NASM without Shear Stregnth

[flyforever85]

Pretty new in writing reports and I have to check the bearing strength of a screw NASM24694. The document does not report the actual value of the shear strength, just the axial strength. Do I have to calculate the shear strength myself? Should I use sigma equivalent i.e. Von Mises?

Thank you

 

[rb1957]

assuming steel ... Fsu = 0.57*Ftu is a typical relationship

another day in paradise, or is paradise one day closer ?

 

[flyforever85]

Thank you, I guess it comes from Von Mises since sigma eq = sqrt (3) tau so tau= 1/sqrt(3) sigma eq = 0.57 Ftu

 

[ATSE]

von Mises is true for isotropic materials. Mild steel comes close to this.
Fasteners are less isotropic than you might think.
von Mises may or may not be a good predictor. von Mises is a yield criteria, not a fracture criteria. Fracture and yield do not always correlate well - especially for fasteners.
In the absence of test data, I suggest using Tresca instead. But fasteners really need standardized test data to establish all modes of failure.

 

[SWComposites]

Huh?
Bearing strength is not fastener shear strength.
Does this fastener have a solid or threaded shank?
This is better posted in the Aircraft Engineering forum.

 

[tham1]

rb1957,

Just a quick clarification unless I’m missing something...

Should it not Fsu=0.75*Ftu and Fsy=0.57*Fty ?

 

[rb1957]

i've used 0.57 for both yield and ultimate. Maybe conservative for ultimate, better to have something in the back pocket. Better yet to have proper fastener allowables !

another day in paradise, or is paradise one day closer ?

 

[flyforever85]

I agree with you rb1957 but NAS24694 does not report the shear strength, just the ultimate tensile strength which is this case is 1700 lbf. so I calculate the shear strength for yield as .57 x 1700. Lucky, the application has a very low load applied and the margin of safety is above 20. So I'm not too worried.

Thank you all for the interesting answers!