"Oil Canning"

[WKTaylor]

Guys..

We all know sheet-metal structure "oil-canning" when we see/hear/feel-it... and that it can be a "bad" thing for load distribution and fatigue cracking a fasteners.. especially in a high load reversal or sonic environment. Obviously, for thin-sheet riveted structure this can be a very significant issue.

However, just recently I was asked by an inspector... to provide reference to "a well known, respected or authoritative document, that defines the phenomena... and what is typically aceptable and what is excessive" [requiring continuous inspection or repair]. This inspectior felt that some new vendor supplied assemblies were slightly distorted... allowing the rib webs to oil-can between stiffeners. The QA guy had NO way to judge the acceptability of these thin sheet metal assys.

I can find snippets of info here/there about "what it is", "why it is not good" and "what to do about-it"... but not how to judge the relative severity of oil canning for individual cases [IE: monor, mild, significant, severe, etc]. The "best I can tell" is that each aircraft maintenance manual should have this info provided as a background statement for inspections... but I guarantee You this info is rarely provided.

NOTE.
What REALLY bugs me is that the USAF aircraft structural repair T.O.(s) and the base-structural repair T.O. [1-1A-1] are virtually silent on the subject.

My "final answer" was an unsatisfying reference to the assembly drawing contour limits. The area in question must remain within drawing flatness tolerances when "force-canned" using light finger pressure... deliberately forcing the web area(s) to "snap-through" to opposite-max waivieness position... and then measuring absolute web wavieness difference [max-to-max]; and compare this to drawing contour limits.

Anyone else have any better references, data or approach???

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[berkshire]

Will,
In the Douglas process specifications for the DC 10 and MD 11
There were some references to oil canning.
They essentially re- iterate what you just wrote, with some pressure limits and tolerances. They were issued to the hand straighteners in the manufacturing sections of Douglas and General dynamics aircraft Co.s and other sub contractors who made parts. A shop floor fix for an oilcanning part was frequently to shrink an edge or gently planish the center thereby stretching it very slightly, while it was still in the AQ condition.
I no longer have access to that data, but it may point you in the general direction of someone who does.
B.E.

 

[Compositepro]

I'm certainly no expert here but it seems that the issue is fatigue. Fatigue failures depend on where the material is on its stress-strain curve during cycling. The material strain during oil-canning can be severe to almost nothing so you just can't make any general statement. It all depends on specific geometry.

 

[WKTaylor]

berkshire... thanks.

I have a few contacts in the DC/MD world... worth a shot.

Most times, T.O. data requires "one or more stiffeners be fastened across the oil can" [when allowed]. IF that works, then great... IF it doesn't, then replacing the skin, web, panel etc is the primary repair option. However, there is obvious maintenance/management pressure to minimize work effort/time/cost... so it takes serious convincing to allow major repairs... especially if there is no "immediate need" for repairs [no cracking, deep aligned wrinkles or permanent buckles, etc].

New parts/assys really present a challenge to determine best course of action... especially when inspection data is ambiguous and costs are high for new-old-design parts.

NOTE: I suspect that everyone in the 1950s just knew what was right/wrong for thin sheet metal. Most current design practices, rarely rely on thin riveted structure: new designs lean heavily on aluminum or composite sandwich structure.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[thruthefence]

Being a little aircraft guy, my comments are probably without any real merit here, but in my business, "oil canning" especially across bays between ribs, has always been a "canary-in-the-coalmine" indicator, of possible excessive loads being place on the airframe.