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MODULE 3 ASPECTS OF MATERIALS FAILURE

MODULE 3 ASPECTS OF MATERIALS FAILURE. FAILURE OF MATERIALS Modes of failure Mechanical properties and behavior Failure (yield) theories Factor of safety Overview of fracture mechanics Stress states at crack tip Stress intensity factor Fracture toughness

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MODULE 3 ASPECTS OF MATERIALS FAILURE

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  1. MODULE 3 ASPECTS OF MATERIALS FAILURE

  2. FAILURE OF MATERIALS Modes of failure Mechanical properties and behavior Failure (yield) theories Factor of safety Overview of fracture mechanics Stress states at crack tip Stress intensity factor Fracture toughness Ductile-to-brittle transition behavior

  3. Failure Versus Fracture Failure Inability of a component to perform according to its intended function. Fracture Separation of a component into two or more parts.

  4. Modes of Failure • Gross Yielding • Fatigue Fracture • Creep Rupture • Buckling • Static Delayed Fracture

  5. Fracture of a Specimen in a Tension Test Load cell Extensometer Crosshead Specimen grips Specimen Data acquisition system

  6. Properties Values y0.2%(MPa) 429 u (MPa) 604 E (GPa ) 208 n 0.0935 K ( MPa ) 682.65 r2 0.9857 Mechanical Properties and Behavior Stress-strain curves for Type 316 SS

  7. Mechanical Properties of Some Materials

  8. Theory of Failure Yield Criterion The material of a component subjected to complex loading will start yielding when the (parametric stress) reaches the (characteristic stress) in an identical material during a tensile test. f() = Other parameters: Strain Energy Specific stress component (shear stress, maximum principal stress)

  9. Theory of Failure Maximum-distortion-energy theory

  10. Theory of Failure Maximum-normal-stress theory

  11. Factor of Safety Ffailis determined from experimental testing of the material Fallowis the allowable or working load F.S. ≥ 1.0 to avoid failure

  12. Fracture Mechanics - Overview WWII Liberty Ships Photo by Neil Boenzi, The New York Times.

  13. Mechanics of Materials A branch of mechanics that studies the relationships between external loads applied to a deformable body and the intensity of internal forces acting within the body. Fracture Mechanics The mechanics that describes the response of materials to loading in the presence of crack or crack-like defects.

  14. Scope of Fracture Mechanics

  15. Linear Elastic Fracture Mechanics (LEFM) Fracture mechanics within the confines of the theory of linear elasticity. Analytical procedure that relates the stress magnitude and distribution in the neighborhood of a crack to: the nominal applied stress crack geometry (size, shape) and orientation material properties An underlying principle is that unstable fracture occurs when the stress-intensity factor at the crack tip reaches a critical value.

  16. Basic Loading of Cracked Bodies

  17. Stress Field Ahead of Crack Tip Complex state of stress exists in the vicinity of a crack tip

  18. Crack-tip Stress Stress magnitude at the crack tip approaches (mathematically) an infinite value

  19. Crack-tip Plasticity There is always a plastic zone at the crack tip

  20. σ Crack 2a r θ σ σyy τxy σxy Stress Intensity Factor The stress intensity factor, KI describes the crack tip stresses.  (or Y) – geometry factor

  21. SIF – Finite Width Correction For 2a<<W,

  22. Values of KI for different loading conditions and geometries Adapted from Fig. 8.8, Callister 6e.

  23. Stress Intensity Factor

  24. Stress Intensity Factor

  25. Fracture Toughness of Some Materials Fracture toughness represents the resistance of materials to resist cracking. Fracture toughness values are determined from fracture toughness tests. Based on data in Table B5, Callister 6e.

  26. Fracture Toughness of Some Materials

  27. Condition for Fracture Fracture occurs when the applied stress intensity factor, KI reaches the value of the fracture toughness, KIC of the material

  28. (Charpy) Impact Test

  29. Impact Energies of Some Materials

  30. Fracture Surfaces of Impact Test Specimens

  31. ductile BCC FCC Energy absorbed Brittle TDBT Temperature Ductile-to-Brittle Tansition (DBT) Temperature, TDBT Transition Temperature (ASTM specification) The temperature at which specimens show a fracture of 50 pct. shear and 50 pct. cleavage. Nil-ductility temperature (NDT) The ref. point in the transition range giving the limiting condition of temperature-stress combination under which catastrophic fracture can occur.

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