Analysis of Creep in a Variable Thickness Rotating Disc Made of Functionally Graded Composite

Abstract

Metal matrix composites consisting of aluminum/aluminum alloy matrix reinforced with ceramics like silicon carbide (SiC) exhibit higher specific strength and stiffness, and superior thermal resistance and hence may be employed in rotating disc of turbine rotor and disc brakes. The conventional metal matrix composites sometimes fail under extreme service conditions of temperature and mechanical loads. To meet such stringent loading condition, a new class of composites, known as Functionally Graded Materials (FGMs), have been developed. The contents of constituent phases in FGM are gradually varied with respect to position coordinates to attain smooth and continuous variation in the desired properties. The present study is an attempt to investigate the steady state creep behavior of a variable thickness rotating disc made of aluminum or its alloy matrix reinforced with SiC (particles or whiskers). The first segment of the study deals with the analysis of the steady state creep in a rotating FGM disc yielding according to Tresca criterion. The creep behavior of the disc has been described by a threshold stress based law, with the value of stress exponent equals to 5. The disc thickness and distribution of SiCp reinforcement are assumed to decrease linearly on moving from the inner to outer disc radius. The stresses and strain rates are estimated from the analysis and compared with those available in literature for a similar FGM disc yielding according to von Mises criterion. The study reveals that the use of Tresca criterion gives much safer design of a variable thickness rotating FGM disc as compared to FGM disc designed on the basis of von Mises criterion. The second segment of the study investigates the effects of varying disc geometry, radial distribution of SiCp and radial thermal gradient on the steady state creep behavior of a rotating disc, yielding according to Tresca criterion. The study indicates that by varying the disc thickness gradient from 0 to 29.9 mm, the strain rates in a uniform composite disc reduce over the entire disc radius. On increasing the SiCp gradient in a variable thickness disc, the radial stress increases throughout and the tangential stress increases near the inner radius but decreases towards the outer radius. The strain rates are observed to reduce significantly over the entire disc radius with the increase in the SiCp gradient. The imposition of radially increasing temperature profiles (linear, parabolic and exponential), with increasing radius, leads to considerable reduction in the radial and tangential strain rates in a variable thickness FGM disc, with the lowest and relatively more uniform distribution of strain rates observed for exponential temperature profile. The third segment of the study examines the effect of varying materials’ anisotropy on the creep performance of rotating disc made of functionally graded 6061Al-SiCw composite. The disc thickness and distribution of SiCw reinforcement are assumed to decrease non-linearly with increasing disc radius. The yielding of the disc material is described by Hill’s criterion and the extent of anisotropy is defined by the ratio of yield strength of the disc material along the radial, tangential and axial directions (i.e. sry :sq y :s zy ). The study reflects that an orthotropic disc in which the yield strength decreases in the order s zy >sry >sq y exhibits the lowest radial stress over the entire radius and the lowest tangential stresses near the inner and outer radii. However, the strain are observed to be the lowest in an orthotropic disc wherein the yield strength decreases in the order s zy >sq y >sry . The last segment of the study uses Seth’s transition theory to analyze the steady state creep response of a variable thickness rotating FGM disc. The effect of varying the radial distribution of reinforcement (SiCp) has been investigated on the stresses and strain rates. The study indicates that the stresses as well as strain rates in the FGM disc reduce noticeably with the increase in SiCp gradient in the disc. The study evolves a better understanding of the effects of geometrical, material and operating parameters, such as disc profile, distribution of reinforcement, materials’ anisotropy and thermal gradient, on the creep response of a rotating FGM disc. The study also highlights the advantage of using Tresca yield criterion for the design of rotating FGM disc. Besides using classical approach, the use of Seth’s transition theory to analyze the creep problems in rotating FGM disc has also been demonstrated. The results obtained in the study may assist the designers of rotating disc for creep applications.