The impossibility of comminuting small particles by compression

Abstract

COMMINUTION and crushing are important processes designed to break large particles into smaller ones by compression. Under such compression, cracks can propagate through a particle to split it into two or more fragments of smaller size and increase surface area if the material is sufficiently brittle. The appreciation of brittleness as applied to solids under compression is, therefore, basic to an understanding of comminution. This note analyses the brittleness of an ideally shaped particle subjected to compressive forces. New equations, based on the old ideas of Rittinger¹ and Griffith² are presented to explain the fracture of such a particle and these are verified by experiments on a model material, specially chosen to demonstrate the effects at the macro-level. The equations state explicitly that compressed brittle particles should increase in strength as they are made smaller, a fact well known from macroscopic fracture results. In addition, the equations predict a critical particle size, verified by experiment, below which crack propagation is impossible under compressive forces. Bodies smaller than this appear ductile and are squashed flat when attempts are made to comminute them by crushing.