Chapter 2: Fracture Mechanics Theory
2.2 Crack Initiation and Propagation
There are two main theories with respect to crack initiation and propagation. The first is related to the strength of the material and the second is linear elastic fracture mechanics (LEFM). Both theories assume that continuum behaviour is adhered to. Initiation by definition is “to begin or originate” but within the field of rock mechanics it is the point at which a pre-exiting crack starts to grow. Propagation is closely connected and is the subsequent growth of an initiated crack. Crack propagation ultimately leads to the catastrophic failure of the material.
2.2.1 Strength of Material Theory
This method uses two assumptions that can be considered unsatisfactory [Atkinson, B. K. (Ed), 1987]. Namely that the material assumes elastic-brittle failure, leading to the thought that initiation is instantaneous. Also no information is available as to the size of the crack itself. The main oversight of this method is that no account is made for the energy that is still absorbed after the peak stress has been induced.
2.2.1 Linear Elastic Fracture Mechanics (LEFM) Theory
The major aspects of LEFM are associated with stress-intensity (KI) and critical stress-intensity (KIC) of the material. Crack growth only occurs when:
KI = KIC
The crack will stop propagating when stress-intensity is less than the critical stress-intensity. A limitation to this method is its inability to cope with cracks that are smaller than theoretically calculated.