There are two parameters: Elongation and reduction of area.
Elongation and reduction of area measure different types of shear stud behavior. Provided the gage length is not too short, elongation is chiefly influenced by uniform elongation and thus is dependent on the strain-hardening capacity of the shear stud. Reduction of area is more a measure of the deformation required to produce fracture and its chief contribution results from the necking process.
Because of the complicated stress state in a neck, values of reduction of area are dependent on specimen geometry and deformation behavior, and they should not be taken as true material properties. However, reduction of area is the most structure-sensitive ductility parameter, and as such it is useful in detecting quality changes in a material.