Kalyan Kumar Phani

Young's modulus of porous brittle solids

A new equationE =E0 (1 −aP)n whereE andE0 are the Youngs moduli at porosity,P, and zero, respectively, a andn are material constants, has been derived semi-empirically for describing the porosity dependence of Youngs modulus of brittle solids. The equation satisfies quite well the exact theoretical solution for the values of Youngs moduli at different porosities for model systems with ideal and non-ideal packing geometry. The equation shows excellent agreement with the data Onα- andβ-alumina over a wide range of porosity. Unlike the existing porosity-elastic modulus equations, the proposed equation satisfies the boundary conditions and is inherently capable of treating isometric closed pores as well as non-isometric interconnected pores. The parameters a and n provide information about the packing geometry and pore structure of the material.

Temperature dependence of hydrothermal ageing of CSM-laminate during water immersion

Abstract Random glass fibre reinforced polyester resin specimens were exposed to hot water at different temperatures. Effects of the environmental history on strength characteristics of the composite were investigated using the three-point bend tests. The study shows that reduction of the strength of random-fibre composites due to hydrothermal effects is a rate process for which the temperature influences only the rate constant. The rate constant follows the Arrhenius equation 1/τ = 1/τ 0 exp [−E/RT], where 1/τ is the rate constant, E and R are activation energy and gas constant respectively; τ 0 is a constant and T is absolute temperature. The time/temperature superposition principle is applicable for the degradation process and a master curve for strength can be obtained by shifting the data along the logarithmic time scale by a shift distance, ln a D . This allows the estimation of strength at any temperture from experiments conducted only at one temperature.

Strength and elastic modulus of a porous brittle solid: an acousto-ultrasonic study

The strength and elastic modulus of a porous brittle solid such as gypsum have been studied using an acousto-ultrasonic technique. Acousto-ultrasonics has been found to be a sensitive indicator of strength and porosity which are linearly related to some powers of a stress wave factor. New equations for porosity dependence of ultrasonic velocity, elastic modulus and strength of brittle solids have been proposed.

Ultrasonic evaluation of elastic parameters of sintered powder compacts

The variation of elastic moduli, M, of sintered powder compacts with porosity, p, has been analysed in terms of an equation M = M0 (1−p)n, where M0 is the elastic modulus of non-porous material and n is a constant. The variation of ultrasonic velocities has also been described in terms of a similar equation derived from the relations given by physical acoustics theory. It has been shown that the parameter n is related to a stress concentration factor around pores in the material and is dependent on pore geometry and its orientation in the material. The observed variation in moduli and velocities with porosity has been compared with the theoretically predicted values based on self-consistent oblate spheroidal theory.

An analysis of microstructural parameters in the minimum contact area model for ultrasonic velocity–porosity relations

The relationship between the normalised ultrasonic velocity (v=v0) and the volume fraction (P) of pores in porous materials has been derived on the basis of a minimum solid area of contact model. It considers the development of the minimum solid area of contact during the sintering of an assembly of monosized spheres stacked in simple cubic packing. It is shown that by using a single model parameter ‘‘c’’, related to the eAective aspect ratio ‘‘’’ of spheroidal pores; it is possible to predict the trends in variation of the experimental (v=v0 versus P) data of hot pressed silicon carbide (SiC), hot pressed silicon nitride (HPSN), reaction bonded silicon nitride (RBSN), porcelain, ceramic superconductors based on YBCO system, sintered iron and tungsten powder compacts as well as alumina. In addition, it was found that a single relation describes the behaviour of both longitudinal and transverse wave velocity as a function of pore volume fraction in all but the ceramic superconductors of the aforesaid materials. Finally, it is proposed that for all practical purposes, the analytical relation derived in the present work, can be eAciently approximated by an empirical relationship: v=v0a 1ˇ a1P OU n , where the subscript zero refers to the material of theoretical density and a1, n are fitting

Mechanical characterization of microwave sintered zinc oxide

The mechanical characterization of microwave sintered zinc oxide disks is reported. The microwave sintering was done with a specially designed applicator placed in a domestic microwave oven operating at a frequency of 2.45 GHz to a maximum power output of 800 Watt. These samples with a wide variation of density and hence, of open pore volume percentage, were characterized in terms of its elastic modulus determination by ultrasonic time of flight measurement using a 15 MHz transducer. In addition, the load dependence of the microhardness was examined for the range of loads 0.1–20 N. Finally, the fracture toughness data (KIC) was obtained using the indentation technique.

Young's modulus—porosity relations: an analysis based on a minimum contact area model

The Youngs modulus–porosity relation of porous ceramic materials has been analysed based on a minimum solid area of contact model. The minimum solid area of contact developed during sintering of an assembly of monosized spheres stacked in simple cubic packing is calculated by approximating the neck area by two sine-wave functions. The first function represents the shape of a sphere and the second function signifies the shape of the neck between neighbouring spheres. The model shows excellent agreement with 12 sets of relative Youngs modulus, E/Eo, versus pore volume fraction, P, data from literature on five different polycrystalline ceramic oxides, namely Lu2O3, Sm2O3, Yb2O3, Al2O3 and ThO2, whose porosities are reasonably represented by such idealized packing.

Strength distribution and gauge length extrapolations in glass fibre

Tensile strength data of E and S-994 glass fibres, reported by previous workers, have been analysed using a modified Weibull distribution. The function provides an upper and a lower limit of strength and is characterized by two shape parameters. Based on the strength data at two gauge lengths (15 and 120 mm), predictions of the strengths at other gauge lengths are in good agreement with experimental data.

Hydrothermal ageing of CSM-laminate during water immersion — an acousto-ultrasonic study

Acousto-ultrasonics represents a very attractive and promising technique for detecting flaws and studying degradation of composite material. Studies of damage under hydrothermal ageing incurred by random fibre-reinforced laminates have been carried out with this technique. It is shown that the “stress wave factor” is a sensitive indicator of random fibre composite strength reduction due to a hydrothermal effect. The same study can be used for the estimation of strength from the data generated under accelerated conditions.

A new modified Weibull distribution function for the evaluation of the strength of silicon carbide and alumina fibres

The strength distributions of silicon carbide and alumina fibres have been evaluated using a modified Weibull distribution function. The function provides an upper and lower strength limit and is characterized by two shape and location parameters. The sum of squares was used as a measure of fit between the distribution function and the data. The result showed good agreement between the two. In addition, the strength distribution and the average value at a different gauge length were extrapolated from the parameters estimated at the original gauge length. In this case also, the proposed function accurately predicted the data points.