Split cell testers

As the name implies, the split cell testers use a cell, usually in the form of a ring similar to that used in the Jenike shear cell or the wall friction testers, but both the cell and supporting plate are split vertically, across the diameter. The sample is compacted vertically in the usual way (by application of normal loads via a lid) and the sample is then pulled apart by moving the split halves away. There have been some research devices designed on this principle, for example by Boden33, who suspended the cell supports on air bearings to minimize friction, but such devices are too expensive for general use. 

There is a split cell tester available commercially, however, which uses an ingenious mechanical system to counter-balance friction so that the tester is inexpensive and field-portable. The machine is a refinement of the device developed by Warren Spring Laboratories for research use the assembly has been made more compact and robust, suitable for site, works or lab use. 

The moving cell portion may be removed from the machine for cleaning and checking the density of the sample after failure by weighing the contents. 

The following machine preparation and test procedure is recommended by the manufacturer further details are given in the instructions with the instrument. 

In operation, the sample is first compacted into the cell under a known normal load, using the plunger provided if the mass of the material compacted into the volume of the cell is known, the density of the sample may be determined—the sample can be weighed after the test by brushing it out of the cell. 

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It remains to be seen whether this method gains greater popularity than the split cell testers at the moment, the latter are more widely used simply because there are commercial models available. 

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. 

Split cell testers Tensile strength measurements may also evaluate internal adhesion properties, independent of the mechanical interaction of shear plane, because T is measured at zero shear. Warren Springs Laboratories (WSL) designed two instruments one to measure the cohesive strength of powders at varied states of compaction and the other to measure tensile strength. The tensile strength measurement equipment, developed by WSL, was the WSL tensile tester, a diametrically divided split shallow circular cell. 

The paper does not describe the sample preparation in much detail but it appears that the test mould is a split cell like the one used with the Compression Tackiness Tester except that the L/D ratio is always equal to two. The mould is not split into section horizontally like the one used by Williams et al,24 and the material is reported to be filled in the mould in three layers. Three different... 

Test equipment used included load cell hardness testers, in-house designed computer operated systems and commercial crush test apparatus. Representative samples were provided and a test portion of 50 to 200 pellets was obtained by riffling or splitting. The samples were then heated at 400°C for 3 hours and cooled in a desiccator, since moisture absorption by hygroscopic catalytic materials has been shown to affect crushing strength. (3)... 

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