The ductility factor μ measures deformation capacity of specimens.
It can be defined as the ratio of axial shortening corresponding to 85% ultimate resistance during descending stage (u85) to the yielding axial shortening (u Y) in Eq. (3).
The u is the axial shortening corresponding to decrease of slope coefficient of the axial load–axial shortening curve, that is, the materials evolve from elastic stage to plastic stage: