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Ductile fracture
Accompanied by significant plastic deformation absorbing high energy before fracture
Brittle fracture
little or no plastic deformation; catastrophic, rapid crack propagation
Fracture process steps
crack formation
crack propagation
Pipe failures
ductile: one piece, large deformation, brittle failure: many pieces, small deformations
Moderately ductile vs brittle failure
cup and cone vs flat surface
Moderately Ductile failure stages
necking, void nucleation, void growth and coalescence, shearing at surface, fracture
Brittle fracture cleavage
crack propagation(cleavage), either transgranular(through grains), intergranular(along grain boundaries)
longer the wire, the smaller the load for failure
flaws cause premature failure, larger samples contain longer/more flaws
Flaws are…
stress concentrators
Griffith Crack
elliptical hole through plate perpendicular to the applied stress
stress amplification is not restricted to microscopic defects
may occur at macroscopic internal discontinuities, at sharp corners, notches in large structures
How to drive crack growth
opening or tension mode, sliding mode, tearing mode
crack propagates if…
applied stress exceeds critical stress
crack propagation brittle materials
creates new surfaces, crack tip remains sharp, retain a high stress concentration factor Kt
crack propagation for ductile materials
plastic deformation blunts crack tip(reduces Kt)
Fracture Resistance: Bending test
3 or 4 point bending, easy to control experimental conditions, directly and quantitatively measure fracture toughness
Fracture Resistance: Impact testing
severe testing case, material more brittle during impact test, measure impact energy
impact energy
only qualitative indication of material toughness, rather than quantitative measure
fracture behaviors are dependent on loading conditions:
materials are more brittle under high loading rates, lower temperatures, subject to certain triaxial stress states
Fatigue
form of fracture, can occur below yield strength, structures subjected to cyclic loads, fracture occurs after millions of cycles of loading
fatigue fracture
brittle, no shape change, fatigue deteriorates material
Fatigue=
failure under applied cyclic stress
Fatigue responsible for
90% of mechanical engineering failures
Improving fatigue life
impose compressive surface stresses(to prevent surface cracks from growing), remove stress concentrators(fillet)
shot peening
improves fatigue life by shooting balls on surface of material causing compression on the surface.
creep
very slow strain rate plastic deformation, conditions: applied stress less than yield stress, elevated temperatures
primary creep
slope decreases with time
secondary creep
steady state, linear slope
tertiary creep
slope increases with time
creep(fracture)
brittle
creep is dependent on…
temperature
polycrystalline materials more prone to creep because…
grain boundary migration/slip at elevated temperature
Note 
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