New alloys for advanced metallic fighter-wing structures

Abstract

This paper summarizes the materials test portion of a program sponsored by the U.S. Air Force Flight Dynamics Laboratory (AFFDL) to develop lightweight/low-cost fighter-wing structures. Fatigue-crack propagation-rate curves are presented which compare aluminum alloys 7475-T7651, 7050-T73651, 7050-T7651 plates and 7050-T736 forgings, and titanum alloys Ti-6-4 £MA, Ti-6-2-1-1 A plates, Ti-6-4 A and STA castings, and Ti-6-22-22 STA forgings. Standard mechanical properties are also reported. For most aircraft applications, these new aluminum alloys appear better than 7075. Except for Ti-6-2-1-1 A plate and Ti-6-4 STA castings, the titanium alloys tested are better than conventional, wrought Ti-6-4 and Ti-6-6-2. I. Introduction T^HIS alloy evaluation program was undertaken as JL a portion of an Advanced Development Program sponsored by the Air Force Flight Dynamics Laboratory under Contract F33615-72-C-1891 (Project 486U) with the Northrop Corporation for the design of advanced metallic fighter-wing structures. Part of the objective was to design an F-5E fighter-wing using new structural concepts and new alloys which would meet specific damage tolerance criteria. The materials evaluation portion of the program concentrated on the mechanical properties needed for a thin, lightweight fighter-wing structure.! Figure 1 shows that for a wing of this type, the upper wing skin is usually loaded in compression so that compression yield and compression modulus values are of primary importance. The lower wing skin is subjected to tensile forces; therefore, the design is based on the criteria of tension yield, tension modulus, S-Nfatigue, corrosion behavior, plus new design criteria based on stress corrosion (K /5CC), fracture toughness (K Ic and K c), and fatigue-crack propagation (FCP) rates. Ribs and spars need properties which are compatible with both skins. Several new alloys and heat treatments emphasizing improved fracture toughness have been developed in the last few years. The objective for the materials program was to select and evaluate pertinent alloys, heat treatments, and product forms which could become useful for fighter aircraft in the near future. Wing skins of thin-wing fighters are structural members carrying a large portion of the loads. To do this with minimum weight, tapering or sculpturing of the wing skins is required. Aluminum wing skins are normally machined from plate material. Therefore, important mechanical properties were measured as a function of thickness and orientation within the plate material. In this program, 1-in. to 11A in.