In product design and development it is necessary to compare operating stresses to material strength properties in order to estimate the durability of the various components. The effects of material hardness, surface finish, section size, and presence of stress concentrations on the fatigue properties of steel, cast irons, sintered irons and steels, and aluminum are discussed.
Because a material specification normally includes a range of strength properties, a method is suggested for constructing an estimated S-N curve based upon a selected strength level. The method is extended to construct an S-N curve for an approximate failure rate of the selected strength level.
The construction of failure diagrams for irons and steels is discussed. Use of the true fracture strength instead of the ultimate tensile strength is suggested for construction of modified Goodman diagrams for ductile materials.