Foamig

Source: HANDBOOK OF DIE DESIGN

CHAPTER 8

BENDING AND FORMING OPERATIONS

8-1 STRESS, STRAIN, ELONGATION, COMPRESSION
The process of material forming (i.e., deformation) depends on several laws, closely related to those of physics. First of all, the law of constant volume applies here. No matter how much we shrink or stretch a part, no matter how much we form it, draw it, or compress it,the basic volume we had at the beginning will always be there. True, minor volumnar changes may take place during compressive forming, but these are so minute, so small with regard to the bulk of the part that they can easily be considered irrelevant. Another rule is pertaining to the distribution of particles in formed material. During changes of material structure, all affected segments willattempt to relocate into areas of least resistance. In other words, the material will always tend to flow where it is not obstructed, or to fill the gaps located nearby, or to conform to shapes exerting pressure upon it. Rule number three: Every permanent deformation takes place after the changes in the material structure exceeded the maximum elastic limit of that material. However, this is not thefinal deformation achieved, as, after release of the applied forming pressure, the material makes an attempt to return to its previous location; we say it springs back. The complete amount of deformation is therefore equal to the sum of the elastic segment and the plastic segment of the operation, or Etotal = EEL + EPL (8-1)

There are two types of deformation that can be observed during anyforming process. These can be either localized, or affecting the whole part: • Equal deformation, which is fairly even, free from excessive deviation from its mean values, and unaffected by axial orientation. • Unequal deformation, where the shape and the size of the formed part are changed unequally. Here the differences between mean values and marginal values of the process are of greater span.During this type of deformation, many additional stresses, beneficial or detrimental, may develop. The emergence of localized stresses within the material during unequal deformation is caused mainly by • Unequal friction between the forming tool and the part • Unequal temperature distribution within the part
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BENDING AND FORMING OPERATIONS 322
CHAPTER EIGHT

• Too complex a product • Chemical differences within the material • Mechanical properties of the material These stresses sometimes equal each other out, for which reason they are not always observable on thesurface of the part. Anyway, the human eye is mostly capable of observing only volumnar changes; those occurring on the structural or substructural level, taking place between the crystals of material, are not visible to us. The deformative processes applied to the fabricated sheet-metal material can be numerous and their sources can be easily grouped into but several categories: • • • • • •Cutting Simple bending Forming Drawing Compressive forming Combinations of the above

Bending and forming are quite similar operations, the only difference being the presence of the drawing action in forming. In simple bending, a portion of the part is flexed along a straight line until a bend is obtained. In forming, the bend line may be curved, circular, or otherwise shaped. The variability of thebend line contour causes the material to expand on one side and be compressed on the opposite. For these reasons, there are basically two types of forming operations, one producing shrink flanges and the other is used for making stretch flanges. In shrink flanges, as the name implies, the material of a flange is squeezed or compressed during forming, whereas stretch flanges are stretched and...