TABLE OF CONTENTS Deep drawing and stretching are the two main processes involved in most sheet-metal-forming operations. An excellent introductory overview on sheet-metal forming is provided by Hecker and Ghosh. [23] In a stamping operation, one part of the blank might be subjected to a deformation process similar to deep drawing (thickness increasing with time). In deep drawing the material is required to contract circumferentially, while in stretching the stresses applied on the sheet are tensile in all directions. Sheet-metal forming has evolved from an art into a science, and important material parameters have been identified. These material properties are obtained in special tests and allow a reasonable prediction of the blank in the actual sheet-forming operation.
The work-hardening rate n is important, because it determines the onset of necking (tensile instability), an undesirable feature. According to Consid re s criterion (see Section 3.2.2), n is equal to ? u, the uniform strain. Hence, the higher n, the higher ? u. The strain-rate sensitivity m is an important parameter, too, because it also helps to avoid necking. If m is positive, the material becomes stronger at incipient necks because the strain rate in the necked region is higher. (See Section 3.2.3.) The parameter R (the through-thickness plastic anisotropy) is also important; it is equal to the ratio between the strain in the stretching direction and the strain in the thickness direction. The greater the resistance to thinning in stretching, the...
