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