Image correlation
The image correlation or digital image correlation ( English digital image correlation or short DIC) is a camera-based method for the contactless deformation measurement . It is mainly used for material testing and mechanical component tests.
Working principle
During the deformation , images of the component are recorded with one or more industrial cameras. A camera is used to measure flat components, and displacements and expansions in the component plane are recorded. With two cameras, 3D displacements and expansions can be measured on any object geometry.
The object surface to be measured is provided with a random point pattern. The point pattern is either generated from the natural surface structure of the sample or applied using various methods.
The algorithms for image correlation work in a similar way to pattern recognition processes and identify the pattern shifted and deformed in the image in the recorded camera images. Displacements and strains in the component surface are calculated over a large area and with high precision from the measured pixel coordinates of the pattern .
The image correlation determines the “true” or logarithmic expansion based on the pattern flow observed at the measuring points . Measurement methods that directly determine the distance between measurement marks, on the other hand, record the technical strain based on an initial distance between the measurement marks.
3D measurement with stereo setup
A 3D image correlation system is based on the combination of correlation methods with photogrammetry . As a result, the corresponding 3D coordinates of the object and the 3D deformation (displacement vector) are determined at each camera pixel or surface point of the component . In a stereoscopic setup with two cameras, several correlations are required to measure the geometry and the deformation / displacement of the component:
- The 3D coordinates x, y, z are calculated from the stereo correlation between the left and right camera image.
- From the time correlation, e.g. B. between the left reference image and the left image in the deformed state, the 3D displacement (u, v, w) is calculated.
See also
Strain distribution measured by means of 3D image correlation on a model wing in a bending test.