Surface energy is not one of the energy losses in rock comminution

Abstract

It has been argued that fragmentation is an energy-consuming process, which cannot increase landslide mobility. This argument fails on three counts. (1) Most energy in landsliding is expended in friction between grains in grain flow, but grain flow is the origin of landslide mobility. Thus, if fragmentation were shown to reduce frictional resistance between grains, it would increase their mobility if it were less energy-consuming than is resistance without fragmentation. A companion paper contends that the overall frictional resistance when grains are being broken is less than when no grains are being broken. (2) Energy calculations for relevant landslide processes, whether on not they be energy sinks, shows that the cumulative energy in repeatedly stressing and de-stressing grains in a grain flow is orders of magnitude larger than the cumulative increase in surface energy through comminution. (3) Surface energy is inherent in the existence of material interfaces and is not a landslide energy loss, because the increase in surface energy arises because there are more atoms exposed at surfaces; it does not arise because any form of “latent heat” has been added. Griffith fracture theory takes an energy-balance approach to present a criterion for fracture propagation; to induce a “Griffith crack” to propagate in the interior of a material, it proposes that a force must be applied within the material to pull molecules apart against the force of their mutual attraction. Griffith loosely stated that this force “went to surface energy”, which stems from molecular attraction across material interfaces. In point of fact, it goes to elastic strain energy in the material, to exactly oppose the molecular attraction across the future crack plane. If this elastic-strain energy is increased above the minimum needed to balance the attraction, the crack enlarges; thus it provides a quantitatively accurate criterion for failure to occur. Griffith fracture theory is incorrect about what happens to elastic energy upon completion of a through-going fracture, when the separation is far too great for any molecular attraction to need opposing. The elastic energy is almost fully recovered in kinetic energy, and is not lost to surface (interfacial) energy.