Power transmission shafts — on motors and gearboxes, for example — are subjected to torque loads that result in torsion, or twisting of the shaft about its axis. Similar to structures under tension or compression, two important mechanical properties of shafts under torque loads are shear stress and shear strain.
Stress is a material’s resistance to an applied force, and strain is the deformation that results from stress. Shear stress and shear strain (which are caused torsional loads) occur when a force is applied parallel or tangent to an area. Normal stress and normal strain (which are caused by tension and compression) occur when a force is applied normal (perpendicular) to an area.