A steel rod with a fixed and standardized size and shape, is subjected to tensile force until it breaks. At first the rod will stretch elastically. At some point the tension is so high, that the deformation is pemanent. At that moment the yield strenght of the material is exceeded. When the tensile force is further increased the moment will be reached that the rod shows a constriction. The tensile stress in the constricted cross-section will rapidly increase and the rod will break. The tensile strength is the maximimun tension force the rod can withstand till the moment of constriction. This maximum tensile force, divided by the original diameter is called the tensile strength.