secondary bonding and aircraft primary structure certification 1

[RPstress]

Does anyone have experience of important airframe that's been secondarily bonded?

My understanding was that things like fuse, wing covers and the skins of fins and tailplanes couldn't have stiffeners secondarily bonded on. Co-curing was ok, but any joint made after the parts were cured had to be mechanical.

This is because (I thought) bonded joints are theoretically impossible to inspect for strength, unless a proof test is done (ultrasonic NDT says it's fine, but it could still be a 'kissing' bond, with no strength).

I notice that the 787 fuse has co-cured hat-section stiffeners and mechanically fastened frames.

-RP.

 

[CompositeGeek]

I am aware of some bonded structures in Helicopter empennages. MD Helicopter has secondary bonds in use for their horizontals on NOTAR models and in the tail booms themselves. The horizontals have solid laminate to honeycomb verified by NDT (very painful NDT). The bond line was created using film adhesive that is very persnickity.

I have worked a number of years on Brand A and Brand B primary and secondary structures and I do not recall a secondary bond line that did not have fasteners.

I know in General Aviation Lancair has secondarily bonded wing skins, to ribs and spars on the columbia 300. They used NAIR specifications for the bonding.

Good Luck

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[SWComposites]

Its been done; in several cases by fudging the definitions of bonding and cocuring: precured stiffeners have been "co-bonded" to uncured skin laminates during cure of the skin. If you want more info that what I am willing to post here, give me a call (# is at my web site:

http://www.swcomposites.com).

Steve

 

[MechanicalChef]

Sorry I can't help your search, but for what it's worth, laser-shearography (NDT) can find kissing bonds in limited cases. Given your ply depth is smaller than your minimum defect size (1/8" or larger defect under 1/8" of laminate) for example. It can find them, but since shearography is a subjectively analyzed test-method, and it can't find porosity, it's a hard sell.

 

[RPstress]

Many thanks to all three of you.

It's becoming clearer that secondary bonds for primary structure may well be acceptable to the authorities provided you can show damage tolerance, e.g., the structure can still carry limit load with (perhaps) a stiffener disbonded.

Co-bonding (for instance, pre-curing the stiffener and bonding it on in the same op as the skin is cured) seems to help, as well as saving an operation.

Natch, all processes, partic. surface prep, have to be very highly controlled.

Any additional comments are still most welcome.

-RP.

 

[berkshire]

RPstress (Aerospace)
Look at the German sailplane industry and light airplanes made in Germany such as those by Grob GMBH and the Diamond aircraft also Extra aircraft. they use secondary bonding under LBA supervision and have rules regarding damage tolerance. It may not help you under FAR's but it may give you some ideas.
B.E.

 

[RPstress]

Berkshire - do you know if the LBA is involved on a day-to-day basis, or is it more of an approvals and occasional inspection type of thing?

 

[CompositeGeek]

SW reminded me! Thanks!

I used to do a co-bond similar to the one described by SW composites for the A330/340 shroud box. Precured channel bonded into an uncured laminate. The process was abandonded later due to cost not due to any engineering issues. It was fully vetted through recertification to the new design.

There was another example of this in the wing panel forward of the shroud box on that aircraft over the Main Landing Gear storage area. That work went to (UK) a while ago so the process or product may have changed since then.

This particular application also had some very tough failsafe modes to meet for tire burst and other considerations. I had forgotten about these application in my earlier response. It involved a precured solid laminate I beam co-cured with a sandwich laminate. Pretty large part too. Easily 10'x10' range.

The design was qualified on the initial production, and modified several times but fully qualified through all regulatory requirements.

RPStress, I think you are correct in that the FAA and other agencies will look at this kind of design solution, I think they will want a lot of data collection on issues like damage tolerance, latent defect tolerance (NDT types), along with material qualification data for basis etc. In each case I identified above we had extensive testing (including full scale destructive etc) to complete to satisfy everyone.


Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[berkshire]

RP stress.
It is more of an approvals and occasional inspection thing, most of the inspections are done by the factories own inspectors. with audits by the LBA.
B.E.

 

[berkshire]

RP Stress,
The following link is to the Glaser Dirks web page which has some info and some videos of building sailplanes.
This may provide some information, or not, on secondary bonding.

http://www.dg-flugzeugbau.de/index-e.html

B.E.

 

[RPstress]

Thanks 'Geek. Interesting.

Thanks for the update Berkshire. Currently my net-nanny won't let me get at that site, but I'll get them to give me access.

Another snippet I've collected in the meantime is that (apparently) the A400M wing has pre-cured stiffeners which are co-bonded to the skin when the skin is cured. This may have a lot to do with workshare: the stiffeners are made in Spain...

Also, apologies for delays in response. I'm currently working off-site with net access on occasional Fridays.

-RP.