Bending of Sheet and Plate

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Bending of Sheet and Plate

• Bending is done to form flanges, etc.
• Also for giving stiffness to a sheet part

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• Length of neutral axis

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• In bending outer fibers are in tension and inner
  fibers are in compression
• Strain is usually equal in magnitude
• But in effect e0 is larger than ei because of
  neutral axis shift to inner surface
• With R/T e0-ei

Rbend radius Tthickness
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Minimum Bend Radius

• We can see that as R/T decreases tensile strain
  at the outer surface increases.
• Radius R at which cracks appear on the outer
  surface is called minimum bending radius
• Min. bending radius is expressed in terms of T
  (2T, 3T, etc.)

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• True strain at outer fiber for cracking true
  fracture strain in tension test.
• Spring back
• Elastic recovery after the removal of load.
  (Finite modulus of elasticity) Rods, sheets,
  bars, etc.
• Radius of bending increases when load is removed.
• Springback factor Ks.

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• Bend Allowance
• Since recovery depends on the stress level and
  modulus of elasticity E we can recalculate
  Ri/Rf.

Where Y is the yield stress of the material
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• Bendability can be improved
• Heating the area
• Applying hydrostatic pressure
• Reducing outer tensile strain by compressive
  force
• As R/T decreases, narrow sheets (smaller length
  of bend) crack at the edge and move towards
  center. Wide sheet crack at center.
• Rough edges can also cause reduction of
  bendability (stress raisers).

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Operations

• Beading
• Flanging
• Hemming
• Roll Forming
• Tube Bending

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Bead Forming
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TUBE BENDING
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STRETCH FORMING
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FLANGING
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Spinning

• Conventional Spinning
• As large as 6m (20 ft)
• Shear Spinning
• Missile nose cones, rocket parts.
• Tube Spinning

uspecific energy of deformation
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High Energy Rate Forming
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• Diffusion and Super plastic formation
• A hot research area
• Honeycomb material
• Deep Drawing
• Pure drawing
• Stretching
• ironing

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Superplastic Forming

• Same fine grained alloys can elongate as much as
  2000
• E.g. Zn-Al, titanium can be formed into very
  complex shapes.
• High ductility, low strength
• Very strain rate sensitive
• Extremely slow forming
• 10-4 to 10-2 s
• Some times forming can take hours

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HONEY COMB STRUCTURE
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Super Plastic Forming