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Articles - Effect of Sealant on Behavior of Structural Joints
2

Articles - Effect of Sealant on Behavior of Structural Joints

Articles - Effect of Sealant on Behavior of Structural Joints

(OP)
Previous thread thread2-441923: Highest Recommended Edge Distance is now closed, leaving me unable to add to it.

With regard to some of the later discussion, I have found some great articles which people may find interesting:

Fatigue behaviour of double lap riveted joints assembled with and without interfay sealant
Fatigue & Fracture of Engineering Materials & Structures 34(1):60 - 71 · June 2010
https://onlinelibrary.wiley.com/doi/abs/10.1111/j....

Analysis of the Effects of Interference and Sealant on Riveted Lap Joints
Journal of Aircraft 44(2):353-364 · March 2007
https://arc.aiaa.org/doi/abs/10.2514/1.18320?journ...

Analysis of the Effects of Thin Sealant Layers in Aircraft Structural Joints
AIAA Journal 41(11) · November 2003
https://arc.aiaa.org/doi/abs/10.2514/2.6814?journa...

Effects of thin adhesive layer and riveted-pitch distance on the stress concentration factor of riveted lap joints
Songklanakarin Journal of Science and Technology 29(4):1069-1092 July 2007
https://www.researchgate.net/publication/289074216...

Thanks.

Keep em' Flying
//Fight Corrosion!

RE: Articles - Effect of Sealant on Behavior of Structural Joints

Thank you LD!
I really need an Elsevier subscription

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Articles - Effect of Sealant on Behavior of Structural Joints

LD... some caution/background knowledge RE these research papers is required.

They were written by researches that made some fundamental errors in the 'best practices' of riveted assembly with fay-sealant; and other researchers seemed to lack a fundamental understanding of how the early Boeing acft fuselages and wings were assembled [707, 720, 727, 737 and KC-135] for air-pressure and fuel tightness. For instance...

Air pressure tightness was assured by wide lap joints [at least 4-fastener-rows] that were tightly fastened together [primer finish only] "otherwise dry'... and that sealant was filleted along the edges of panels as a moisture barrier and secondary air-seal. Also, there was a report... RE the 727 fuselage tear-down fatigue/crack/corrosion findings... that I read in the distant past... there were remarkable differences between the LH and RH fuselage sections that none of the researchers could immediately understand. An old production guy solved the dilemma: there were two [2] separate crews assigned to [hand] build the fuselage cabin sections: a LH crew and a RH crew... who did these jobs for years without mixing. The variation boiled down to minor quality/workmanship practices as emphasized by their crew supervisors.

In the early 1970s my dad converted his Thorp T-18 to 'wet-wings' [for added fuel capacity/range/endurance]. In doing so I was able to have 'first-hand' understanding of the process from the aircraft designer John Thorp... who was already thought thru the process [for Dad] and who was already experienced in the science/art of incorporating integral fuel tanks into GA and military aircraft. After information go-out about use of integral tanks in his [Dad's] aircraft, there were several articles about how bad this was structurally. YES JT was forced to make several structural adjustments to the wing that were not well understood by others... and JT gave very strict instructions 'as to what' specific finishes, pre-cleaning practices, LOW VISCOSITY [PRC] sealants were to be used, specified directions for sealant mixing/use/application/etc... including the need for access doors to allow fillet-seal along part edges and over installed DRIVEN rivets. ETC...

NOTE. My sister was the only one in our family who could actually reach-in and do the hand fillet-seal. She still remembers/hates the smell of uncured sealant and how it stuck to her hands/arms... and the solvents to clean it off.

NOTE.
There are three relatively important documents on sealing processes that help define 'how-to' for best structural durability and long-term sealing durability... along with a host of other 'obscure but critical factors to consider. In the two AIAA JA papers I able to download, NONE of these were cited as fundamental 'how-to references'. I suspect that there would be similar lack of practical understanding/references in the Wiley and Researchgate articles.

AFWAL-TR-87-3078 AIRCRAFT INTEGRAL FUEL TANK DESIGN HANDBOOK

SAE-AIR4069 Sealing of Integral Fuel Tanks [DoD accepted]

SAE-AIR5774 Composite Fuel Tanks, Fuel System Design Considerations

NOTE. MOST companies that pressure-seal cabins and integral tanks have very strict controls to the process... and yes... probably have reduced allowables [static strength and fatigue] even when the process is done properly.

BTW... pressurizing a double-row lap-splice-skin cabin structure is structurally unwise... three or four rivet rows is much better practice. MANY reasons.

Reverse riveting by the 'NACA Method'...brazier heads inside, bucked-tails into countersinks, then mill flush-to-skin [over-coat with CCC]... has been proven 'more fatigue durable' than conventional practices in flush-riveted lap-joints... at greater expense [time]... many reasons.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Articles - Effect of Sealant on Behavior of Structural Joints

I can only speak for the one major OEM whose aircraft I regularly stick my head inside...
The pressure sealing and fuel tank sealing procedures at that OEM are virtually identical. In either case, the fastening process is usually done after the sealing process, allowing the sealant to cure before driving rivets and torquing bolts. During the cure process, they hold components together with temporary fasteners and use various tools to ensure consistent thickness of the bondline. The temporary fasteners are not installed into every hole; they are installed skipping many holes and alternating from one side to the other of a multiple-fastener row.
Installation of permanent fasteners (rivets/hiloks) after this sealing procedure, considering the space between faying sheets and the "springiness" of the sealant between them would make me wonder about the joint strength allowables. But I've seen those allowables in their reports - standard MMPDS/Mil-Hdbk-5 values. However, I've only seen the OEM static (ultimate) strength analysis, not the damage-tolerance analysis they do. Perhaps the stiffness of the joint would have a more pronounced effect on the DTA and elastic load distributions than the static margins?

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Articles - Effect of Sealant on Behavior of Structural Joints

Tmoose,
Wrong link?
Or do you mean some other kind of "squeeze out"?

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Articles - Effect of Sealant on Behavior of Structural Joints

I meant to imply " all rivets, no bolts" should be sung along to that "all bass, no treble " former pop hit song.

Sorry, it seemed like a good idea at the time.
I really should tone it back sometimes. At least wait a few hours before posting my more arcane (insipid) posts.

RE: Articles - Effect of Sealant on Behavior of Structural Joints

Military and civil applications where 'wet-sealant assembly is required are typically done, thus...

Wet-sealant* assembly of pre-drilled/deburred structure... fitted/mate-drilled/reamed tightly together ‘dry’...

Structure is re-assembled with a wipe-coat of fay-surface sealant [along fastener-lines on flanges] and the use of adequate quantities of temp fasteners and clamps to re-align structure, place it in pull-up forces which begin ‘settling’ it tightly together.

NOTE. The hole engagement parts of temp fasteners [draw-clecos, etc] should be coated with parting compound to avoid substantial sealant adhesion.

At this point, installation of bolts-nuts/Hi-Loks/lock-bolts begins in earnest... followed by blind bolts/rivets then [lastly] solid rivets... using typical cross-pattern sequences.

This sequence for 'wet-sealant assembly' is used through-out the military. MAX sealant* [joint] 'squeeze-out' is the optimum goal... with excess 'squeeze-out' often used to fillet along the edges/corners, of parts [no-waste].

*CAUTION/WARNING. Sealants MUST have 'long-enough working life’ to ensure that the uncured-sealant DOES NOT expire... IE: loose it ability to flow and adhere by on-going chemical cure processes... before a high majority of fasteners have drawn the structure tightly together... just like when it was dry-assembled/drilled structure.

*Fay surface sealants MUST have relatively low viscosity for brush/squeegee application, ensuring easy flow/squeeze-out within it’s working life. Sealant working life vary widely depending on formulations. Thes sealants come with designators that imply then ‘nominal working life. Sealant can come with working-life’s of 1/2-Hr, 2-Hrs, 4-Hrs [for small jobs]... working-life’s of 8-Hrs, 12-Hrs [for large-jobs]... and so-on... up-to working life’s up-to 48-Hrs for major structure Assy [huge jobs], depending on ambient conditions.

Note. Sealant adhesion and cure-time can be influenced by the use of ‘adhesion promoters’ [for painted and base finishes].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

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