Elasticity is the property of materials to resist an applied external force and return to their original shape and size when the force is removed. Stress is defined as the resistive force per unit area within a material in response to an applied load. Strain is the change in dimension of a material under stress divided by the original dimension. According to Hook's law, within the elastic limit stress is proportional to strain. The stress-strain curve shows this proportional relationship and defines the elastic limit and proportional limit of a material. Brittle materials fracture more abruptly than ductile materials, which experience plastic deformation beyond the yield point before fracturing.