Brittle Fracture You can observe a lot just

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 Deck • How should it be fixed? – Who will perform repairs? • Who is at fault? – State/City/Contractor? • What are the ramifications? – Cost – Inconvenience – Other bridges 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 strength % elongation Sequential tearing of bonds ef < 1%

Brittle Fracture (Limited Dislocation Mobility) Ø No appreciable plastic deformation Ø Crack propagation is very fast Ø Crack propagates nearly perpendicular to the direction of the applied stress Ø 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 [100] [010] [001]

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? )