1. Sheet Metal Forming Processes

2. Introduction
Ratio – Surface Area:Volume is very high for sheet metal process.
Plates – thickness > 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.

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.’

9. Yield Point Elongation
Low carbon steel
Certain points elongate more in the specimen.
Strain rate  elongation 
Grain size  Y.P. elongation 
Leuder’s 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 Springback Wrinkles
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)