Riveted joints / structures

[Rat5]

Hi, i have a few questions about the riveted joints that hold a aircraft together :

-Does the rivet retain axial preload from being bucked ? if yes how to calculate the value ?

-Does a riveted joint provide strength from the friction between the 2 sheets jointed together and normal load on them ? Or is it purely from the rivets in shear ?

-In glued/riveted joints, how is the adhesive thickness made even ? is it with the use of "bonding beads" for controlling the thickness, just like when asssembling composite parts ?

Thank you

 

[rb1957]

"Does the rivet retain axial preload from being bucked ?" ... yes (the two sheets are clamped togther), but never analyzed in 40+ years (not a critical design load.

"Does a riveted joint provide strength from the friction between the 2 sheets jointed together and normal load on them ? Or is it purely from the rivets in shear ?" ... again, yes, but never analyzed ... we don't count on friction (nor fay surface sealant acting as an adhesive).

"In glued/riveted joints, how is the adhesive thickness made even ? is it with the use of "bonding beads" for controlling the thickness, just like when asssembling composite parts ?" ... very carefully !? "bonding beads" ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Rat5]

Hello rb1957, thank you for your answers, that is very helping.

can you recommend me a few books on the subject ?

yes bonding beads, i dont know the real name in english, they're calibrated glass balls you mix with the adhesive, they limit how thin the adhesive can go.

thanks again

 

[SWComposites]

-Does the rivet retain axial preload from being bucked ? if yes how to calculate the value ? > yes, there is some preload; no, there is no way to calculate it, and it has never been used.

-Does a riveted joint provide strength from the friction between the 2 sheets jointed together and normal load on them ? Or is it purely from the rivets in shear ? > only count on the rivets in shear; never rely on friction.

note, some bonded aluminum aircraft structures have had problems over the years with environmental degradation.

 

[rb1957]

books ? ... Bruhn, Niu, Flabel.

Adhesive bonding ? ... two forms ...
1) composites ... usually adhesive is included in the "pre-preg" laminae; you can do "wet lay-up" but I don't think any major composite supplier does this.
2) metal-metal bonding ... here I think it is typical to use a film of adhesive, like a separate ply.
As mentioned above, processing and process control are critical to a good bond. I'm not particularly current on this "art" but I think PAA (Phosphoric Acid Anodise) developed by Boeing is typical.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Stress_Eng]

During my days on the shop floor (A/C Fitting Apprentice and Detail Fitter) we used a fretting compound (yellow in colour) or a thick brown tank sealant. No bonding beads were involved. On an analysis point of view, shear allowables are usually in table form, based on rivet type, sheet material type and thickness (bearing or shear critical). You can also get tension allowables.

 

[Ng2020]

I think you might need to give some more details of your proposed design so we can 'calibrate' our answers.

Rivet preload and friction is never quantified or used in static strength justification. However these effects are inherent in some fatigue methods, which use data derived from coupon tests of representative joint specimens.

Adequacy of joint clmap-up for sealant squeeze-out wouldnt be quantified, but again would be knowledge derived from testing.

Sometimes phenolic microballoons are used as a filler, mixed with resin for potting applications. But I don't believe they would ever be used in structural bonding applications.

 

[WKTaylor]

Eubet...

FYI. For paste adhesive repairs we have been using 'fresh glass beads' for paste adhesive bondline control. We add [thoroughly mix-in, when stir-mixing the 2-part adhesives] 3-to-5% glass beads 'by weight'... typically 0.007 [0.005-to-0.010-inch] Dia. The glass beads are usually MIL-PRF-9954 or AMS2431/6. Under certain conditions we can also use '3M glass bubbles' or 'phenolic micro-spheres'.

Riveted aircraft structure question You are asking is actually a very broad topic with many elements to consider.

Engineering understanding of the metallic and composite materials being assembled.
Engineering design/analysis of riveted joints.
Consistent [industry-standard] good riveting-Assy practices.
Assembly tools/clamp-up to hold everything tightly/aligned together
Corrosion protection... deburring, surface treatments, primers, sealants, etc.
To name a few of the 'big-ticket aspects'.

NOTE. There are good/practical books for sheet metal assembly... and a few ancient books specifically for factory riveting practices.

Regards, Wil Taylor
o Trust - But Verify!
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", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Compositepro]

I did a test bond once of glass microscope-slides using a paste adhesive with glass beads added. In the cured samples after a 350F cure I could observe interference fringe lines around every bead indicating a local bond line failure. This is due to the fact that adhesives shrink slightly when curing and also from cooling after cure. Glass beads are very hard with a low cte, so at the bead, the adherends are prevented from getting closer together to accommodate the resin shrinkage. I would suggest that if beads are used, they should be plastic rather than glass. Also, at the glass bead the bondline thickness is actually zero, so the strain capacity of the adhesive is also zero at that location.

Micro-balloons are hollow and are used to make syntactic foams. Being hollow, they have limited compression strength and should not be confused with solid beads. Film adhesives usually have some type of fabric carrier, which also serves as bondline control.

 

[WKTaylor]

CP...

My application would NOT be a 'production practice'... repairs only. Use of 'filled-paste' or 'film-adhesive' [woven or un-woven scrim] are typical for production... especially high temp service.

Use of 'sparse quantity of glass beads' as described... is intended for Room Temp [RT] cure 2-part paste adhesives, ONLY... at relatively modest elevated temperature [-65F to 250F-to-350F] service. NO idea how they would function in a 2-part paste adhesive with high temp cure bondline. However, in reality, there would be very few glass beads in the bondline.

In prior years, we used... no kidding... nylon mono-filament fishing line* [small diameters] woven-loosely across a thick wipe of paste adhesive on a repair part... then apply vacuum-bagged-pressure to crush-down/squeeze-out the excess paste... for bondline control until cure. *wipe the mono-filament line with IPA on cheesecloth and dry-wipe to remove residues. In truth... 'Fresh glass beads' are a more elegant solution.

OK, glass or phenolic microspheres, at lower density, would have more particles/surface-area/bondline... but still relatively sparse.

Regardless, the bondline thickness is established/controlled and there is adequate adhesive shear strength for most repairs for modest elevated-temp service.

NOTE. 'Service Temp', as I define it, is the long-duration temperature where ~xxxx-PSI shear strength/toughness is reliably constant. I prefer service temp defined for 1000-PSI... although some 'rules' allow as low as 500-PSI [hence a few degrees hotter].

Of course, metallic surface preparation, not discussed here, is an absolutely vital element for great bonds.

It's late and I feel like I'm punch-drunk.

Regards, Wil Taylor
o Trust - But Verify!
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", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov