---
term: "Extensometer"
category: "material-testing"
shortDefinition: "Strain-measuring device attached to a specimen gauge section so tensile machines report elongation and modulus independent of crosshead displacement and grip compliance."
formula: "ε = ΔL / L0"
relatedStandards: ["ISO 6892-1","ASTM E8"]
url: "https://vectorbtc.com.tr/resources/glossary/extensometer/"
---

## What it measures

An **extensometer** measures **strain in the gauge length** rather than total crosshead travel. That separation is essential for accurate **Young's modulus**, **yield/proof stress**, and **elongation at break** on metals and polymers.

## How it is tested

Clip-on, axial, or **video extensometers** are mounted per manufacturer guidance. Strain rate control (ISO 6892-1 Method A) uses extensometer feedback to regulate loading speed.

## Standards and reporting

Classify extensometer gauge length and calibration traceability. Report whether strain is engineering or axial, and whether the device was removed before fracture per method limits.

## Common errors

Knife-edge slip, gauge length outside the parallel section, and leaving a long gauge extensometer on through necking can invalidate high-strain readings.