The Mechanics of Fine Fragment Formation in Coal


The following four step mechanism is proposed: (1) development of a crush zone, (2) macrocrack propagation, (3) shear movement along macrocracks and (4) additional fragmentation from shear. The cutting tool interacts with coal material and its inherent flaw structure, producing fine coal fragments near the tool grading into coarser fragments further away. Two fine fragment sources exist: the crush zone and the rough fracture surfaces. Fine particle creation is controlled in part by fracture toughness and also the inherent flaw geometry. Basis relationships are developed between the flaw geometry and fine fragment generation. Experimentation centers on extensive mixed mode fracture toughness testing. The system developed herein provides a record 117 measurements concluding that K[subscript Ic] is a reliable material property despite coal's heterogeneity. Strain energy density theory adequately describes the onset and direction of crack growth under mixed mode loading conditions. Tentative relations exist between K{sub Ic} and fracture surface roughness and fracture velocity. Fracture toughness values are applied in crush zone size calculations with a boundary element program containing a failure criterion based on strain energy density theory. From postulates about the role of inherent flaws in fragmentation, the crushing extent is the locus of active 10 micron flaws. The calculations demonstrate the effect of attack angle and tool geometry on crushing extent and fine fragment formation. Future research should measure the fracture toughness of coal constituents at a microscopic scale and quantitatively describe their inherent flaw structure.


Mining Engineering

Document Type

Technical Report

Document Version


File Type





© 1988 Pennsylvania State University, All rights reserved.

Publication Date

01 Jan 1988

This document is currently not available here.