protruding head bolt in csk hole bearing stress

[namylem]

in general, bolt hole bearing stress=P/dt, where P=bolt load, d=bolt dia. t=matl thk regardless the hole is straight (protruding head bolt) or csk (countersunk head bolt). what if the hole is excessively chamfer/fillet relief so the result looks almost like csk hole, then what is t if protruding head bolt is used in csk hole, thk = matl thk - csk depth?

 

[SWComposites]

t=0

Simply not an acceptable condition.

 

[dik]

Cannot you rely on the material yielding a bit? Check the depth of material utilised based on yield stress, and maybe with a bit of a safety factor... like 0.8 or 0.9? or, you don't do that with aircraft? I'd look at it that way and see if the deformation was acceptible. As it yields, the connection will only gain more strength.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[namylem]

i built 3d fe model of plate with 1/3 csk 2/3 straight hole, rigid straight pin and define contact elem b/t hole and pin. run static load case, i noticed flat wall region of hole is deformed and radial stress is higher then max brg stress of ~1.27*P/dt (avg is P/dt). no deformation and very low stress are seen in the csk wall region. i believe the round hole will turn into slot hole before full thk can be utilized?

 

[rb1957]

I struggle to understand the problem, I understand "ESL" (the OP's native language is not English).

It sounds like someone CSK a rivet hole intended for a protruding head rivet.
Are you installing a CSK rivet ? (I hope so, and not trying to buck a protruding head rivet into the CSK).

It sounds like the CSK obeys the standard rules of thumb (1/3rd thickness), which is good.

Why not treat as a CSK rivet ? (why model this ??) The bearing allowable for CSK rivets is well understood.

What materials are we talking about ... sheet and rivet ??

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

 

[dik]

Does that mean that they can countersink to 1/3 the thickness of the material?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SWComposites]

well, it all depends on the criticality of the fastener. a LOT more detail is needed. And a FEM is certainly not needed. please fill in the blanks:
the sheet material is ____
the fastener type is ___
the fastener head outer diameter is ___
the hole diameter is ___ and tolerance is ___
the (incorrect) countersink/fillet depth is ___ and diameter on the sheet surface is ___
the max applied bearing stress under static loads (based on total sheet thickness) is ___
the max applied bearing stress under fatigue loads (based on total sheet thickness) is ___
the allowable bearing stress is ___ for ultimate and ___ for fatigue
the fastener connects ___ part to ___ part of ___ aircraft

is this a one off MRB situation? or a bunch of fastener hole defects? or something else?

and a 3D FEM with a rigid pin is completely unrepresentative of the actual joint stress state.

 

[SWComposites]

rb/dik - they want to put a protruding head bolt into a countersink'd hole. UGH.

dik - yes, in aerospace we typically allow and have test data for countersink depths up to 70% of sheet thickness for bolt and deeper for rivets. But a flush head bolt/rivet is installed in a countersunk hole.

 

[rb1957]

yeah, I misspoke ... I had his comment "1/3 csk 2/3 straight hole".

normal rule of thumb is a CSK less than 2/3 thick is ok from fatigue.

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

 

[WKTaylor]

OK... Hope this makes sense.

In a situation like this... using a protruding head bolt at a countersink is an ideal set-up for a countersink filler [countersink-washer, conical washer] and a thin-flat washer under the protruding [lockbolt, bolt, rivet head] head for routine repair, shear application, for one or two holes in a pattern. Install the countersink filler and fastener(s) 'wet' with sealant or epoxy primer [epoxy or sealant with bond/seal the Csk filler in place]. OR similar application with an over-lay repair doubler with countersink fillers [as needed] where shear and bearing are important... and flushness is not.

NOTE1: Countersink fillers should be made like a rivet head, but from high strength alloys... have a small 'pilot hole' and MUST be anodized or alodined for corrosion resistance. Also... the filler must be made so that they protrude 0.001-to-0.004 ABOVE FLUSH... when installed in a flush-tension countersink.

NOTE2. One different case would be where a flush-shear-countersink fastener install [like NAS1097 head-style rivet or similar lock-bolt in a shallow/small Dia countersink] installation has to be replaced by a OS or next-larger-Dia fastener. In this case drilling the hole oversize may remove ~30-to-70% of the countersink [Dia/depth]... depending on hole Dia oversize]... leaving a small effective chamfer at the edge of the hole that is difficult to fill. NO problem: a washer under the protruding head would be adequate and there will be plenty of bearing/shear capability. ONLY problem is that the chamfer presents a ring where moisture can collect... so filling the ring-chamfer with epoxy paste adhesive [cure solid/hard]... or sealant... pliable and space-filling... during fastener installation is essential.

Most liaison/service-engineering experienced stress weenies can deal with either option...


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

 

[namylem]

standard shop operations after drilling (1) deburr (2) fillet relief/chamfer on head side of bolt. operator excessively did step 2 so it looks like csk hole. plz refer to attached pic for any clarification.
they went on install the bolt citing the fillet radius region (R=015-025) between bolt head and shank (pin) will fill up csk area thus full t1 can be used to compute brg stress in plate 1. i am now tasked to challenge the engineer's decision.

 

[SWComposites]

does not look acceptable.
please provide rest of the requested info in my posting above.
a lot depends on the structure related to this joint, whether the joint is fatigue or static critical, how many fastener holes in the overall joint are affected.

 

[rb1957]

agreed, completely misses why the bolt has a shoulder radius ...

I would not find this acceptable.

My preferred solution (on the basis of what I can see) is to up size the fastener (and clean out the damage),
2nd solution would be a "CSK repair", like Will mentioned above.

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

 

[namylem]

swc, idk if the joint is fatigue critical but lowest margin of safety (ms) at this hole is declared bearing critical (static) ms=p,ult/p-1~.31
p,ult=Fbru*dt Fbru=ult bearing allowable of plate 1 d=bolt dia and t=t1 (no reduction is applied due to deep fillet relief/chamfer portion!)

 

[dik]

"Alodine coating is an aluminum metal surface finish that helps to protect the metal from corrosion. Because of how common the use of Aluminum is today in industry parts, it is essential to have basic knowledge about alodine, especially when dealing with precision parts. Aside from being a fantastic corrosion-resistant finish, the application ..." never heard of the coating before...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[WKTaylor]

All...

All male fasteners have a shank to head fillet radius... the size of which is ~based on the fastener intended use: 'primary shear', 'mixed-shear-tension' or 'primary tension'. In most cases, a 45-Deg deburr [light or deep] at the edge of the hole is sufficient... with exceptions, which washers can 'accommodate'. This 'rule' holds true for both protruding and flush-head fastener styles.

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

 

[rb1957]

interesting, that you're saying holes should be chamfered to accommodate the shank radius. I thought these were meant to plastically form the plate, to improve fatigue. Really, the hole preparation should be whatever the fastener spec calls for ... either an interference or a neat fit.

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

 

[dik]

Is that to improve fatigue resistance? ... minimise stress risers?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[rb1957]

yes, plastic yielding the plate under the head reduces the Kt. It also improves the fastener ... think what a sharp corner under the head would be like.

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

 

[WKTaylor]

Fastener head-to shank fillet radius is a subtle, but important feature for bolt integrity.

Most bolts in aerospace have forged heads and slightly oversized shanks... which are finish-machined [ground] to precision diameters. The 'smooth' forged-grain-flow between the head and shank... and a fillet-radii that is 'cold-worked/sized by rolling' [after shank/fillet grinding]... provides tremendous dimensional consistency, durability and reliability unique to aerospace.

Also, although the idea of using the head-to-shank fillet radius of the bolt to crush-cold-work the lip of the hole... in practice it is rarely effective. Often the head ends-up 'floating a few thousandths of an inch' above the mating surface... NOT in solid contact. In-service the bolt install tends to loosen in tension-preload under cyclic loading... and becomes a moisture trap. Bad and bad.

The ONLY way to have a almost sharp lip of the hole mating with any bolt fillet radius is by use of a flat-washer with a (a) 'loose ID hole' or a (b) 'countersunk ID edge'... otherwise the edge of the hole needs to chamfered.

NOTE1. EVEN for flush-head fasteners, the hole-to-countersink edge needs to be chamfered or radiused to accommodate the bolt fillet radius... allowing the fastener head to sit in tight contact with the countersink. Notably, this hole-to-fillet radius is usually built-into the countersink cutter [at the pilot].

NOTE2. There are a few 'special' ways around the fillet-radius-to edge conflict-dilemma. These methods are especially useful, when material thickness is 'low' and edge chamfers will remove too-much bearing-material for satisfactory shear. I hesitate to describe 'how' to do this since I have to get back to work. Hint: undercut fillet radius in the fastener head... under flanged bushes... etc.



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