Torsional Stress

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Problem statement

A solid circular shaft has a diameter of 3 in. and a length of 24 in. It is subjected to a torque T=2kp in. Determine the maximum torsional stress in the shaft.


To determine the state of stress, we select an element of the bar d and radius rho. Due to the torque, a straight line parallel to  will be deflected or twisted by , and so the shear strain is

This equation clearly shows that the shear stress is directly proportional to the radius of the element and hence is largest at the surface of the bar.

Next we have to relate the shear stress to the torque which produced it. Any element of the cross section is subjected to a tangential force due to the external torque . The torque is the integral over all these moments.

We know that . Using equation (a), we have

Thus the twist per unit length is

Thus, for a bar of length  inches,

where theta; is the total angle of twist in radians.

To obtain the shear stress, we note that

Then, since ,

And, because of equation (a),

The cross section element dA is rhodrho for circular cross sections, therefore

Rewriting this result in Mathematica syntax, we get a function for the stress.

For the above example, we get

Converted by Mathematica      September 1, 1999