Powders are complex systems. This is ostensibly due to the combination of solids, liquids and gases that comprise a powder. The interaction of these phases contributes to the changeability of powders, and as a consequence the processing and handling of powders is not straightforward or easily predictable.
Historically, a suite of semi-empirical testing techniques (poured and/or tapped bulk density; angle of repose; flow through an orifice or funnel) have been used to characterise the flow properties of powders but often with very limited success. This is due to restrictions imposed by the manual measurement process; operator bias; and, perhaps most importantly, relevance of the test to way the powder is actually processed. Individually, they don’t represent all the conditions that powders experience in either manufacture or application.
A key relationship is that between the levels of stress to which a powder has been subjected and the force required for it to be re-mobilised. This can be evaluated by uniaxial testing which involves measurement of the force needed to break or fracture a free standing column of powder consolidated by a known applied stress – this determines the unconfined yield strength of the powder. Cohesive powders have relatively strong inter-particulate forces, which encourage the particles to stick together rather than move easily with respect to one another. In contrast, in free-flowing powders, the forces between particles tend to be much weaker. Uniaxial testing is arguably the simplest, most intuitive technique based on this aspect of powder characterisation. Historically, however, the practicalities of uniaxial testing have inhibited its use. Shear testing has become a more accessible way of deriving the unconfined yield strength of powders, but it does this in a less direct way and delivers a theoretically derived value which can be subject to various limitations. The major challenge for the uniaxial tester is to find a proper solution which is user friendly, ensures a free-standing powder column and guarantees a homogeneous density and stress distribution across the full powder column.
For many years the routine use of uniaxial testing for powder characterisation has been compromised by the practical difficulties associated with its operation. Steps taken to address these issues have resulted in the development of a compact solution that makes it easier to obtain accurate and reliable data using this simple technique.
This paper describes the challenges associated with the development of a uniaxial tester, both in terms of mechanical design and operational methodologies, and compares the data it generates with rotational shear testing.
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