Project ended 1991, report dated Feb 1992
EXECUTIVE SUMMARY
We have developed a model of impact attrition of particulate solids with a semi-brittle failure mode. Observations of the impact damage by high speed photography show that attrition is caused by platelets chipping off from faces adjacent to the impact site. Detailed examinations by optical as well as scanning microscopy, and more recently by confocal laser scanning microscopy show that the platelets are produced by propagation of sub-surface lateral cracks. Therefore, the analysis of impact attrition is based on the fracture mechanics of sub-surface lateral cracks. A dimensionless parameter, representing the volume fraction of materials lost from a single particle by the formation of such cracks, is derived:
where p is the density, 1 is the linear dimension, v is the velocity, H is the hardness, Kc is the critical stress intensity factor, and 4 is the constraint factor given by the ratio of the hardness to the plastic yield stress. This parameter quantifies the attrition propensity, and it includes all the relevant material properties. The fractional loss per impact is considered to be function o f Y/. In the first instance the existence of a simple linear relationship has been explored.
A series of tests on a number of model materials were carried out to verify the theoretical predictions, in particular for the effect of velocity, particle size, and material properties. It is shown that, overall as a first attempt, the trend of the data agrees reasonably well with the theory. However, there are intricacies in the experimental data, the description of which requires further refinement of the theoretical model.