“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

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 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

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

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)

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?)