Fracture Overview Fall 2011 Rectangular Plate with Hole
Figure 6.1 Rectangular plate with hole subjected to axial load. (a) Plate with cross-sectional plane; (b) one-half of plate with stress distribution; (c) plate with elliptical hole subjected to axial load. Thermal Stress:
Stress = E.. T Flaws Make the World Beautiful! Were it not for the flaws, rocks and mountains would have been perfectly boring
Griffith (1920) Griffith (1920), realized the significance of microcracks in reducing the fracture strength Applied the mathematical work of Inglis (1913) for an elliptical hole, and developed a
theoretical criterion of rupture based on the concept of minimum potential energy of classical mechanics and thermodynamics which seeks a minimum total free energy of a system
Griffith Theory In the Griffith theory, the theoretical strength is the microscopic fracture stress which is actually reached in a very small volume of the rock while the mean stress may remain very low
Griffith's work, which has since been known as the Griffith energy balance approach, and has served as a foundation for fracture mechanics, deals with the equilibrium state of an elastic, solid body, deformed by specified surface
forces Griffith (1920) Griffith extended the theorem of minimum energy by accounting for the increase of
surface energy which occurs during formation of cracks He assumed that the equilibrium position is one in which rupture of the solid occurs if the system is allowed to pass from an unbroken to
a broken state through a process involving continuous reduction of potential energy Griffith (1920) Griffith (1920) argued that brittle solids fail by
incremental propagation of a multitude of randomly-oriented, small pre-existing cracks Griffith cracks are common in rocks and include intragranular and intergranular microcracks (grain boundaries) and larger transgranular
cracks In a larger scale, the Griffith flaws include joints, faults, and bedding planes Fracture Strength
Brittle fracture strength depends largely on the elastic properties of the elastic rock and the length and sharpness of the flaws Stress concentrators such as low aspect
ratio cavities, inclusions, material property mismatches, and fossils, give rise to tensile stresses that may fracture rocks even when applied stresses are compressive provided they
are non-hydrostatic Griffith (1920) The intensification of stress depends on the: Length and orientation of the crack with respect
to the applied stress Radius of curvature at their tips, rendering certain cracks more vulnerable than others The propagation of these Griffith cracks involves the creation of two new incremental crack surfaces which
is a process that absorbs energy Griffith (1920) The creation of these new surfaces in the interior of a solid (by crack propagation) leads
to an increase in potential energy as work must be done against the cohesive forces of the molecules on either side of the crack The work is part of the potential surface energy of the system. Thus bounding surfaces posses a
surface tension and a corresponding amount of potential energy Griffith Energy-Balance Concept If we subject the outer boundary of a rock to tension (such that
boundary moves out) This decreases the potential energy (i.e., dWR<0), of the loading device (Fig. 3.2 Engelder). R designates rock The work to propagate the crack is positive, and is defined as an
increase in surface energy (dUs) Griffith Energy-Balance Concept As the crack propagates, the rock undergoes a change in strain energy (dUE).
The total change in energy for crack propagation is: UT = Us - WR + UE Griffith energy balance concept:
Propagation occurs without changing the total energy of the rock-crack system. i.e., for an increment of crack extension (dc), d UT /dc = 0
Griffith Energy-Balance Concept This means that the mechanical and surface energy terms within the rock-crack system must balance
The motion of the crack walls represents a decrease in mechanical energy While work (as surface energy) must be done to remove the restraints across crack increment