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Introduction to Brittle Deformation

Introduction to Brittle Deformation. Outline. Principal stresses Stress ellipse and stress ellipsoid Fundamental fracture modes Crack propagation Extensional joints Interpretation Problem. Principal Stresses. s 1 - greatest principal stress s 2 - intermediate principal stress

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Introduction to Brittle Deformation

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  1. Introduction to Brittle Deformation

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

  3. 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

  4. Principle Stresses • Principal stresses are normal stresses only

  5. Stress Ellipse • In 2-D, s1 and s3 are mutually perpendicular and form the axes of a stress ellipse s3 s1

  6. 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

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

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

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

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

  11. Extensional Joints • Joint surface oriented perpendicular to s3 • Mode I fracture - opening, no shear, no scissoring Opening direction s3

  12. Growth of Joints Growth direction

  13. Joints in Sedimentary Rocks

  14. Joints in Igneous Rocks

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