Limit loads on rivets (fasteners) 1

[dcascap]

Hi all,

Out of curiosity, in general when you evaluate a joint, in general you check ultimate strength of fasteners, plus limit/ultimate of the connected parts.
Why limit load of the fastener itself is not checked? Do you have any reference explaining this?
My thought is the following:
Most of rivets only have info on the ultimate load allowed. If I see the material used for rivets and bolts the ultimate to yield values according to MMPDS (material itself not fastener), have a ratio ultimate/yield>1.5, therefore, it seems to be conservative for aerospace.

 

[SWComposites]

Its because there is no "yield" shear allowable defined or published for fasteners, and fastener allowables are typically derived empirically from fastener tests not from material properties. Why no yield data? probably because there is no clearly defined yield point and because the test standards for fastener shear don't measure displacement (though I haven't looked at those standards in a very long time) and probably because its never been an issue.

 

[rb1957]

you could do a bearing yield check on the material around the fastener.

you could fashion your own yield shear allowable, by scaling the ultimate shear allowable by fty/ftu of the fastener material.

but like SWC says ... hasn't been a problem in some 50+ years ... in an industry that looks for problems.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[dcascap]

I totally agree with you guys, you confirmed my thoughts. I just wondered if there was a reference for it, even though that from experience is not used and it has never been a problem in industry as rb1957 mentioned.
It came today in a discussion with a colleague but I said I didn't not have a reference for it but it didn't make sense to look at yield except maybe for bearing of the connecting part, as sometimes you want to remove thr bolt and you don't want any deformations at limit load

 

[SWComposites]

Also, bolts should not be the critical failure mode in an aerospace joint, the sheets in bearing should be. That allows load redistribution within the joint and should avoid a two part fracture of the structure.

Fuse pins are an exception but a very specialized type of joint.

 

[WKTaylor]

BTW... my 2-cents-worth. hope this makes sense.

Solid-driven rivets, installed per the book, typically exhibit very high reliability/statistical-consistency for many reasons. I usually accept the installed yield and SHEAR strength values per MMPDS/MIL-HDBK-5... or my company's design manuals. Straight holes with collapsed/expanded shank and bucked/driven 'tails'... 100% solid rivets... are remarkably efficient/cost-effective low strength fasteners.

Blind Rivets [BRs] on the other-hand, tend to exhibit installed shear strength/yield variations that can be difficult to design for. The BR OEMS strive for consistency in manufacturing for consistent installation... when 'installed by the book'... in-tolerance hole sizes, grip-lengths, chamfering/deburring and countersink etc. For this reason I deliberately design blind rivet installs for the lower strength of the 'yield allowable'. However if the BR install replaces a solid Rivet install... that is when I step-up the diameter to next larger... or at least +1/64[0.016]-OS or 1/32[+0.032]-OD larger than nominal BR diameter... and try NOT to deepen countersinks... and still try to evaluate the OS BR install VS yield... VS solid ultimate. Usually a CRES/Steel pull-pin for the BR gives the rivet a higher shear and tension yield.

BTW... +40-years ago when I started, solid rivet TENSION allowables were published. I have some of those allowable tables in ancient data... You can't find these allowables published anywhere that is 'open source'. Some companies maintain this data for use ONLY where tensile fatigue on solid rivets is NEVER An issue [rare]. Conveniently Blind Rivets, Bling bolts, Lock-Bolts and bolts/nuts/collars do have some degree of allowed shear/tension allowable loading [within prescribed limits].

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

too much "ink" and "brain-effort" has gone into this already. If your colleague thinks you need to do a limit load check, ask him "how? what calculation?" then "why?". if he says "why not ?" say "Generally Accepted Analysis Practices".

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[LiftDivergence]

Failure of joints is general in what we call "transitional" regime... neither pure bearing or pure shear failure.

As mentioned it is commonplace to check bearing yield MoS.

If your joints are shear critical and you are worried about yielding of the shank in shear, you have bigger problems.

Keep em' Flying
//Fight Corrosion!

 

[mfgenggear]

Mighty nice for these gentlemen sharing their knowledge. Many years of experience. Don't for get that that is by the book. Go down on floor and watch
The guys that incorporate thar assembly by the book.

 

[dcascap]

Thanks for the answers, it's funny that you mentioned that rb1957, it is literally how the conversation went. But then I thought to myself, well it does trigger my mind a bit and I could ask for an external opinion to verify I'm not missing something

 

[mfgenggear]

During Ww2 and after all designs were extensively tested, my opinion it's not done enough to many bean counters get involved.

 

[rb1957]

my general approach to this sort of "silliness" is to start expounding on how complicated the analysis (and testing) will be to Prove it good, and whether he (or the project manager) has budget for this "nonsense". Or would he rather we say "GAAP", "PBI", and have done with it.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[dcascap]

Yeah exactly...

 

[mfgenggear]

No worries, you signed it. It's all good.

 

[WKTaylor]

MFG... Design, ground-test, make changes, flight/service test, modify/fix A-model configuration and begin production... was a valid recognized process for airframes... 1940s to the early/mid 1970s... Then the process shifted towards design based on specifications... Analysis tools got the design 'close'... but testing was the poof of the puddin'...which often needed beef-up and re-test.

Let's reflect on, say the B-29. After the initial flight tests and early production. There were hundreds of mandatory/critical changes instituted in production and post production... mostly for safety or serviceability. The 'old process' came with 'speed'... but was burdened with problems... which delayed entry late in WWII... and it only served a short while doing intense bombing of Japan. After WWII... it was a much better Acft and served into the jet-age and the Korean war... although many were destroyed by jet-MiGs. A old friend of our family was a navigator on a B-29 in the Korean war. Loss rate was horrible and he had low expectations of survival... but he made it! One thing he never forgot was that the B-29s [still] had many service issues... which made them seem unreliable.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

The B-29 is an amazing story ... the program cost (AIUI) more than the Manhattan project !

And wartime is not peacetime ... some corners are cut to get the mission accomplished, to get the capability into service. The loss rate may be "high" but that can be fixed.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[mfgenggear]

I guess it, the B29 was reliable enough to get it done. Remember the initial testing on the f16. Lots of bugs.

 

[rb1957]

yeah, that and enough "bloody mindedness" on the part of the USAAF upper management ... "we can't fail, then they'd turn to the Navy to get the job done ... and that won't do". Daylight bombing in Europe, heck night bombing in Europe ... all cost an enormous number of young men's lives for questionable gains. What daylight bombing did do is it forced the Luftwaffe fighters up, and eventually annihilated them, gave the allies air supremacy, and "the rest is history".

war is politics by other means

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[mfgenggear]

Amen brother