surface is usually not possible due to surface pores.

Reviewer 2 Report

The paper is based on their previous work, where the effect of the pre-compaction on the density and the mechanical behaviour was investigated.

Remarks

Page 2, Line 84: some more information about the expanded perlite particles should be provided, e.g. size; were they sieved? If not, what was the distribution of the size of the EP particles?

Page 2, Line 87-88, there is an extra space at the end of line 88

Page 3, Fig. 1 is misleading: on Fig1a the denser part is on the top, while on Fig 1b it’s on the bottom

Page 4, Line 128 suggests that one casting is one sample. Is it so? More information should be added about the original size of the material and the size of the sample after machining. (Fig 1b suggests that the surface of the sample exhibits a higher density than the inner part of the sample)       

According to page 5, Line 173, the volume of the non-compacted layer is larger than the volume of the other layer, therefore an error is introduced both in eq. 3 and 4. It would have been better to calculate the volume for each sample. (It’s possible due to the radiographs.)

Page 4, Mechanical testing: some more information about samples and the testing condition should be added (e.g. size of the samples; how many samples; was the friction between the grip and sample reduced by something)

Page5, Fig 2 and Fig 3 not consistent: in Fig 2 the denser part is at the bottom, so at 0 mm sample-height the grey value should be lower than at 45 mm. Change fig 3 accordingly.

Page 5, Fig 3, according to page 4, line 134-135, the height of the radiograph is 1000*35.32 micron=35.32mm, but on fig. 3 this height is 45 mm, why????

Page 5, Line 179, two dots at the end of the sentence

Use either Fig (e.g. Page 2, Line 157) or Figure (e.g. Page 5, Line 169) through the paper

Put the data of Fig 5 (matrix volume fraction) in the table in the appendix

Page 8, Line 239, stating that “minor scattering of the quasi-elastic gradient” is not valid, see the data from the Appendix: 1255 MPa, 521 MPa, 845 MPa, mean: 873 MPa, deviation: 367 MPa, approx.  40% of the mean value – it is not minor scattering, actually, it cannot be characterized as reproducible.

Page 9, Line 268, not all of the mechanical properties were repeatable (see the previous remark)

Page 9, Fig. 8b, 4 samples for 250 N? Fig 8a, next to the sign “o” a zero is missing, after 150, 250, 350 put “N”

The discussion should contain some comparisons of the quasi-elastic gradient, the 1% offset yield stress and volumetric energy absorption between the FG-MSF and the MSF containing only compacted PE particles. What is the advantage of having two layers?

Is the scatter in the properties partly due to some pores in the matrix developing during casting?   (On the surface of the sample on Fig 1b pores can be seen.)

Author Response

We want to thank all reviewers for their valuable time and inputs into our manuscript. Their remarks have indeed helped us to further improve our paper.

*** Reviewer II

Remarks

1) Page 2, Line 84: some more information about the expanded perlite particles should be provided, e.g. size; were they sieved? If not, what was the distribution of the size of the EP particles?

Our reply: The following information has been added:

The particles used in this study were sieved between the mesh sizes of 2 mm (lower limit) and 2.8 mm (upper limit).

2) Page 2, Line 87-88, there is an extra space at the end of line 88

Our reply: This formatting error has been removed.

3) Page 3, Fig. 1 is misleading: on Fig1a the denser part is on the top, while on Fig 1b it’s on the bottom

Our reply: The figure has been updated.

4) Page 4, Line 128 suggests that one casting is one sample. Is it so? More information should be added about the original size of the material and the size of the sample after machining. (Fig 1b suggests that the surface of the sample exhibits a higher density than the inner part of the sample)   

Our reply: The machining has been explained in more detail:

Following casting, the upper and lower surfaces of all samples were machined to ensure equal heights of the compacted and non-compacted layers. The curved surfaces were not machined and each cast yielded one sample.

The samples indeed exhibit a higher surface density. This effect is identical to the formation of the high-density interphase (see Fig. 4), i.e. particle cannot intersect the outer surface.

5) According to page 5, Line 173, the volume of the non-compacted layer is larger than the volume of the other layer, therefore an error is introduced both in eq. 3 and 4. It would have been better to calculate the volume for each sample. (It’s possible due to the radiographs.)

Our reply: The authors were unable to find the statement “the volume of the non-compacted layer is larger than the volume of the other layer” in the text. In fact, it was attempted to ensure a near identical size of each layer. Figure 3 suggests that this aim was not exactly achieved for all samples. However, due to the associated cost only a small number of samples were subjected to radiography and a correction based on this method is thus not possible. The reviewer is correct and this deviation will introduce an error to Eqs. 3 and 4. This has been clarified in the text.

An additional inaccuracy stems from the fact that the height of both layers slightly deviates in most samples due to the difficulty of precisely locating the interface on the outer sample surface.

6) Page 4, Mechanical testing: some more information about samples and the testing condition should be added (e.g. size of the samples; how many samples; was the friction between the grip and sample reduced by something)

Our reply: Following the reviewer’s comment, the following information has been added

Samples were compressed between two steel platens lubricated with CRC® 5-56 (CRC Industries, NSW, Australia) multipurpose lubricant in order to minimize frictional effects.

In addition, Table A1 has been added to provide geometric sample information.

7) Page5, Fig 2 and Fig 3 not consistent: in Fig 2 the denser part is at the bottom, so at 0 mm sample-height the grey value should be lower than at 45 mm. Change fig 3 accordingly.

Our reply: Thank you for pointing out this inconsistency. For efficiency, Fig. 2 has been modified instead.

8) Page 5, Fig 3, according to page 4, line 134-135, the height of the radiograph is 1000*35.32 micron=35.32mm, but on fig. 3 this height is 45 mm, why????

Our reply: The figures have been rescaled during image processing and slightly cropped to remove surface artefacts.

9) Use either Fig (e.g. Page 2, Line 157) or Figure (e.g. Page 5, Line 169) through the paper

Our reply: The expression has been unified throughout the document (Fig. X is used)

10) Put the data of Fig 5 (matrix volume fraction) in the table in the appendix

Our reply: The data has been added to the new Table A1

11) Page 8, Line 239, stating that “minor scattering of the quasi-elastic gradient” is not valid, see the data from the Appendix: 1255 MPa, 521 MPa, 845 MPa, mean: 873 MPa, deviation: 367 MPa, approx.  40% of the mean value – it is not minor scattering, actually, it cannot be characterized as reproducible.

Page 9, Line 268, not all of the mechanical properties were repeatable (see the previous remark)

Our reply: The statement relates to Fig. 8a and is indeed misleading. Due to the high sensitivity of this material property with respect to density, the absolute values exhibit a significant deviation. The statement has been replaced by the following sentence:

A clear correlation between the quasi-elastic modulus and the sample density is observed. However, this is superimposed by a strong scattering between individual samples.

12) Page 9, Fig. 8b, 4 samples for 250 N?

Our reply: Thank you, the incorrect data point has been removed.

13) Fig 8a, next to the sign “o” a zero is missing, after 150, 250, 350 put “N”

Our reply: The legend has been corrected.

14) The discussion should contain some comparisons of the quasi-elastic gradient, the 1% offset yield stress and volumetric energy absorption between the FG-MSF and the MSF containing only compacted PE particles. What is the advantage of having two layers?

Our reply: As suggested by the reviewer, the following comparisons were added

In [24], samples containing only precompacted particles (150 N) were investigated. Their quasi-elastic gradient is between 830-1140 MPa and thus below the corresponding values of the functionally graded structures (2191-2964 MPa). However, as the entire particle beds were compacted, the sample densities are also significantly lower, i.e. 0.72-0.76 g/cm3. Their lower stiffness is likely related to density gradients within these samples resulting in high porosity subvolumes.    

A previous study on completely compacted samples [24] yielded offset yield stresses of 4.5-7.1 MPa for a compaction force of 150 N. These values are significantly below the findings of the current study (13.54-16.65 MPa) and are again attributed to distinct density gradients within the completely compacted samples.

The mechanical benefits of functionally graded structures typically include:

* Advanced tailoring of compressive response, e.g. for impact engineering

* Enable a lower aspect ratio within layers to counteract catastrophic shear failure

* non-uniform resonance frequency, e.g. for structural damping

The following information has been added to the text:

An advantage of two volumes with different particle volume fractions is thus the increased control over the deformation behaviour. Shear bands can be supressed to prevent catastrophic failure of the sample (smaller aspect ratio). This is of particular interest for materials showing brittle deformation and shear failure.

15) Is the scatter in the properties partly due to some pores in the matrix developing during casting?   (On the surface of the sample on Fig 1b pores can be seen.)

Our reply: Whilst this possibility cannot be excluded, this effect is likely small. The volume fraction of closed pores within the metallic struts is typically very low. Solidification shrinkage is typically concentrated near the contact points of adjacent particles where narrow flow channels exist. The scatter is much more likely due to the random meso-porosity introduced by the weak expanded perlite particles.

Reviewer 3 Report

The paper describes the production of gradient syntactic metal foams by menas of melt-infiltration and partial compaction of the filler material.

The presentation and language of the paper are of good quality, the reviewer has only few remarks as listed in the following.

• Page 2: information on the original height of the layers and the height after compaction would be helpful
• Page 2: information on size distribution of the filler would be helpful
• Figure 1: the reversed photograph on the right side (in comparison to the sketch) is unfavourable and should be replaced
• Page 3: the reviewer had problems to understand the discussion of equations 3 and 4. Several questions popped up: E.g. what is the meaning of phi_V (line 120)? In line 122 the term “non-compacted layer” refers to what? The upper non-compacted layer? The lower part of the first layer which is – according to figure 1 – not influenced by the compaction? What is the thickness of the “influence zone” of the compaction in the first layer? Would it not be easier to cut one specimen into several parts and measure the density?
• Page 4, line 158: no electrons penetrate through the sample but X-rays!
• Figure 2: there seems to be a systematic effect in the upper layer: from left to the right there is an increase of bright areas at the edge of the specimen. How can this be explained?
• Figure 6: maybe the y-axis should be limited to 120MPa as the more interesting part of the diagram is at the beginning.     

Author Response

We want to thank all reviewers for their valuable time and inputs into our manuscript. Their remarks have indeed helped us to further improve our paper.

*** Reviewer III

The paper describes the production of gradient syntactic metal foams by menas of melt-infiltration and partial compaction of the filler material.

The presentation and language of the paper are of good quality, the reviewer has only few remarks as listed in the following.

1) Page 2: information on the original height of the layers and the height after compaction would be helpful

Our reply: Unfortunately, this information is not available.

2) Page 2: information on size distribution of the filler would be helpful

Our reply: The following information has been added

The particles used in this study were sieved between the mesh sizes of 2 mm (lower limit) and 2.8 mm (upper limit).

3) Figure 1: the reversed photograph on the right side (in comparison to the sketch) is unfavourable and should be replaced

Our reply: The image has been updated.

4) Page 3: the reviewer had problems to understand the discussion of equations 3 and 4. Several questions popped up: E.g. what is the meaning of phi_V (line 120)?

Our reply: Phi_V has been removed from the text.

5) In line 122 the term “non-compacted layer” refers to what? The upper non-compacted layer?

Our reply: The text has been clarified to “the non-compacted layer in FG-MSF” where required throughout the document.

6) The lower part of the first layer which is – according to figure 1 – not influenced by the compaction? What is the thickness of the “influence zone” of the compaction in the first layer? Would it not be easier to cut one specimen into several parts and measure the density?

Our reply: There should be no “influence zone” as the non-compacted layer is only added following the compaction of the first layer. Indeed, the samples could be cut for a density characterisation; however, this would have prevented their mechanical testing. Hence, a non-destructive estimate was selected as the preferred approach.

7) Page 4, line 158: no electrons penetrate through the sample but X-rays!

Our reply: Thank you – this mistake has been corrected.

8) Figure 2: there seems to be a systematic effect in the upper layer: from left to the right there is an increase of bright areas at the edge of the specimen. How can this be explained?

Our reply: The following information has been added

Page5, line 173: (…) left and right edges of the projections. It can additionally be noted that the brightness towards the edges, as well as, towards the interface of the compacted volume increases with increasing compaction force. This can be attributed to the higher particle volume fraction caused by the particle compaction. In the caste of the non-compacted sample (…)

9) Figure 6: maybe the y-axis should be limited to 120MPa as the more interesting part of the diagram is at the beginning.    

Our reply: The image has been rescaled as suggested.