For ductile materials, the largest value of tensile stress that can be sustained by material before breaking is known as:

The stress-strain curve for ductile material is shown below So it is evident from the graph that the strain is proportional to stress or elongation is proportional to the load giving a straight line relationship. This law of proportionality is valid up to a point A. Point A is known as the limit of proportionality or the proportionality limit.For a short period beyond point A, the material may still be elastic in the sense that the deformations are completely recovered when the load is removed. The limiting point B is termed as Elastic Limit.Beyond the elastic limit plastic DEFORMATION occurs and strains are not totally recoverable. There will be THUS permanent deformation or permanent set when the load is removed. These two points are termed as upper and lower yield points respectively. The stress at the yield point is called the yield strength.A further increase in the load will cause marked deformation in the whole volume of the METAL. The maximum load which the specimen can withstand without failure is called the load at the ultimate strength. The HIGHEST point ‘E' of the DIAGRAM corresponds to the ultimate strength of a material.Beyond point E, the bar begins to form the neck. The load falling from the maximum until a fracture occurs at F.