Research on the Short-Term Compressive Creep Behavior of the Bamboo Scrimber Based on Different Zener Models

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents some research on the compressive creep behaviour of the bamboo scrimber.

1. Comments on the title:

-It is known that creep behaviour has three main stages. In the manuscript only the primary or transient creep stage is presented and this should be specified (usually a creep test lasts 1000 hours, but the one in the manuscript only 10 hours). For engineering practice the second stage is more important;

-the viscoelastic model used by the authors is the standard or Zener model, which consists of two springs and a dashpot. (see Kelvin representation).

2. It is usually considered that in creep tests the axial strain depends on time, temperature and stress. But in materials of the type studied in the manuscript, humidity also has an influence. The variation of temperature and humidity during the tests has an influence on the axial strain (see Fig. 9, reference 24 in the manuscript). Authors must present the measures taken to reduce the influence of temperature and humidity variations.
3. The authors state that residual stresses influence creep tests and for this purpose they performed a "pretreatment process" to reduce residual stresses, which would have been done according to reference [24]. But this reference does not mention residual stresses! Do the authors have any information about the influence of residual stresses on creep and methods for reducing them? Attention is drawn to the fact that residual stresses in composite materials cannot be completely eliminated, because the different materials that form a composite have different thermal expansion coefficients (see Barsanescu P., Grimberg R and Carlescu P., Generation of Residual Stresses in Composites, PROCEEDINGS OF THE 9TH BIENNIAL CONFERENCE ON ENGINEERING SYSTEMS DESIGN AND ANALYSIS - 2008, VOL 1, pp.623-628, Technion, Haifa).
4. The following statement is deficient: “Figure 3 shows the detailed information of the load history for the pretreatment, in this way the residual stress can be released to a certain degree and the accuracy of the creep strain measurement can be guaranteed [24]”. The authors do not measure the residual stresses. In these conditions, how can they guarantee that after the "pretreatment process" the remaining residual stresses are so low that they do not influence in any way the accuracy of the creep strain measurement? The indicated reference [24] is not related to residual stresses.
5. What measures did the authors take to avoid eccentric application of the load?
6. It is known that there are several methods for compressive testing and the method has an influence on the compressive strength. What standard and which test method were used for this test? What type of testing machine was used for the tests?
7. In Fig. 4 there are only 2 springs (with Young's moduli E₁ and E₃) and a damper (with viscosity coefficient η₂). However, when explaining the notations involved in equation (2), the notation η₄ appears, which does not exist in Figure 4.
8. The writing is quite sloppy:

- „For the Kelvin body established in Figure 3,…” but Figure 3 does not refer to Kelvin body and in the absence of the appropriate figure, understanding the notations in the system of equations (9) is difficult;

- In the system of equations (9) the last equation is no longer necessary;

- The commentary on the diagrams in Figure 6 is general for the creep phenomenon and does not bring anything specific to the tested material;

- Table 1 should be compressed, writing in the "Specimen number" column groups of 3 specimens on a single row, etc.

9. Have measures been taken to reduce friction between the steel discs and the specimen?

Other minor observations and suggestions:

1. In Figure 1, the other side of the square should also be dimensioned.
2. In the sentence “Figure 1 shows the cuboid specimen…” the word "prism" should be used instead of "cuboid".
3. The writing in Figure 2 is difficult to read, due to the low contrast.
4. The letters of the Greek alphabet appear in the text written as if they were exponents, although this is not the case.
5. Instead of naming the first, second, third or fourth stress level, it would be better to indicate the value of these stresses in the text.
6. The acronym BUCs needs to be explained.
7. The level of English could be improved. For this purpose, authors can use the services of a native English speaker.
8. The authors are asked to perform the following experiment: place the extensometer on the table, under tension, at the initial length of 50mm. In the absence of axial strain, does the extensometer reading remain unchanged after 24 hours (for example)?

Comments on the Quality of English Language

The authors use some inappropriate terms, such as "cuboid" instead of "prism" and some sentences should be rephrased. The authors should use the services of a native English speaker authorized in the field.

Author Response

Comment 1. It is known that creep behaviour has three main stages. In the manuscript only the primary or transient creep stage is presented and this should be specified (usually a creep test lasts 1000 hours, but the one in the manuscript only 10 hours). For engineering practice the second stage is more important;

Response 1: Thanks for your comment. In the revised paper, the title was changed with more detailed information (short term creep), please check.

Comment 2. The viscoelastic model used by the authors is the standard or Zener model, which consists of two springs and a dashpot. (see Kelvin representation).

Response 2: Thanks for your comment. We have changed the name of the model throughout the whole manuscript, please check.

Comment 3. It is usually considered that in creep tests the axial strain depends on time, temperature and stress. But in materials of the type studied in the manuscript, humidity also has an influence. The variation of temperature and humidity during the tests has an influence on the axial strain (see Fig. 9, reference 24 in the manuscript). Authors must present the measures taken to reduce the influence of temperature and humidity variations.

Response 3: Thanks for your comment. In the revised paper, more detailed information of the experiment process has been added, including the temperature and humidity, please check.

Comment 4. The authors state that residual stresses influence creep tests and for this purpose they performed a "pretreatment process" to reduce residual stresses, which would have been done according to reference [24]. But this reference does not mention residual stresses! Do the authors have any information about the influence of residual stresses on creep and methods for reducing them? Attention is drawn to the fact that residual stresses in composite materials cannot be completely eliminated, because the different materials that form a composite have different thermal expansion coefficients (see Barsanescu P., Grimberg R and Carlescu P., Generation of Residual Stresses in Composites, PROCEEDINGS OF THE 9TH BIENNIAL CONFERENCE ON ENGINEERING SYSTEMS DESIGN AND ANALYSIS - 2008, VOL 1, pp.623-628, Technion, Haifa).

Response 4. Thanks for your comment. After carefully consulting the literature and communicating with the experiment operating personnel, we finally confirmed that the main purpose of the pretreatment is to eliminate the microscopic gaps between the specimen and the equipment. We are sorry for the mistake in the previous version. In addition, the reference has also been changed to support the conclusion.

Comment 5. The following statement is deficient: “Figure 3 shows the detailed information of the load history for the pretreatment, in this way the residual stress can be released to a certain degree and the accuracy of the creep strain measurement can be guaranteed [24]”. The authors do not measure the residual stresses. In these conditions, how can they guarantee that after the "pretreatment process" the remaining residual stresses are so low that they do not influence in any way the accuracy of the creep strain measurement? The indicated reference [24] is not related to residual stresses.

Response 5. The same to the above.

Comment 6. What measures did the authors take to avoid eccentric application of the load?

Response 6. Thanks for your comment. In fact we are a bit puzzled by this sentence. Do you mean that how to guarantee that the load is just applied on the central of the specimen? If yes, before the load application, the specimen is just located at the central of the press head, as well as the steel base. In addition, both the upper and bottom surfaces of the specimen are large enough to guarantee the vertical installation of the specimen. We have added corresponding information in the revised paper. If the comment is not in this case, please give us more detailed information.

Comment 7. It is known that there are several methods for compressive testing and the method has an influence on the compressive strength. What standard and which test method were used for this test? What type of testing machine was used for the tests?

Response 7. Thanks for your comment. In this paper, the standard used for the compressive experiment is GB T1041-2008. We have added corresponding information in the revised paper. In addition, the testing machine was also introduced.

Comment 8. In Fig. 4 there are only 2 springs (with Young's moduli E1 and E3) and a damper (with viscosity coefficient η2). However, when explaining the notations involved in equation (2), the notation η4 appears, which does not exist in Figure 4.

Response 8. Thanks for remind. In the revised paper, the notation has been deleted, please check.

Comment 9. The writing is quite sloppy:

- „For the Kelvin body established in Figure 3,…” but Figure 3 does not refer to Kelvin body and in the absence of the appropriate figure, understanding the notations in the system of equations (9) is difficult;

Response9. Thanks for your comment. In the revised paper, the figure number and notations have been revised to make more clear expression, please check.

Comment 10. In the system of equations (9) the last equation is no longer necessary;

Response 10. Thanks for your advice. In the revised paper, the equation has been deleted.

Comment 11. The commentary on the diagrams in Figure 6 is general for the creep phenomenon and does not bring anything specific to the tested material;

Response 11. Thanks for your advice, the figure was deleted for more clear expression.

Comment 12. Table 1 should be compressed, writing in the "Specimen number" column groups of 3 specimens on a single row, etc.

Response 12. Thanks for your advice. In the revised paper, the table was compressed.

Comment 13. Have measures been taken to reduce friction between the steel discs and the specimen?

Response 13. Thanks for your comment. In fact during the experiment process, the load condition during the steel discs and the specimen is just the compressive type, so we are a bit puzzled by the friction. Please give us more detailed information.

Other minor observations and suggestions:

 

Comment 14. In Figure 1, the other side of the square should also be dimensioned.

Response 14. Thanks for your advice. In the revised paper, the label has been added in the other side, please check.

Comment 15. In the sentence “Figure 1 shows the cuboid specimen…” the word "prism" should be used instead of "cuboid".

Response 15. Thanks for your advice. In the revised paper, the word has been changed.

Comment 16. The writing in Figure 2 is difficult to read, due to the low contrast.

Response 16. Thanks for your comment. In the revised paper, the color of the words has been changed to black, which can make more clear expression.

Comment 17. The letters of the Greek alphabet appear in the text written as if they were exponents, although this is not the case.

Response 17. Thanks for remind. We have modified the written, please check.

Comment 18. Instead of naming the first, second, third or fourth stress level, it would be better to indicate the value of these stresses in the text.

Response 18. Thanks for your advice, we have added the stress value in the revised paper, please check.

Comment 19. The acronym BUCs needs to be explained.

Response 19. Thanks for your advice. In the revised paper, we have made corresponding modifications, please check.

Comment 20. The level of English could be improved. For this purpose, authors can use the services of a native English speaker.

Response 20. Thanks for your advice, the whole paper has been polished by the MDPI system, please check.

Comment 21. The authors are asked to perform the following experiment: place the extensometer on the table, under tension, at the initial length of 50mm. In the absence of axial strain, does the extensometer reading remain unchanged after 24 hours (for example)?

Response 21. Thanks for your advice. In fact we are a bit puzzled by this comment. Do you mean that we should add corresponding tensile creep experiment? If yes, please give us more detailed information (for example, the load amplitude).

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript "Research of the compressive creep behavior of the bamboo scrimber based on different Boltzmann models" investigates the compressive creep behavior of bamboo scrimber using different Boltzmann-type viscoelastic models, including a memory-dependent (MD) derivative formulation. The paper presents a combination of experimental results and analytical modeling, which provides potential contributions to the prediction of time-dependent mechanical behavior of bamboo-based materials. However, some adjustments are needed to improve the quality of the manuscript:

1) How this modified model differs from fractional or nonlinear viscoelastic models previously applied to bamboo or similar composites? What is the novelty of the study? These points need to be highlighted in the text.

2) The description of specimen preparation (Section 2.1) can be shortened and organized more clearly. Please specify number of replicates per stress level and environmental conditions (temperature, humidity).

3) In the modeling section (2.3), several equations lack proper numbering and consistency in notation. Ensure all symbols are defined upon first use.

4) Consider merging Figures 7 and 8 into one comparative figure with two columns for clarity.

5) Include a comparison with previous studies on bamboo creep modeling to highlight advancements.

6) The manuscript needs thorough language editing by a native or professional scientific editor. Frequent grammatical issues (verb tense, article use, pluralization) and awkward phrasing affect readability.

Author Response

The manuscript "Research of the compressive creep behavior of the bamboo scrimber based on different Boltzman models" investigates the compressive creep behavior of bamboo scrimber using different Boltzman-type viscoelastic models, including a memory-dependent (MD) derivative formulation. The paper presents a combination of experimental results and analytical modeling, which provides potential contributions to the prediction of time-dependent mechanical behavior of bamboo-based materials. However, some adjustments are needed to improve the quality of the manuscript:

 

Comment 1. How this modified model differs from fractional or nonlinear viscoelastic models previously applied to bamboo or similar composites? What is the novelty of the study? These points need to be highlighted in the text.

Response 1. Thanks for your comment. In fact, the memory-depend derivative has the similar expression with that of the fractional-order derivative. The main advantage of this approach is that derivative comprised intuitive physical meanings and is expressed as a weighted average of ordinary derivatives over a certain past period of time. It corrects the defect that the memory effect of the fractional derivative was weakened with time. We have added these in the revised paper, please check.

Comment 2. The description of specimen preparation (Section 2.1) can be shortened and organized more clearly. Please specify number of replicates per stress level and environmental conditions (temperature, humidity).

Response 2. Thanks for your advice. In the revised paper, the serial numbers of the specimens have been changed, please check.

Comment 3. In the modeling section (2.3), several equations lack proper numbering and consistency in notation. Ensure all symbols are defined upon first use.

Response 3. Thanks for remind. In the revised paper, the equations have been revised, please check.

Comment 4. Consider merging Figures 7 and 8 into one comparative figure with two columns for clarity.

Response 4. Thanks for your advice. In the revised paper, the figures are merged together to make more clear expression, please check.

Comment 5. Include a comparison with previous studies on bamboo creep modeling to highlight advancements.

Response 5.  Thanks for your advice. At present, most of the creep modeling research in the bamboo material related objects is based on the Burgers model, which brings four elements in all. The proposed MD model can accurately simulate the compressive creep strain evolution process of the bamboo scrimber with relatively less element for more clear expression, which makes it valuable for application. We have added this in the revised paper at the discussion part, please check.

Comment 6. The manuscript needs thorough language editing by a native or professional scientific editor. Frequent grammatical issues (verb tense, article use, pluralization) and awkward phrasing affect readability.

Response 6. Thanks for your advice, the whole paper has been polished by the MDPI system, please check.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have made many changes that have significantly increased the value of the manuscript. The authors have responded well to the comments, except for those below, for which the reviewer provides additional details.

Comment 6

For compression testing, a spherical joint is usually used on one of the dikes. This is necessary because the ends of the specimen, which are in contact with the discs, cannot be perfectly parallel. If the contact surfaces between the specimen and the machine dikes are not parallel, eccentric compression may occur. Did the authors use a spherical joint for the compression test?

Comment 13

During compression testing of ductile metals, the specimen acquires a barrel shape and thus errors are introduced. This is due to the friction between the ends of the specimen and the machine disks. To reduce this phenomenon, for ductile materials, a lubricant is introduced between the ends of the specimen and the disks (paraffin, graphite, etc.). In the case of brittle materials (as is the case here) the barrel shape does not occur, so lubrication is no longer necessary. However, reducing friction could still be necessary if very precise measurements are made.

Comment 21

Some sensors are accurate for short-term measurements but for long-term measurements they can introduce some errors due to changes in environmental factors over time (temperature, pressure, humidity, etc.). Usually extensometers are used for static tests, which last a few minutes. The suggestion was that the authors check that the extensometer used does not have zero drift when it is in operation for several hours, without external strain.

Author Response

The authors have made many changes that have significantly increased the value of the manuscript. The authors have responded well to the comments, except for those below, for which the reviewer provides additional details.

Comment 6

For compression testing, a spherical joint is usually used on one of the dikes. This is necessary because the ends of the specimen, which are in contact with the discs, cannot be perfectly parallel. If the contact surfaces between the specimen and the machine dikes are not parallel, eccentric compression may occur. Did the authors use a spherical joint for the compression test?

Response 6: Thanks for your comment. In fact, there is no spherical joint in our equipment. During the experiment process, some other approaches are applied in avoiding the eccentric compression load. As shown in the revised figure 2(a), the pressing head was installed on a steel beam connected with two arms. Before the experiment process, the levelness of both the beam and the bottom plate has been checked to ensure that they are parallel to each other. In addition, the upper and bottom surfaces of the specimen are also parallel to each other due to the cutting technology. In this way the eccentric compression can be avoided in advance. We have added corresponding explanation in the revised paper, please check.

On the other hand, the size of the specimen in our study is relatively small, thus corresponding operation according to the given experiment standard can already fulfill the demand of accuracy.

Comment 13

During compression testing of ductile metals, the specimen acquires a barrel shape and thus errors are introduced. This is due to the friction between the ends of the specimen and the machine disks. To reduce this phenomenon, for ductile materials, a lubricant is introduced between the ends of the specimen and the disks (paraffin, graphite, etc.). In the case of brittle materials (as is the case here) the barrel shape does not occur, so lubrication is no longer necessary. However, reducing friction could still be necessary if very precise measurements are made.

Response 13: Thanks for your comment. In this paper, the accuracy demand of the experiment does not belong to the very precise measurement case, thus corresponding lubricant work is missing. However on some related compression experiments on other construction and building materials such as the concrete, we have added corresponding lubricant in advance.

Comment 21

Some sensors are accurate for short-term measurements but for long-term measurements they can introduce some errors due to changes in environmental factors over time (temperature, pressure, humidity, etc.). Usually extensometers are used for static tests, which last a few minutes. The suggestion was that the authors check that the extensometer used does not have zero drift when it is in operation for several hours, without external strain.

Response 21: Thanks for your comment. In this paper, the brand name of the electronic extensometer is Reliant, and the serial number of the instrument is SN: R2489. According to our previous check, this type of electronic extensometer has no obvious zero drift property within relatively short creep experiment process (no more than 10 days). In addition, the temperature and humidity during the experiment is generally steady based on the operation of the condition control system of the test room. Thus the accuracy of the recorded strain can be guaranteed. We have added corresponding explanation in the revised paper, please check. If detailed zero drift experiment is necessary, please give us longer revision time to prepare.

Reviewer 2 Report

Comments and Suggestions for Authors

I appreciate the authors' efforts in addressing my comments, and that's sufficient for now.

Author Response

Thanks for the work of the reviewer.