Lect 6 Strain Hardening

8/13/2019 Lect 6 Strain Hardening

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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



Unloading

Deformation

Reversal


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TimeGroundDisp.

F

F

G



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!

Lect 6 Strain Hardening

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