Jenike yield locus

Jenike carried out many experimental measurements on free-flowing and cohesive materials. He found that the yield locus of a dry material would be a straight line passing through the origin, as shown in... 

Figure 16(a). The term Cohesionless was therefore used to describe materials which have a negligible shear strength under zero normal load (an = 0). On the other hand, Jenike found that the yield loci of cohesive materials differ significantly from a straight line and have a nonzero intercept, indicated by C. Moreover, the position of the locus for a cohesive powder is a strong function of the interstitial voidage of the material. Fig 16(b) shows the typical yield locus for cohesive materials. 

Jenike developed the idea that no single line represents the yield but rather a curve called the yield locus. The yield behavior depends on the packing density of the powder when it is caused to flow under the action of normal and shear stress. Figure 12.36 shows a yield locus for a given porosity, e. A Mohr circle for the stage when yielding starts is characterized by the principal stresses i and 2-The points at the end of the yield locus lies on the Mohr circle pertains to... 

FIGURE 12.37 Jenike s yield locus for a particular porosity, e. 

As with the Jenike cell, this process is time-consuming. Amidon and Houghton, " however, used a single yield locus with this cell for comparative purposes. Hiestand et al., ° in comparing this apparatus to the Jenike cell, claimed that this simple shear cell can be used to provide characterization of the unconfined yield strengths of powders. The results from the two devices are not identical. However, as much as the Hiestand device requires less powder and the consolidation step is more automated and consistent, it provides an inexpensive alternative to the Jenike-type cell to characterize pharmaceutical powders. Amidon and Houghton used the cell to examine the effect of moisture on the powder flow properties of microcrystalline cellulose. ... 

Shear strength of a bulk material in differing states of dilation is a key property of interest for flow considerations. The conventional hopper design method for mass flow is based upon critical state theory, and a Jenike shear cell is used to secure yield locus values upon which a design procedure is based. This technique is universally accepted, but not widely used for small hoppers for various reasons. Significant cost and expertise is required to obtain accurate values, compared with full-scale trials and... 

A powder is characterized running an experiment in a Jenike s shear cell. The results are given in Table 2.4. Determine the failure properties that can be derived from the yield locus of the powder. 

A better method for determining the cohesive and frictional effects of particles is by using a shear cell (48,51,52). There are various cell configurations, the most popular proposed by Jenike (51). In the Jenike cell (Fig. 13), a powder is loaded and then compressed by twisting the lid of the cell. The number of twists required to load the powder to the point at which the resistance to shear (measured as stress applied to ring around the bed) is constant. This phase of the test is known as shear consolidation. The load is reduced and the resistance to shear is then recorded. A yield locus of this shear stress vs. the reduced load is obtained and used to calculate various flow-related parameters (47,48,51). Numerous parameters can be... 

The Jenike effective angle of friction is the angle of the straight line drawn through the origin of a normal stress-shear stress plot and tangential to the Mohr semi-circle, which inscribes the equilibrium, or end point of the yield locus when failure occurs at no sample volume change. The Mohr semi-circle represents the stresses in a powder consolidated under a major principal stress. 

Although the Jenike effective angle of internal friction can be determined fi om one yield locus the internal angle of friction requires a family of yield loci, each locus corresponding to a different state of compaction (ac) (Figure 1.17). 

Measurement of tensile stress cannot be measured directly with a Jenike shear cell or an annular shear tester, although one approach in the measurement of tensile strength of a powder is to determine the yield locus of the material and then to extrapolate part of this locus to zero normal stress. The negative intercept on the normal stress axis is the tensile strength of the material under investigation (Figure 1.18). 

Although estimation of tensile strength, adhesion and cohesion from a Jenike shear test yield locus is the easiest and less demanding way of assessing powder stresses, there are other types of equipment which attempt to measure cohesion and tensile strength. 

In view of the experimental errors normally affecting shear cell measurements and the amount of personal judgement required to draw Mohr stress circles tangential to a curved yield locus, there is always some uncertainty in the flow function derived from the Jenike-type shear yield loci method. A direct measurement therefore offers considerable advantage and, besides possibly giving better accuracy, may prove to be more rapid and reproducible. 

The well-known failure criterion according to Jenike [3] was the first, which could describe the behaviour of particulate solids at stress levels relevant in powder technology. It was modified e.g. by Schwedes [4], Molerus [5] and Tomas [6,7]. For Coulomb fnction in the interesting positive compressive stress range, an yield locus can be approximated by a straight line [7] ... 

Experiments were performed using the same humid salt as used in [2], sheared in an annular ring shear tester. We present here a temporal study of its yield locus and of its cohesion. Two types of experiments have been performed, using the classical procedure of Jenike [1] where the shear stress is removed during the consolidation time, and relaxation experiments without removing the shear stress. In both cases, the dilatancy of the powder has been recorded in order to follow its compaction. 

The well-known failure hypotheses of Tresca, Coulomb-Mohr and the yield locus concept of Jenike [1] and Schwedes [2] as well as the Warren-Spring-Equations [3 to 7] were specified from Molerus [8, 9] by the cohesive steady-state flow criterion. The consolidation and non-rapid, frictional flow of fine and cohesive particulate solids was explained by acting adhesion forces in particle contacts [8]. 

From it, the stress dependent effective angle of internal friction effective yield locus follows obviously [28], see Fig. 4 ... 

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