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Short Paper Proc. of Int. Conf. on Recent Trends in Mechanical, Instrumentation and Thermal Engineering 2011

A Model to Show the Relaxation of Thermal Stresses When Using Functionally Graded Material (FGM) in Place of Laminar Composite K.K.S.Mer1*, B.K.Mishra2, S.Ray3 1

Associate Professor, Mechanical Engineering Department. Mechanical Engineering Department, G.B.P.E.C. Pauri Garhwal, Uttarakhand. Email: kks_mer@yahoo.comTel: 91-9412957015(M) Fax: 91-1368-228093 2 Professor, Mechanical and Industrial Engg. Deptt. Indian Institute of Technology, Roorkee. 3 Professor, Metallurgical and Materials Engg. Deptt. Indian Institute of Technology, Roorkee. Thus, surface modification or coating evolved for imparting the required properties at the surface, distinctly different from that of the bulk. But this type of combination of surface and bulk often has different deformation characteristics resulting in development of intense shear stress leading to failure. The recent catastrophe in the space shuttle Columbia due to detachment of ceramic tiles reminds us of danger of combining two materials of dissimilar thermal and mechanical characteristics. Composite materials may alleviate the situation in certain circumstances by combining dissimilar materials in the bulk and not along a specific interface in order to synergize them to obtain unique combination of properties while compensating the deficiencies of one another. This process of material optimization can release a designer from the constraints associated with the conventional materials. One can make use of tougher and lighter materials, with properties that can be tailored to suit particular design requirements. Complex shapes can also be manufactured easily using composite materials. Hence a complete rethinking of an established design in terms of composites can often lead to both cheaper and better solutions. The basic attributes of metals reinforced with hard ceramic particles (Particle Reinforced Metal Matrix composites or PMMCs) or fibers (Fiber Reinforced Metal Matrix Composites or FMMCs) are improved strength and stiffness, improved creep and fatigue resistance, increased hardness, better wear and abrasion resistance and higher operating temperatures. The increasing interest for research in the area of aluminium based metal matrix composites (MMCs) is due to the increasing demand of lightweight components from aerospace, automobile and other industries. In automobile industries, the potential application of MMCs and its benefits have been demonstrated in cast engine blocks, cylinder liners, pistons, valves and other components. Some automobile components are already being manufactured using MMCs. The aluminium and magnesium based MMCs lead to lightweight components and there is, consequently, weight savings and lower consumption of fuel, the price of which has escalated sharply over the past few years. Normal composite materials consist of a continuous phase, which forms the matrix, where one or more phases called dispersoids

Abstract:-The use of Functionally Graded Materials (FGMs) is important in context of application in aerospace industry. The outer surface temperature of the space vehicles is estimated to reach 2100 K while the inner surface is to be maintained at normal temperature for personnel, machines and storage of fuel inside. The outer surface of these space vehicles is exposed to very high temperature due to friction with air while moving through earth’s atmosphere. To withstand this high temperature, these vehicles are covered with heat resistant ceramic tiles, which may be said as laminar composite. These ceramic tiles are bonded to the metal surface with some adhesives. Thus the layered composites are being used in these space vehicles to withstand high temperatures. Due to the high temperature gradients, very high thermal stresses develop in the outer shell of these space vehicles. Due to mismatch in the properties of the metal and ceramic, there is a steep change in the thermal stresses at the metal ceramic contact surface and due to this steep change in the thermal stresses, the outer ceramic tiles become prone to delamination or cracking. Functionally graded metal-ceramic metal matrix composites or functionally graded materials (FGMs), selectively reinforced at one surface by ceramic particles are a promising response to these specific application requirements. FGMs provided with heat-resistance and hard ceramics on the high-temperature side and tough metals with high thermal conductivity on the low-temperature side and gradual change from ceramic to metal in between, are capable of withstanding severe thermo-mechanical loadings. In the present work FEM model has been used to show how thermal stresses may be relaxed by using FGMs.

I. INTRODUCTION: FUNCTIONALLY GRADED MATERIALS AND THEIR MODELLING FOR THERMAL STRESSES In many demanding applications such as aviation, spacecraft, automobile, electronics and cutting tools, there are often contradictory property requirements in components like very high hardness, heat resistance and high wear resistance along with fairly good ductility and toughness. Homogenous materials like monolithic alloys may not be suitable because different properties may be required in different locations in the components. For example, wear resistance is required in the surface region which may require high hardness but if the same hardness is there even in the bulk the toughness and ductility may be seriously impaired. Š 2011 AMAE DOI: 02.MIT.2011.01.505

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