Video extensometer

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A video extensometer is a camera-based measuring system for non-contact strain measurement during tensile tests . It is mainly used for material testing of flat or round specimens using tensile testing machines. During material testing, a tensile specimen is loaded with an increasing force . During the test, the force is recorded by a load cell in the tensile testing machine and continuously recorded together with the sample strain measured by the video extensometer. A stress-strain diagram is created from the measured values , from which material parameters such as modulus of elasticity and Poisson's ratios are derived.

Measuring principle

Camera image with detected line markings

The expansion determination of a video extensometer is based on precise and accuracy-optimized image processing algorithms for edge detection . This requires two high-contrast line markings on the surface of the tensile specimen . The markings are usually applied with a stencil and colored pencil or line sticker.

During image processing (image evaluation), the distance between the two line markings in the camera image is determined with subpixel accuracy before and during exercise. From the consequent change in length .DELTA.l and the original distance l 0 which is strain calculated:

The system does not need to be calibrated because only relative changes in distance are required to calculate the expansion. After calibration, a video extensometer can also output the distances of the line marking, scaled in millimeters, and thus becomes a displacement sensor . The transverse strain is measured by means of transmitted light and by determining the sample geometry (sample width).

properties

  • The non-contact determination of the longitudinal strain leads to a significant improvement in the measurement accuracy , especially with soft materials, compared to the distance measurement of the traverse .
  • The measuring range is independent of the sample size and is up to 1000% elongation.
  • The sample surface must not move or move only slightly in the direction of the camera during the experiment.
  • The measured values ​​are transferred to the tensile testing machine as an analog signal or digitally in real time and further processed directly by its control .
  • Measurement accuracy: up to 20 µm / m ( microstrains ) for elongation and better than 1 µm for paths.
  • A sampling rate of up to 4 kHz allows use in and control of dynamic applications.
  • A video extensometer works without wear compared to a clip-on transducer .

See also