Modeling of Falling Ball Impact Test Response on Solid, Veneer, and Traditional Engineered Wood Floorings of Several Hardwoods

Round 1

Reviewer 1 Report

The paper presents original research work.

The authors provided enough information about wood flooring types, but there is a lack of information on hardness testings of wood floorings. Also, the information about investigated species' anatomical and physical wood properties is missing and their relation to wood hardness.

The research design is appropriate regarding dynamical testings, but it seems there is a lack of comparison with statical testings (Brinell or Janka test).

The results are original and statistical analysis is provided in significant volume (maybe even too extensive), but it seems the results were expected. When we talk about the hardness of veneer floorings, the thickness of wood is too tiny to determine the hardness of the surface layer. There is a question of early and late wood proportion in this layer. Is it all early, all late wood or mix? That is the reason there is no difference between species. The authors obtained the hardness of composite, much influenced by HDF, adhesive, and surface finish.

Figures 3. and 4. are great but only for a knowable audience. They are too extensive. It would be better if more figures showed differences between species in one row.

The question is, what did the authors want to show in this research. For example, was it the possibility of using different species in wood floorings; the difference in the wood hardness between types of wood floorings; or the size of the ball which we should use to determine the dynamical wood hardness?

English language and style are acceptable, although the authors should adjust the text to British or North American spelling.

Few recommendations for authors:

1. Row 18 – It would probably be better to write that hardness is a key surface "mechanical" property.
2. Row 120 - EN 13183:2002 is the standard for measuring the moisture content of the wood. "EN 13183- 1 Moisture content of a piece of sawn timber – Part 1: Determination by oven dry method". "EN 13183- 2 Moisture content of a piece of sawn timber – Part 2: Estimation by electrical resistance".

 

3. Row 143 - ASTM D1037-99 is not based on measuring the diameter of the impact footprint left by a 50 mm diameter steel thus it specifies only: "record the heights of drop that produce the visible fractures on the two surfaces". On the other hand, the Brinell hardness test records the diameter of the footprint.
4. Row 171 – How did you measure footprint diameter? What equipment did you use?

 

 

Comments for author File: Comments.docx

Author Response

The paper presents original research work. The authors provided enough information about wood flooring types, but there is a lack of information on hardness testings of wood floorings. Also, the information about investigated species' anatomical and physical wood properties is missing and their relation to wood hardness.

 

Q1. The research design is appropriate regarding dynamical testings, but it seems there is a lack of comparison with statical testings (Brinell or Janka test).

Response: Please kindly note that classical laboratory methods for the determination of hardness (e.g., Brinell, Janka, Meyer or Monnin tests) are not the subject of this paper. In this work we intend to use only the impact test to assess both the behavior of different EWF architectures and the suitability for industrial utilization of new hardwood species, and we would like to keep the focus intact. Relationships between hardness and impact tests were already investigated by our group in previous works (e.g., please refer to [F. Sepliarsky, R. Tapias-Martin, and L. Acuña-Rello. "Eucalyptus globulus and Quercus robur engineered wood flooring. Impact resistance for several compositions." Maderas. Ciencia y tecnología 20.1 (2018): 103-116. http://dx.doi.org/10.4067/S0718-221X2018005001901]). Further, the use of the impact test is supported by the fact that it is the only tool available in the industry (it is difficult to find a certified testing machine in the wood processing industry that can accurately determine Brinell hardness or any other mechanical properties).

 

Q2. The results are original and statistical analysis is provided in significant volume (maybe even too extensive), but it seems the results were expected.

Response: We do not share the Reviewer’s view on the volume of statistical analyses. Please kindly note that they are much more concise than what would be requested in other journals (e.g., see the practical guides in https://www.nature.com/collections/qghhqm). We simply present a robust general methodology to appropriately perform descriptive, comparative and predictive analyses. Taking into consideration that many works in the forest engineering and wood technology area of knowledge apply inadequate statistical techniques, which leads to doubtful results and lack of scientific rigor, we consider that the statistical sections are particularly relevant and should be kept in their current form. We hope that the Reviewer will agree with us that statistics is an essential tool in scientific works and that meeting some minimum requirements is ‘a must’ for any author who would like to present results with scientific guarantees.

 

Q3. When we talk about the hardness of veneer floorings, the thickness of wood is too tiny to determine the hardness of the surface layer. There is a question of early and late wood proportion in this layer. Is it all early, all late wood or mix? That is the reason there is no difference between species. The authors obtained the hardness of composite, much influenced by HDF, adhesive, and surface finish.

Response: As noted by the Reviewer, the values obtained for veneer wood floorings are those of the composite, which is the subject of the study. Indeed, as evidenced by the presented results, in thin top noble wood layer EWFs, the top layer has very little influence on the final impact footprint, regardless of the species, density, type of growth and industrial cut, etc. That is precisely one of the main outcomes of the study, which would support the use of these products in the European industry.

 

Q4. Figures 3. and 4. are great but only for a knowable audience. They are too extensive. It would be better if more figures showed differences between species in one row.

Response: Given that large number of variables involved in the analysis, plotting the data according to the Reviewer’s suggestion (i.e., organizing the subplots to show differences between species) would make the figures even more difficult to read. Please kindly note that we have made an attempt to organize data in that way, but, unless we use four separate graphs for the footprint diameter + four separate graphs for the indentation depth (which would make the manuscript noticeably lengthier), the Reviewer’s suggestion cannot be addressed. Hence, we have made the decision of keeping Figure 3 and Figure 4 in their current configuration, albeit with some corrections suggested by one of the other Reviewers in Figure 3. Nonetheless, it should also be taken into consideration that a comparison between species is already provided in Figures 7 and 8, in which the model results are presented.

 

Q5. The question is, what did the authors want to show in this research. For example, was it the possibility of using different species in wood floorings; the difference in the wood hardness between types of wood floorings; or the size of the ball which we should use to determine the dynamical wood hardness?

Response: The last paragraph of the introduction has been re-written to state the goals of the study in a clearer manner.

 

Q6. English language and style are acceptable, although the authors should adjust the text to British or North American spelling.

Response: Corrected.

 

Few recommendations for authors:

Q7. Row 18 – It would probably be better to write that hardness is a key surface "mechanical" property.

Response: Corrected.

 

Q8. Row 120 - EN 13183:2002 is the standard for measuring the moisture content of the wood. "EN 13183- 1 Moisture content of a piece of sawn timber – Part 1: Determination by oven dry method". "EN 13183- 2 Moisture content of a piece of sawn timber – Part 2: Estimation by electrical resistance".

Response: Corrected.

 

Q9. Row 143 - ASTM D1037-99 is not based on measuring the diameter of the impact footprint left by a 50 mm diameter steel thus it specifies only: "record the heights of drop that produce the visible fractures on the two surfaces". On the other hand, the Brinell hardness test records the diameter of the footprint.

Response: Corrected. The sentence now reads: “[…] According to ASTM D1037-99 [31] and ASTM D2394-83(1999) [32], with the modifications of some parameters of the tests indicated in AITIM recommendations [33], the FD and ID of the impact footprint left by different diameter steel balls upon free fall from a given height were measured. […]”.

 

Q10. Row 171 – How did you measure footprint diameter? What equipment did you use?

Response: The footprint diameter and indentation depth were measured using a digital micrometer (with a ±0.001 mm error) and a back plunger dial indicator (with a ±0.01 mm error), both from Mitutoyo (Takatsu-ku, Kawasaki, Kanagawa, Japan). This information has been included at the end of section 2.2, below Figure 2.

Reviewer 2 Report

Dear Authors,

It is a well-prepared manuscript expanding the knowledge of the methodology of the dynamic hardness of different wood flooring elements.

I have only one comment to complete the information about the steel balls used (Line 148).

As I calculated on the basis of the data in Table 1, steel balls have a density of approx. 7774 kg/m³. I think that the type of steel should be given, which determines the elastic properties of the balls. In dynamic hardness tests, the elastic properties of the balls may have significant importance and influence the obtained results. In particular, this information may be important when interpreting the test results relating to Figure 10 (Lines 304-314).

Yours sincerely
Reviewer

Author Response

Dear Authors,

It is a well-prepared manuscript expanding the knowledge of the methodology of the dynamic hardness of different wood flooring elements.

 

Q1. I have only one comment to complete the information about the steel balls used (Line 148). As I calculated on the basis of the data in Table 1, steel balls have a density of approx. 7774 kg/m³. I think that the type of steel should be given, which determines the elastic properties of the balls. In dynamic hardness tests, the elastic properties of the balls may have significant importance and influence the obtained results. In particular, this information may be important when interpreting the test results relating to Figure 10 (Lines 304-314).

Response: We thank the Reviewer for pointing out this relevant detail that we had missed to clarify. We have now included a table with the characteristics of the type of steel used in the balls. As it can be observed in Table 3, the hardness and rigidity of the chosen steel are so high that the elastic deformation suffered by the impact energy resulting from a fall from a low height is negligible. Further, the following sentence has been added to the original text: “In the tests, chromed steel balls, of 100Cr6 steel type (SKF, Gothenburg, Sweden), with the indicated hardness according to ISO 3290-1:2014 [34] standard were used. Table 3 shows the characteristics of the steel balls used in the tests.”

Table 3. Steel ball properties.

Maximum Nominal Diameter Deviation (μm)	Hardness (HRC)	Elastic Modulus (MPa)	Compressive Breaking Stress (MPa)
±11.4	62-65	200,000	2,500-2,600

 

Reviewer 3 Report

Dear Authors,

your study is interesting and well written. But there are some minor issues that shoud be solved.

Precise the aim of the study. What do you want to prove or disprove? For what the model was prepared?

L. 96-105 method description. Reorganize the text structure.

Provide 3mm top layer technology. How it is made? Also 0.6 veneer  - rotary or slicing technology?

Provide the same Y-axes in the figure 3.

Good luck!

I think the authors must discuss earlier works in this topic published in this journal. Its crucial for scientific discuss. For example: doi.org/10.3390/f11080878

 

Author Response

Dear Authors,

Your study is interesting and well written. But there are some minor issues that shoud be solved.

Response: We thank the Reviewer for his/her time and thorough review. His/her recommendations have certainly contributed to improving our research paper. An itemized response to each query is provided below.

 

Q1. Precise the aim of the study. What do you want to prove or disprove? For what the model was prepared?

Response: This study arose in response to the growing commercialization of engineered wood flooring with 0.6 mm top layer in North America and its possible introduction into the European market. The main objective of the study was to evaluate the performance of EWF with a 0.6 mm top noble wood layer and to compare it with traditional products using the same testing procedure and conditions. In addition, the possibility of using new fast-growing timber species in the top layer was explored. Regarding the need for the prepared models, we expect them to be a valuable tool for wood flooring factories, facilitating the choice of appropriate combinations of wood flooring typology and thickness of the top noble layers to meet market exigencies in terms of impact hardness. The final paragraph of the introduction has been entirely re-written to state the goals of the study in a clearer manner.

 

Q2. L. 96-105 method description. Reorganize the text structure.

Response: The indicated paragraph (lines 96-105) has been re-written (please refer to the response to previous query). Further, the method description in subsection 2.2 has also been modified.

 

Q3. Provide 3mm top layer technology. How it is made? Also 0.6 veneer  - rotary or slicing technology?

Response: The 0.6 mm veneer was manufactured by slicing and the 3 mm by thin-cutting frame saw. This information has been clarified in the revised manuscript: “[…] a 3-mm hardwood wear layer obtained by thin-cutting frame saw, a 9-mm […]” and “[…] consisting of a 0.6-mm thick top layer of veneer obtained by slicing, a 9-mm […]”.

 

Q4. Provide the same Y-axes in the figure 3.

Response: Figure 3 has been replotted according the Reviewer’s suggestion.

 

Q5. I think the authors must discuss earlier works in this topic published in this journal. It’s crucial for scientific discuss. For example: doi.org/10.3390/f11080878

Response: Following the Reviewer’s recommendation, the discussion has been improved by including references to several research works: “Similar conclusions were reached by other researchers [43,44], although from the comparison of different hardwood species with similar density ratios”; “These results evidence that hardness is more closely related to density than to other wood properties [45], which explains why HDF board properties are more representative than those of the hardwood layer.”

The following references have been included:

43. Laine, K.; Rautkari, L.; Hughes, M. The effect of process parameters on the hardness of surface densified Scots pine solid wood. European Journal of Wood and Wood Products 2013, 71, 13-16, doi:10.1007/s00107-012-0649-0.
44. Sydor, M.; Pinkowski, G.; Jasińska, A. The Brinell method for determining hardness of wood flooring materials. Forests 2020, 11, doi:10.3390/f11080878.
45. Kollmann, F. Untersuchungen über den Abnutzungswiderstand von Holz, Holzwerkstoffen und Fußbodenbelägen. Holz als Roh- und Werkstoff 1963, 21, 245-256, doi:10.1007/BF02616315.

Reviewer 4 Report

IThe article deals with the widely discussed issue of the composition of the covering layer of floor systems. The authors focus on the composition of the layer as well as the actual material of the upper decorative layer. Even though several works have already been written on the issue, I consider the article to be interesting and beneficial. I appreciate the synthesis of the two views, engineering (well-constructed experiment), the article has the potential direct transfer of knowledge into practice and scientific verification of the engineering problem of adequate scientific methods. At the same time, the article is innovative from the point of view of the examined tree species, I appreciate the effort to expand the for  "ecological dimension". From the point of view of the form of processing, I did not notice any formal errors that would reduce the value of the article. From a professional point of view, the authors use an adequate methodology for obtaining as well as evaluating the results. The conclusions reached are correct and sufficiently substantiated by the presented results. 

Author Response

The article deals with the widely discussed issue of the composition of the covering layer of floor systems. The authors focus on the composition of the layer as well as the actual material of the upper decorative layer. Even though several works have already been written on the issue, I consider the article to be interesting and beneficial. I appreciate the synthesis of the two views, engineering (well-constructed experiment), the article has the potential direct transfer of knowledge into practice and scientific verification of the engineering problem of adequate scientific methods. At the same time, the article is innovative from the point of view of the examined tree species, I appreciate the effort to expand the for  "ecological dimension". From the point of view of the form of processing, I did not notice any formal errors that would reduce the value of the article. From a professional point of view, the authors use an adequate methodology for obtaining as well as evaluating the results. The conclusions reached are correct and sufficiently substantiated by the presented results. 

Response: We thank the Reviewer for his/her time reviewing the manuscript and for his/her positive feedback.