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Component Failure in road trafic accidents
By : AYOUB EL AMRI.
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How do Materials Break?
Chapter Outline: Failure
Ductile vs. brittle fracture
Principles of fracture mechanics
Stress concentration
Impact fracture testing
Fatigue (cyclic stresses)
Cyclic stresses.
Crack initiation and propagation
Factors that affect fatigue behavior
Creep (time dependent deformation)
Stress and temperature effects
Alloys for high-temperature use
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Brittle vs. Ductile Fracture
• Ductile materials - extensive plastic deformation
and energy absorption (“toughness”) before
fracture
• Brittle materials - little plastic deformation and
low energy absorption before fracture
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Brittle vs. Ductile Fracture
A. Very ductile: soft metals (e.g. Pb, Au) at
room T, polymers, glasses at high T
B. Moderately ductile fracturetypical for metals
C. Brittle fracture: ceramics, cold metals,
A B C
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Steps : crack formation
crack propagation
Fracture
Ductile vs. brittle fracture
• Ductile - most metals (not too cold):
Extensive plastic deformation before
crack
Crack resists extension unless applied
stress is increased
• Brittle fracture - ceramics, ice, cold
metals:
Little plastic deformation
Crack propagates rapidly without
increase in applied stress
Ductile fracture is preferred in most applications
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Ductile Fracture (Dislocation Mediated)
(a) Necking, (b) Cavity Formation,
(c) Cavities coalesce form crack
(d) Crack propagation, (e) Fracture
Crack
grows
90o to
applied
stress
45O -
maximum
shear
stress
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Ductile Fracture
(Cup-and-cone fracture in Al)
Scanning Electron Microscopy. Spherical
“dimples” micro-cavities that initiate crack
formation.
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Crack propagation is fast
Propagates nearly perpendicular to
direction of applied stress
Often propagates by cleavage -
breaking of atomic bonds along specific
crystallographic planes
No appreciable plastic deformation
Brittle Fracture (Low Dislocation Mobility)
Brittle fracture in a mild steel
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A. Transgranular fracture: Cracks pass through
grains. Fracture surface: faceted texture because of
different orientation of cleavage planes in grains.
B. Intergranular fracture: Crack propagation is
along grain boundaries (grain boundaries are
weakened/ embrittled by impurity segregation etc.)
A B
Brittle Fracture
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Fracture strength of a brittle solid:
related to cohesive forces between atoms.
Theoretical strength: ~E/10
Experimental strength ~ E/100 - E/10,000
Difference due to:
Stress concentration at microscopic flaws
Stress amplified at tips of micro-cracks etc.,
called stress raisers
Stress Concentration
Figure by
N. Bernstein &
D. Hess, NRL
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Stress Concentration
0 = applied stress; a = half-length of crack;
t = radius of curvature of crack tip.
Stress concentration factor
2/1
t
0m
a2
Crack perpendicular to applied stress:
maximum stress near crack tip
2/1
t0
mt
a2K
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Two standard tests: Charpy and Izod. Measure the
impact energy (energy required to fracture a test piece
under an impact load), also called the notch toughness.
Impact Fracture Testing
CharpyIzod
h’
h
Energy ~ h - h’
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As temperature decreases a ductile
material can become brittle
Ductile-to-Brittle Transition
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Low temperatures can severely embrittle steels. The
Liberty ships, produced in great numbers during the WWII
were the first all-welded ships. A significant number of
ships failed by catastrophic fracture. Fatigue cracks
nucleated at the corners of square hatches and propagated
rapidly by brittle fracture.
Ductile-to-brittle transition
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Under fluctuating / cyclic stresses,
failure can occur at lower loads than
under a static load.
90% of all failures of metallic
structures (bridges, aircraft, machine
components, etc.)
Fatigue failure is brittle-like –
even in normally ductile materials.
Thus sudden and catastrophic!
Fatigue
Failure under fluctuating stress
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Fatigue: Cyclic StressesCharacterized by maximum, minimum and mean
Range of stress, stress amplitude, and stress ratio
Mean stress m = (max + min) / 2
Range of stress r = (max - min)
Stress amplitude a = r/2 = (max - min) / 2
Stress ratio R = min / max
Convention: tensile stresses positive
compressive stresses negative
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Fatigue: Crack initiation+ propagation (I)
Three stages:
1. crack initiation in the areas of stress
concentration (near stress raisers)
2. incremental crack propagation
3. rapid crack propagation after crack
reaches critical size
The total number of cycles to failure is the sum of cycles
at the first and the second stages:
Nf = Ni + Np
Nf : Number of cycles to failure
Ni : Number of cycles for crack initiation
Np : Number of cycles for crack propagation
High cycle fatigue (low loads): Ni is relatively high.
With increasing stress level, Ni decreases and Np
dominates
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Fatigue: Crack initiation and propagation (II)
Crack initiation: Quality of surface and sites
of stress concentration
(microcracks, scratches, indents, interior
corners, dislocation slip steps, etc.).
Crack propagation
I: Slow propagation along
crystal planes with high
resolved shear stress.
Involves a few grains.
Flat fracture surface
II: Fast propagation
perpendicular to applied
stress.
Crack grows by repetitive
blunting and sharpening
process at crack tip.
Rough fracture surface.
Crack eventually reaches critical dimension and
propagates very rapidly
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Factors that affect fatigue life
Magnitude of stress
Quality of the surface
Solutions:
Polish surface
Introduce compressive stresses (compensate for
applied tensile stresses) into surface layer.
Shot Peening -- fire small shot into surface
High-tech - ion implantation, laser peening.
Case Hardening: Steel - create C- or N- rich
outer layer by atomic diffusion from surface
Harder outer layer introduces compressive
stresses
Optimize geometry
Avoid internal corners, notches etc.
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Factors affecting fatigue life
Environmental effects
Thermal Fatigue. Thermal cycling causes
expansion and contraction, hence thermal stress.
Solutions:
change design!
use materials with low thermal expansion
coefficients
Corrosion fatigue. Chemical reactions induce
pits which act as stress raisers. Corrosion also
enhances crack propagation.
Solutions:
decrease corrosiveness of medium
add protective surface coating
add residual compressive stresses
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Creep
Creep testing
Furnace
Time-dependent deformation due to
constant load at high temperature
(> 0.4 Tm)Examples: turbine blades, steam generators.
Creep test:
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Stages of creep
1. Instantaneous deformation, mainly elastic.
2. Primary/transient creep. Slope of strain vs.
time decreases with time: work-hardening
3. Secondary/steady-state creep. Rate of straining
constant: work-hardening and recovery.
4. Tertiary. Rapidly accelerating strain rate up to
failure: formation of internal cracks, voids,
grain boundary separation, necking, etc.
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Parameters of creep behavior
Secondary/steady-state creep:
Longest duration
Long-life applications
Time to rupture ( rupture lifetime, tr):
Important for short-life creep
t/s
tr
/t
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Creep: stress and temperature effects
With increasing stress or temperature:
The instantaneous strain increases
The steady-state creep rate increases
The time to rupture decreases
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Creep: stress and temperature effects
Stress/temperature dependence of the steady-state
creep rate can be described by
RT
QexpK cn
2s
Qc = activation energy for creep
K2 and n are material constants
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Alloys for High-Temperatures(turbines in jet engines, hypersonic
airplanes, nuclear reactors, etc.)
Creep minimized in materials with
High melting temperature
High elastic modulus
Large grain sizes
(inhibits grain boundary sliding)
Following materials (Chap.12) are especially
resilient to creep:
Stainless steels
Refractory metals (containing elements of
high melting point, like Nb, Mo, W, Ta)
“Superalloys” (Co, Ni based: solid solution
hardening and secondary phases)
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Summary
Brittle fracture
Charpy test
Corrosion fatigue
Creep
Ductile fracture
Ductile-to-brittle transition
Fatigue
Fatigue life
Fatigue limit
Fatigue strength
Impact energy
Intergranular fracture
Stress raiser
Thermal fatigue
Transgranular fracture
Make sure you understand language and concepts:
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Thank you for your attention.
Any questions?