1791. Structural Efficiency of High Temperature Materials

Paper

Abstract

The purpose of this technical paper is to provide awareness and insight into the choices of high temperature structural materials now available to designers. Increased knowledge of the material choices available will assure that maximum weight efficiency is achieved in each application. Materials for high temperature airframe structural application were limited until recently to titanium and steel alloys. For hypersonic speeds, where temperatures exceed the limits of steel, protective insulators or ablators have been used. Titanium and high temperature steel alloys were used in the XB-70 bomber, the X-15 research airplane, and the SR-71 reconnaissance aircraft. The space shuttle orbiter uses ceramic insulation bonded over an aluminum structure. Higher temperature leading edge components are fabricated from carbon/carbon material. Other spacecraft use ablative coatings to protect their structures from re-entry aerodynamic heating. Now, new classes of lightweight composite materials are emerging as weight saving alternatives to steel and titanium. These materials have the potential for providing significant weight savings as graphite/epoxy composites now do when used in place of aluminum construction. Two categories of high temperature materials are described, and their relative advantages and disadvantages are discussed. The first category (T = 300? – 600?F) includes glass/bismaleimide, graphite/bismaleimide, glass/polyimide, and graphite/polyimide composites. The second category (T> 600?F) includes 4130 steel, 17-4PH and PH15-7MO stainless steel, Rene 41 alloy, 6A1-4V titanium, and ACC-4 carbon/carbon composites. Baseline materials (< 300?F) used for comparison are aluminum (7075, 2024), glass/epoxy, and graphite/epoxy. Carbon/carbon material has the potential for use as hot, primary structure in future Mach 15 to 25 aerospace vehicles, for temperatures up to 3000?F. Structural efficiency (strength/density and stiffness/density) comparisons are presented for tensile strength, tensile modulus, and bearing strength. Maximum use temperatures of each material is discussed. High temperature composite materials are already in limited use in production aircraft, and they will be used more extensively in the next generation of military fighter and attack aircraft. These applications are discussed. Graphite/polyimide materials are now used in production jet engine applications. Carbon/carbon material is currently used in aircraft brakes, rocket nozzle liners, and in the space shuttle leading edge structure. Future applications of polymer matrix and carbon matrix composite materials to supersonic and hypersonic aircraft and aerospace planes is discussed.