Fragmentation of plastic materials

Ferenc Kun

Department of Theoretical Physics
University of Debrecen, Hungary

Wednesday, 04. June 2014, 16.00
WW8, Room 2.018-2, Dr.-Mack-Str. 77, Fürth

Fragmentation phenomena are ubiquitous in nature and play a crucial role in numerous industrial processes related to mining and ore processing. A large variety of measurements on the rapid breakup of heterogeneous materials have revealed the existence of a striking universality in fragmentation phenomena: fragment mass distributions exhibit a power law decay, independent on the type of energy input (impact, explosion, …) and on the relevant length scales. For heterogeneous brittle materials the value of the power law exponent is mainly determined by the dimensionality of the system.

Important questions of broad scientific and technological interest are how plasticity, and the emergence of complicated stress states like shear affect the fragmentation process, how robust the universality classes are with respect to mechanical properties and whether there exist further universality classes of fragmentation of solids. We investigate the fragmentation process of plastic materials by impacting spherical particles made of polypropylene (PP) against a hard wall. Our experiments show that the mass distribution of plastic fragments exhibits a power law behavior with an exponent close to 1.2, which is substantially different from the one of bulk brittle materials in three-dimensions. In order to understand the physical origin of the low exponent, a three-dimensional discrete element model is developed where the sample is discretized in terms of spherical particles connected by elastic beams. To capture the fracture mechanisms of plastic materials in the model, broken particle contacts are able to reconnect when compressed against each other leading eventually to the healing of cracks and to a plastic energy dissipation. By adding two novel ingredients into a DEM model, namely healing of broken bonds and breaking under compression, we are able to simulate the plastic behavior of PP and reproduce in fact the observed new power law distribution.