Stress compacting

Figure 4.8 Schematic of a lateral stress compaction testing cell [2]...

Bjdrlykke, K. and H0egh, K. 1997. Effects of burial diagenesis on stresses, compaction and fluid flow in sedimentary basins. Mar. Pet. Geol., in press. 

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

Probably the largest compound vessels built were two triple-wall vessels, each having a bore diameter of 782 mm and a length of 3048 mm designed for a pressure of 207 MPa (30,000 psi). These vessels were used by Union Carbide Co. for isostatic compaction unfortunately the first failed at the root of the internal thread of the outer component which was required to withstand the end load (40). A disadvantage of compound shrinkage is that, unless the vessel is sealed under open-end conditions, the end load on the closures has to be resisted by one of the components, which means that the axial stress in that component is high. 

Roll friction angle deg deg (Sr Pow der compaction normal stress kg/cm psf... 

Compaction Rate Processes The performance of compac tion techniques is controlled by the ability of the particulate phase to uniformly transmit stress and the relationship between apphed stress and the compaction and strength characteristics of the final compacted particulate phase. The general area of study relating compac tion and stress transmission is referred to as powder mechanics (Brown Richards, Principles of Powder Mechanics, Pergamon Press Ltd., Oxford, 1970). 

FIG. 20-76 The yield loci of a powder, reflecting the increased shear stress required for flow as a function of apphed normal load. YLl through YL3 represent yield loci for increasing previous compaction stress. EYL and VYL are the effective and wall yield loci, respectively. 

A powder s strength increases significantly with increasing previous compaction. The relationship between the unconfined yield stress/, or a powder s strength, and compaction pressure is described by the powder s flow function FE The flow function is the paramount characterization of powder strength and flow properties, and it is calculated from the yield loci determined from shear cell measurements. [Jenike, Storage and Flow of Solids, Univ. of Utah, Eng. Exp. Station Bulletin, no. 123, November (1964). See also Sec. 21 on storage bins, silos, and hoppers.]... 

Transmission of Forces As pressure is applied to a powder in a die or roll press, various zones in the compact are subjected to differing intensities of pressure and shear. Compaction stress decreases with axial distance from the applied pressure [Strijbos et al.. Powder Tech., IS, 187 209 (1977)] due to frictional properties of the powder and die wall. For example, the axialpressure experienced within a cyhndrical die with an applied axial loaa Oq may be estimated to a first approximation by... 

FIG. 20-78 Reaction in compacts of magnesium carbonate when pressed (P = 671 kg/cnr ). (a) Stress contour levels in kilograms per square centimeter, (h) Density contours in percent solids, (c) Reaction force developed at wedge responsible for stress and density patterns. [Tf ain, Trans. Inst. Cbem. Eng. (London), 35, 258 (1957).]... 

Fig. 6.4. At relatively low pressures the shock speeds observed for stress waves in low density powder compacts are dominated by the crush-up of the powder toward solid density. The figure shows measured wavespeeds for a range of densities and fits to the data based on Herrmann s P-a model on Fe. Note the unusually low wavespeeds compared to solid density (after Herrmann [69H02]).

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