Knife-edge fasteners in bonded stackups 1

[sdar79]

On typical sheet metal structures, we want a fastener countersink depth to be no more than 2/3 of the part thickness to avoid a knife-edge condition. Let's say I have a single part made of three sheets of .040 thick material that are bonded together to give a total stack up of .120 thick. I want to install a fastener that has a .063 deep countersink. Do I call that knife-edge because the fastener head is larger than the .040 outer layer? Or do I say it's good because .063 is less than 2/3 of the total stack up thickness of .120?

 

[rb1957]

depends on the bonding ...

good hot metal-metal bonding makes the two sheets work as one ... DHC-8 construction is typically 2 thin 0.032" sheets hot bonded with BB4 rivets so the CSK penetrates the outer sheet ... fatigue performance is fine.

i woundn't trust a room temp cure bond.

i'd plann on some coupon tests to validate your design.

Quando Omni Flunkus Moritati

 

[WKTaylor]

MG201...

I agree with rb1957 in principle, except real-live experience is a buggar.

This practice is typical on several old acft I work on; and would work indefinitely, except for one major problem.

Environmental attack of the knife-edge skin(s) and adhesive layers by cappilary action of moisture around the fastener head/shank, is the enemy of this stack-up.

We discovered that after drilling/countersinking this stack-up, strongly urge You do the following for long-term environmental protection of the joint to avoid delaminations, corrosion ,etc

Apply "no-rinse" chemical conversion coating [CCC, MIL-DTL-5541 Cl 1A type opt] to the bare aluminum edges in the hole and countersink.

Apply [chromated or non-chromated corrosion-inhibitive] epoxy primer to the CCC hole and countersink... and onto the surronding surfaces, both sides; then allow the primer to dry.

Install fastener [FH Bolt, FH blind bolt/rivet or FH rivet, etc] "wet" with corrosion-inhibitive polysulfide or polythioether structural sealant.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[rb1957]

i'll go along with "the devil's in the details"

Quando Omni Flunkus Moritati

 

[blakmax]

The above comments are correct in relation to dependency on the integrity of the bond. Despite the best "sales job" by Boeing that this failure was due to "Multi Site Damage", the Aloha 243 incident was actually caused by bad bonding practice, combined with knife edge countersunk holes. The MSD would never have occurred if the bond had not failed first.

Boeing chose a room temperature curing film adhesive to bond the joints. If the adhesive was allowed to warm to room temperature before use it cured, so it had to be used frozen. That caused condensation to settle on the adhesive prior to assembly. The result was disbonding and corrosion which then led to cracking at the knife edges in the fasteners. [Reference: Aloha 243, Are You Still Up? by Macarthur Job, Google it.]

I strongly urge you to not only verify the strength of the bond in question but also to verify the bond durability. See DOT/FAA/AR – TN06/57, Apr 2007. Durability of adhesive bonds to metals not only depends upon initial strength, it depends most upon resistance of the interface to hydration which causes a loss of bond strength over time. Short-term strength and fatigue tests do not discriminate between processes which produce good initial bond strength and fatigue resistance and those which later fail in service due to interfacial hydration.

If you are to rely on the bond, then you must know that the surface preparation process will provide adequate bond durability. And please, do not rely on "moisture conditioned" specimens. Moisture conditioning only influences the bulk adhesive properties, not the hydration resistance of the interface.

Regards

Blakmax