Torsion force is a twisting force that causes deformation or failure in materials. It acts when one end of an object is twisted while the other end is held fixed. Examples include twisting a ruler or turning a key in a lock. Torsion is caused by applied torque which promotes rotation. Circular objects subjected to torsion experience shear stresses that increase towards the outer edges, potentially causing yielding or fracture. The maximum shear stress and angle of twist can be calculated using formulas involving the applied torque, polar moment of inertia, and material properties. Ductile materials fail in shear planes while brittle materials fail in tension planes under torsional loads.