4

• 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

6

• 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

14

• 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

15

• 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

16

• Pre-WWII: The Titanic • WWII: Liberty ships

• Problem: Used a type of steel with a DBTT ~ Room temp.

DESIGN STRATEGY:STAY ABOVE THE DBTT!