The increase in a sample’s gauge length measured after a rupture or break divided by the sample’s original gauge length is referred to as elongation. The greater the elongation, the higher the ductility or elasticity of the material.
Elongation cannot be used to predict the behaviour of materials subjected to sudden or repeated loading. Some non-rigid materials like rubber and some plastics have very high elongations prior to break. Cross-head travel can be used to measure elongation of specimens with uniform width dimensions. If the specimen is in a dog-bone or dumbbell shape with a reduced cross-sectional region called gage length, an extensometer will be required to measure elongation within the gage length region by attaching it directly to the specimen and tracking movement as the material is stretched to failure.
Typical graph showing an elongation/ensile test on a rubber dumbbell sample: