I am trying to calculate the torsion in a rectangular bar.

I have found closed form calculations in Shigley, Roark, and a NZ structural code which all agree (within 10% or less).

However, the solid works simulation of the same geometry is higher:

b = 4"

t = 1"

L = 3"

T = 10,000 in*lbf

Tau (Shigley) = 8.63 ksi

Tau (Roark) = 8.63 ksi

Tau (NZS 4404) ~9 ksi

Tau (FEA) = 11.65 ksi

The FEA results are 35% higher.

Has anyone every experienced this

Results are attached below:

Apparently, the torsional shear stress is influenced by the stresses in bars with a short length.

I increased the length of the bar to 12" long and checked the torsion at a plane located at the mid length and the stresses

agree closely (with ~ 2%) with Roark and Shigley.

Initially, I made 2 mistakes.

1. I was checking stresses at the ends instead of the mid length.

2. I was checking the shear stresses in the wrong directions.

Here are my new results:

TauYZ is the shear in the Z-direction acting along the X-Z plane.

The Y axis is orthogonal to the X-Z plane, thus TauYZ notation.