Powder shear testing

In friability tests the material s susceptibility to attrition is evaluated. But it is not as simple as it may seem at first to select the suitable test procedure. In this context Pell (1990) gave a simple thought experiment to illustrate the difficulties If we took a batch of rubber stoppers and a batch of diamonds, and rubbed them on abrasive paper, we would conclude that the diamonds were more attrition resistant. If we instead struck the particles with a hammer we would conclude that the rubber were more attrition resistant. So, different test methods can rank materials differently with respect to their attritability. This effect was for example observed by Knight and Bridgwater (1985). They subjected spray-dried powders to a compression test, a shear test and a test in a spiral classifier. They found that each test gave a different ranking of the materials. Obviously, there is no... 

The angle of repose of a powder blend, effective angle of internal friction (EAIF) from shear cell measurements, and the mean time to avalanche (MTA) in powder cohesivity tests are useful for assessing the flow of a tableting mixture at various scales (15 18). 

Bilgili E, Yepes J, Stephenson L, Johanson K, Scarlett B. 2004. Stress inhomogeneity in powder specimens tested in the Jenike shear cell Myth or fact Part. Part. System Charact. 21(4) 293-302. 

Carson, J. W. and Wilms, H. (2006), Development of an international standard for shear testing, Powder Technol., 167,1-9. 

Studies have also been conducted comparing the Jenike direct shear test method to other test methods used to measure a powder s flowability (3), including the Hosokawa Micron Powder Characteristics Tester (Hosokawa Micron, Osaka, Japan), Peschl shear tester, and Johanson Hang-up Indicizer (Johanson Innovations, San Luis Obispo, California, U.S.A.). This is not an exhaustive list of all the powder testers available. In general each tester has its own test method, which measures some property of the powder that changes as the flowability changes. As stated previously, however, the term flowability must be taken in context. 

The so-called shear cells are used for direct shear tests, where the powder specimen is consolidated in the vertical direction and then sheared in a horizontal plane. There are basically two types of shear cells in use today the Jenike shear cell (sometimes referred to more generally as the translational shear box) and the annular (or ring) shear cell (the rotational shear box). As the equipment needed is highly specialized (and hence outside the scope of this Guide) and as manufacturers instructions are usually adequate, the following contains only an outline description of both the hardware and the test procedures. 

Miyanami, K., and Terashita, K., Direct Shear Test of Powder Beds, Kona, Powder Science and Technology in Japan, 1, 1983, pp. 28-39. 

ABSTRACT This research project aims to use reactive powder concrete, RPC. as a new repair material and evaluate its bond strength and bond durability to existing concrete. One accelerated aging environment, namely a freeze-thaw cycle acceleration deterioration test, was selected for the evaluation of bond durability of the repair materials. Before and after aging, the samples were evaluated by the compressive strength, bond strength (slant shear test), steel pull out strength, and relative dynamic modulus NDT tests. 

Cake strength. The cake strength as measured by a uniaxial shear test is often used as a measure of flowability and granule stickiness. These are important properties for both postprocessing and consumer acceptance. The cake strength tends to be an intrinsic function of the formulation and blown powder moisture. Apart from this, the process conditions tend to have only a minor effect... 

The technique of shear testing has traditionally been used to determine the shear behavior of consolidated powders. As discussed previously the Jenike shear cell testing is generally accepted as an established method for measuring powder flowability and standards based on it have been issued, both in Europe and the United States. Some disadvantages of the Jenike shear cell have also been mentioned. The process is extremely time consuming and tedious and correct consolidation of sample is difficult to achieve. It can be hardly used for all types of bulk solids due to its limited shear... 

The mathematical stress analysis of the flow of unaerated powders in a hopper requires the use of principal stresses. We therefore need to use the Mohr s stress circle in order to determine principal stresses from the results of the shear tests. 

If the wall shear stress line is curved, a Mohr circle may be drawn in contact with the powder yield locus of a separate shear test obtained at a pre-shear normal stress identical or similar to that of the wall friction test (Akers 1992). The intersection of the curved WYL with the superimposed Mohr circle is then extrapolated to the origin to give the angle of wall friction. 

Jenike (1964, 1970) has published a fundamental and widely used definition and shear testing protocol on the flowabiUty of powders. The failure properties of powders and thus flow are measured and calculated from a family of yield loci obtained from a number of shear tests. 

Enstad and Maltby (1992) concluded, from a series of shear tests carried out by five different laboratories on the certified material, CRM 116, that reproducible flow function results can be obtained only with skilled and experienced operators. Harwood (1971) applied the technique of Jenike to evaluate the flow of a range of pharmaceutical powders and found that as size of the powder increased the flowability became more free flowing (Table 1.12). 

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. 

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. 

Akers, R.J. (1992) Certification of a Limestone Powder for Jenike Shear Testing. CRM116. Commission of European Committees, BCR, Office for Official Publications of the European Communities, Brussels, Luxembourg. 

Peschl, I.A.S.Z. Colijn, H. (1977) New rotational shear testing technique. J. Powder Bulk Solid Technol., 1,... 

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