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

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
slide9
Parameters Affecting Fracture
  • Load Rate
  • Nature of Loading
    • Triaxiality
    • Cyclic
  • Material
  • Temperature
  • Corrosion
  • Fabrication Cracks
  • Design Features
    • Notches
    • Holes
    • Fillets
    • Uneven surface Roughness
slide10
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 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.
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 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.
slide20
Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Loading

d

dG

F

slide21
Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Unloading

d

Deformation

Reversal

dG

F

slide22
Behavior Under Seismic Excitation

(Inelastic Response)

F

Ground Disp.

Time

d

Reloading

d

dG

F

slide23
Definition of Ductility,m

Stress or Force or Moment

Strain

or Displacement

or Rotation

du

dy

Hysteresis

Curve

slide24
Definition of Energy Dissipation,Q

Stress or Force or Moment

Area = Q = Energy Dissipated

Units = Force x Displacement

Strain

or Displacement

or Rotation

slide25
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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