Why is necessary to state the gage length when repoting the percentage of elongation?

Answer 1

Elongation is defined as the increase in the gage length 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 (%) = 100 x ∆L/Lo Eq. 1
where Lo is the original gage length.
DL is the change in length of the original gage length when measured after the specimen fractures and the specimen is fitted together.
The original gage length, Lo, as specified in E8 is usually 1, 2, 4 or 8 inches and is dependent on the size of the specimen. A punch is often used to apply the gage marks to each specimen. The change in gage length, DL, is determined by carefully fitting the ends of the fractured specimen together and measuring the distance between the gage marks.
In reporting the elongation, ASTM E8 requires both the original gage length and percentage increase. If any portion of the fracture occurs on or outside a gage mark, the elongation and reduction of area may not be representative of the material.

Answer 2

Gage Length