n Introduction to Brittle Deformation 1 Outline Principal

n. Introduction to Brittle Deformation 1

Outline • Principal stresses • Stress ellipse and stress ellipsoid • Fundamental fracture modes • Crack propagation • Extensional joints • Interpretation Problem 2

Principal Stresses • s 1 - greatest principal stress • s 2 - intermediate principal stress • s 3 - minimum principal stress • Stress is a force/area • Principal stress directions are mutually perpendicular 3

Principle Stresses • Principal stresses are normal stresses only 4

Stress Ellipse In 2 -D, s 1 and s 3 are mutually perpendicular and form the axes of a stress ellipse • s 3 s 1 5

Stress Ellipsoid In 3 -D, s 1, s 2 , and s 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 • 6

Stress Ellipsoid This stress ellipsoid could represent the stress at a point located in the suture of a collision zone between two continents • 7

Fracture Modes n. Direction crack tip propagates relative to sense of slip defines Mode type Mode I Opening Mode II Sliding Mode III 8 Scissoring

How to Stop a Propagating Crack… Extremely high stress concentration 9

How to Stop a Propagating Crack… Extremely high stress concentration Drill a hole and increase the area of the crack tip Lower stress 10

Extensional Joints • Joint surface oriented perpendicular to s 3 • Mode I fracture - opening, no shear, no scissoring Opening direction s 3 11

Growth of Joints Growth direction 12

Joints in Sedimentary Rocks 13

Joints in Igneous Rocks 14