jorisdebont From Netherlands, joined Sep 2012, 10 posts, RR: 0 Posted (8 months 13 hours ago) and read 2693 times:
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?
Dalmd88 From United States of America, joined Jul 2000, 2370 posts, RR: 15 Reply 1, posted (8 months 12 hours ago) and read 2653 times:
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.
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.
KPWMSpotter From United States of America, joined Dec 2006, 331 posts, RR: 2 Reply 4, posted (7 months 4 weeks 1 day 8 hours ago) and read 2159 times:
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...