Which Bearing Allowable for Countersunk Fastener

[magom]

Dears,

I know that this topic has been appraoched several times through many named threads but there is still some confusion (to me) on the subject...I would like to know if anyone is used to following bearing allowable stress: Sbr=Fsu/(d*(t-0.5*CSK)

where t is the sheet thickness, d is the fastener diameter, CSK is the Coutersink Heigth.

Actually I have seen this formula a long time ago but I can not remember where exactly (reference)?

Thank you in advance and best regards,

 

[RPstress]

This formula does not make sense to me. As written it appears to find a bearing stress by dividing the allowable shear stress by an area. Apart from the resulting units not being stress, I also can't think of a good reason to base the bearing strength on a shear allowable. Also, is this meant to be a yield allowable or an ultimate?

 

[rb1957]

i guess Fsu is the ult. shear allowable load (in which case the units make sense).

but i wouldn't make an allowable bearing stress out of an allowable shear load (i think your allowable is way too low).

you Could do Fbr = Sbru*(d*(t-0.5*CSK)), which says that the CK is not fully effective in bearing.

why aren't you using the tables in MIL-HDBK 5 ?

 

[magom]

@RPstress : Sorry I did not point out to ultimate shear allowable load Fsu...

@rb1957: Actually, the most (may be all) of the shear load is transferred based on the "vertical" part of the rivet shank (also for shear area: pi*d^4/4, d is the diameter of the rivet shank). The reason whey only one half of the thickness has been considered in the above formula. This seems more physical/realistic to me!

To my knowledge tables from MIL-HDBK 5 are dedicated to protruding rivets.

http://www.aluminum.org/Content/NavigationMenu/The_Industry/Industry_Standards/4October05.pdf

@I-B-62

Rgd.

 

[rb1957]

magom ...
my post was concerned with calculating a bearing allowable stress based shear allowable load ...

pi*d^4/4 is something, but it isn't the area of anything.

MIL-HDBK 5 considers many types of rivets, the tables are most useful for CSK rivets (since it is pretty easy to calculate allowables for protruding head rivets, being Fbru*d*t).

your equation as written doesn't match your reference.

i'm really surprised that if you're working on a airplane why are you using what looks like a civil (building) code. if you aren't working on an airplane, please post in the right forum, maybe "structural engineering".

 

[specialmission]

Allowables for countersunk fasteners used in aircraft are developed from test data. To my knowledge, the relationships between fastener stiffness, hole fill, sheet properties and geometry have not been accurately modeled for countersunk fasteners. If I recall correctly, with few exceptions, the FAR's require allowables for 99.9% accuracy with 95% confidence and is typically achieved using a large number of test specimens.

The large airframers will use proprietary allowables developed from their own tests as well as Mil5 data. I would advise caution when applying the formula you have found without test data to support it. It may be conservative for one application and un-conservative in another.

 

[WhiteRabbit]

Yes...I agree. I would not use Fsu (Psu) formulae to calculate Bearing Allowables for bearing. They are two very separate and different failure modes.

As Specialmission said, "The large airframers will use proprietary allowables developed from their own tests as well as Mil5 data."

But from the limited aerospace companies I've worked for I've never seen a reduction factor less than 50% of the csk thickness (based on 66%-70% csk depth). They were mostly around 70% (+/-5%) of the protruding head fastener based on Tension of Shear Head (obviously Tension Head Csk having a lower allowable then Shear Head in Bearing).

Anyhow...good luck. My advice is to use Mil-HDBK-5 or your specific company allowables.

I would not use the Psu allowables because they are different failure modes.