Powder failure properties measurement

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 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. 

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. 

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... 

An instrument which attempts to measure cohesion was originally developed at Warren Springs Laboratory and is now available from Ajax Equipment (Bolton) Ltd. It is designed to aid the assessment of flow properties of bulk solids in that it measures the cohesive strength of samples of powders in varied states of compaction, from lightly settled conditions to firm compacts. It attempts to measure directly cohesion as defined in section 4.1.2, i.e. the shear stress at failure, with no normal load acting upon the surface of failure. 

Linear thermal expansion testing helps to determine if failure by thermal stress may occur in products and materials. Precise knowledge of the CTE can be utilized to estimate the thermal stresses. This aspect makes CTE to an important property of the used fiber for composite materials. A rule of mixtures is sufficient for calculating the CTE of polymers filled with powder or short fibers. In case of long libers, the rule of mixtures is valid perpendicular to the reinforcing fibers. Molecular orientation affects the thermal expansion of polymers. Processing also affects CTE, for semicrystalline polymers this fact is very important. For that reason, CTE measurements are often used to predict shrinkage in injection moulded parts. 

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