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Fracture and Failure Theory

Fracture and Failure Theory. Defining Failure. Failure can be defined in a variety of ways: Unable to perform the to a given criteria Fracture Yielding …… ……. Failure of Structural Materials. Yielding is often considered “impending failure”

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Fracture and Failure Theory

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  1. Fracture and Failure Theory

  2. Defining Failure • Failure can be defined in a variety of ways: • Unable to perform the to a given criteria • Fracture • Yielding • …… • ……

  3. Failure of Structural Materials • Yielding is often considered “impending failure” • The deformations associated with yield provide a warning • The service life is effectively ended if a materials yields under service, in most cases.

  4. Tresca Yield Criterion • Ductile Materials • Assumes slip occurs during yielding • “yielding begins when max. shear stress reaches the shear stress obtained in a simple tension test at yield” tmax= (σ1 –σ2)/2

  5. Tension in x and y directions t s sx sy

  6. Tension in x and y directions t s sx sy

  7. von Mises Yield Criterion • Separates “hydrostatic pressure” from “distortion stresses” • Bases on “Energy of distortion is equal to energy of distortion at yield of a simple tension test”

  8. Comparison of Theories

  9. Surface Energy and Fracture • Energy required to generate new surfaces • Griffith crack theory • Crack will propagate if elastic strain energy is equal to energy of newly created surface

  10. Real Distribution r r 2a Plastic Zone 

  11. Stress Intensity  The Stress Intensity Factor is a constant that defines the stress distribution near a crack r r 2a r = distance from crack tip. 

  12. Fracture Toughness, KIc • Fracture toughness, KIc • Material property • Used to predict the crack size that can be tolerated • sc = KIc/(pa)1/2 • KIc increases with thickness • KIc decreases with temperature increase

  13. Stress Concentrations  Stress risers occur at discontinuities SCF is related to hole shape and size smax save r r 2a 

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