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Rivet factory head failure - can anyone tell me how this happens?

mill2978

mill2978

Well Known Member
While I was riveting ribs to rear spar on my RV8 wings and 5 AN470AD4-8 factory heads failed in the same way.

RivetHeadPoppedOff2.jpg


RivetHeadPoppedOff.jpg


I was hand bucking these with a double offset cup and a tungsten bucking bar. I was using ~45PSI and the shop head was well formed. I had an assistant who was holding the rivet gun perpendicular to the work so I could focus on squeezing the trigger and forming the shop head. When we lifted the rivet gun the factory head just fell out of the cup. leaving the rest of the rivet in place.

Any ideas how this happens? Was this technique or could I have other rivets that are ready to fail in a similar way? I inspected(including some light prying on the rest with no failures) and don't see any indication of problems.
 
defective rivet?

I don't know much about rivets but could it be a defect in the rivet itself?
 
I would say bad rivets, I would probably drill out some or all of them and see what they look like. I am sure someone else will chime in on this but if you do not get to the bottom of it you will always wonder. The only time I have seen rivet heads come off like that it was due to salt water corrosion on some old over seas containers.
 
Close ups of 3 heads

When I was cleaning up I mistakenly dumped the heads in the trash, i dug through it and found 3 of the 5, here they are in close up.

Top of factory head pre clean:
IMG_0274.JPG


Bottom of factory head pre clean:
IMG_0272.JPG


Top after rinse in MEK:
IMG_0276.JPG


Bottom after rinse in MEK:
IMG_0278.JPG
 
Your offset set is clearly sideloading the mfg head. You can see where they sheared in the upside down shot. Also looks like the cup is slightly undersize leaving an extruded rim around the head. Drill em all out and try another setup.
 
Yup, exactly what Arehed said. Even though the factory head appears ok (sans the ring) the bottom of the head show signs of shear; the area that has uniform parallel lines. The other part of the fracture area is the ductile failure after enough of the cross sectional area was reduced by the shearing action.

Corrective action; ensure the set is perpendicular (or in axial alignment with the rivet) to the piece being riveted.
 
Aerhed and Dan thanks for the help. I would much rather this be an issue of technique than of defective rivets.
 
Yup, this is technique - the force was not perpendicular to the rivet head, there is some side load there that is shearing the rivet.
 
Can you squeeze these rivets instead of driving them? Much less likely to shear the heads, IMHO.

I personally prefer to squeeze a rivet than buck it, if I can.

As always: YMMV!

BTW: Very sharp eyes, AERHED!! Well done. :cool:
 
This is a good read for a newbie like me.

Sorry you are having some difficulty with that set but thanks for posting it up for new guys like me to read and learn about.
 
Double offset rivet sets are extremely difficult to use correctly to get all the force perpendicular to the rivet. As the others said, the rivet heads are sheared off. Ditch the double offset wherever you can.

Might not be a bad idea to replace all rivets done so far with the double offset.
 
Careful

I agree that you don't want to add any side load when riveting, but I'm not convinced it's still not a rivet issue. I've experimented today and tried to induce a side load and snap the rivet head off and I have not been able to successfully do it; the rivet head deforms before enough side load is induced to snap the rivet head off. What pressure are you using on your rivet gun?

Bill
 
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A double offset rivet set will usually require a bit more than 45 psi. The length and mass of the rivet set suck up some of the oomph of the rivet gun. Try bumping up the pressure to more like 60 psi. I bet you are work hardening the rivet and the bit of side pressure cracks the head off. You shouldn't be pulling the trigger for more than about 2 seconds to set the rivet completely. Every time during my build that I had difficultly with rivets setting correctly, it was an air pressure issue (too much or too little).
 
Thanks all for the help in understanding what it happening. This forum is invaluable!

I was able to squeeze most of the rivets on the spar, it was an order of operations issue that got me in a spot that I needed to hand buck a few. I was trying to "work smarter" and squeeze all the rivets of the same length on the spar at one time to reduce resetting the squeeze length.

On the second wing I ditched that methodology and was able to squeeze all the rivets.

The pressure I was using was 65 until I broke the first head, then I dropped it to ~45-50 to see if that helped.

In retrospect I think that a contributing factor was the tool rotating in the gun as I bucked, this rotation could be the cause of the shear. Last time I used the double offset I put some duct tape on it to prevent it from rotating.
 
I think it is time to put down your rivet gun and call in another local builder. Check around and find if there is a multiple builder close by, they can be a great resource. Clearly there is something wrong with your technique and this forum, although a great source of information, will not solve this one for you. I would suspect that you may have issues with all your driven rivets. That is not likely something you want to hear but now is the time to call in some help. If there is not a local builder then perhaps your airport might have a maintenance shop that does structural repairs. There will be someone there that can get you going down the right road.
 
Quote "Also looks like the cup is slightly undersize leaving an extruded rim around the head. "

+1 on this. The heads look like they have a ring around them. I think your rivet set is one size too small.
 
Wow thanks for this!

I would agree with the other responses in that it appears that a side load caused a shear of the head from the shank.

Having said that I am afraid I would suggest drilling out all rivets done in this manner to be sure that you will not experience a failure in a component under load while flying or maneuvering.

I have had disfigured heads on AD470's largely because I allowed the cup to shift while riveting. When I drilled them out none came off like this so it appears to me that perhaps you are not doing something right with the rivet gun? Sometimes I swear I drill out more than I shot in a day but I try to be Very comfortable with these things as in critical areas and under load, if they fail, it could get real uncomfortable in the cockpit fast in my opinion.
 
Drill out? I think that is an opportunity for more/worse failure

I appreciate the suggestions, and I have already engaged other repeat builders and my EAA tech inspector.

I hand bucked 7 rivets, never more than one per rib, the rest were squeezed. I've inspected them closely 1 has a moderate smiley on it's face the others do not.

To drill these out and then be able to squeeze them would require drilling out most if not all rivets holding the flap brace to the spar. To me that would introduce more risk of future damage/failure. I am off base? :confused:

I'll be calling the mother ship later today to see what they say.
 
I had a heck of a time with my double offset rivet set. I thought that would be easier to use since..... well...... it just seems like it would make more sense. Nope. I bought a single offset and duct taped it in the gun. Just smash a few on the bench first. It takes a bit different pressure so you don't mess the head up, but I was much more proficient with the single offset set.
 
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