1. “You can observe a lot just by watchin’.” Yogi Berra
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. 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
Deck
Approach

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. Brittle Fracture ef < 1% strength % elongation
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. 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
[010]
[001]
[100]

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?)