Material allowables for altered sheet thickness
Material allowables for altered sheet thickness
(OP)
In the aircraft world, we commonly "reverse" engineer using ultimate material properties found in Mil-Hdbk, MMPDS, or equivalent source.
Which allowable do you use if a part is made by altering a sheet thickness. Example a 0.090" thk 2024-T3 sheet is chem-milled down to 0.050" thk in order to fabricate a fuselage skin. Do you use the ultimate allowables for the original 0.090" thk sheet or the 0.050" thk in order to reverse engineer the max load capabilities?
My question pertains to sheet or plate at this time, however, I do have a similar question for extrusion which is machined which I may post after I do some further research.
Which allowable do you use if a part is made by altering a sheet thickness. Example a 0.090" thk 2024-T3 sheet is chem-milled down to 0.050" thk in order to fabricate a fuselage skin. Do you use the ultimate allowables for the original 0.090" thk sheet or the 0.050" thk in order to reverse engineer the max load capabilities?
My question pertains to sheet or plate at this time, however, I do have a similar question for extrusion which is machined which I may post after I do some further research.





RE: Material allowables for altered sheet thickness
If there were any doubts however which one you should be using, you could always stay safe and take the larger of the two values for say, the structure before repair and use the lower value for your repair materials.
RE: Material allowables for altered sheet thickness
debodine
RE: Material allowables for altered sheet thickness
The only exception would be if the reduced thickness panel was subsequently cold worked or heat treated in some way. I'm not sure what I'd do, then, apart from try to make a safe assumption as per Bazzo's advice.
I've never seen reduced static allowables for chem milling (aka etching). However, post-chem milled sheet with no further work on the surface usually has significantly reduced fatigue allowables. Allegedly this is due due to the relatively poor surface finish which chem milling can leave, but I've never seen any proof of this such as micrographs. The edge of the etched-away area can also be a bit ropey, with a degree of undercutting and a lack of a clear radius (though this should be minimised with a good process). Some people lightly shot peen the etched surface (obviously care and experience are needed for thin material). Boeing shot peen just about every piece of light alloy, even if it's quite thin (well, they did 10 years ago...), but I don't remember them peening the etched flap skins. Anybody know different?
Because of additional manufacturing cost and being a messy process, chem milling is generally avoided in more recent designs, even at a small weight penalty. Also, of course, a lot of secondary structure things which would have been thin aluminum are now slightly less thin carbon, which can be very easily varied in thickness.
Also, with thicker parts there can be significant internal stresses, especially with forged or rolled material. Exposing tensilely stressed internal material to the environment could give stress corrosion problems. Again, peening might be advisable.
RE: Material allowables for altered sheet thickness
This applies for extrusions as well.
SuperStress
RE: Material allowables for altered sheet thickness
This may also impact the allowable membrane stresses under pressure - the outer surface's Ftu isn't the same as the inner surface's Ftu.
Also, I thought I'd point out that I have observed extensive chem-milling in Beech twin-turboprops. Even frames in the tail, where the chem mill might remove only about 0.05 pound. (There's probably a beneficial CG effect).
Steven Fahey, CET
RE: Material allowables for altered sheet thickness
I was involved in the production of Chem milled fuel tanks for a fighter, the tanks were rolled to shape prior to Chem etch. When the excess material was removed, the tanks changed shape quite considerably, requiring cold working (hand straightening) to get them back to shape.
B.E.
RE: Material allowables for altered sheet thickness
I had to dig into my old "materials class" books and notes from my college days, but I think I have enough data to convince him now. I am looking forward to seeing what he has come up with.
Anyone know of any other good articles or text books on this issue?
RE: Material allowables for altered sheet thickness
In thin sheet/plate I would say you are okay to go with original. Personally I would use 0.100" as a cutoff before I started to get conservative. Plus at some point you have a "step" when secondary bending becomes an issue.
Surface finish would be my biggest concern. There have been alot of studies due to scribe mark issues. As well as alot of research of various lap joint issues. NRC did a study based on clad layer finish due to corrosion and local yeilding. Title "Damage Characterization of corroded 2024-T3 fuselage lap joints".
All my data is lap joint related though not much of the out in space (mid panel) stuff.
RE: Material allowables for altered sheet thickness