Post on 03-Jun-2018
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Strain Hardening,
Ductile/Brittle Fractures
UAA School of Engineering
CE 334 - Properties of Materials
Lecture # 6
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First Cycle: A structural
element is loaded beyond theelastic range and experiences
permanent set (1). Second Cycle: The structural
element is loaded to fracture.Experienced strain=- 1<
Strain History: The finalsketch shows the true strain
history of the element.
How does pre-loading affect theresults obtained from the
second loading?
Strain History
00
0
11
1
0
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What is Strain Hardening?
Strain history in plastic range: The history ofprevious loading and unloadingbeyondthe yield stress.
Apparently lose ductility. Hardening due to strain
Distinguish withHardness: Hardness isa measure
of a materials resistance to scratchingor indentation.
How to select the
unloading points
in Lab2?
-20
-2
16
34
52
70
Stress
(ksi)
0 0.05 0.1 0.15 0.2 0.25 0.3Strain (in/in)
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More Strain Hardening
Mechanical Hysteresis: is a loading and unloadingprocess beyond elastic range
Energy dissipation:A loss of energy from the heatproduced by internal friction as strain energy is dissipatedduring unloading.
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Effects of Strain Hardening
Loss of Ductility.
Decrease in Modulus of Toughness.
Apparent increase in Yield Strength.
Ultimate Tensile Strength is unaffected.
Modulus of Elasticity is unaffected. Hardness increase ? ?
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Strain Hardening in Metal
Processing Hot-Working:
milling, rolling: to its final shape
Cold-Working:A process of strain hardeningat roomtemperature
to deform the material beyond the elastic rangeto
obtain a desired property. Examples of cold-working:rolling, drawing,
extruding, cutting, pulling, indenting
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Purpose of Cold-Working
To make its final shape
To alter its structure and properties:
Increase yield strength
Decrease ductility
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Fracture
BrittleFracture
DuctileFracture
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Parameters Affecting Fracture
Load Rate
Nature of Loading
Triaxiality
Cyclic
Material
Temperature
Corrosion
Fabrication Cracks
Design Features
Notches
Holes
Fillets
Uneven surface
Roughness
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Fracture Mechanics
A specialization within both Structural and
Mechanical Engineering.
The study of how structures fracture.
Difficult in mechanics and mathematics.
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Characteristics of BrittleFracture
in Tension
Underuniaxialtension
loading, fracture occurs at
90 degreeswith the axis of
loading.
There is no plastic
deformation (i.e. there is
no necking). The failure plane has a
granular appearance.
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Mechanics of BrittleMaterial
Fracture in Tension
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Mechanics of BrittleMaterial
Fracture in Tension Thetensilecomponent of stress pulls the crystal
apart:
= [] Shear strengthof the material isrelatively
higher.
< [
] Fracture surfaceis orthogonal to the direction of
maximum principle tensile stress.
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What isBrittleFailure ?
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Ductile Fracture
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Characteristics of Ductile Fracture
Failure:
Failure begins when micro-cracking causing a
fibrous surface to develop. This is followed by arapid fracture orientedat 45owith the axis of
loading.
Necking in roundspecimens:Asneckingoccurs, atri-axial
state of stress develops in the
region of necking. This is most
popular inroundspecimens.
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Mechanics of DuctileMaterial
Fracture in Tension
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Mechanics of DuctileMaterial
Fracture in Tension The SHEARcomponent of stress shears the
crystal apart:
= [] < [] Ok Shearstrength of the material isrelatively
lower.
Fracture surface is45otothe direction of
maximum principle tensile stress.
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What isDuctileFracture ?
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Time
GroundD
isp. F
Behavior Under Seismic Excitation
(Inelastic Response)
F
G
Loading
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TimeGround
Disp.
F
F
G
Behavior Under Seismic Excitation
(Inelastic Response)
Unloading
Deformation
Reversal
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TimeGroundDisp.
F
F
G
Behavior Under Seismic Excitation
(Inelastic Response)
Reloading
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Stress or Force or Moment
u
y
y
u
d
d
Definition of Ductility,
Strain
or Displacement
or Rotation
Hysteresis
Curve
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Stress or Force or Moment
Definition of Energy Dissipation,
Strain
or Displacement
or Rotation
Area = = Energy DissipatedUnits = Force x Displacement
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Basic Earthquake Engineering
Performance Objective
(Theoretical)
SupplyDemand
SuppliedDemand QQ
An adequate design is accomplished when a structure
is dimensioned and detailed in such a way that the
local ductility demands(energy dissipation demands)
are smaller than their corresponding capacities.
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Lab this week is the strain hardening
lab.... Read it in advance. Remember that the 1st lab write up is
due at the start of the lab class.
Bibliography Durrant, Olani and Holiday, Brent, An Introduction to the
Properties of Materials, Brigham Young University, 1980.
Shackelford, James F., Introduction to Material Science for
Engineers, Macmillan Publishing Co., New York, 1985.
The End!