introduction introduction to brittle deformation
TRANSCRIPT
Introduction Introduction
to Brittle to Brittle
DeformationDeformation
OutlineOutline
• Principal stresses
• Stress ellipse and stress ellipsoid
• Fundamental fracture modes
• Crack propagation
• Extensional joints
• Interpretation Problem
Principal StressesPrincipal Stresses
• 1 - greatest principal stress
• 2 - intermediate principal stress
• 3 - minimum principal stress
• Stress is a force/area
• Principal stress directions are mutually perpendicular
Principle StressesPrinciple Stresses
• Principal stresses are normal stresses only
Stress EllipseStress Ellipse
• In 2-D, 1 and 3 are mutually perpendicular and form the axes of a stress ellipse
1
3
Stress EllipsoidStress Ellipsoid
• In 3-D, 1, 2 , and 3 are mutually perpendicular and form the axes of a stress ellipsoid
• The length of an axis is related to the magnitude of the stress
• The stress ellipsoid to the right might represent the stress at a point in the shallow subsurface
Stress EllipsoidStress Ellipsoid
• This stress ellipsoid could represent the stress at a point located in the suture of a collision zone between two continents
Fracture ModesFracture Modes
Mode I
Opening
Mode II
Sliding
Mode III
Scissoring
Direction crack tip propagates relative to Direction crack tip propagates relative to
sense of slip defines Mode typesense of slip defines Mode type
How to Stop a Propagating How to Stop a Propagating
Crack…Crack… Extremely high stress concentration
How to Stop a Propagating How to Stop a Propagating
Crack…Crack… Extremely high
stress concentration
Drill a hole and increase the area of the crack tip
Lower stress
Extensional JointsExtensional Joints
• Joint surface oriented perpendicular to 3
• Mode I fracture - opening, no shear, no scissoring
Opening direction
3
Growth of JointsGrowth of Joints
Growth direction
Joints in Sedimentary RocksJoints in Sedimentary Rocks
Joints in Igneous RocksJoints in Igneous Rocks