Unconfined yield stress

Unconfined yield stress of powder kg/cm psf u,v Granule volumes 3 cm in ... 

Flow Property Tests Flowahility of the produc t granules can be charac terized by unconfined yield stress and angle of friction by... 

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.]... 

The unconfined yield stress is a measure of the stress necessary to cause a material unsupported in two directions to fail in shear. This is what must happen when an arch fails within the powder or at the hopper opening. The effective... 

It is convenient to introduce the concepts of material flow function, FF, and flow factor, ff. The material flow function, FF, relates the unconfined yield stress, To, to the corresponding major consolidating stress, cri, and is determined experimentally from the yield locus of the material, as shown in Fig. 8.9. The material flow function is presented as a plot of To versus flow factor, ff, is defined by... 

Considering the equilibrium of a stable arch, show that the minimum diameter of the orifice in a conical hopper, d,ma, which is defined to account for the arching, can be related to the unconfined yield stress of the bulk material by the relation... 

One may compare the flowability of powders at similar pressures by comparing their unconfined yield stress at a single normal stress, or one point off a flow function. In this case one should clearly state the pressure of comparison. Flow indices have been defined to aid such one-point comparisons, given by the ratio of normal stress to strength, or... 

Uniaxial Compression - Williams Method This method was developed by Williams, Birks and Bhatta-charya24. A compact is first formed in a split mould by applying an axial compressive force, the mould is then removed to leave a cylindrical specimen with its axis vertical. The compressive vertical stress needed to cause failure of the specimen is then found and this is the unconfined yield stress for the consolidating stress used in the compaction of the specimen. The failure function is found by forming a number of compacts under different consolidating stresses and finding the unconfined yield stress for each specimen. 

In order to eliminate the effect of wall friction during compaction, Williams et al.2A compacted the powder in a number of shallow increments and proposed a method of extrapolation to estimate the unconfined yield stress for an infinite number of increments, i.e. for a uniformly compacted specimen. 

For specimens formed by the method proposed, the unconfined yield stress is uniquely determined by its average bulk density, independent of the number of increments in which it is formed. 

Comments on the Compression Tackiness Tester This is clearly a fingerprinting method of a proven practical value. The result does not represent, however, the unconfined yield stress corresponding to the compression load because the compression stress varies within the height of the briquette. This is because the load during the sample compression is partly taken up by wall friction and the stress (and the bulk density) therefore reduces in... 

The Cohesion Tester has been used in industry, mainly as a quality control-type test. It can be useful as an aid to assessment of flow properties and of power requirements in mixers, bulk conveyors and feeders. The original developers of the tester, Warren Spring Laboratories35, have even tried to correlate the cohesion value with unconfined yield stress determined with the Jenike shear cell and found a good correlation for some powders (fc = 6 x C). The tester is, quite obviously, only useful with fine, cohesive powders because ... 

Stress, or force per unit area (SI units Pa or N/m2), has been defined as the intensity of the internal components of forces in a certain point through a given plane of a body. Compressive stress (or pressure) refers to the perpendicular components toward a normal plane on which compressive forces act. Different denominations can be used for stresses that characterize compression of a certain volume of powder mass natural and engineering stress, compressive, tensile, or shear stress, yield stress, unconfined yield stress, and principal stresses. 

The unconfined yield test is a conventional technique that is easily applied to cohesive powders (Buma, 1971 Head, 1982). It is generally used to determine the unconfined yield stress of a specific material. The method is based on preparing consolidated plugs of powder, generally with a cylindrical shape, and then applying an axial load until the powder fails. This load is defined as the unconfined yield stress. 

For example, the cohesion of dairy powders has been studied with an unconfined yield test by preparing cylindrical plugs of powder at different particle sizes and moistures. Unconfined yield stress values were obtained as an index of cohesion for whole milk powder and skim milk powder. Dry... 

Hollenbach et al. (1982) showed compressibility to be a more reproducible index than parameters such as angle of internal friction or unconfined yield stress obtained from the Jenike shear test (ASTM D 6128). 

Conversely, Ehlermann and Schubert (1987) sustained that compressibility results from materials of different composition cannot be compared and that flowability characterization through compressibility must be made specifically for each food variety. Moreover, confined uniaxial compression is a simple compression test that provides an approximate measure of the flowability of powders. Therefore, it is not suitable for silo design but may prove to be a convenient method for process control in any food laboratory (e.g., to evaluate particle cohesion). Table II offers a range value definition for flowability classification by comparing flow function (ratio between the maximum consolidation stress and unconfined yield stress) with compressibility. 

A powder s strength increases significandy with increasing previous compaction. The rdationship between the unconfined yield stress/,., or a powder s strength, and compaction pressure is described by the... 

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