Chapter 2: Fracture Mechanics Theory
2. Fracture Mechanics Theory
The study of fracture mechanics has been applied to a wide range of fields, mainly metals and rock but it can be applied to any substance that is subjected to some form of stress. In most cases the information gained is used to predict a products life span and its working limits. Within the field of rock breakage behaviour, the values obtained are used to quantify the comminution characteristics of an ore sample and hence predict energy requirements for crushing and grinding.
Giffith [1920] conducted the original work in this field and most of today’s models have been based on the conclusions of his work. A summary of what the term ‘Fracture Mechanics’ means is given by Irwin and de Wit [1983]:
“…the fracture of materials in terms of the laws of applied mechanics and the macroscopic properties of materials. It provides a quantitative treatment, based on stress analysis, which relates fracture strength to the applied load and structural geometry of a component containing defects”.
2.1 Modes of Fracture
Three basic modes of fracture exist and are illustrated in Figure 2.1 where A, B and C are modes I, II and III respectively. In Mode I the body is in a tensile or opening regime where the crack and the crack surface are perpendicular to one another. Mode II has the characteristics of being sheared or slid and Mode III is in a state of tearing or anti-plane. These modes of fracture can exist independently or combined with one or two of the others, with the most complex being a I, II and III combination.
Figure 2.1 Three basic modes of fracture