Strain hardening ductile brittle fractures
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Strain Hardening, Ductile/Brittle Fractures. UAA School of Engineering CE 334 - Properties of Materials Lecture # 6. Strain History. First Cycle: A structural element is loaded beyond the elastic range and experiences permanent set (  1 ) .

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Strain Hardening, Ductile/Brittle Fractures

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Strain hardening ductile brittle fractures

Strain Hardening, Ductile/Brittle Fractures

UAA School of Engineering

CE 334 - Properties of Materials

Lecture # 6


Strain history

Strain History

  • First Cycle:A structural element is loaded beyond the elastic range and experiences permanent set (1).

  • Second Cycle:The structural element is loaded to fracture.

    Experienced strain=- 1< 

  • Strain History:The final sketch shows the true “strain history” of the element.

  • How does pre-loading affect the results obtained from the second loading?

0

1

1

1

0

0

0


What is strain hardening

What is Strain Hardening?

How to select the unloading points in Lab2?

  • Strain history in plastic range:The history of previous loading and unloadingbeyond the yield stress.

  • Apparently lose ductility. Hardening due to strain

  • Distinguish with“Hardness”:Hardness isa measure of a material’s resistance to scratching or indentation.


More strain hardening

More Strain Hardening

Mechanical Hysteresis:is a loading and unloading process beyond elastic range

Energy dissipation:A loss of energy from the heat produced by internal friction as strain energy is dissipated during unloading.


Effects of strain hardening

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 ? ?


Strain hardening in metal processing

Strain Hardening in Metal Processing

  • Hot-Working:

    milling, rolling: to its final shape

  • Cold-Working:A process of strain hardening at room temperature to deform the material beyond the elastic rangeto obtain a desired property.

  • Examples of cold-working:rolling, drawing, extruding, cutting, pulling, indenting…


Purpose of cold working

Purpose of Cold-Working

  • To make its final shape

  • To alter its structure and properties:

    Increase yield strength

    Decrease ductility


Fracture

Fracture

  • BrittleFracture

  • DuctileFracture


Strain hardening ductile brittle fractures

Parameters Affecting Fracture

  • Load Rate

  • Nature of Loading

    • Triaxiality

    • Cyclic

  • Material

  • Temperature

  • Corrosion

  • Fabrication Cracks

  • Design Features

    • Notches

    • Holes

    • Fillets

    • Uneven surface Roughness


Strain hardening ductile brittle fractures

Fracture Mechanics

  • A specialization within both Structural and

  • Mechanical Engineering.

  • The study of how structures fracture.

  • Difficult in mechanics and mathematics.


Characteristics of brittle fracture in tension

Characteristics of Brittle Fracture in Tension

  • Underuniaxialtension loading, fracture occurs at90 degreeswith the axis of loading.

  • There is no plastic deformation (i.e. there is no necking).

  • The failure plane has a granular appearance.


Mechanics of brittle material fracture in tension

Mechanics of Brittle Material Fracture in Tension


Mechanics of brittle material fracture in tension1

Mechanics of Brittle Material Fracture in Tension

  • Thetensilecomponent of stress “pulls” the crystal apart:

     = []

  • Shear strengthof the material isrelativelyhigher.

     < []

  • Fracture surfaceis orthogonal to the direction of maximum principle tensile stress.


What is brittle failure

What isBrittle Failure ?


Ductile fracture

Ductile Fracture


Characteristics of ductile fracture

Characteristics of Ductile Fracture

  • Necking in round specimens:

  • Asneckingoccurs, atri-axialstate of stress develops in the region of necking. This is mostpopular inroundspecimens.

  • Failure:

    Failure begins when micro-cracking causing a

    fibrous surface to develop. This is followed by a

    rapid fracture orientedat 45owith the axis of

    loading.


Mechanics of ductile material fracture in tension

Mechanics of Ductile Material Fracture in Tension


Mechanics of ductile material fracture in tension1

Mechanics of Ductile Material Fracture in Tension

  • The SHEARcomponent of stress “shears” the crystal apart:

     = [] < []Ok

  • Shearstrength of the material isrelatively lower.

  • Fracture surface is45o tothe direction ofmaximum principle tensile stress.


What is ductile fracture

What isDuctile Fracture ?


Strain hardening ductile brittle fractures

Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Loading

d

dG

F


Strain hardening ductile brittle fractures

Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Unloading

d

Deformation

Reversal

dG

F


Strain hardening ductile brittle fractures

Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Reloading

d

dG

F


Strain hardening ductile brittle fractures

Definition of Ductility,m

Stress or Force or Moment

Strain

or Displacement

or Rotation

du

dy

Hysteresis

Curve


Strain hardening ductile brittle fractures

Definition of Energy Dissipation,Q

Stress or Force or Moment

Area = Q = Energy Dissipated

Units = Force x Displacement

Strain

or Displacement

or Rotation


Strain hardening ductile brittle fractures

Basic Earthquake Engineering

Performance Objective

(Theoretical)

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.


Bibliography

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!

  • 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.


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