Torsional shear stress for square bars

You need to study 'shear flow' in relation to this. This is a mechanics of materials property not the shear flow regime we see in fluids, although they are related.

Try this pdf

http://www.aerostudents.com/files/mechanicsOfMaterials/shearStressAndShearFlow.pdf

 

It depends on your ratio of height and width of the bar. The equation you would use to calculate the stress is as follows.

σ=(Ta)/J

a = shortest side

The trick is figuring out what J is. There are a couple methods. The first method which isn't always accurate would be to use this equation.

J=C(1/3)(b)(a^3)
b= longer side

C = .91 if , b<10a
C= 1 of b>10a

There are also tables out there that you can use to help you solve for the stress.

For a quick reference you can also look at this webpage:
http://www.sbainvent.com/strength_of_materials/noncircular-torsion.php