Comparison of Aluminum Alloys from Aircraft of Four Nations Involved in the WWII Conflict Using Multiscale Analyses and Archival Study
Round 1
Reviewer 1 Report
Corrections were made where my concerns were identified. Look forward to seeing the work published.
Author Response
Thanks for your comments.
Reviewer 2 Report
Dear Authors,
thank you for resubmitting the paper and addressing the comments provided.
Author Response
Thanks for your comments.
Reviewer 3 Report
Thank you for taking my comments into consideration based on your last submission. I appreciate that you went in and added context to the aircraft that were part of this study, which helps the reader understand the historic and archaeological importance of these wreck sites.
However, I do not think enough revision was done on this manuscript. There are extensive English writing errors and poor word choice that needs to be addressed and corrected before the paper can truly be reviewed.
In addition, I do still think the study itself is lacking in the alloys selected and low number of samples. If this work is to be published, I would recommend to the authors that they make it more of a technical note about applying the microscope methods to the study of metal aircraft components.
If you are to try and study the differences in metal alloys from aircraft wreck sites, much more information needs to be provided on how the planes crashed, what environments the wreckage has been in since the crash, what the metal alloys are and background context from materials science work on what those alloys are and how environments affect them in addition to what they were designed for, such as why certain alloys were selected for certain aircraft parts. Examining four pieces from four different aircraft is not enough to begin to compare these sites.
Author Response
Thank you for taking my comments into consideration based on your last submission. I appreciate that you went in and added context to the aircraft that were part of this study, which helps the reader understand the historic and archaeological importance of these wreck sites.
However, I do not think enough revision was done on this manuscript. There are extensive English writing errors and poor word choice that needs to be addressed and corrected before the paper can truly be reviewed.
The article was read and editted by a native speaker colleague. The additions made to the paper are highlighted in yellow, so that the reviewer can easily see the changes.
In addition, I do still think the study itself is lacking in the alloys selected and low number of samples. If this work is to be published, I would recommend to the authors that they make it more of a technical note about applying the microscope methods to the study of metal aircraft components.
Following the recommendations of the reviewer, the full corpus including 15 aircraft remains are now presented in the paper. Their background history is detailed in the supplementary table S1. Several plates (from each aircraft) had their chemical composition analyzed: the results are now presented in table 1.
On the other hand, thanks to an extensive archival research, we were able to understand that the identified alloys are representative of the ones used for the plates’ construction in each nation. Some information retrieved in the archives of aircraft constructors are added in the discussion of Part A.
p.10 line 22:
“Focke-Wulf apparently used the alloy with higher magnesium content, while a Messerschmitt archive document proves that the alloy used for plates seem to be AlCuMg1 (low magnesium content) …”
With the presentation of the alloys constituting the remains of 15 aircraft (all from the same period) and from additional information retrieved from the constructors of aircraft in archives, we hope that the reviewer will be convinced that the 4 presented parts are representative of the alloys used by each nation for the construction of plates in aircraft.
If you are to try and study the differences in metal alloys from aircraft wreck sites, much more information needs to be provided on how the planes crashed, what environments the wreckage has been in since the crash, what the metal alloys are and background context from materials science work on what those alloys are and how environments affect them in addition to what they were designed for, such as why certain alloys were selected for certain aircraft parts. Examining four pieces from four different aircraft is not enough to begin to compare these sites.
The aim of this study is not to compare the differences from aircraft wreck sites but to compare representative alloys used in the plates’ construction in each nation. We agree that the information on the conditions of the crash should be provided, so this was added in the supplementary table S1. Additional information was also given regarding the storage conditions.
p3. line 3 The remains have stayed buried for decades and once they were excavated, they were stored indoor.
But, although the environment in which the parts are left can have an influence, for instance a long stay in the ground could trigger corrosion and changes in the composition, in this article only bulk material was analyzed for composition, avoiding surface and corroded areas. Changes in composition are only linked to the fabrication of the part, not to post-wrecking events. In some cases, (for example the D.520), the crash conditions can have an influence on the mechanical properties, and the microstructure of the alloys. This was thoroughly discussed in part B.
Round 2
Reviewer 3 Report
Thanks for addressing the previous comments. I do find the manuscript much improved.
Author Response
Thank you very much for your valuable comments on our mansucript. We have sent the paper to a native English speaker for edition. We highly appreciate your help for us.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
This is an interesting and useful paper which outlines the beginnings of an interesting area of study on the history and use of duraluminum in WWII. It will be useful to archaeologists, heritage managers, and conservation scientists. I only have a few comments and suggestions which are attached in the PDF.
Comments for author File: 
Comments.pdf
Reviewer 2 Report
Comments to the paper "Comparison of aluminium alloys from aircraft of four nations involved in the WWII conflict using multiscale analyses and archival study" by T. Ouissi et al.
GENERAL COMMENTS.
The paper presents some material analyses (namely hardness measurements and microstructural imaging) on aluminium alloy plates coming from wrecks of some WWII aircraft, and compares the results with information about the nature of the alloys, obtained with the inspection of historical documentation. The idea beyond the paper is interesting and fascinating, and the physical analyses conducted on the findings are well performed and presented; the section about microstructure is especially convincing.
However, some weak points can anyway be identified in the work. These are in my opinion:
1) the limited amount of investigated findings.
2) some gaps into the historical apparatus that need to be filled to complete the logical steps of the article.
Point 1): the limited number of specimens is a common problem for whoever is interested in the scientific examination of CH. Indeed, this kind of studies are really significant only when they consider a relatively large number of specimens, in order to overcome obvious problems due to non-uniformity. I understand, however, that this paper is just the first publication related to a newly-undertaken project, and that the authors are willing to extend their analyses to more, already available findings. For this reason, I encourage the publishing of this paper as their first effort on the subject, hoping to see soon their further results. However, I suggest the authors to underline this point at the beginning of the paper (not only at the conclusion) , and that this is mainly a methodological paper.
Point 2): the authors present, in section 3.3, a very interesting historical account of the (possible) nature of the used aluminium alloys for the four aircraft of interest. However, what is not clearly stated into the references is the EFFECTIVE USE of the named alloys by the different factories, and the use of the named alloys for BOTH STRUCTURE AND SKIN of the aircraft. The latter point is to me especially important: I understand that the information about effective use of specific alloys can no longer be recovered (and this is somehow the reason behind the whole project), but can the authors find any documental evidence that, within a single company, the SAME alloy was used for both structure and skin? Or maybe that the two kinds of identified alloys (high-Mg and low-Mg) were (preferably) used in different parts of the aircraft? I can imagine that documentation from Bayerische Flugzeugwerke and SNCAM/SCASE is hard to be recovered, but not so for A.V. Roe & C. and North American Aviation! Moreover, a relatively high number of P-51 is still preserved both in Europe and USA, either in museum or in flying conditions as "warbirds". It should thus be possible to obtain this piece of information from a number of different sources, at least for the P-51 case. If this supporting piece can be recovered and discussed, the whole meaning of the paper would be in my opinion highly improved.
COMMENTS TO SPECIFIC POINTS OF THE PAPER
line 97 and following: I suggest to give more details and add some comments to the identification of the investigated planes. In particular, it is known that the Bf 109 was mass-produced by many factories in more than 33000 examples (as correctly stated) in at least 7 main "marks" (B to K) and dozens of sub-models (B-1 to K-4), with major differences between models. Moreover, due to the war effort, late copies were realised with inferior materials than the early ones. Thus, just stating that the examined plane is "a Bf 109" is not a comprehensive information for the expert. From the "werknummer" 5145, the present reviewer may imagine that the plane was an E-4 model manufactured by Bayerische Flugzeugwerke, but this must be confirmed (a research based on werknummers should be relatively easy for the authors). If this is the case, the example should be representative of an early, high-quality production standard.
Similar information should be given, at least as an educated guess, also for the Lancaster, P-51 and D.520 (although the P-51 and D.520 production should be, for different reasons, characterised by a remarkably high uniformity).
line 108: I assume that the correct wording would be "The North American P-51 Mustang.....", referring to the manufacturer (North American Aviation). The correct nickname of the specific aircraft is "Fools Paradise IV" (see further for details).
Figure 1: the quality of the images is poor, at least in my example. If possible, it should be improved in terms of resolution. In panel a) an early-mark Bf 109 is shown (likely a B-1); for sake of clarity, it is advisable to show an image of the same version of the plane (see previous comments).
The plane shown in panel d) is a late-mark, Griffon-engine Spitfire, and it is DEFINITELY not a P-51! As a matter of fact, on the web (and namely on the "aerocherche" website) some pictures not only of a P-51D but indeed of the same specimen (S. N. 413309) are available. By the way, from these pictures it is possible to see that the complete nickname of the plane was indeed "Fools Paradise IV".
lines 116-119: it seems that the choice of specimens to be examined was led by the thickness of the specimens themselves only, based on the consideration that the similar thickness of the specimens is a guarantee of uniformity of properties. This looks somehow weak. Moreover, someone could comment that between 1.1 mm and 2.2 mm there is a factor 2 difference. The choice would be more convincing if the authors could state a similar "origin" of the different specimens, i. e. the fact that all of them seem to be part of a plate (i.e. not from a rod, profile etc.) Given the previous comments about the use of alloys for structure and/or skin, could the authors state that all the specimens come from parts of the skin of the respective planes? This looks a reasonable hypothesis, given the shape and thickness of the findings, and it would strengthen the choice of the specimens. Some comments should thus be added about this hypothesis.
Line 152: it is not completely clear to the common reader which method has been used for the hardness measurements, and which standard. I deduce the Vickers method has been used, and I assume that standard ASTM E 384 was followed, while ASTM E 140 is just used for the correct conversion Vickers-> Brinell. Please clarify on this point and spend some words to better describe the technique.
Table 1: are the reported values from a single measurement per specimen or the result of an averaging on multiple measurements? Please specify.
Final comments on style: the reviewer noticed some unevenness in lettering and denominations along the paper (e. g. P51 or P-51 or Mustang P-51); similarly, he noticed some inaccuracies in punctuation and some misprinting (e. g. line 354 " present", line 439 "conventionam" etc.). The authors are encouraged to have the final version of their paper carefully read and amended by a native English speaker.
Reviewer 3 Report
This study is very interesting and presents some very good geochemical methodology and results. However, as written, this manuscript needs major revisions. Firstly, the English writing needs to be edited for language and writing style.
In addition, I have some concerns regarding the samples. The authors say there were 15 samples investigated but only 4 were analyzed, one per type of aircraft. This is not ideal for scientific work or statistical analysis. Is it possible to analyze more? Multiple samples from each aircraft? Samples from multiple aircraft of the same type? I understand that acquiring samples from these types of materials is not easy, and there is value in a small sample study, but further sampling and analysis would strengthen this work immensely.
The authors chose a fighter, but instead of comparing fighter to fighter for England, they compared a bomber.
Table 1. Inconsistency in comma/period nomenclature for decimals. Should be "."
It needs to be very clearly described and specified what exactly was sampled on the aircraft. Parts of the skin, structure, stringers, etc? There are at least 4-6 different aluminum alloys in each WWII aircraft depending on what they were supposed to do and what part of the planes were being manufactured. They had rivets from different alloy aluminum to skin, just as an example. Also, fabrication method counts, if you compared a piece of cast aluminum to cold-rolled aluminum it's going to appear different under a microscope. Not only do the authors need to be very specific about what materials and pieces of the aircraft the samples were acquired from, adding further samples to the study from different parts of the aircraft would be needed. Main point that they do need to discuss is that an aircraft was not made of a single alloy or type of metal, and the singular samples here sort of imply that they were.
The authors do provide a very good discussion of historical development of alloys and their results, tables, graphs, and microscope images are good.
