Correlation for Deformation measurement
Digital Image Correlation is a full-field image analysis method,
based on grey value digital images, that can determine the contour
and the displacements of an object under load in three dimensions.
Due to rapid new developments in high resolution digital cameras
for static as well as dynamic applications, and computer technology,
the applications for this measurement method has broadened and
Digital Image Correlation techniques have proven to be a flexible
and useful tool for deformation analysis.
The dynamic range is wide, with the capability to measure large
strains (>100%). The resolution depends on the field of view
and is therefore scalable.
Material testing (Young's Modulus,
Poisson's Ratio, Elasto-Plastic Behaviour)
High Speed applications (Dynamic
Advanced materials (CFRP, wood,
fibre injected PE, metal foam, rubber etc.)
Component testing (Displacements,
Measurement area: Flexible -
mm2 up to m2
Measurement results: Surface
contour, 3D displacement and strains
Measuring sensitivity: down to
1/100,000 of the field of view
Using a stereoscopic sensor setup each object point is focused
on a specific pixel in the image plane of the respective sensor.
Knowing the imaging parameter for each sensor (intrinsic parameter)
and the orientation of the sensors with respect to each other
(extrinsic parameter), the position of each object point in three
dimensions can be calculated.
Using a stochastic intensity pattern on the object surface, the
position of each object point in the two images can be identified
by applying a correlation algorithm.
The correlation algorithm is based on the tracking of the grey
value pattern G(x,y) in small local neighbourhood facets. Due
to a loading of the object this pattern is transformed into
Within the correlation algorithm the difference
of these patterns is minimized.
By varying the illumination parameters ()
and the parameters of the affine transformation (),
a matching accuracy of better than 0.01 pixel can be achieved.
The quality of the measurement relies on exact know-ledge of
the intrinsic and extrinsic parameters of the system. The calibration
is easily done by taking images of a calibration panel under
different perspective views.
A bundle-adjustment algorithm calculates the in-trinsic parameter
(focal length, principal point, distortion parameter) for each
camera and their respective orientation, as well as the extrinsic
parameter (translation vector and rotation matrix).
An object point will be identified in the images of the two cameras
by applying the correlation algorithm and finding homologous
points. Taking the imaging parameters into account the contour
of the object can be calculated.
Calculating the transformation parameters for images under different
loading conditions, both the displacement vector and deformation
for each facet can be deter-mined.
Considering the curvature of the object, the strain can be calculated
by the parameter of the affine transformation and by the gradients
of the deformation.
Digital Image Correlation offers characterization of material
parameters far into the range of plastic deformation. Its powerful
data analysis tools allow the determination of the location and
amplitude of maximum strain, which are important functions in
Digital Image Correlation is also ideal for fracture mechanics
The full-field measurement delivers exact information about local
and global strain distribution, crack growth, and can be used
for the determination of important fracture mechanics parameters.
The software offers convenient data handling, reliable evaluation
and extensive post-processing and analysis capabilities (e.g.
determination and visualisation of principal strain).
Live image (left), maximum principal strain (middle), principal
Crack growth. Live image (left) and principal strain (right).
Deformation analysis on a CFRP (carbon fibre reinforced plastics)
structure. Live image (left) and out-of-plane deformation (right).
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