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Brittle Fracture

Brittle Fracture. “You can observe a lot just by watchin’.” Yogi Berra. All graphics from ASM Metals Handbook unless otherwise noted. Case Study: Paseo Bridge – Kansas City. The Bridge Suspension bridge Built in 1957 Carries I-35, I-29, & US-71 Crosses Missouri River

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Brittle Fracture

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  1. Brittle Fracture “You can observe a lot just by watchin’.” Yogi Berra All graphics from ASM Metals Handbook unless otherwise noted

  2. Case Study:Paseo Bridge – Kansas City • The Bridge • Suspension bridge • Built in 1957 • Carries I-35, I-29, & US-71 • Crosses Missouri River • Major artery north of K.C. • 94,000 vehicles/day

  3. Case Study:Paseo Bridge – Kansas City • The problem • Expansion joint misalignment (23 Jan 03) • Deck rose 9 inches above approach on one end • 1 inch step on another • Guardrails snapped • Bridge closed for 2 weeks

  4. Approach Deck Case Study:Paseo Bridge – Kansas City • What happened? • Cause(s) • Mitigating circumstances • How should it be fixed? • Who will perform repairs? • Who is at fault? • State/City/Contractor? • What are the ramifications? • Cost • Inconvenience • Other bridges

  5. Case Study:Paseo Bridge – Kansas City

  6. How Material Breaks? • Ductile vs. brittle fracture • Principles of fracture mechanics • Stress concentration • Impact fracture testing • Fatigue (cyclic stresses) • Cyclic stresses, the S—N curve • Crack initiation and propagation • Factors that affect fatigue behavior • Creep (time dependent deformation) • Stress and temperature effects • Alloys for high-temperature use

  7. Fracture Separation of a body into pieces due to stress, at temperatures below the melting point. Steps in fracture: • crack formation • crack propagation Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle Ductile fracture - most metals (not too cold): Extensive plastic deformation ahead of crack Crack is “stable”: resists further extension unless applied stress is increased Brittle fracture - ceramics, ice, cold metals:Relatively little plastic deformation Crack is “unstable”: propagates rapidly without increase in applied stress Ductile fracture is preferred in most applications

  8. strength % elongation Brittle Fracture Sequential tearing of bonds ef < 1%

  9. Brittle Fracture (Limited Dislocation Mobility) • No appreciable plastic deformation • Crack propagation is very fast • Crack propagates nearly perpendicular to the direction of the applied stress • Crack often propagates by cleavage – breaking of atomic bonds along specific crystallographic planes (cleavage planes)

  10. [100] [010] [001] Brittle Fracture • Cleavage occurs primarily in BCC and HCP crystals • Only in FCC materials at low temp • Cleavage occurs with in grains on specific planes

  11. Brittle Fracture

  12. Brittle Fracture • Macroscopic • Flat fracture face • Little/No necking • “Crystallized” fracture surface

  13. Brittle Fracture • Low-Magnification • “Chevron Marks” • Chevrons point back to origin

  14. Brittle Fracture • Microscopic (SEM) • “River Pattern” • Crack progressed “downstream” • These are not fatigue striations! (How can you tell?)

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