MECHANISMS OF FRACTURE IN METALS

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MECHANISMS OF FRACTURE IN METALS
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DUCTILE FRACTURE VOID NUCLEATION, GROWTH, and
COALESCENCE
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Mechanism for ductile crack growth
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CLEAVAGE FRACTURE
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MECHANISMS OF FRACTURE IN FATIGUE
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EXAMPLE Striation width vs. da/dN
Fracture surface of high-strength Al 2024 - T3
that failed by cycling fatigue. Test specimen
was a Centre Notch-panel 610 mm x 229 mm, 10 mm
thickness with initial crack lenght 13 mm.
Arrow indicates direction of crack
growth. Image corresponds to a position 20 mm
from de center of the plate.
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Block A, R 0.5 DKeff 0.75 DK
DK 17 MPa m1/2, Ds?, smax?, smin?
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INTERGRANULAR FRACTURE
Ductile metals usually fail by coalescence of
voids formed at inclusions and second phase
particles Brittle metals typically fail by
transgranular cleavage
Under special circumstances, HOWEVER, cracks can
form and propagate along grain boundaries
resulting in intergranular fracture

• There is no single mechanism for intergranular
  fracture. Rather, there are a variety of
  situations that can lead to cracking on grain
  boundaries, including
• Precipitation of a brittle phase on the grain
  boundary
• Hydrogen embrittlement and liquid metal
  embrittlement
• Enviromental assisted cracking
• Intergranular corrosion
• Grain boundary cavitation and cracking at high
  temperatures

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Examples
(1) Brittle phases can be deposited on grain
boundaries of steel as a result of improper
tempering tempered martensite embrittlement
(tempering at 350 C). Involves segregation of
impurities (P, S) to prior austenite grain
boundaries (blue brittleness!!!).
(2) Atomic hydrogen apparently bonds with the
metal atoms reducing the cohesive energy strength
at grain boundaries. Sources H2S, hydrogen gas.
Important problem in welding of steels cracking
in the Heat Affected Zone (HAZ). Hydrogen is a
problem when welding high strength steels
special care!!!