3.2.2.3 Ultra Fast load Cell
This device has been developed at the Utah Comminution Centre [Bourgeois 1992] and has proved to be very successful in recording impact fractures of particulate material. It allows the generation of force time histories when a particle is subjected to an impact. Dropping a steel ball of known mass and diameter from a known height onto the test particle, which is resting on a rod, generates the impact. At the base of the rod there are strain gauges which record the compression wave resulting from the impact. From the generated output the minimum force to fracture can be determined. From this using the relationship below the particle strength can be calculated:-
Sigmap = 4Fi / pi2d
Where Fi is the fracture initiation force and sigmap is the measured particle strength.
There were some doubts concerned with the reliability of the results achieved, due to the uncertainty from the fact that the initial fracture may not always be clearly defined and that particles below 2mm gave the greatest concern regarding accuracy.
The shape and orientation of the particle are factors that are not addressed in the testing method. For example a smaller particle gives the ball further to drop, therefore removing the standardisation of the method. The presentation of the particle is not considered with respect to the fact that a particle may break more easily in one plane of orientation than another.