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Introduction to Brittle Deformation Outline • Principal stresses • Stress ellipse and stress ellipsoid • Fundamental fracture modes • Crack propagation • Extensional joints • Interpretation Problem Principal Stresses • s1 - greatest principal stress • s2 - intermediate principal stress • s3 - minimum principal stress • Stress is a force/area • Principal stress directions are mutually perpendicular Principle Stresses • Principal stresses are normal stresses only Stress Ellipse In 2-D, s1 and s3 are mutually perpendicular and form the axes of a stress ellipse • s3 s1 Stress Ellipsoid In 3-D, s1, s2 , and s3 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 Ellipsoid This stress ellipsoid could represent the stress at a point located in the suture of a collision zone between two continents • Fracture Modes Direction crack tip propagates relative to sense of slip defines Mode type Mode I Mode II Mode III Opening Sliding Scissoring How to Stop a Propagating Crack… Extremely high stress concentration How to Stop a Propagating Crack… Extremely high stress concentration Drill a hole and increase the area of the crack tip Lower stress Extensional Joints • Joint surface oriented perpendicular to s3 • Mode I fracture - opening, no shear, no scissoring Opening direction s3 Growth of Joints Growth direction Joints in Sedimentary Rocks Joints in Igneous Rocks
