The Principle

The term "digital image correlation" (DIC) refers to optical systems for the contactless detection and analysis of displacements. In this process, an artificially applied, stochastic surface pattern is optically tracked with high-resolution cameras. Displacements in 2D or 3D are calculated from the individual images taken by two cameras by tracking the surface pattern - starting from a reference image of the unloaded test object. From these displacement values, full-surface strains or strain rates can subsequently be calculated and visualised.

The system consists of two high-speed cameras that can record at up to 1000 full frames per second. By reducing the optical range, the frame rate can be increased even further. A separate light source is also available for high-speed measurements. With the DIC, even large displacements (up to the cm range) can be recorded and analysed. This applies not only to harmonically excited structures, but also to static or even transient loads (e.g. shock wave propagation along a surface after shock excitation). From the temporal course of the determined 3D displacements, 3D velocities and 3D accelerations can also be calculated. DIC is a good complement to vibrometry.

Two cameras on a tripod and light above and below

Figure: Setup of the measuring system

Application in Practice

  • Use in material and component testing
  • Analysis of deformations, strains and movements on non-planar structures

A Combination with other measuring systems (e.g. thermographic camera) is possible.

Video: Measurement of the deformation of a component during cooling

The following video shows a possible application of DIC. The deformation of a component during cooling is examined. This is a preliminary test to check the measurement setup. The test object was heated up to 85°C. After the test object was placed, the actual measurement was started at a surface temperature of about 73°C. The test object was then cooled down. A data set (one video image per camera and the surface temperature) was recorded every 15". The cooling process took just under four hours and 957 data sets were recorded.

Based on the optical calibration of the camera setup, the displacements along the surface were calculated in relation to a defined reference point. The temporal course of these displacements was exported as an animated video. The result shows that with this setup displacements in x- and y-direction can be captured very well. However, the chosen method of positioning the test specimen is less suitable for deformation investigations in the z-direction.

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Measurement of the deformation of a component during cooling



Assistant Prof. Dipl.-Ing. Dr.techn. Florian Toth

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