Bulk solid failure properties

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. 

With the mechanistic approach in the background, there are two objectives in this paper. One is to present a method by which component stress data for Jenike loci (t, ctn), can be analysed to retrieve mechanistic failure property coefficients, assuming a statistically significant number of data. These coefficients may then be used in three ways to characterise the bulk solid, to numerically determine the data required for silo design or, to convert Jenike loci into principal stress loci presented in principal stress space. 

This paper compares the outlet dimensions predicted by the Jenike silo design method [2], with those measured experimentally, in a plane flow silo test facility. These results represent a small portion of an experimental program outlined in [9], which involved the technique of laser ranging for the measurement of critical cohesive arch profiles. These experimental arching tests, and failure property measurements have been conducted with three cohesive bulk solids, namely a fly ash, hydiated lime, and olivine sand (mixed with a glycerol solution to provide cohesion). 

Because fluid pressures within the flowing bulk were inevitable, a brief period of standing, (10 seconds) was necessary to bring the bulk solid state closer to the conditions under which the failure properties were measured. 

In many cases of traditional tribology, friction and wear are regarded as the results of surface failure of bulk materials, the solid surface has severe wear loss under high load. Therefore, the mechanical properties of bulk material are important in traditional friction and wear. However, in microscale friction and wear, the applied load on the interactional surface is light and the contact area is also under millimeter or even micrometer scale, such as the slider of the magnetic head whose mass is less than 10 mg and the size is in micrometer scale. Under this situation, the physical and chemical properties of the interactional surface are more important than the mechanical properties of bulk material. Figure 1 shows the general differences between macro and micro scale friction and wear. 

The theory indicates that the mechanical properties of the foam are dependent on the properties of the cell wall materials and their size and shape. By relating the density of the foam to its bulk mechanical properties, the slope of the fitted line (n) can give us information about the type of failure mechanism (Figure 20.19). This also indicates that the size and shape of the bubbles in a foam will have a predictable effect on the strength and fracture of the foam. Bread and extruded cereal foams have been considered as cellular solids using the Gibson and Ashby analysis, and have been shown to follow the Gibson and Ashby prediction (Keetels et al. 1996 Hayter etal. 1986). 

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