Rivet grip length too short 1

[Gann]

Hello, if a rivet was incorrectly installed with a shorter than nominal grip length what would be the standard way of determining if it was strong enough or not. I know of ways to look at the final dimension of the rivet from measurements taken after the fact, but if measuring every single rivet was not an option is there another way of determining what the worst case scenario strength would be for these under-length rivets? Or are rivets just too unpredictable for something like this to be done? Thankyou for your help!

extra info: I have dimensions for a few of these under-length blind rivets, but not all of them of course. I'm a recent graduate so I'm thinking along the lines of a students T curve or something like that. But if some data exists for under-length rivets and their strength it would be very helpful. These rivets were installed on hydraulic brackets with a small handle load. So in theory this should be a simple analysis to do. I'm very new to this so I'm not sure how to properly approach it though. Thankyou very much again.

 

[berkshire]

Gann,
Are these solid rivets or pulled rivets ?
Either way the buck tail is going to be short.
With a driven rivet the thickness of the remaining head , and whether you have enough metal to achieve the tensile strength required of that rivet would govern.
With a pulled rivet , the size of the formed head , and whether it is big enough to adequately resist pull through would govern.
Either way I would suggest replacing them , unless removing and replacing the rivet results in further deterioration of the structure.
B.E.

You are judged not by what you know, but by what you can do.

 

[Gann]

Yes that's really what I'm wondering. Is there any way to accurately determine the worst case scenario size of the formed head in this case? Are the handful of specimens I have enough to do this? Or should I use a different approach? They are blind rivets. There was a bad length callout on the rivets and that's what caused this. I agree with replacement if that seems like the best approach. This is not a critical component though so if there was a method of determining if these are strong enough I'd like to be able to find it.

 

[rb1957]

if the upset tail is not to spec, then you have a problem.

it sounds like it's a research project to show it good,
or just drill them out and start again.

another day in paradise, or is paradise one day closer ?

 

[tbuelna]

If this is an aircraft structural application, then your question is purely academic. If you have reason to believe that there is an issue with some blind rivets installed in your aircraft structure, even if you don't consider it to be critical, the appropriate corrective action is to replace the rivets.

You mentioned the source of this problem was "a bad length callout on the rivets", but it is not clear exactly what this means. Were the rivets themselves incorrectly marked? Was the rivet callout on the engineering drawing/ECN/work order incorrect? Aerospace quality blind rivets typically have the grip length dash number marked on the head (refer to p.10 of the document linked below). This is to eliminate problems identifying loose rivets or mis-marked packaging. It would be highly unusual for the rivets themselves to be incorrectly marked. If there was an incorrect rivet callout on some engineering documentation or work order used, then you should submit the discrepancy to whoever issued the engineering documentation or work order, and request instructions on what corrective action to take.

It would also be a good idea to review the procedure you are using to install these blind rivets. It only takes a couple seconds to verify the material stack-up at each hole location is within tolerance limits for the grip length of the blind rivet being installed. It can be checked using a simple gauge (like the one shown on p.5 of the document linked below) that you can get for free from the rivet manufacturer. You should also perform periodic in-process inspection of the blind rivets to verify they are being installed properly. With the flush fracture spindle type blind rivets commonly used for aircraft structural applications, a quick visual inspection of the stem break and collar height after installation (refer to p.24 of the document linked below) would have revealed problems like the one described where the rivet grip length was too short.

Here's a good technical reference for installation of aircraft quality structural blind rivets:

http://www.cherryaerospace.com/docs/catalogs/CA-1005.pdf

 

[SWComposites]

The is no way to analytically predict the strength of the discrepant rivets.

If you really want to know you will have to run some tests on specimens made to the same (incorrect) config.

And as others said, just replace them.

 

[Gann]

Thank you all so much! I was expecting that there was no way to analytically predict the strength of them but I just wanted to make sure. Looks like testing is the only option besides replacement. My only issue with that is how consistent will the formation of the formed head really be in this case? Would a standard test sample of 5 to 10 really be sufficient? I am not familiar with testing of rivets.

The maximum value of the 1/16" grip length range from the NAS spec is 125 thou but our stack up was 140. Is there any extra tolerance on this grip range? I can't find any spec that talks about something like this. Fastener is an NAS9301B-4-02.

I'm all for replacing these things. I just would like a justification from a spec. Again, thank you so much!

 

[rb1957]

how does the head look ? if it looks "wimpy" then drill them out, if it is within spec then it's ok. The NAS should define the correct head, if not try Cherry (equivalent to CR3213).

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Gann...

A few minor comments on top of the sage advice from the other 'old guys'...

Following applies to blind rivets [BRs], and to blind bolts [BBs]

Proper deburring of the hole is very important to BR head-seating and tail-bulb forming. Proper deburring is to [barely] break the sharp/ragged edge of the hole. Any burrs on the hole can substantially interfere with head-seating or the tight BR tail formation and sheet take-up. HOWEVER, excessive deburring [chamfering] can also cause loose head-seating and sloppy tail-bulb formation.

Rivet grip lengths [GL] can be very tricky to define, especially when the material stack-up/fit-up tolerances [min-max total hole depth] is on-the-edge of two distinct GLs. In-general, when this GL situation occurs, I always recommend the longer GL be specified on the drawing... with a BOLD FACE CAUTION NOTE to measure with the GL-gage before actual installation [trust but verify]. AND ALWAYS visually inspect the BR installation [both sides] whenever possible: tight head seating and symmetrical tail-bulb formation are essential for good fatigue performance.

Cherry, Huck [ALCOA] and AllFast make BRs to the same NAS specs. TRY NOT TO MIX/MATCH BRs FROM VARIOUS VENDORS, if at all possible. Each vendors BRs tend to have subtle internal/external tolerance and material variations which affect installation and long-term performance that will drive Your mechanic [structural tech], and service engineers crazy.

IN GENERAL, BRs are harder to remove ['drill-out'] than solids... which tends to damage/oversize the close-tolerance holes for various reasons. When a LOT of BRs need to be replaced I recommend drilling-for, and installing, 1OS [1st oversize, 0.0156-larger Dia] BRs. The oversize for NAS9301 is NAS9304B-04-*. Recommended 'Hole Limits' for most [and these] BRs are usually specified in the NAS specs for each Dia rivet. I specify [explicitly state] that holes be drilled as small as possible to get a tight-as-possible installation. Hence...

For a 1/8 [nominal] 9301B-4-* rivet, the hole limits are given as 0.129--0.132-Dia. I usually specify to use a #30 [0.1285"] Dia drill bit.

For a 1/8 [1OS] 9304B-4-* rivet, the hole limits are given as 0.143--0.146-Dia. I usually specify to use a #27 [0.1440"] Dia drill bit.

Regards, Wil Taylor

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