Testing of round circular samples is another method of determining a basic engineering relationship in structural materials. Unlike tensile testing, torsional tests are not complicated by the phenomenon of necking and reduction in areas.
Torsion occurs when any shaft is subjected to a torque. This is true whether the shaft is rotating (such as drive shafts on engines, motors and turbines) or stationary (such as with a bolt or screw). The torque makes the shaft twist and one end rotates relative to the other inducing shear stress on any cross section. Failure might occur due to shear alone or because the shear is accompanied by stretching or bending.