Elongation at Break or Elongation is one measure of ductility. The conventional means by which we obtain Elongation is by pulling a specimen in tension until fracture. Elongation is defined as the increase in the gage length, Lo, of a test piece subjected to tensile forces divided by the original gage length. Elongation is expressed as a percentage of the original gage length and is given by:
Elongation (%) = ΔL/Lo x 100
Lo is the original gage length
ΔL is the change in length of the original gage length measured after the specimen fractures and the specimen is fitted together
A punch is often used to apply the gage marks to each specimen. The change in gage length, ΔL, is determined by carefully fitting the ends of the fractured specimen together and measuring the distance between the gage marks. In reporting Elongation, both the original gage length and percentage increase are to be reported. If any portion of the fracture occurs on or outside a gage mark, Elongation may not be representative of the material.
ASTM E8 governs the determination of Elongation for Metals