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  #21  
Old 08-31-2011, 03:50 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by morristull View Post
....
A deburred hole has a much higher tolerance to stress and cracking than a square hole that has been deburred by rubbing a piece of scothbrite over it. You do not find square fishing rods as they would would break very quickly when flexed and the same applies by putting a small radius on rivet holes achiving the same increased crack tolerance. cheers Morris.
Not really correct.

The Mil-Spec a couple of posts above just references "burr removal" not chamfering or radiusing the edges of rivet holes.

The Cherry-Max specifications (approved aircraft rivets) specifically show a diagram that says radiused edges are bad.

Page 19 here - http://www.cherryaerospace.com/files...og/CA-1015.pdf

Remove the burrs, sharp edges meet all of the process specifications.
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  #22  
Old 04-06-2018, 04:16 PM
TASEsq TASEsq is offline
 
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Seems the b787 is devolving cracks because holes did not receive A “demurring chamfer”...

SUMMARY:
We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 787-8 airplanes. This AD requires revising the maintenance or inspection program, as applicable, to include an airworthiness limitation for repetitive inspections of the web fastener holes in the overwing flex-tees. This AD was prompted by a report that certain web fastener holes in the overwing flex-tees at the wing-to-body interface might not have been deburred properly when manufactured. Fastener holes without the deburr chamfer applied can develop fatigue cracking. We are issuing this AD to detect and correct cracking in the web fastener holes in the overwing flex-tees, which can weaken the primary wing structure so it cannot sustain limit load.


https://www.federalregister.gov/docu...pany-airplanes
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  #23  
Old 04-07-2018, 12:21 AM
rv8ch rv8ch is offline
 
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Default radius vs. square, rivet

It does seem to make sense that a hole edge that has been perfectly radiused would be less likely to have a stress fracture. The tools we use to deburr help create a more radiused hole edge - although not perfectly radiused - if used correctly.

It's not clear to me if the rivet that's put into the hole and then compressed, expanding to fill the hole and more, is happier with a more square hole edge or a radiused edge, and would this in any way affect the likelihood of a stress fracture?

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  #24  
Old 04-07-2018, 12:45 AM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by TASEsq View Post
Seems the b787 is devolving cracks because holes did not receive A “demurring chamfer”...

SUMMARY:
We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 787-8 airplanes. This AD requires revising the maintenance or inspection program, as applicable, to include an airworthiness limitation for repetitive inspections of the web fastener holes in the overwing flex-tees. This AD was prompted by a report that certain web fastener holes in the overwing flex-tees at the wing-to-body interface might not have been deburred properly when manufactured. Fastener holes without the deburr chamfer applied can develop fatigue cracking. We are issuing this AD to detect and correct cracking in the web fastener holes in the overwing flex-tees, which can weaken the primary wing structure so it cannot sustain limit load.


https://www.federalregister.gov/docu...pany-airplanes
If they are fasteners as opposed to rivets, as the AD seems to imply, then a burr at the hole edge could certainly stop any bolt or similar fastener from sitting flush and in full contact with the material surface. This would prevent the fastened joint from developing the design strength characteristics.
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  #25  
Old 04-07-2018, 04:47 AM
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Always get worried when I see an old post resurrected...... Thought I had done something wrong
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  #26  
Old 04-07-2018, 08:10 AM
David Paule David Paule is offline
 
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What I wanted to see, as an aerospace stress engineer, was square edges, round holes, joints in flat contact to each other with no debris between the pieces.

Especially for hole-filling fasteners like driven rivets, that let the rivets actually fill the hole. Over deburring would have the rivets see more bending moment than otherwise, which is bad for fatigue. Fatigue life was increased by the preload force that the rivets create between the parts, as that provides some shear to transfer through friction.

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  #27  
Old 04-07-2018, 09:26 AM
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Snowflake Snowflake is offline
 
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Quote:
Originally Posted by rv8ch View Post
You're assuming that a rivet when driven will expand to fill the filleted edges. You're also overlooking the fact that this graphic only shows one sheet, when in fact you'll have two sheets stacked, with back-to-back fillets that would need to be filled in.

To fill in the fillet under the manufactured (round) head, the rivet shank would have to swell a lot starting right under the head. That's not going to happen, and because the rivet is unsupported by the skin at that point it's more likely to crack at the sharp corner where the shank meets the head.

On the shop head side, I could see the rivet expanding to fill the space. But nowhere else with any consistency.
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  #28  
Old 04-07-2018, 10:26 AM
rv8ch rv8ch is offline
 
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Default 2 sheets

Quote:
Originally Posted by Snowflake View Post
You're assuming that a rivet when driven will expand to fill the filleted edges. You're also overlooking the fact that this graphic only shows one sheet, when in fact you'll have two sheets stacked, with back-to-back fillets that would need to be filled in....
Correct - I missed that critical detail, thanks Rob!

Here's the modified picture - I think you are right, the rivet will probably not expand properly to fill in the gaps.

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  #29  
Old 04-07-2018, 08:47 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by rv8ch View Post
Correct - I missed that critical detail, thanks Rob!

Here's the modified picture - I think you are right, the rivet will probably not expand properly to fill in the gaps.

I have done quite a bit of analysis of riveted joints by milling away 45% of the hole diameter, polishing, and then viewing under magnification.

A rivet actually does a very good job of swelling into the irregular areas caused by excessive deburring or the voids that result from nested dimples.

Where the rivet doesn't swell as well is directly below the manufactured head. All of the additional material in the larger diameter rivet head causes this

That is why the one with sharp corners will be stronger. The small amount of shank swelling that happens in this location is more likely to tighten up in this part of the hole.

The one with radiused corners will result in less baring area against the rivet, of the sheet directly under the manufactured head.

This issue gets worse as the stack-up gets thicker.

BTW, this is why some specialty rivets have a raised nub or dimple in the center of the manufactured head. When the rivet is squeezed or driven, the extra material is pushed down the center of the rivet shank helping the portion of the shank just below the manufactured head to swell more than it would have otherwise.
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