jorisdebont
Topic Author
Posts: 10
Joined: Sat Sep 15, 2012 11:39 am

Why Use Chicken Rivets?

Sat Oct 20, 2012 3:48 pm

I was at the Delft University of Technology last week and someone there told me about chicken rivets. Some parts of the wing can be glued and/or welded into place, no need for rivets. But they still put a few rivets into the wing to secure the part that has al ready been welded there. The rivets serve no purpose except for giving the self reasurance that there are some rivets there. Why do engineers put in these rivets?
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Dalmd88
Posts: 2395
Joined: Fri Jul 28, 2000 3:19 am

RE: Why Use Chicken Rivets?

Sat Oct 20, 2012 4:53 pm

I've never worked on any structure that has these but a quick search yielded redundant load path and peel back prevention in case of adhesive failure. Sounds like they are common in some newer biz jets like Gulfstreams.
 
Horstroad
Posts: 353
Joined: Thu Apr 08, 2010 8:19 pm

RE: Why Use Chicken Rivets?

Sat Oct 20, 2012 6:00 pm

fail safe
if the welding/bonding fails the rivets take the load. will probably never happen, but better be safe than sorry
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Why Use Chicken Rivets?

Sun Oct 21, 2012 7:12 pm

Quoting jorisdebont (Thread starter):
Some parts of the wing can be glued and/or welded into place, no need for rivets.

Yes. It's typically referred to as "cold bonded" or "hot bonded."

Quoting jorisdebont (Thread starter):
But they still put a few rivets into the wing to secure the part that has al ready been welded there.

Yes.

Quoting jorisdebont (Thread starter):
The rivets serve no purpose except for giving the self reasurance that there are some rivets there.

No. They have a purpose.

Quoting jorisdebont (Thread starter):
Why do engineers put in these rivets?

As horstroad said, if it's a fail-safe design the rivets provide backup in case the bond fails. Unlike a bond, rivets tend not to individually fail all at once unless you're talking about multi-site damage (like the 737 "convertible").

If it's a damage tolerant design, which is a lot more likely for today's designs, then the rivets greatly simplify your inspection plan. Assuming the rivets can carry limit load (which is the normal design criteria) then you only need to inspect the joint about every 1/3 to 1/2 of the *rivet* fatigue life. If you don't have the rivets, you have to do the much more difficult bonded joint inspection more often.

Tom.
 
KPWMSpotter
Posts: 451
Joined: Tue Dec 26, 2006 1:01 am

RE: Why Use Chicken Rivets?

Sun Oct 21, 2012 8:41 pm

The "chicken rivet" methodology is a hold-over from the early years of structural adhesive technology and realistically is no longer necessary. In the era of the 707 and DC-9 very little was known about adhesive reliability and generally the quality of bonding was quite poor. Rivets and mechanical fasteners were well known and were added for redundancy.

These days, structural adhesives are usually much stronger than the metals skins they are holding together, are corrosion resistant, and are fatigue resistant. Structural bonding can out-perform mechanical fasteners in many cases. In some cases, the addition of mechanical fasteners actually detracts from the effectiveness of the bonded technology.

Most MD-80 and 737-series flight controls (spoilers, ailerons, flaps) are constructed of bonded honeycomb wedges. Some of these surfaces (especially on the MD-80 series aircraft) also make use of redundant rivets which fasten into the honeycomb core. One of the biggest problems with honeycomb core is moisture ingression (which leads to disbonding, weight gain, corrosion, etc). The "chicken rivets" on these flight controls actually accelerate the failure of the parts, since the rivet holes provide a path for moisture to enter the honeycomb. Boeing has issued a number of standard repairs for these parts which replace the skins with adhesive only (no rivets) to improve the service life of the parts.

Initially, yes, chicken rivets were installed for redundancy. In modern composite structures they have very little use, and often cause more problems than they solve. That being said, improper use of adhesives can also cause problems (the aforementioned "737 convertible" was partly due to improperly installed adhesive interfering with a rivet joint), but that's a different topic altogether...
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