Extensometers

An extensometer is a device used to measure changes in length of a specimen under load, primarily in materials testing. It directly measures elongation or strain by comparing distances between two points on the specimen. The basic quantity is strain ε = ΔL/L0, where L0 is the original gauge length.

There are several types. Contact extensometers include clip-on, knife-edge, or needle or lever types that attach to the specimen and track relative displacement as it deforms. Non-contact optical extensometers use imaging or laser techniques to measure surface displacement without touching the sample; methods include laser triangulation, digital image correlation, and video extensometry. Some devices combine LVDT or other transducers for displacement measurement.

In practice they are used in tensile, compression, creep, and fatigue tests to obtain stiffness, yield, and ultimate elongation data, and to monitor deformation in structures. They require proper gauge length, alignment, and mounting to avoid bending errors. Temperature and surface conditions can affect readings; calibration and traceability are important. They can be integrated with data acquisition systems and standards govern their calibration and use, such as ISO standards for extensometry.

In civil and geotechnical engineering, extensometers monitor deformation in materials and structures, including rock, soil, and concrete, sometimes through boreholes or surface mounts; in these contexts they measure settlement, crack opening, and subsidence.