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Fundamentals of Material Science
CHAPTER 9
Fracture, Fatigue and Creep
Fundamentals of Material Science Dr. Gamal Abdou
Fracture
WHY STUDY Failure?
Breaking two or pieces- external
•
• load
• Two steps in the process of fracture:––
Crack
Crack
initiation
Propagation
Brittle
Fracture
Ductile
Brittle vs. Ductile Fracture
• Ductile materials - extensive plastic deformation
energy absorption (“toughness”) before fracture
and
• Brittle materials - little plastic deformation and low
energy absorption before fracture
3
Ductile fracture
a)
b)
c)
d)
e)
Necking
microcracks formation
Crack formation
Crack propagation
fracture
Brittle Fracture
•
Exhibits little or no plastic deformation and low energy absorption
before failure.
Crack propagation spontaneous and rapid
Occurs perpendicular to the direction of the applied stress,
forming an almost flat fracture surface.
Crack propagation corresponding to Successive and repeated
•
•
•
breaking of atomic bonds along specific crystallographic planes
called cleavage
is
•
•
This type of fracture
This type of fracture
FCC
is called cleavage fracture
are generally found in BCC and HCP, but not
Transgranular and Intergranular
Fracture
Crack propagation across grain boundaries is known as
transgranular
•
• While propagation
Intergranular
along grain boundaries is termed
Ductile – Brittle Transition
Ductile materials fracture abruptly and with little plastic
deformation
Crack propagation takes precedence over plastic deformation
•
•
•
1.
2.
3.
Ductile – Brittle transition occurs when
Temperature is lowered
Rate of straining increased
Notch or stress raiser is introduced
Ductile-Brittle Transition Temp
The temperature at which the stress to propagate a crack бf is equal to
the stress to move dislocations бy .
When бy < бf material is ductile
When бy > бf material is brittle
This transition is commonly observed in materials having BCC and
HCP structures.
For ceramic materials, the transition takes place at elevated
temperatures.
For polymers the transition occurs over a narrow range, below room
temp.
•
•
•
•
•
•
Griffith theory of fracture
Measured fracture strength of most brittle materials are
significantly lower than theoretical strength- what is the
reason?
• Stress concentration
• Brittle materials contains a population of fine cracks which
produce a stress concentration
• Stress amplification is assumed to be at the crack tip
• Magnitude of this amplification depends on the crack
orientation and geometry
• It is assumed that the crack is elliptical in shape and is
oriented with major axis perpendicular to the applied stress
Protection against fracture
Introducing compressive stresses
Polishing surfaces
Avoiding sharp corners
•
•
•
•
•
•
Improving purity of the
Grain refinement
materials
Avoid precipitation of second phase
Fatigue: CyclicStresses (I)
Random
stress
fluctuations
Periodic and
asymmetrical
about zero
stress
Periodic and
symmetrical
about zero
stress
Fatigue: Crack ini t iat ion and propagat ion ( I I )
• Crack initiation at the sites o f stress concentration
(microcracks. scratches. indents. interior corners.
dislocation
important.
slip steps. etc.) . Quality o f surface is
•
:;.....
Crack propagation
( T
Stag�e I: initial
along
slow
crystalpropagation
planes with high resolved
shear stress. Involves just a
few grains, and has flat Stage II
fracture surface
> Stage II: f aster propa gation
perpendicular to the applied
stress. Crack grows by
andrepetitive
sharpening
blunting
process
fracture
at crack
surface.tip. Rough o
• Crack eventually reaches critical dimension and29propagates very rapidly
• Permanent deformation of materials on the application
of a load can be either plastic deformation or creep.
• The permanent deformation at temperature below
0.4 Tm is called PLASTIC DEFORMATION.
Amount of deformation occurring after the application
load is negligible. Rate at which material deformed
determines deformation characteristics
• of
• At temp above 0.4 Tm permanent deformation
function of time too. This behaviour is CREEP.
is a
CREEP
• Materials are often placed under steady loads forlonger periods of time
Without increase in load materials undergoes
deformation
Creep is predominant at higher temperature, ie. An elevated temperature effect.
Creep is a time-dependent and permanent
deformation of materials when subjected to a
constant load at a high temperature over a longer
periods. (> 0.4 Tm).
•
•
•
Creep TestTo determine the continuing change in the deformation of
materials at elevated temperatures
•
• Four variables measured during a creep test are stress, strain,
temperature and time.
Creep resistant materials
Materials of high melting point like
refractories, superalloys, ceramics etc.
Alloys with solutes of lower diffusivity
Coarse grained materials
•
•
•
• Directionally solidified alloys
grains
Single grained materials
with columnar
•
Factors affecting creep
1.
2.
3.
4.
5.
6.
Thermal stability and melting
Grain size and shape
Precipitation hardening
Dispersion hardening
point
Cold working
Formation of
or work hardening
substitutional solid solution
Structural changes
Deformation by slip
Sub-grain formation
during creep
1.
2.
3. Grain boundary sliding
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