Powder failure properties

This is the simplest powder failure property to test as it depends little on the state of consolidation of the powder. The equipment required includes a shallow, open-ended ring or a square frame... 

Table 13 Worked example E5 failure properties of the powders...

It is well known that particle shape affects many secondary properties relevant to powder handling such as the bulk density, failure properties or particle-gas interaction. For non-spherical particles, the results obtained with different methods of particle size measurement are, in general, not comparable. From the point of view of powder handling, flaky or stringy particles like wood shavings, mica or asbestos fibres are known to be difficult because they interlock and form obstructions to flow. 

The three failure properties above are those related directly to flow of powders as required in hopper design, for example. There are another two properties related to powder flow ... 

Another important point to make here is that the failure properties of powders are affected strongly by humidity and to a varying degree also by temperature and time of consolidation. It is important, therefore, that those properties are tested under controlled conditions using sealed powder samples or air-conditioned rooms or enclosures. Time-consolidated samples must be tested to simulate the storage conditions. 

The angle of wall friction is the simplest of the five failure properties it is equivalent to the angle of friction between two solid surfaces except that one of the two surfaces is now a powder. It describes the friction between the powder and the material of construction used to confine the powder, e.g. the hopper wall. The wall friction causes some of the powder weight to be supported by the walls of a hopper. 

The remaining two failure properties, the cohesion and the ultimate tensile stress are not used in hopper design directly but are used by many industries as general measures of powder... 

Flowability is a difficult thing to measure as it depends very much on the design and characteristics of the delivery device as well as on the powder itself. The procedures for the free flow time are far from standard at the moment and improvement might be needed here. It seems, however, that it is far better to measure flowability by the failure properties which are functions of the powder only and for which theories exist to be used in equipment design and scale-up. 

Kocova, S., and N. Pilpel. 1972. Failure properties of lactose and calcium carbonate powders. Powder Technol. 5 329-343. 

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. 

Laboratory Exercise Determination of Some Failure Properties of Powders... 

York P. Powder failure testing—pharmaceutical applications. Int J Pharm 1980 6 89. Train D. Some aspects of the property of angle of repose of powders. J Pharm Pharmacol 1958 10 127T. 

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. 

Tensile strength is a fundamental failure property but tensile strength testing is dependent upon the direction of force necessary to cause separation of a bulk structure with respect to the direction of compaction or consolidation. Split cell testers pull the sample apart at 90° to the direction of compaction whilst the lifting lid or vertical shear testers pull in the same direction as the compaction/consolidation stress was applied. Results obtained from both methods differ greatly because tensile testing has a poor record of reproducibility, possibly due to the fact that consolidated powders in the tester cells may not be isotropic. 

In a survey of the various bulk powder tests developed and in use for the determination of the failure properties and measurement of the degrees of flowability of powders, Schwedes (2003) indicated that the equipment available varied from highly theoretical and technical... 

Pilpel, N. Walton, C.A. (1974) The effect of particle size and shape on the flow and failure properties of procaine penicillin powders. J. Pharm. PharacoL, 26, IP-IOP. 

Stanley-Wood, N.G., Sarrafi, M., Mavere, Z. Schaefer, M. (1993) The relationships between powder flowability, particle-arrangements, bulk densities and Jenike failure properties. Adv, Powder Technol., 4(1), 33-40. Stephens, D.J. Bridgwater, J. (September-October 1978) The mixing and segregation of cohesionless particulate materials. Part I Failure zone formation. Powder Technol., 21(1), 17—28. 

The second objective is to examine how different functions of the equilibrium stress for the failure property coefficients, when inserted in the simple powder equation, affect the form of the failure function and explore the implication for sizing hopper outlets. 

The onset of powder motion in a hopper is due to stress failure in powders. Hence, the study of a hopper flow is closely related to the understanding of stress distribution in a hopper. The cross-sectional averaged stress distribution of solids in a cylindrical column was first studied by Janssen (1895). Walker (1966) and Walters (1973) extended Janssen s analysis to conical hoppers. The local distributions of static stresses of powders can only be obtained by solving the equations of equilibrium. From stress analyses and suitable failure criteria, the rupture locations in granular materials can be predicted. As a result, the flowability of granular materials in a hopper depends on the internal stress distributions determined by the geometry of the hopper and the material properties of the solids. 

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