why use average shear stress instead of maximum?

[Merullo]

I'm selecting a bolt to carry a shear load.

I'll be calculating shear stress and then comparing that to the bolt's shear strength to get my FOS for this static application.

My question is - why is "average" shear stress used instead of "maximum" shear stress?

average shear stress = transverse shear force/stress area

max shear stress (cylinder) = 4/3*average shear stress

As I understand it this is because the stress profile goes from zero at the perimeter to a maximum in the center of the bolt for transverse shear.

My criteria for failure is when the bolt yields.
Couldn't the bolt yield in the center before the average shear stress reached the shear strength?

 

[plasgears]

If yield is the failure criterion, then stress redistr will cause average stress to govern. Visulaize the form of the deformed bolt in extreme shear.

 

[Merullo]

plasgears: thanks for the explanation.

a follow thru question - therefore max shear may be used:
- for cyclic loading
- if a brittle material were used in static instead of a standard fastener?

 

[JStephen]

The 4/3 factor is derived assuming a long beam of cylindrical cross section, and would give the shearing stress away from any concentrated loads. It would not be applicable to a direct-shear type loading such as in a bolt under shear.