Question

What is meant by pure Torsion?

Answer 1

A torsion force is a load applied to a material by twisting it. A shear stress is created by the torque applied. During elastic and plastic deformation, a torsion force of sufficient magnitude can cause a material to twist. Torsion force is usually expressed in feet-pounds or Newton-meters.
Because mechanical parts, such as shafts, are frequently subjected to torsion forces, material selection is a critical factor in preventing failure.

Complete answer:
In general, a shaft is subjected to two types of stresses.
1. Torsional Shear Stresses Caused by Torque Transmission
2. Bending stresses caused by the weight of pulley gear and other components mounted on the shaft.
When a circular shaft is only subjected to torque and not acted upon by any bending moment or axial force, it is said to be in a state of pure torsion. OR, the shaft is said to be in pure torsion if it is subjected to two opposite turning moments. It will also have a tendency to shear off at every cross-section perpendicular to the longitudinal axis.

Note:
A torsion test, which monitors the amount of force applied in torsion as well as the degrees of rotation that have occurred, is often used to calculate the amount of torsion force a material can handle. When torsion forces are applied to ductile materials, they can deform plastically. Although hard, brittle materials can withstand greater torsion forces, they are less likely to deform before final fracture.