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Sheet Metal Forming Processes

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Sheet Metal Forming Processes Introduction Ratio Surface Area:Volume is very high for sheet metal process. Plates thickness 0.25 inches Boilers, bridges ... – PowerPoint PPT presentation

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Title: Sheet Metal Forming Processes


1
Sheet Metal Forming Processes
2
Introduction
  • Ratio Surface AreaVolume is very high for
    sheet metal process.
  • Plates thickness gt 0.25 inches
  • Boilers, bridges, ships, etc. use relatively
    thick plates
  • Usually sheet forming is done on relatively thin
    material.
  • Shapes can be of various size
  • Beverage can, car body
  • Sheet metal forming includes many widely used
    processes.

3
  • Sheet Metal Characteristics
  • Formability
  • Methods
  • Sheet metal is formed as coils of sheets (by
    rolling) or plates.
  • Blanks are created for required shapes.
  • Shearing sheets
  • Flame cutting - plates

4
Sheet Metal Characteristics
  • Main forces acting in sheet metal forming
    tensile
  • No (or very low) compressive load. Can lead to
    buckling or folding and wrinkling.
  • What is the big difference between bulk
    deformation and sheet metal?
  • What factors affect stretching and bending?
  • Elongation, Yield point elongation, anisotropy,
    grain size, residual stresses, springback,
    wrinkling.

5
Elongation
  • Uniform elongation region
  • Postuniform elongation region
  • True stress, true strain
  • Necking begins at
  • Uniform strain ? strain hardening index n
  • Large n, uniform elongation is better.

6
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7
  • Necking occurs at an angle ?
  • Localized
  • Diffuse
  • Depends on strain rate sensitivity m of the
    material.
  • Postuniform elongation is higher with higher
    values of m.
  • Total elongation ? n and m.

8
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9
Yield Point Elongation
  • Low carbon steel
  • Certain points elongate more in the specimen.
  • Strain rate ? elongation ?
  • Grain size ? Y.P. elongation ?
  • Leuders bands stretcher strain marks
  • May not be acceptable in many cases
  • To avoid skin rolling (cold rolling)
  • Reduces thickness of sheet 0.5 to 1.5
  • Strain aging can cuase the Y.P elongation to
    appear after a few days.

10
  • Residual Stresses
  • Because of non uniform deformation. Cause
    distortion if some parts are removed.
  • Stress corrosion cracking
  • Springback
  • Thin considerable springback
  • Wrinkles
  • Due to compressive load in certain areas.

11
  • Anisotropy
  • Previous history of the sheet metal can cause
    anisotropy.
  • Crystallographic grain orientation
  • Mechanical Fibering - alignment of impurities,
    inclusions
  • Grain Size
  • Mechanical properties
  • Appearance coarse grain causes rough surface
    appearance (orange peel)
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