MONOLITHIC AIRFRAME STRUCTURE: MATERIALS AND METHODS FOR REDUCED COST, REDUCED WEIGHT AND IMPROVED DAMAGE TOLERANCE

Integral metallic structure can reduce the cost of airframes significantly. It is very weight efficient for designs driven by compression stability. However, the damage tolerance of the first generation of monolithic designs is not as good as that of built-up structure. Recent studies of potential monolithic fuselage structure concluded that a machined panel with mono-directional stiffening was one of the most cost effective and technically most feasible solutions. However, the aluminum alloy selected at first (7050) was found to have insufficient toughness for the required residual strength. Alloy 7475 did improve this situation, at the expense of some stability and strength, but the damage tolerance aspects still proved the principal design drivers – especially at low temperatures, where fracture toughness of 7xxx-series alloys is known to decrease. Natural aged 2xxx-series T3 do not have this problem, but they are much less strong, and at the thick gauges needed for integral machined