Brittle Fracture

“You can observe a lot just by watchin’.”

Yogi BerraAll 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 – Major artery north of K.C.

• 94,000 vehicles/day

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

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

Case Study: Paseo Bridge – Kansas City

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

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 crackCrack is “stable”: resists further extension unless applied stress is increased

Brittle fracture - ceramics, ice, cold metals:Relatively little plastic deformationCrack is “unstable”: propagates rapidly without increase in applied stress

Ductile fracture is preferred in most applications

Sequential tearing of bonds

Brittle Fracture

strength

% elongation

f < 1%

Brittle Fracture (Limited Dislocation Mobility)

No appreciable plastic deformationCrack propagation is very fastCrack propagates nearly perpendicular to

the direction of the applied stressCrack often propagates by cleavage –

breaking of atomic bonds along specific crystallographic planes (cleavage planes)

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]

Brittle Fracture

Brittle Fracture

• Macroscopic• Flat fracture face• Little/No necking• “Crystallized” fracture

surface

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

Brittle Fracture

• Microscopic (SEM)• “River Pattern”• Crack progressed

“downstream”• These are not fatigue

striations! (How can you tell?)