DuctilityAn important behaviour observed during a tension test is ductility-the extent of plastic deformation that the material undergoes before fracture.1.Elongation 2. Reduction of area

The second measure of ductility of the reduction of area.

where AO and Af are, respectively, the original and final (fracture) cross-sectionalareas of the test specimen. Reduction of area and elongation are generally interrelated,as shown in Fig. 2.4 for some typical metals.Tensile reduction of area has been shown to predict the bendability(sec 16.5) and spinnability(sec 16.11) of metals.

There is an inverse relationship between bendability and the tensile reduction ofthe area of the material (Fig. 16.18). The minimum bend radius, R, is, approximately,

Where’ r ‘ is the tensile reduction of area of the sheet metal. Thus, for r = 50, the minimumbend radius is zero; that is, the sheet can be folded over itself (see hemming,Fig. 16.23) in much the same way as a piece of paper is folded. To increase the bendability

of metals, we may increase their tensile reduction of area either by heating or bybending in a high-pressure environment (which improves the ductility of the material; see hydrostatic stress, Section 2.2.8). Bendability also depends on the edge condition of the sheet. Since rough edges are points of stress concentration, bendability decreases as edge roughness increases.

The spinnability of a metal in this process generally is defined as the maximumreduction in thickness to which a part can be subjected by spinning without fracture.

Spinnability is found to be related to the tensile reduction of area of the material,just as is bendability (see Fig. 16.18). Thus, if a metal has a tensile reduction of areaof 50% or higher, its thickness can be reduced by as much as 80% in just onespinning pass. For metals with low ductility, the operation is carried out at elevatedtemperatures by heating the blank in a furnace and transferring it rapidly to themandrel.