Lifting-lid testers

This method is shown schematically in Fig. 25 it is easier to use and, according to Williams35, gives results with less scatter. Williams recommends it for monitoring of cohesivity of a material, or for comparisons. Boden33 describes the system he developed at Bradford and this is briefly described in the following no commercial instruments based on this principle are known to the author. 

A mould in the form of a ring, like the Jenike shear cell ring for example, is used and a lid which just fits inside it. The base of the cell and the lower face of the lid are covered with sellotape on which a glue is spread. The cell is filled with the powder to be tested and it is scraped level with top of the cell the lid is placed in position, on top of the sample. 

A compacting load is applied to the lid and left in position until the glue has hardened. The lid is then slowly lifted via a tensile load cell, by an electric motor. The stress required to break the specimen is noted and both the lid and base of the cell are examined after failure to make sure that both are covered with powder, showing that failure occurred within the powder specimen and not at the surface. If both surfaces are not covered with powder, the test is rejected. 

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. 

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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. 

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