Download - MODERATELY DUCTILE FAILURE
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• Evolution to failure:necking void
nucleationvoid growth and linkage
shearing at surface fracture
• Resulting fracture surfaces (steel)
50 m
particlesserve as voidnucleationsites.
50 m
100 m
MODERATELY DUCTILE FAILURE
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• Stress-strain behavior (Room T):
E/10
E/1000.1
perfect mat’l-no flawscarefully produced glass fiber
typical ceramic typical strengthened metaltypical polymer
TS << TSengineeringmaterials
perfectmaterials
• DaVinci (500 yrs ago!) observed... --the longer the wire, the smaller the load to fail it.• Reasons: --flaws cause premature failure. --Larger samples are more flawed!
IDEAL VS REAL MATERIALS
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• Increased loading rate... --increases y and TS --decreases %EL• Why? An increased rate gives less time for disl. to move past obstacles.
initial heightfinal height
sample
y
y
TS
TSlarger
smaller
(Charpy)• Impact loading: --severe testing case --more brittle --smaller toughness
LOADING RATE
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• Increasing temperature... --increases %EL and Kc• Ductile-to-brittle transition temperature (DBTT)...
BCC metals (e.g., iron at T < 914C)
Impa
ct E
nerg
y
Temperature
FCC metals (e.g., Cu, Ni)
High strength materials (y>E/150)
polymers More Ductile Brittle
Ductile-to-brittle transition temperature
TEMPERATURE
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• Pre-WWII: The Titanic • WWII: Liberty ships
• Problem: Used a type of steel with a DBTT ~ Room temp.
DESIGN STRATEGY:STAY ABOVE THE DBTT!