Materials Science

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Fracture, Toughness, Fatigue, and Creep

• Materials Science
• Manufacturing PROCESSES

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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
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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?
Computer chip-cyclic thermal loading.
Hip implant-cyclic loading from walking.
Ship-cyclic loading from waves.
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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 stepscrack
  formation and propagationin response to an
  imposed stress.

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Fracture Modes

• Ductile fracture
• 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

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Ductile vs Brittle Failure
Classification
Ductile fracture is usually desirable!
Ductile warning before fracture, as increasing
is required for crack growth
Brittle No warning
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Example Failure of a Pipe
Ductile failure --one/two piece(s)
--large deformation
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Moderately Ductile Failure- Cup Cone Fracture
Evolution to failure
crack occurs perpendicular to tensile force
applied
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Ductile vs. Brittle Failure
cup-and-cone fracture
brittle fracture
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Transgranular vs Intergranular Fracture
Intergranular Fracture
Transgranular Fracture
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Brittle Fracture Surfaces
Transgranular (within grains)
Intergranular (between grains)
304 S. Steel (metal)
316 S. Steel (metal)
160 mm
4 mm
Polypropylene (polymer)
Al Oxide (ceramic)
3 mm
1 mm
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Stress Concentration- Stress Raisers

• 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 (sm) occurs at crack tip
• where ?t radius of curvature
• so applied stress
• sm stress at crack tip
• Kt Stress concentration factor

sm so
?t
Theoretical fracture strength is higher than
practical one Why?
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Concentration of Stress at Crack Tip
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Engineering Fracture Design
Avoid sharp corners!
s
Kt
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Crack Propagation

• Cracks propagate due to sharpness of crack tip
• A plastic material deforms at the tip, blunting
  the crack.
• deformed region
• brittle
• Effect of stress raiser is more significant in
  brittle materials than in ductile materials. When
  sm exceeds sy , plastic deformation of metal in
  the region of crack occurs thus blunting crack.
  However, in brittle material, it does not happen.

plastic
When sm sy
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Fracture Toughness Design Against Crack Growth
Crack growth condition
Largest, most stressed cracks grow first!
sc
sc
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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

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Design Example Aircraft Wing
Material has Kc 26 MPa-m0.5
Two designs to consider...
Design B --use same material --largest flaw
is 4 mm --failure stress ?
Design A --largest flaw is 9 mm --failure
stress 112 MPa
Key point Y and Kc are the same in both
designs.
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 -gt 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

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A. Charpy Test
Impact energy Kinetic energy energy absorbed
by specimen
Energy absorbed during test is determined from
difference of pendulum height
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b. Izod Test

• Izod test varies from charpy in respect of
  holding of specimen

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Effect of Temperature on Toughness
Increasing temperature... --increases EL
and Kc
Ductile-to-Brittle Transition Temperature
(DBTT)...

Low strength FCC metals (e.g., Cu, Ni)


Low strength BCC metals (e.g., iron at T lt 914C)
polymers

Impact Energy
More Ductile

Brittle

s

High strength materials (
gt E/150)
y
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

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Types of Cyclic Stresses
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Types of Cyclic Stresses
Random Stress Cycle
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Terms Related to Cyclic Stresses

• Mean stress
• Range of stress
• Stress Amplitude
• Stress Ratio

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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.

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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

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Stress Temp Effects on Creep

1. Time to rupture decreases as imposed stress or
   temperature increases
2. Steady creep rate increases with increase of
   stress and temperature

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• Good Luck