Material allowables for altered sheet thickness

[737eng]

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.

 

[Bazzo]

I would tend to use the material allowable for the base thickness, i.e. before chem. milling or other type of processing.
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.

 

[debodine]

I would wonder what effect the chem milling had on the properties of the material (if any, I don't know myself).

debodine

 

[RPstress]

The mechanical properties depend mostly on the as-supplied sheet or extrusion thickness. The thickness affects the rate of cooling from solution treatment, and is also a function of how much work has been done on sheet or plate to reduce it in thickness. Both these things have a significant effect on microstructure, and will affect the mechanical properties after aging (natural or artificial).

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.

 

[SuperStress]

The quick answer is that the analysis convention is that you use the original material thickness for static strength mechanical property data, even for chem-milled skins.

This applies for extrusions as well.

SuperStress

 

[Sparweb]

Wouldn't cladding also play a role? If the 0.090" sheet was clad before chem-milling, then the cladding is gone by the time it's been milled down to 0.050".

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

 

[berkshire]

One thing to consider with chemilled parts is the effect of the removal of metal on a pre stressed part.
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.

 

[737eng]

Thanks for all the answers. Years back I was taught to use the as supplied sheet thickness for the same reasons tht RPStress mentions. Since it made sense when explained to me and seemed to be conservative, I never questioned this process or even researched it. Now a few years later, I was just asked the same question by a young engineer and I told him to use the as supplied condition. This younger engineer isn't so willing to take the advice as blindly as I did when I was younger and he hit me up with about 10 other "why" questions. So I told him to do some research on his own and I will do my research and then compare notes at the end of this week, in the meantime since using the as-supplied allowables is more conservative than using the milled thickness allowables to continue on as instructed.
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?

 

[planedr]

Using the original would seem acceptable but I would be concerned as to where the material was removed from and the remaining surface finish issues. Plus the clad no clad issue. I have had to take a hit on fatigue life when we milled down some extrusion. Due to the grain direction and sizes. That was the justification anyway.

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.

 

[planedr]

We had an approval on material milled from plate shot down 3 days ago (day after my previous post). I should have been sheet stock but a gentleman decided to save time and mill it from plate. Since they had no sheet in stock.