Fracture, Toughness, Fatigue,
and Creep
MATERIALS SCIENCE
&
MANUFACTURING PROCESSES
Why Study Failure
In order to know the reasons behind the occurrence of failure so
that we can prevent failure of products by improving design in
the light of failure reasons
2
Mechanical Failure
ISSUES TO ADDRESS...
• How do flaws in a material initiate failure?
• How is fracture resistance quantified; how do different
material classes compare?
• How do we estimate the stress to fracture?
• How do loading rate, loading history, and temperature
affect the failure stress?
Ship-cyclic loading
from waves.
Computer chip-cyclic
thermal loading.
3
Hip implant-cyclic
loading from walking.
What is a Fracture?
 Fracture is the separation of a body into two or
more pieces in response to an imposed stress that is
static and at temperatures that are low relative to the
melting temperature of the material.
 The applied stress may be tensile, compressive,
shear, or torsional
 Any fracture process involves two steps—crack
formation and propagation—in response to an
imposed stress.
Fracture Modes
 Ductile fracture
5
Occurs with plastic deformation
 Material absorbs energy before fracture
 Crack is called stable crack: plastic deformation
occurs with crack growth. Also, increasing stress
is required for crack propagation.

• Brittle fracture
– Little or no plastic deformation
– Material absorb low energy before fracture
– Crack is called unstable crack.
– Catastrophic
Ductile vs Brittle Failure
• Classification:
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Fracture
behavior:
Very
Ductile
Moderately
Ductile
Brittle
Moderate
Small
(%EL)=100%
Large
• Ductile
fracture is usually
desirable!
Ductile:
warning before
fracture, as
increasing is
required for crack
Brittle:
No
warning
Example: Failure of a Pipe
• Ductile failure:
--one/two piece(s)
--large deformation
• Brittle failure:
--many pieces
--small deformation
7
Moderately Ductile Failure- Cup & Cone
Fracture
• Evolution to failure:
necking
s
• Resulting
fracture
surfaces
void
nucleation
void growth
and linkage
shearing
at surface
fracture
50
50mm
mm
(steel)
100 mm
particles
serve as void
nucleation
sites.
crack occurs perpendicular to tensile force applied
8
Ductile vs. Brittle Failure
9
cup-and-cone fracture
brittle fracture
Transgranular vs Intergranular Fracture
Transgranular
Fracture
Intergranular Fracture
Brittle Fracture Surfaces
• Transgranular
• Intergranular
(between grains)
(within grains)
304 S. Steel
(metal)
316 S. Steel
(metal)
160 mm
4 mm
Polypropylene
(polymer)
Al Oxide
(ceramic)
1 mm
11
3 mm
Stress Concentration- Stress Raisers
12
σm › σo
t
Suppose an internal flaw (crack)
already exits in a material and it is
assumed to have a shape like a
elliptical hole:
The maximum stress (σm) occurs at
crack tip:
1/ 2
a
s m  2so 
 t



 K t so
where
t = radius of curvature
so = applied stress
sm = stress at crack tip
Theoretical fracture strength is higher
Kt = Stress concentration factor
than practical one; Why?
Concentration of Stress at Crack Tip
13
Engineering Fracture Design
• Avoid sharp corners! 14
so
sm
Stress Conc. Factor, K t =
so
w
smax
r,
fillet
2.5
h
Kt
2.0
increasing
w/h
radius
1.5
1.0
0
0.5
sharper fillet radius
1.0
r/h
Crack Propagation
15
Cracks propagate due to sharpness of crack tip
 A plastic material deforms at the tip, “blunting” the crack.
deformed
plastic region
When σm › σy
brittle
Effect of stress raiser is more significant in brittle
materials than in ductile materials. When σm exceeds σy ,
plastic deformation of metal in the region of crack occurs
thus blunting crack. However, in brittle material, it does
not happen.
Fracture Toughness: Design Against
Crack Growth
• Crack growth condition:
Kc
=
Ys c a
• Largest, most stressed cracks grow first!
--Result 1: Max. flaw size
--Result 2: Design stress
dictates design stress
(max allowable stress).
dictates max. allowable flaw
2
size.
Kc
s design 
Y amax
σc
amax
1  K c

  Ys design
amax
fracture
no
fracture
fracture
no
fracture
amax
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σc




Fracture Toughness
 Brittle materials do not undergo large plastic
deformation, so they posses low KIC than ductile ones.
 KIC increases with increase in temp and with reduction in
grain size if other elements are held constant
 KIC reduces with increase in strain rate
Design Example: Aircraft Wing
• Material has Kc = 26 MPa-m0.5
• Two designs to consider...
Design A
--use same material
--largest flaw is 4 mm
--failure stress = ?
--largest flaw is 9 mm
--failure stress = 112 MPa
• Use...
sc 
Design B
Kc
Y a
• Key point: Y and Kc are the same in both designs.
--Result:
112 MPa 9 mm
s a   s a 
c
A
c
4 mm
B
Answer: (sc )B  168 MPa
• Reducing flaw size pays off!
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Impact Tests
 A material may have a high tensile strength and yet




be unsuitable for shock loading conditions
Impact testing is testing an object's ability to resist
high-rate loading.
An impact test is a test for determining the energy
absorbed in fracturing a test piece at high velocity
Types of Impact Tests -> Izod test and Charpy
Impact test
In these tests a load swings from a given height to
strike the specimen, and the energy dissipated in the
fracture is measured
A. Charpy Test
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(Charpy)
Impact energy= Kinetic energy + energy absorbed by
specimen
Energy
absorbed
during test is
determined
from
difference of
pendulum
height
final height
initial height
b. Izod Test
 Izod test varies from charpy
in respect of holding of
specimen
Effect of Temperature on Toughness
• Increasing temperature...
--increases %EL and Kc
• Ductile-to-Brittle Transition Temperature (DBTT)...
Impact Energy
Low strength FCC metals (e.g., Cu, Ni)
Low strength BCC metals (e.g., iron at T < 914°C)
polymers
Brittle
More Ductile
High strength materials ( s y > E/150)
Temperature
Ductile-to-brittle
transition temperature
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Fatigue Test
 Fatigue is a form of failure that occurs in structures




subjected to dynamic and fluctuating loads (e.g. bridges,
aircrafts, ships and m/c components)
The term Fatigue is used because this type of failure
occurs after a lengthy period of repeated stress of strain
cycling.
Failure stress in fatigue is normally lower than yield
stress under static loading.
Fatigue failure is brittle in nature even in ductile metals
The failure begins with initiation and propagation of
cracks
Types of Cyclic Stresses
Types of Cyclic Stresses
Random Stress Cycle
Terms Related to Cyclic Stresses
Mean stress:
Range of stress:
Stress Amplitude:
Stress Ratio:
4. Creep
 Creep is defined as time dependent plastic
deformation under constant static load/stress (steam
turbines blades under centrifugal force, pipes under
steam pressure) at elevated temperatures
 At relatively high temperatures creep appears to
occur at all stress levels,
 But the creep rate increases with increasing stress
at a given temperature.
4. Creep Test
 A creep test involves a tensile specimen under a
Constant Load OR Constant Stress maintained at a
constant temperature.
 Temperature: Greater than 0.4Tm
Stress & Temp Effects on Creep
Time to rupture
decreases as
imposed stress or
temperature
increases
2. Steady creep rate
increases with
increase of stress
and temperature
1.
Good Luck