A video extensometer is a camera- based measuring system for non-contact strain measurement in tensile tests. It is mainly used in materials testing of flat or round specimens by tensile testing machines. In materials testing out a pull is loaded with an increasing force. During the test, the force is measured by a load cell in the tensile testing machine and recorded together with the measured samples from the video extensometer strain continuously. From the measurement results, a stress-strain diagram is prepared to be derived from the material properties such as modulus and Poisson's ratio.

Principle of measurement

The determination of elongation of a videoextensometer based on precise accuracy and optimized image processing algorithms for edge detection. Prerequisite for two contrasting line marks on the surface of the tensile specimen. The marks are usually applied with a stencil and colored pencil ( Edding ) or line label.

In the image processing (image processing ) the distance between the two line markers in the camera image with subpixel accuracy before and during exercise determined. From the resultant change in length DELTA.l and the original distance l0 the strain is calculated:

Calibration of the system is not required since only relative changes in the distance needed to stretch calculation. A video extensometer may, after calibration, the distances of line marking, scaled in mm, output, and thus becomes the transducer. The transverse strain is measured by transmitted light and determination of sample geometry ( sample width ).


  • The non-contact determination of the longitudinal elongation leads especially for soft materials to a significant improvement in the measurement accuracy compared to the displacement measurement of the traverse.
  • The measuring range is independent of the sample size and is up to 1000 % elongation.
  • The sample surface must not or only slightly move in camera direction while trying.
  • The measurements are digitally transferred as analog signal or in real time to the tensile testing machine and processed directly from their control.
  • Accuracy: up to 20 microns / m (micro trains ) at elongation and better than 1 micron in ways.
  • A sampling rate of up to 4 kHz allows for the use and control of dynamic applications.
  • A video extensometer is working towards a clip-on wear-free.

See also

  • Tensile test
  • Strain sensor
  • Fine strain measurement
  • Material testing
  • Optical Metrology