Amelia Earhart Search Forum > Artifact Analysis
A Piece Of The Electra?
Ric Gillespie:
--- Quote from: Walter Runck on February 13, 2011, 07:01:24 AM ---Has NAA been performed on any of the pieces found to date? You could start building a database of known heats based on what you already have. First check for repeatability from area to area on the same artifact, then to similar artifacts, then dissimilar (different gauge but still Alclad) artifacts, then other aircraft (Grace McGuire), etc..
--- End quote ---
No NAA has been done to date. Building a database would be interesting but expensive and, as you say, ultimately inconclusive. The bottom line is that we need to find the rest of the airplane.
Walter Runck:
Alclad is a co-rolled or clad material used for aircraft skin and other sheet applications. It is two sheets of different types of aluminum alloy smashed together during the hot rolling process. Take a coil of structural grade aluminum (aluminum alloyed (melted) with some copper, magnesium and manganese). This is a strong material, but not the most corrosion resistant. Now take a coil of relatively pure aluminum, which forms an oxide layer that protects itself, but does not have much strength (I have a 1/2 inch bar of 99.999 pure aluminum that I could bend with my hands). Lay one on top of the other, heat and run them through rollers under pressure. Viola, an open faced aluminum sandwich! Strong on one side, shiny on the other! But right from the start you will have different NAA signatures from each side of the sheet.
For our purposes, batches correspond to mill heats. This is the last time the material is melted and mixed with various elements to improve various properties. It's like making soup. The size of the batch is determined by the size of the pot. Probably on the order of hundreds of cubic feet or tens of thousands of pounds each. but you would have at least two heats for each sheet of Alclad used in construction. Aluminum is fluffy stuff, so this is a lot of product by volume.
Fab shops using this kind of material would probably buy it by the sheet rather than the coil (heavy) and it would be packed on pallets. Handling stuff way bigger than you need is a pain, my guess is that Lockheed was taking 4 x 8 or 5 x 10 mill sheets and starting from there.
Walter Runck:
--- Quote from: Ric Gillespie on February 13, 2011, 08:19:58 AM ---Factors we have considered in contemplating NAA:
- There are many reasons that a non-match between Niku aluminum and a known piece of NR16020 could be a false negative, but it's difficult to construct a credible explanation for how a match could be a false positive. You'd have to say that aluminum from a batch (or "heat") that was being used in 1936 was still being used to build or repair airplanes during the explosion in aircraft production that began in 1939.
We've tried, but have so far been unable, to learn how much aluminum was a single "heat" represented in 1936/37.
--- End quote ---
If all we are getting from artifacts and other samples is basic chemistry, there is no reason why a mid war heat from Kaiser might not have the same composition as a pre-war heat from ALCOA. The specs for 2024 allow for minimum and maximum percentages of the desired alloying elements as well as TOA (Total of All Others), so any combination within those limits would have been possible. Manufacturing processes strive for repeatability, so having two different heats with the same chemistry is not only possible, it was desirable and probably strove for.
To our advantage, each piece of Alclad has two signatures, so the odds of matching both sides of two samples would greatly reduce, but cannot eliminate, the chances of a false positive (believing that two samples were drawn from the same lot when they were drawn from different lots).
Ric Gillespie:
You know a lot more about the manufacturing process than I do. Are you also familiar with neutron activation analysis? I'm no expert, but my understanding of is that NAA yields a highly individual "fingerprint" that includes specific trace element profiles that is not likely to be duplicated in a different batch of metal.
Walter Runck:
--- Quote from: Ric Gillespie on February 13, 2011, 05:30:19 PM ---You know a lot more about the manufacturing process than I do. Are you also familiar with neutron activation analysis? I'm no expert, but my understanding of is that NAA yields a highly individual "fingerprint" that includes specific trace element profiles that is not likely to be duplicated in a different batch of metal.
--- End quote ---
My experience with metallurgy is from a commercial perspective (recertifying tool steel to support a marketing effort), rather than research or artifact analysis, so I only know as much as I've had to learn to get the job done in the past. That said, if NAA provides sufficient resolution on the TOA contributors, you'd have to set up a statistical test to see whether the variability within NAA analyses of a sample is small enough to differentiate one specimen from another. I'll ask around.
BTW, I owe the forum some corrections on ALCLAD and request permission to revise and extend my remarks. It's a full sandwich, not an open face, with one sheet of structural material between two sheets of CP (Commercially Pure) aluminum. Hot rolling, heat treating and cold working all contribute to diffusion (mixing) of the alloying elements in the structural center into the CP outer layers. Significance to us, I would expect NAA surface testing results to varying depending on the life history of the sample, not just the original heat chemistry. The more heating and beating, the more homogenous the material. Or, the longer the trip, the soggier the sandwich!
You make a good point; find the plane and all will be revealed (or overcome by events). Hope I can help.
Navigation
[0] Message Index
[*] Previous page
Go to full version