Many of the materials that are used in motor vehicle engineering are elastic, that is to say that they change shape under load and then return to their original shape when the load is removed. Elasticity is one of the properties that allow materials and components to experience much stress and strain during their working lives. Stress is calculated by dividing the applied force by the cross-sectional area of the bolt. There are three basic forms of stress: tensile stress; compressive stress; and shear stress – torsional stress is a form of shear stress. In materials testing, a standard size specimen of steel is stretched in a tensile testing machine until it breaks. During the test, the applied force and the corresponding extension are recorded. In terms of stress and strain, Robert Hooke's law states that the strain produced in an elastic material is directly proportional to the stress that produces it.