extensometry

Extensometry is the measurement of extension, strain, or deformation in materials and structures under mechanical loading. It involves tracking the relative change in length of a specimen, typically expressed as strain ε = ΔL/L0. Extensometry provides quantitative data on material stiffness, plastic deformation, and failure behavior and is central to materials testing and structural analysis.

The methods are broadly categorized as contact and non-contact. Contact extensometers attach to the specimen ends and record the displacement between reference points; variants include clip-on, needle, and mechanical or servo-controlled devices with a defined gauge length L0. Non-contact approaches use optical techniques, such as cameras with target patterns or speckle fields, laser-based systems, or digital image correlation, to measure deformation over local or full-field areas. Fiber-optic and holographic methods also provide non-contact options, especially at high temperatures or in restricted spaces.

Extensometry is applied in tensile, compressive, bending, creep, and fatigue testing to determine properties such as Young's modulus, yield strength, ultimate tensile strength, and Poisson's ratio when combined with other measurements. It is used across metals, polymers, composites, ceramics, and biomaterials, and supports standards and quality control in aerospace, automotive, civil engineering, and manufacturing.

Practical considerations include choosing an appropriate gauge length, ensuring proper alignment, and accounting for temperature effects and clamp-induced slip. Calibration and traceability are essential for accuracy, and the choice between contact and non-contact methods depends on specimen geometry, temperature, speed, and surface finish.