Showing papers on "Functionally graded material published in 1995"

Thermal residual stresses in a functionally graded material system

Abstract: A one-dimensional calculation of thermal residual stresses, arising from the fabrication of a Functionally Graded Material (FGM) system, is presented. As a first step, calculations have been limited to the linear elastic case. The FGM system consists of ceramic (Al2O3) and metal (Ni) phases varying with distance in one direction. Several functional forms of gradation of constituents were examined to arrive at the optimum profile giving the minimum residual stress level. A linear variation in composition from fully ceramic to fully metal showed the least residual stress. Residual stresses were found to increase when fully ceramic and/or fully metal regions are included in the structure, adjoining the graded zone. The character (tension or compression) of stresses is dependent on the functional form of gradation. A low residual stresses state close to that of the continuously graded structure can be achieved also in a multilayer FGM system having several layers (typically > 11) of constant composition. The effects of temperature dependent elastic and thermal expansion characteristics of constituents on residual stress were found to be small.

A study of crack in functionally graded material under dynamic loading

Abstract: The paper addresses a numerical treatment of a fracture occurring in the functionally graded materials (FGM) under a dynamic load. The FGM is composed of a titanium alloy as an inclusion and zirconia as the matrix, where a generation of microcracks is considered to occur in the ceramic phase of the high stressed area. A spherical grain model is used to describe thee elastic constitutive law for the FGM composite, in which the nonlinear effects due to the microcracking are accounted for. The most appropriate fracture parameter, T*, is used to assess the crack-tip severity in the highly inhomogeneous materials such as the present.

Reduction of the stresses in a joint of dissimilar materials using graded materials as interlayer

Abstract: If two dissimilar materials are joined at high temperature, very high residual stresses develop near the free edge of the interface during cooling to room temperature. In most cases, the stresses are singular for elastic material behaviour. One method to avoid the stress singularities is to introduce a graded interfacial zone with a continuous change of all material properties between the two joined components. In this paper the distribution of the stresses in FGM has been calculated by FEM. The interlayer has continuous material properties, which means that the graded interlayer has not been treated as N layers. The value of the peak stress and the location of the peak stress in the joint have been investigated.

Applicability of Fracture Mechanics in Strength Evaluation of Functionally Graded Materials.

Abstract: Elastic and elastic-plastic analyses of a crack in a particulate-dispersed functionally graded material (FGM) have been carried out using a newly developed finite element method based on Tohgo-Chou-Weng's(1994, 1996) constitutive relation for particulate-reinforced composites. By setting the mechanical properties of particles and a matrix and their content graded in the thickness direction, FGMs and non-FGM are designed. From comparison of the numerical results for the FGMs and non-FGM, the influence of the gradient of the mechanical properties on a stress intensity factor and the crack tip field is discussed. The following conclusions are derived: (1) The stress intensity factor of a crack under constant boundary conditions is considerably affected by the gradient of the mechanical properties. (2) The elastic and plastic stress singular fields around a crack tip in a FGM are basically described by the fracture mechanics parameters(K 1 and J 1 ) as well as in a non-FGM, using the mechanical properties of the material at the crack tip. (3) The size of the singular field decreases with an increase in the gradient of the mechanical properties. This means that the applicability of fracture mechanics, such as the small-scale-yielding condition and the validity of the J-integral, is affected by the gradient.

Fabrication of Ti/SUS/Mo Functionally Graded Material by Slurry Dipping Process

Abstract: Molybdenum/stainless steel/titanium functionally graded materials (FGM), which will be used as long lire cladding tubes in a fast breeder reactor, have been fabricated by slurry dipping and sintering process, emphasizing the increase in corrosion resistance to liquid sodium and fission products. Slurries of different composi-tions were prepared by mixing the appropriate amount of Mo, SUS304 and Ti powders in water with polyvinyl alcohol. Cylindrical substrates were formed by die pressing of stainless steel and alumina powders. The sub-strates were dipped in the slurry to coat the surface subsequently dried in air and debinded in Ar. They were encapsulated in Pyrex glass tubes and then HIPed 2h at 1050°C at a pressure of 200MPa.The microstructural observation in the cross section of the sintered compacts has revealed that the dipped layers were formed, in which no defects such as small cracks and residual pores were observed. Some intermetallic compounds were observed in the FGM layer as detected by EPMA.

Correlation Between Hardness and Residual Stress in ZrO2(3Y)/metal Functionally Graded Materials

Abstract: Dense FGMs were fabricated from ZrO2(3Y) and metals (Ni and stainless steel) by spark plasma sintering. These FGMs exhibited two kinds of residual stress (a local stress and a layer stress), local stress is stored in each layer, while layer stress is dispersed throughout each layer of the whole FGM. The local stress was estimated by measuring Vickers hardness.