I would define wrinkling as a form of buckling of a panel, regardless of its means of being fastened. The wrinkles can extend beyond any fastener group and may not be associated with any fasteners at all. Wrinkling can be produced by shear, not necessarily compression.
Inter-rivet buckling, on the other hand, involves an area of material that has deflected significantly between the fasteners, regardless of the overall shape or size of the panel. IR buckling is usually associated with compression in the joint with the fasteners.
Do you have a copy of Bruhn? There are several discussions, with diagrams & photos, in C7, C8, C11 and probably elsewhere.
Inter-rivet buckling is an instability that occurs between two adjacent rivets in a stiffener, the strap of skin between two rivets buckles as if it were a wide column supported by a rivet on each end. Wrinkling is a skin buckle that continues through a stiffener attach flange (possibly) to the adjacent panel, the behavior is mostly driven by rivet spacing and the distance between the rivets and an upstanding stiffener web. Which is more critical depends on the panel.
The classic reference for wrinkling is NACA-TN-1255, figure 1 is a good illustration of the wrinkling/inter-rivet buckling relationship (pdf):
Something to consider when talking about inter rivet buckling, is the observed item truly inter rivet buckling, or is it something caused by displaced material from bucking the rivets too hard. I've seen this lots of time where the skin layout was done correctly, but when assembled, the skin puckered up between the rivets after the rivets were driven too hard and actually displaced material under the head of the rivet.
Sometimes this condition gets set up when a "cleco",a common fastener slips or looses it's grip on the two different work pieces.Its not common but when noticed it was assumed that it was stress induced,after much concern and study,it was reazlized that it was shop practices that were to blame.The spring in the cleco becomes worn and losses its ability to hold tightly the two pieces being joined,and slippage occurs,rather than stopping at that point most riveters continue the operation,all looks pretty normal,but after a little while the excess material that had bunched up shows up as a ripple ot wrinkle.It was latter realized that worn out springs in the cleco's were the cause.Some of these temporary holding or clamping devices have been arounr since before ww-2,ans are still in use.
After my first post I did some digging into brugn and also with a research paper. My findings are follows:
- At IRB, the skin between fasteners buckle locally. This do no deform the attached flange.
- At wrinkling, the skin has to deform in a way that it also deforms the attach stiffener. Thus wrinkling failure is a combination of skin and flange failure and could occur due to high shear stress as well.
At both these instances, these loads will induce tension loads on attached fasteners and fasteners need to be checked in tension
Thank you for your replies.
Bruhn is in all my digging about the best reference book / text around.My only problem with it is the way it's organized,it may seem logical for some readers,but not for me.things that would seem to be or needed to be placed in one area are some place else.I at one time thought about re doing it in my logic pattern to make it easier to use or more systematic,but it may well and probably is my logic pattern that is not logical,so I left well enough alone,probably be a legal nightmare in itself.
Your conclusions are concise and to the point,and I can see no error it their logic,good rules to go by.those old NACA/NASA documents are a real treasure trove if anybody has the time and patient to go thru them,I'm wondering if they are printed and published somewhere,anybody know ?
RE: ANSI H35.2 Dimensional Tolerances for Aluminum Mill Products
Perfect flatness of raw material [sheet, plate, etc] is a fairytale. Per H35.2, aluminum sheet is 'allowed' an as-manufactured out-of-flatness tolerance that is substantial for large flat skins, webs, etc... but perfectly adequate for small flat, flat-formed [bent or hydroformed] parts such as frames, ribs, intercostals, formed-spar channels, etc. Just look at the average wide/long sheet of clad aluminum from a shallow angle, and this phenomena is easy to observe!
Just about ever GA aircraft out there has waviness induced by the natural-out-of-flat condition of the raw sheet aluminum stock. Perhaps these tolerances can be reduced a bit for 'premium sheet-stock' with extra flatness requirements imposed [$$]; however this is not the rule from most raw sheet/plate aluminum stock. Same goes for any 'hard' alloy made from titanium, CRES, Inconel, etc.
When assembled to rigid structure, there is a tendency for large skins/webs to form shallow waves or pillows between rigid members, such as stiffeners, frames, fittings, etc. Often adjacent open bays may have opposite tendencies: one bay pillowed-out, adjacent bay pillowed in... and natural wrinkles and large pillows may actually pass-thru these rigid edge attachments. Most GA Acft, when taxied at low speed and low power [reduced noise levels] can produce some awful popping/snapping sounds... clearly indicating that the skin/web is in a state of dynamic flex in these 'oil-canned bays'.
NOTE. The technical term for 'oil-canning' is [gasp!!!] 'snap-thru elastic buckling', which describes the nature of the beast: the member snaps from one-state [EX bowed-out] to the opposite-state [bowed-in] elastically, and almost instantly, with very little force [transverse to the sheet plane]. The problem with oil-canning is [generally speaking], fatigue cracking at periphery fastener holes due to the large number of oil canning cycles per flight [which are not usually heard]. Even though flight loading can mitigate the problem for awhile, still experiences various stress states in non-static-flight conditions; and eventually the structure still has-to return to it's normal static state when tied-up on the flight line. NOTE: the poor riveting practices noted in a previous post [excessive localized swelling due to rivet installation pressure] is a great aggravating factor for this phenomena!
NOTE.
I have encountered a phenomena where localized overstress due to poor fastener-fit [and also poor riveting practices - another story], mechanical or flight loads has induce permanent distortions resembling natural oil-canned sheet, that make it 'easier' to 'oil-can buckle' with less/less energy...' at lower and lower outside loading. In worse case instances, fasteners in holes can also loosen/smoke/crack... and worst possible case is where flutter [natural vibration] frequencies can change... usually for the worst [IE flutter at progressively lower airspeeds, etc].
NOTE. The aircraft I work on was designed with secondary wing rib webs allowed to diagonal-tension-wrinkle at/above limit load: rivet-hole cracking is a major problem. I suspect that, even though the thin web sheet used was perfectly adequate by analysis/test, it had enough inherent natural flatness tolerance issues to be a 'set-up' for oil-canning 'as-riveted into position'; hence premature failure.
Regards, Wil Taylor
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