Spring back and Bending Forces

• Spring back is the elastic recovery following plastic deformation during bending

• Bending force is a function of material strength, length of bend (L), sheet thickness (T) and size of die opening (w)

• Bending Sheet Metal
  • ln bending, outer fibers are in tension while inner fibers are in compression.
  • Bend allowance is the length of the neutral axis in the bend, Lb = a(R+kT)
  • k is a constant that ranges from 0.33 (for R<2T) to 0.5(for R>2T)
  • Engineering strain during bending:
  • As R/T decreases, the tensile strain at the outer fiber increases and the material eventually cracks

Bendability and Minimum Bend Radius

A3T minimum bend radius means the smallest radius the sheet can be bent to without cracking is 3 times thickness

·r is the tensile reduction of area of the sheet metal
(a)and (b) The effect of elongated inclusions (stringers) on cracking. as a function of the direction of bending with respect to the original rolling direction of the sheet.
(b)Cracks on the outer surface of an aluminum strip bent to an angle of 90. Note the narrowing of the top surface due to the Poisson effect.