Welding Inspection & Metallurgy API ICP Self Study Notes

When the specimen is subjected to small stresses, the strain is directly
proportional to stress. This continues until the yield point of the material is
reached. If the stress were removed prior to reaching the yield point of the
metal, the specimen would return to its original length and is, considered
elastic deformation. However, stress applied above the yield point will
produce a permanent increase in specimen length and the yielding is
considered plastic deformation.
Continued stress may result in some work hardening with an increase in the
specimen strength. Uniform elongation will continue, and the elongation
begins to concentrate in one localized region within the gage length, as does
the reduction in the diameter of the specimen. The test specimen is said to
begin to “neck down.” The necking-down continues until the specimen can
no longer resist the stress and the specimen separates or fractures. The
stress at which this occurs is called the ultimate tensile strength.