Sheet forming plastic deformation

Fig. 3 A typical situation in which wrinkles occur in the presence of a macroscopic stress is schematically depicted A thin sheet is exposed to a uniaxial macroscopic deformation (1). As a consequence, the sheet is compressed in the direction perpendicular to the elongation axis and the reacts by a buckling instability. Wrinkles are formed, which however relax as the macroscopic strain is released (2), unless plastic deformations occur in the macroscopically stressed state...

The formability is the capability of a material to undergo plastic deformation to a given shape without defects. Formability limits are a hard constraint when sheet metal parts are manufactured, but also in bulk metal forming, formability limits can be reached leading to faulty parts. 

Sheet forming turns the flat metal disk at the left into a three-dimensional structure by plastic deformation. 

When polystyrene foams are compressed, the 1-5 p,m thick, biaxially oriented, cell faces form permanent plastic hinges at intervals (Fig. 8.20), in directions perpendicular to the compression axis. The deformation mechanism is similar to that when thin sheet steel crumples in a car crash. This behaviour contrasts to the crazing and fracture that occurs when 2 mm thick polystyrene sheet is bent. Thirty-two micrometers thick, biaxially oriented polystyrene film, used in window envelopes, yields in tension rather than crazing and fracturing. In closed-cell polyethylene foams... 

A second source for accidental orientation is deformation of the material in the solid state, e.g. with cold-forming or solid-phase forming, in which a plastic sheet or billet is formed into an object at a temperature below the softening point of the plastic. The relaxation process can hardly take place under these conditions and although the article may be stable at room temperature, the deformation is (almost) completely recoverable at elevated temperatures. ... 

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