Mild low-carbon steel is ductile. This means it can be drawn into wires by plastic deformation. A stress-strain graph for a metal wire or spring is shown in the diagram.The graph obeys Hooke's law up to the limit of proportionality. Elastic deformation occurs up to the elastic limit. Plastic deformation occurs after this limit.The upper and lower yield points are points where the material extends rapidly and there is a large strain for a small stress. This part may be absent for non-ductile materials.The ultimate tensile strength is the maximum stress the material can withstand when being stretched before it breaks. Necking starts to occur beyond this point. One part of the material becomes narrower than the rest.