FLOW BEHAVIOUR AND CREEP DEFORMATION IN ENGINEERING COMPONENTS OF COMPOSITES

Abstract

Composites have emerged as a new class of engineering materials, which could be tailored for a given application and designed to attain properties, not achievable in any of its constituents alone. These composites could contain dispersed fibers, particles or whiskers embedded in a matrix which could be metallic, polymer or ceramic material. Metal matrix composites (MMCs) are based on metals or alloys reinforced with fibers, particles or whiskers of hard ceramics. MMCs containing particles and whiskers are generally isotropic and are relatively cheaper. Because of superior high temperature properties compared to the matrix metal or alloy alone, many of these composites are finding increasing application in components exposed to elevated temperatures. The present study investigates the flow behavior and inelastic deformation in MMC components under multiaxial stress. A rotating disc, which has been extensively investigated in solid mechanics, is also chosen as the component under study in this investigation because it is employed in rotating machinery like turbines, rotors, compressors, flywheels etc. The creep deformation taking place due to steady state creep in a rotating disc is marked by nonlinear constitutive equations and the visco-elastic strain rate depends on the effective stress through an empirical creep law involving material parameters. The multiaxial stress developing in a rotating disc and the creep strain developing as a consequence, have been analyzed in isotropic disc with the same yield stress in tension and compression. The difference in yielding under tension and compression is commonly observed in MMC components due to residual stresses developing during processing and its effect on creep behaviour has also been investigated. In MMC wrought components reinforced by whiskers or short fibers, anisotropy is commonly observed (Badini, 1990). The effect of anisotropy on the creep stress and creep strain developing in a rotating disc is also a subject of this investigation. vii Weight saving to reduce the weight of a structural component is a prime motive for application of composites in space, aerospace and automobile industries. From this point of view, the study has chosen lightweight aluminium and its alloys as the matrix material for the rotating disc. The enhanced creep of aluminum and its alloys may not permit their application at elevated temperatures. Experimental study on creep properties under uniaxial condition has demonstrated that steady state creep rate is reduced by several orders of magnitude as compared to aluminium or its alloy when it is reinforced with ceramic particles like silicon carbide. A significant improvement in specific strength and stiffness may also be attained in these composites based on aluminium and aluminium alloys containing silicon carbide particles or whiskers. In this context, a rotating disc made of Al-SiC composite has been chosen for the present investigation on creep under multiaxial stress.