The important factors in selecting alloys for aircraft structures are the useful specific strength of the material, resistance to corrosion, producibility, and cost. Because of the high worth of a pound in aircraft, the major design consideration becomes the strength factor.The useful strength of the material depends not only on the specific static and fatigue strength, but on the reliability required, as well as the ability of the material to resist damage. For reliability, the scatter or statistical strength distribution becomes an important design factor for both static and fatigue-loaded structures. For structures subjected to fatigue loading, the dynamic stress-strain behavior of the material and the notch sensitivity are factors used in determining the useful strength. For compression-loaded structures, the important material properties are the yield condition and the stress-strain relationship beyond yield. The fail-safe design philosophy used in aircraft structures requires that the useful strength be limited by the fracture toughness and fatigue crack propagation rate of the material. For fail-safe structures, the damage tolerance is achieved by designing for residual strength and life.