This conversion is carried out by deforming the metal permanently by the application of forces on it. The desired form or final shape of the metal defines which type of deformation process has to be applied. Physical and mechanical properties of metal such as strength, hardness, brittleness, elasticity, plasticity, malleability, toughness, grain structure, isotropic behavior etc. also play an important role in deciding which kind of manufacturing process is to be used.
Metals are generally ductile materials with a large plastic range on stress strain curve. This is due to the metallic bond present in them (Askeland, 2009: 33). The stress strain curve of mild steel is shown here, (although it will be different for each metal, it will follow more or less the same pattern): The area after the yield point is the plastic range of mild steel. Clearly, it can undergo significant amount of plastic deformation before it finally fractures. Same is true for other materials. Hence, to form a material in to desired shapes, plastic deformation is a desirable process.
There are many different yield criteria which tell us the stress required to cause permanent yielding in a material. Out of these Tresca criterion is considered suitable for ductile materials (Marciniak et al., 2002: 20). It suggests that yielding occurs (or plastic deformation starts) at a point when shear stress crosses a certain limit.
Formability of a metal is its ability to deform in to desired shape or form without failure. Failure can be due to different physical phenomenon like shearing or necking etc. (Kalpakjian and Schmid, 2001: 424).
One of the earlier developed tests is Cupping Test. In this test, a steel ball or any circular profile made of steel is pressed against the sheet with uniform increment of stress. The depth to which the sheet can be deformed is a measure of its formability. This method however has its own limitations as the results obtained are specific to the test conditions. The