If a material is undergoing elastic deformation the graph is a straight line through the origin as the stress is directly proportional to the strain. The gradient of the line is equal to the Young modulus.P is the limit of proportionality.The material is still undergoing elastic deformation but the stress is no longer directly proportional to the strain. The internal structure behaves differently and the graph begins to curve.Beyond E the elastic limit the material is permanently deformed and will not return to its original size and shape when the stress is removed.At the yield point, the material undergoes a sudden increase in extension as its atomic structure is re-organised. There is a large increase in strain for a small increase in stress.Some metals, such as mild steel, show an upper and a lower yield point. Beyond the yield point the deformation is plastic, which is always permanent.The maximum stress the wire can withstand is the ultimate tensile stress (UTS). After this the wire starts to form a neck (it thins at one point) and it then snaps at the breaking point.The limit of proportionality, the elastic limit and the yield point may be in the same place on the graph.