Strain hardening ductile brittle fractures
Download
1 / 26

Strain Hardening, Ductile/Brittle Fractures - PowerPoint PPT Presentation


  • 131 Views
  • Uploaded on
  • Presentation posted in: General

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 ) .

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha

Download Presentation

Strain Hardening, Ductile/Brittle Fractures

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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).

  • 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?

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

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

  • 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

  • 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

  • To make its final shape

  • To alter its structure and properties:

    Increase yield strength

    Decrease ductility


Fracture

  • BrittleFracture

  • DuctileFracture


Parameters Affecting Fracture

  • Load Rate

  • Nature of Loading

    • Triaxiality

    • Cyclic

  • Material

  • Temperature

  • Corrosion

  • Fabrication Cracks

  • Design Features

    • Notches

    • Holes

    • Fillets

    • Uneven surface Roughness


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

  • 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

  • Thetensilecomponent of stress “pulls” the crystal apart:

     = []

  • Shear strengthof the material isrelativelyhigher.

     < []

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


What isBrittle Failure ?


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

  • 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 isDuctile Fracture ?


Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Loading

d

dG

F


Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Unloading

d

Deformation

Reversal

dG

F


Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Reloading

d

dG

F


Definition of Ductility,m

Stress or Force or Moment

Strain

or Displacement

or Rotation

du

dy

Hysteresis

Curve


Definition of Energy Dissipation,Q

Stress or Force or Moment

Area = Q = Energy Dissipated

Units = Force x Displacement

Strain

or Displacement

or Rotation


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

  • 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.


ad
  • Login