Photogrammetrically techniques (Digital Image Correlation) become an important and no contacting measurement method used to measure and evaluate deformations during dynamic loads.
The DIC method consists in recording and analysing displacement and deformation fields on the surface of the tested object using correlation of digital images of the tested object recorded during its displacement or deformation.
DIC displacement and deformation can be measured using a system of one, two or more cameras. The use of one camera allows to determine the displacements and deformations of the tested sample only in the plane parallel to the image plane of the camera matrix. Using at least two cameras to record the image of a test object in different directions, it is possible to measure displacements and deformations in three-dimensional space.
The algorithm used for image analysis consists of three basic steps:
- The system’s digital camera analysing the sample area searches for characteristic points that become reference points for the virtual grid and for discretization of the analysed area.
- On the basis of the recorded changes in the distance between the points and numerical evaluation, a displacement and deformation map is created for the entire surface of the material seen by the camera.
- Two estimation models are used for the DIC method:
- Normalised Cross Correlation (NCC)
- Least-Squares Matching (LSM).