HR: 0800h
AN: T11A-0337    [Abstracts]
TI: No Energy Loss to Surface Energy in Rock Fragmentation
AU: * McSaveney, M J
EM: m.mcsaveney@gns.cri.nz
AF: GNS Science, PO Box 30368, Lower Hutt, 5040, New Zealand
AU: Davies, T R
EM: tim.davies@canterbury.ac.nz
AF: Dept of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand
AB: Rock fragmentation is of industrial importance, and has been researched for some centuries, even before modern energy concepts were formalised. It takes energy to break rock, and all methods for doing it use the same process — a mass is deformed by application of mechanical energy until it breaks. In breaking, new surfaces are created. All material surfaces exhibit an ability to influence the behaviour of matter in their immediate surroundings; it is described in physical chemistry by the concept of surface energy. In fragmentation, the total chemical surface energy of a grain mass increases; but its internal chemical energy decreases by the same amount, because chemical energy is associated with molecules, irrespective of their location. Confusion between the surface energy, and the mechanical energy (termed fracture surface energy) used to create new surfaces through breakage, has led to the notion that fragmentation is an energy sink, consuming energy that might otherwise remain available to affect motion. The notion is incorrect; the increase in chemical surface energy does not arise through an increase in the total energy of the mass; it arises solely because more of the mass is exposed at surfaces. When mass remains unchanged, and surface area is increased, chemical surface energy always increases, but there is no over-all increase in chemical energy associated with the mass. The increase in grain surface area is not where energy is lost in grain fragmentation; the energy loss in fragmentation arises from conventional friction between grains and with imperfect elasticity associated with the deformation needed to cause fragmentation. The mechanical potential energy stored in elastic deformation is all initially converted to kinetic energy when grains break, none goes to chemical surface energy. This kinetic energy is available to further grain motion and significantly affects the dynamics of high-stress granular flows.
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: 2007 Fall Meeting