Modeling of crack initiation, propagation and coalescence in rocks

Abstract

Natural or artificial fracturing of rock plays a very important role in geologic processes and for engineered structures in and on rock. Fracturing is associated with crack initiation, propagation and coalescence, which have been studied experimentally and analytically by many researchers. The analytical models developed to describe the initiation and propagation of cracks in brittle materials can be incorporated in Finite Element (FE) and Displacement Discontinuity (DD) codes. Corresponding research has been going on at MIT and has led to the development of a DDM code -FROCK -which currently uses a stress-based criterion proposed by Bobet (1997) to model crack initiation and propagation. Even though the predictions obtained with this criterion generally correspond to the experimental results, there are cases, in which the results obtained with FROCK are not satisfactory.This thesis proposes and implements new crack initiation and propagation criteria in the DDM code FROCK, namely a strain-based criterion and two stress-dependent criteria. It also studies the crack initiation and propagation processes numerically, using the FEM code ABAQUS. Existing crack initiation and propagation criteria (stress, strain and energy based) are also investigated with ABAQUS. The crack development processes are studied by modeling pairs of pre-existing flaws (double-flaw geometries) embedded in specimens subjected to vertical compressive loads in both ABAQUS and FROCK.For the different flaw arrangements studied, the difference between the stress and strain fields around the flaw tip gradually increases as the horizontal distance between the inner flaw tips increases. In terms of crack initiation, the results obtained with the stress and strainbased criteria studied were more consistent with the experimental observations than the results obtained with the energy-based criterion.The proposed strain-based criterion implemented in FROCK yielded better results than Bobet's stress-based criterion currently used in FROCK, for the five flaw arrangements studied. The results obtained with the two proposed stress-dependent criteria indicate that the critical shear stress at which a crack propagates in rock does not depend upon the normal stress applied, since the best crack propagation results were obtained for very low or zero friction angles.

Influence of inclination angles and confining pressures on mechanical behavior of rock materials containing a preexisting crack

“…In this study, a strain strength criterion, which has significant advantages in distinguishing crack types (wing cracks, secondary cracks, and anti‐wing cracks), will be embedded into the numerical model to describe the crack initiation and gradual propagation processes. This criterion is proposed on the basis of the linear elastic fracture mechanics (LEFM) and employs the similar equations to the Griffith criterion, the maximum tangential stress criterion, the stress‐based criterion, and the maximum circumferential strain criterion . Compared with the previous experimental results, our earlier numerical simulations indicate that the strain strength criterion, which consists of the principal strain criterion and the Mohr‐Coulomb criterion, is capable of accurately predicting crack initiation direction, initiation stress, and peak strength.…”

The joint sets on the Lilstock Benches, UK. Observations based on mapping a full resolution UAV-based image

“…4b). At larger angles the new, and in some cases, the old segments can open and form a transition to pennant veins (Coelho et al, 2006) and wing cracks (Conçalves and Einstein, 2013;Kolari, 2017).…”

Crack Patterns in Heterogenous Rocks Using a Combined Phase Field-Cohesive Interface Modeling Approach: A Numerical Study

“…Exploiting the BEM-based techniques, Einstein and coauthors [19,20] have developed reliable modeling tools to predict crack growth under the loading conditions leading to fracture in Modes I and II, considering the specimens with one or more internal flaws. However, due to the inherent versatility of FEM-based techniques, they are often preferred for fracture applications over alternative methodologies.…”