Relative change in the length of a body under load. Elongation is defined by Eq.1
ΔL = change in length (stretching)
Lo = original length
ε = elongation.
Elongation is either stated as a non-dimensional figurehl or it is multiplied by 100 to be stated as a percentage. The elongation rate that the test rod (s. Tensile test) withstands until the breakage occurs is referred to as elongation at failure.
Elastic elongation (εe) represents a proportional elongation (occurs within the range of Hooke’s line. s. Hooke’s law). When the load is relieved from the specimen, the elongation disappears. If the limit of elasticity of the material is exceeded, the specimen will undergo non-proportional elongation (εp). In this condition the material undergoes plastic deformation. When the load is relieved from the specimen, the elongation will remain permanent (εr).