Showing papers on "Functionally graded material published in 2006"

Modeling bamboo as a functionally graded material: lessons for the analysis of affordable materials

Abstract: Natural fibers are promising for engineering applications due to their low cost. They are abundantly available in tropical and subtropical regions of the world, and they can be employed as construction materials. Among natural fibers, bamboo has been widely used for housing construction around the world. Bamboo is an optimized composite that exploits the concept of Functionally Graded Material (FGM). Biological structures such as bamboo have complicated microstructural shapes and material distribution, and thus the use of numerical methods such as the finite element method, and multiscale methods such as homogenization, can help to further understanding of the mechanical behavior of these materials. The objective of this work is to explore techniques such as the finite element method and homogenization to investigate the structural behavior of bamboo. The finite element formulation uses graded finite elements to capture the varying material distribution through the bamboo wall. To observe bamboo behavior under applied loads, simulations are conducted under multiple considerations such as a spatially varying Young’s modulus, an averaged Young’s modulus, and orthotropic constitutive properties obtained from homogenization theory. The homogenization procedure uses effective, axisymmetric properties estimated from the spatially varying bamboo composite. Three-dimensional models of bamboo cells were built and simulated under tension, torsion, and bending load cases.

Asymmetric flexural vibration and thermoelastic stability of FGM circular plates using finite element method

Abstract: Here, asymmetric free vibration characteristics and thermoelastic stability of functionally graded circular plates are investigated using finite element procedure. A three-noded shear flexible plate element based on the field-consistency principle is used. Temperature field is assumed to be a uniform distribution over the plate surface and varied in thickness direction only. Material properties are graded in the thickness direction according to simple power law distribution. For the numerical illustrations, aluminum/alumina is considered as functionally graded material. The variation in critical buckling load is highlighted considering gradient index, temperature, radius-to-thickness ratios, circumferential wave number and boundary condition of the plate.

Comparison of compressive properties of layered syntactic foams having gradient in microballoon volume fraction and wall thickness

Abstract: The existing functionally graded syntactic foams (FGSFs) are based on creating a gradient of microballoon (hollow particles) volume fraction along the length or thickness of the composite to achieve a variation in density and mechanical properties. However, such an approach has several limitations. Variation in volume fractions of the constituents leads to the possibility of warping or localized swelling of FGSF if it is exposed to varying temperature and moisture conditions. It can also lead to premature fracture due to nonuniform distribution of stress along the microballoon gradient and existence of resin/microballoon rich sides in the material. Hence, a FGSF material needs to be created that is independent of the volume fraction variation. The present study attempts to address this requirement. A FGSF is fabricated that is based on the microballoon wall thickness variation along the specimen length. In this novel approach the volume fraction is available as an additional parameter for controlling the properties of the FGSF. The FGSFs based on volume fraction gradient (VF-type) and on microballoon wall thickness gradient (RR-type) are fabricated in a layered structure, and evaluated for compressive properties. It is observed that the VF-type FGSFs show a sharp drop in stress, on the order of 40–60%, after the peak compressive strength value. However, such feature is not observed in RR-type FGSFs leading to the possibility of gaining better control over strength and energy absorption. The compression of such foams could be continued to 60–75% strain based on the microballoon volume fraction in their structure. The total energy absorption was found to be three to five times higher in RR-type FGSFs compared to VF-type FGSFs and plain syntactic foams.

Application of functionally graded material for solid insulator in gaseous insulation system

Abstract: Functionally graded materials (FGM) have spatial distribution of a material property in order to achieve efficient stress control. An application of the FGM to a solid insulator (spacer) for a gaseous insulation system, like gas insulated switchgear, is expected to improve electric field (E-field) distribution around the spacer. In this paper, we describe the applicability of the FGM spacer to gas insulated power equipment. In the FGM spacer, we gave spatial distribution of dielectric permittivity to control the E-field distribution inside and outside the spacer. This paper includes following key results for the applications of the FGM. Firstly, E-field simulation results when applying the FGM by a finite element method are presented, in which we show the effective reduction of the maximum field strength by applying the FGM. Next, a fabrication technique of the FGM spacer sample with not only step-by-step but also continuous changes of permittivity is presented by use of centrifugal force. Finally, dielectric breakdown tests using FGM samples which are accurately controlled the spatial distribution of permittivity are carried out under lightning impulse voltage applications. The test result indicates the increase of breakdown voltage (BDV). From these results, we verified the applicability and the fabrication technique of FGM spacer for improvement of the dielectric strength in the gaseous insulation system.

Theoretical study on fabrication of functionally graded material with density gradient by a centrifugal solid-particle method

Abstract: Computer simulation was carried out for an estimation of the effect of centrifugal force on the formation of graded distribution of solid spherical particles within molten metal. The difference of migration rate between two kinds of spherical particles with different diameter and density was calculated taking account of the application of a centrifugal solid-particle method. Based on the systematic analysis, the capability of fabrication of the functionally graded materials ring with a unique density gradients which comprise the graded distributions of two kinds of solid particles is discussed. The unique gradient was essentially achieved due to the preferential movement of large particles with low density in comparison to small particles with high density.

Bleustein-Gulyaev waves in some functionally graded materials

Abstract: Functionally Graded Materials are inhomogeneous elastic bodies whose properties vary continuously with space. Hence consider a half-space ( x 2 > 0 ) occupied by a special Functionally Graded Material made of an hexagonal (6 mm) piezoelectric crystal for which the elastic stiffness c 44 , the piezoelectric constant e 15 , the dielectric constant ϵ 11 , and the mass density, all vary proportionally to the same “inhomogeneity function” f ( x 2 ) , say. Then consider the problem of a piezoacoustic shear-horizontal surface wave which leaves the interface ( x 2 = 0 ) free of mechanical tractions and vanishes as x 2 goes to infinity (the Bleustein–Gulyaev wave). It turns out that for some choices of the function f, this problem can be solved exactly for the usual boundary conditions, such as metalized surface or free surface. Several such functions f ( x 2 ) are derived here, such as exp ( ± 2 β x 2 ) (β is a constant) which is often encountered in geophysics, or other functions which are periodic or which vanish as x 2 tends to infinity; one final example presents the advantage of describing a layered half-space which becomes asymptotically homogeneous away from the interface. Special attention is given to the influence of the different inhomogeneity functions upon the characteristics of the Bleustein–Gulyaev wave (speed, dispersion, attenuation factors, depth profiles, electromechanical coupling factor, etc.)

Thermal Post-Buckling of Functionally Graded Material Timoshenko Beams

Abstract: Analysis of thermal post-buckling of FGM (Functionally Graded Material) Timoshenko beams subjected to transversely non-uniform temperature rise is presented. By accurately considering the axial extension and transverse shear deformation in the sense of theory of Timoshenko beam, geometrical nonlinear governing equations including seven basic unknown functions for functionally graded beams subjected to mechanical and thermal loads were formulated. In the analysis, it was assumed that the material properties of the beam vary continuously as a power function of the thickness coordinate. By using a shooting method, the obtained nonlinear boundary value problem was numerically solved and thermal buckling and post-buckling response of transversely non-uniformly heated FGM Timoshenko beams with fixed-fixed edges were obtained. Characteristic curves of the buckling deformation of the beam varying with thermal load and the power law index are plotted. The effects of material gradient property on the buckling deformation and critical temperature of beam were discussed in details. The results show that there exists the tension-bend coupling deformation in the uniformly heated beam because of the transversely non-uniform characteristic of materials.

Transient Thermoelastic Analysis for a Functionally Graded Hollow Cylinder

Abstract: This paper is concerned with the theoretical treatment of transient thermoelastic problem involving a functionally graded hollow cylinder due to uniform heat supply. The transient one-dimensional temperature is analyzed by the method of Laplace transformation. The thermal and thermoelastic constants of the hollow cylinder are expressed as power functions of the radial coordinate. We obtain the one-dimensional solution for the temperature change in a transient state, and thermoelastic response of a functionally graded hollow cylinder. Some numerical results for the temperature change, the displacement and the stress distributions are shown. Furthermore, the influence of the nonhomogeneity of the material upon the temperature change, displacement and stresses is investigated.

Methods for production of FGM net shaped body for various applications

Abstract: Methods for the production of functionally graded material (FGM) are provided. FGM is processed by powder cold spraying ceramic-metal layers, the final shape is pre-pressed by cold isostatic pressing and is then sintered using field activated sintering technique (FAST). The FGM materials can be used for medical applications.

Influence of Functionally Graded Material on Buckling of Skew Plates under Mechanical Loads

Abstract: In this technical note, the critical buckling of simply supported functionally graded skew plate subjected to mechanical compressive loads is evaluated using first-order shear deformation theory in conjunction with the finite element approach. The material properties are assumed to vary in the thickness direction according to the power-law distribution in terms of volume fractions of the constituents. The effective material properties are estimated from the volume fractions and the properties of the constituents using the Mori–Tanaka homogenization method. The effects of aspect ratio, material gradient index, and skew angle on the critical buckling loads of functionally graded material plates are highlighted.

Axisymmetric Mechanical and Thermal Stresses in Thick Long FGM Cylinders

Abstract: The analysis of axisymmetric mechanical and thermal stresses for a long hollow cylinder made of functionally graded material, as functions of radial and longitudinal directions, is developed. The material properties are graded along the radial direction according to power functions of radial direction. The temperature and Navier equations are solved analytically, using the generalized Bessel function and complex form of Fourier integral. A direct method of solution of Navier equations is presented, rather than potential functions method.

Thermal Buckling of Imperfect Functionally Graded Cylindrical Shells Based on the Wan–Donnell Model

Abstract: A thermal buckling analysis of an imperfect functionally graded cylindrical shell is considered using the Wan–Donnell model for initial geometrical imperfections. Derivation of the equations is based on the first-order classical shell theory using the Sanders nonlinear kinematic relations. Results for the buckling loads are obtained in the closed form. The effects of shell geometry and volume fraction exponent of functionally graded material on the buckling load are investigated. The results are validated with known data in the literature.