Tensile fracture of doubly-convex cylindrical discs under diametral loading

Doubly-convex cylindrical gypsum discs have been fractured under the action of two diametrically opposed in-plane forces. The disc diameter was constant throughout the test series. The ratio of cylinder length to diameter ranged from 0.06 to 0.30; the ratio of cylinder diameter to radius of curvature of the disk faces was varied from 0 to 1.43. The fracture loads obtained have been correlated with stress data obtained from the photoelastic analysis of Pitt et al. An empirical equation, valid for any brittle material, relating the tensile strength of the material to the fracture load and dimensions of a doubly-convex disc has also been developed.Nomenclature C, K constants - D diameter of disc - F comparative factor - I _max stress factor - P load - P _s fracture load of convex-faced disc - P ₂ fracture load of plane-faced disc with W/D = 0.2 - R radius of face-curvature - t overall thickness of convex-faced disc - T thickness of plane-faced specimen of convexfaced disc - W cylinder length - _f tensile strength of material - _t maximum tensile stress in convex-faced disc - _x uniform tensile stress in plane-faced disc     

Densification model for powder compacts

The driving force of densification has traditionally been modeled on the basis of local curvature changes between powder particle pairs. Extension of particle pair analysis to powder compacts involving billions of particles has not been successful because of the geometric difference between the two cases. In this paper, a densification stress model for grain boundary and lattice diffusion controlled densification is developed on the basis of a powder compact's thermodynamics and the internal surface area evolution. For compacts with a constant grain size, the model predicts that the densification stress increases as a function of relative density, which is in agreement with experimental trends. With grain growth, the densification stress becomes relatively constant throughout the intermediate stage of densification, in agreement with experimental data in the literature. Comparison of densification rate data with densification rate model employing the developed densification stress relation also gives good functional agreement. These agreements indicate that modelling densification stress and densification rate on the basis of internal surface area captures the essential physics of powder compact densification.     

Densification of raw coke powder compacts

The effects of grinding on densification of a petroleum raw coke were examined. The compact of the powder ground for a short time showed large puffing during calcination below 400 °C, and no densification by subsequent heat treatment. It was found that the puffing could be suppressed by long grinding of the powder. In the compact of the powder subjected to prolonged grinding, the pore volume decreased in two temperature ranges of calcination, at 400–500 and 600–700 °C. The first decrease seems to be attributed to softening and volume reduction due to carbonization of quinoline-soluble components among the coke grains. The second decrease suggests that the sintering due to solid state material transport, such as viscous flow and plastic deformation, takes place in addition to gas-phase material transport. The increase in bulk density at temperatures over 1000 °C was mainly due to contraction of constituent grains in which micropores decreased, not a decrease in pore volume of the compact. The most remarkable effect of grinding on densification of the raw coke was the suppression of puffing during the calcination process.     

Sintering behaviour of powder compacts with multiheterogeneities

The sintering behaviour of a powder compact containing uniformly distributed heterogeneities has been analysed. The results reveal a strong retardation of sintering in the presence of non-sinterable agglomerates, due to the development of a uniform hydrostatic tensile stress in the powder matrix. The variation in stresses and sintering rate depends strongly on the total volume fraction of heterogeneities especially when the volume fraction is small, but is insensitive to the actual number of heterogeneities. Specific results are calculated using densification and deformation laws pertinent to Al₂O₃.     

Hot isostatic pressing of Y-TZP powder compacts

A fundamental study of hot isostatic pressing (HIPing) was performed on Y-TZP powder compacts, and the effect of the various HI Ping process variables (time, temperature and pressure) was examined on the densification behaviour. The results were analysed using Ashby's HIPing model, which was found to be applicable for this system. The densification was found to be governed by the grain-boundary diffusion of cations. The use of grain-boundary transport characteristics determined in this study enabled HIP maps to predict the densification behaviour of HIPing within an accuracy of a factor of three for all stages of densification, except for the results at low HIPing pressure and the region of near full density.     

Electrical and thermal tortuosity in powder compacts

A simple equation to calculate the effective mean path to be travelled by the heat flow or electrical current in a porous powdered material is proposed. In this equation, the effective mean path is a function of the sample porosity degree and the tap porosity of the original powder. This latter parameter determines to a great extent the compact pore structure, since it depends on the powder particle size, shape and distribution. The proposed equation has been validated using real metallographic images from sintered powders compacts. For measurements, a computer program, based on the Pathfinding A* Algorithm, has been developed. Results are in very good agreement with theoretical predictions. The proposed equation can be useful to model the electrical and thermal conductivity of sintered compacts.     

Solid-state reaction behavior of Al-Ti-C powder mixture compacts

The solid-state reaction behavior of the Al-Ti-C powder mixture compacts was investigated. The results showed that the TiC/Al composite was synthesized by the solid-state diffusion reaction in a vacuum electric resistance furnace at a heating temperature of 600 ± 2 °C, with a holding time for 3 h. The TiC particulate was fine and irregular, with a mean size of 2 μm.     

Grain growth in sintering of clustered powder compacts

A theoretical model of grain growth in sintering of clustered powder compacts is outlined, showing that the microstructure evolution is stepwise continuous in time and its general trends can be predicted independently of the particular system and process being considered. The dependence of coarsening on densification can be accounted for by introducing a densification-scaled time variable (intrinsic time). The theory is successively applied to systems where particular local mechanisms of matter transport are supposed to operate, respectively in the initial/intermediate and the intermediate/final stage of sintering. The obtained mathematical models are solved numerically to follow the evolution of three regularly packed clusters. The model predictions are in good agreement with experimental data obtained by other researchers.     

Thermal expansion behaviour of nanocrystalline titanium powder compacts

This investigation is an attempt to understand the expansion behaviour of compacts of nanocrystalline titanium powder during thermal treatments. Entrapped gases cause excessive expansion in the green compacts. The dilatometric study reveals that there is no significant difference in the expansion coefficient of sintered nanocrystalline titanium samples, compared to their micron-size counterpart.     

Characterization of internal structure in Y-TZP powder compacts

The internal structure of yttrium-doped tetragonal zirconia powder compact was characterized using the immersion liquid technique, mercury porosimetry, scanning electron microscopy, and a compaction test. The specimens examined were prepared by pressing spray-dried granules at various pressures from 5–600 M Pa. The immersion liquid technique had numerous potential advantages, over the other methods for evaluating defects of 1–10 μm in size. With this technique, intergranular interfaces and pores were found to be present even in the specimen prepared at 600 M Pa. A feature, which is considered to be an agglomerate, was also found. The significance of the large defects on ceramic processing, as well as a comparison of the technique with the other methods, is presented.     

Processing defects in ceramic powders and powder compacts

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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 = M ₀ (1–p) ⁿ , where M ₀ 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.     

Thermal debinding of Fe3Al-X metal powder compacts

The process of removing a multicomponent binder from a metal powder compact has been investigated. Model experiments of debinding were performed on compounds consisting of less than 40 vol% binders (low molecular weight polyethylene, paraffin and Carnauba waxes) and more than 60 vol% metal content. As typical representatives for injection moulding morphology and meeting all other requirements for optimal powder characteristics, elemental powders of the Fe-Al system were used. Viscosity results over a wide range of shear rates for various plastisols are presented as functions of binder system composition and metal powder content. Based on the rheological response, an optimization of plastisol formulation was performed. Results are reported on three series of debinding modes using heat and fluid wicking in air and in nitrogen. The time dependence of fractional debinding, x(t), during wicking has been estimated using a model. Direct observation by SEM of binder distribution and pore structure evolution at different stages of the debinding process was made. Wick-assisted thermal debinding in nitrogen proved to be an effective debinding method in terms of shape preservation and the absence of defects in the studied material.     

Strength and fracture toughness of indented glass-nickel compacts

Strength and fracture toughness of glass-nickel compacts have been measured in a four-point bend test. Knoop microhardness indentation technique was used to introduce controlled surface flaws in hot-pressed sodium borosilicate glass containing up to 10 vol% spherical nickel powders for the bend test and also to assess the strength of the compacts. It was found that as the number of voids increased, the load to induce indentation flaws, the subsequent fracture strength and toughness all increased.