tailieunhanh - materials handbook 15th 2002

Engineering is the goal-oriented process of designing and making tools and systems to exploit natural phenomena for practical human means, often (but not always) using results and techniques from science. The development of technology may draw upon many fields of knowledge, including scientific, engineering, mathematical, linguistic, and historical knowledge, to achieve some practical result. | McGraw-Hill HANDBOOKS MATERIALS HANDBOOK FIFTEENTH EDITION Foundation for the science of metallurgy Compares the advantages and disadvantages of different manufacturing processes Comprehensive in approach GEORGE s. BRADY HENRY R. CLAUSER JOHN A. VACCARI Part 1 Materials Their Properties and Uses 2 ABLATIVES ABLATIVES. Materials used for the outward dissipation of extremely high heats by mass removal. Their most common use is as an external heat shield to protect supersonic aerospace vehicles from an excessive buildup of heat caused by air friction at the surface. The ablative material must have a low thermal conductivity in order that the heat may remain concentrated in the thin surface layer. As the surface of the ablator melts or sublimes it is wiped away by the frictional forces that simultaneously heat newly exposed surfaces. The heat is carried off with the material removed. The less material that is lost the more efficient is the ablative material. The ablative material also should have a high thermal capacity in the solid liquid and gaseous states a high heat of fusion and evaporation and a high heat of dissociation of its vapors. The ablative agent or ablator is usually a carbonaceous organic compound such as a phenolic plastic. As the dissociation products are lost as liquid or vapor the char is held in place by the refractory reinforcing fibers still giving a measure of heat resistance. The effective life of an ablative is short calculated in seconds per millimeter of thickness for the distance traveled in the atmosphere. Single ablative materials seldom have all the desirable factors and thus composites are used. Phenolic or epoxy resins are reinforced with asbestos fabric carbonized cloth or refractory fibers such as asbestos fused silica and glass. The refractory fibers not only are incorporated for mechanical strength but also have a function in the ablative process and surface-active agents may be added to speed the rate of evaporation. Another .