Annular shear testers

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). 

Figure 7.4 Schulze ring shear tester annular trough.

The ring or annular shear cell, was developed by Carr and Walker as early as 1968. In recent years this tester has undergone a number of modifications. Peschl has developed an annular shear cell in which the sample and shear cell consists of a full circle. This contrasts to the earlier cells that have a band of sample on the outer portion of the circle. This was done to eliminate wall friction. It is also rotated very slowly, since at low speed, velocity variability becomes more negligible in the shear measurement. In this way a full ring can be utilized and speed differences in the outside and inside of the ring become negligible. 

Generally, the annular shear cell is an acceptable alternative to the Jenike tester for engineering design. The advantages of the annular shear cell can be summarised as follows ... 

The limitations of the Jenike shear cell are that it is not very useful for measuring bulk solids with large shear deformations, e.g., plastic powders. The level of consolidation stresses required are inappropriate for pharmaceutical materials, and the quantity of material required is often beyond that available in the early stages of development. Alternative shear cells that have been used include annular shear cells (Nyquist and Brodin 1982 Irono and Pilpel 1982) and ring shear testers (Schulze 1996). 

One way which relies upon the fundamental knowledge of the stress-strain-volume behaviour of bulk solids is dependent upon the development of testers such as the biaxial and triaxial shear testers as well as the now universally accepted Jenike shear cell, or the standard shear test tester. Other instruments, such as the annular shear cells and the cross-sectional Peschl and Colijn (1977) tester, use the same stress-strain-volume principle. These annular shear cells may also be used to evaluate a bulk powder flow function. The powder flow function, having been discussed previously, still requires a family of yield loci before cohesion can be evaluated. 

Figure 6.8 Expanded view of an annular shear cell polymer to metal friction tester. Drawing by Richard Humpidge. 1975, Rapra Technology)...

The issue of non-uniform shear travel in rotational testers can be minimi2ed if the test cell has an annular ring shape instead of a cylindrical one such as in the Peschl tester. While the irmer radius of the ring still has a shorter shear travel than the outer one. the difference is relatively small, particularly if the difference between the two radii is small compared to their average. This concept was developed a number of years ago by Carr and Walker [5]. In our experience, the early models of the device, while very robust from a mechanical standpoint, were too massive for delicate measurements. It also was difficult to clean the cell, particularly the lower ring. This form of ring shear tester uever achieved widespread use when compared to the Jenike cell. 

The Flow Function as the dependence of the unconfmed yield strength Oc on the major consolidation stress oi (at steady state flow) can ordy be determined using testers where both stress states can be realized. Steady state flow can be realized in Jenike s tester, in annular shear cells, in a torsional shear cell, in the true biaxial shear tester and in a very specialized triaxial cell [2]. The unconfined yield strength Oe can be determined by running tests in Jenike s tester, in an annular shear cell [10], in uniaxial testers and in the true biaxial shear tester. Therefore, only Jenike s tester, annular shear cells and the true biaxial shear tester can guarantee the measurement of Flow Functions Oc (cf ) without further assumptions. 

Some bulk solids gain strength, when stored under pressure without movement. Principally this time consolidation can be tested with all testers. Besides the fact that time consolidation can most easily be tested with Jenikes tester and a new version of an annular shear cell [10] -easily with regard to time and equipment - only these testers yield Time Flow Functions which have to be known for applying the doming and piping criteria. 

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 shear tests were performed using the annular ring shear tester (from Schulze SchiittgutmeBtechnik, D-38302 Wolfenbiittel) sketched on Figure 1. 

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