Steel development in the latter part of the 20th century has been remarkable. Modern steel is made of iron with small amounts of carbon, manganese, and traces of other alloy elements added to enhance physical properties. Significant advances in the art and science of steelmaking have occurred since the early part of the 20th century. Many of these advances have been related to the need for steels of increasingly higher strength with improved ductility, fracture toughness, corrosion resistance, and weldability properties. The modern steelmaking process involves continuous casting of the molten steel into slabs or blooms with relatively high cooling rates to discourage segregation of the elements. Yield stress in tension can be measured by simple tensile tests. Localized material failure can occur when applied cyclical stresses are greater than a threshold tensile stress range, but below the elastic yield stress. Ductility is the ability of steel to withstand large strains after yielding and prior to fracture.