Measure of Ductility

It has been stated earlier that a ductile material exhibits considerable deformation in the plastic zone. Naturally, then this deformation or strain can be used as a measure of ductility. The total deformation suffered by a specimen upto fracture divided by the original length (between the gauge points) multiplied by 100 is termed as percent elongation. Percent elongation is taken as in index to explain the ductility of material. It would mean that higher the percentage deformation, more ductile is the material. However, percent elongation shall depend upon what gauge length is selected. To understand this sequence of deformation as depicted in Figure with reference to diagram of Figure will have to be understood well. During elastic deformation the strain is very small and uniformly distributed over entire length of the specimen. Hardly any change in length or diameter can be visually observed. Even beyond the yield point or elastic limit such changes are small, but just before ultimate point the plastic deformation starts to localize in a small part of length and reduction in diameter (in the maximum change in length occurs in this region. Ultimately fracture occurs in the necked region.

                                                               (a)                                                        (b)

                                                                          (C)                                                        (d)

Figure: Sequence of Deformation of a Cylindrical Specimen with Reference to Figure 1.6 (a) Just Before Point, a, (b) Just Before Point, f, (c) Just After Point, f and (d) At Point g