Digital Image Correlation for
Deformation Measurement
Introduction
Digital Image Correlation is a full-field image analysis
method, based on grey value digital images, that candetermine the contour and the displacements of anobject 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 forthis measurement method has broadened and Digital
Image Correlation techniques have proven to be aflexible and useful tool for deformation analysis.

The dynamic range is wide, with the capability tomeasure large strains (>100%). The resolution depends
on the field of view and is therefore scalable.

Features• Material testing (Young’s Modulus, Poisson’s Ratio,Elasto-Plastic Behaviour)• Fracture mechanics• High Speed applications (Dynamic measurements)• Advanced materials (CFRP, wood, fibre injected
PE, metal foam, rubber etc.)• Component testing (Displacements, Strains etc.)• Measurement area: Flexible - mm2 up to m2• Measurement results: Surface contour, 3D
displacement and strains• Measuring sensitivity: down to 1/100,000 of the fieldof viewLoading conditions• Mechanical • Pressure• Thermal• And others• Vibration

Objectunder test

Calibration

Measurement of principal strains(minimum left, maximum right).

ContourLoading

Loading

Resulting data

ContourDataevaluation
Displacement

PRINCIPLES

Contour measurement

Material properties

Using a stereoscopic sensor setup each object point isfocused on a specific pixel in the image plane of therespective sensor. Knowing the imaging parameter foreach sensor (intrinsic parameter) and the orientation of
the sensors with respect to each other (extrinsicparameter), the position of each object point in three
dimensions can be calculated. Using a stochastic
intensity pattern on the object surface, the position ofeach object point in the two images can be identified byapplying a correlation algorithm.

An object point will be identified in the images of the twocameras by applying the correlation algorithm and finding
homologous points. Taking the imaging parameters into
account the contour of the object can be calculated.

Live image (left), maximum principal strain (middle),
principal strain (right).

Correlation

Strain calculationThe correlation algorithm is based on the tracking of thegrey value pattern G(x,y) in small local neighbourhood
facets. Due to a loading of the object this pattern istransformed intoGt (xt ,yt ) where Gt (xt ,yt ) = g0 + g1 G(xt ,yt )Considering the curvature of the object, the strain can becalculated by the parameter of the affine transformation
and by the gradients of the deformation.

x1 = a0 + a1 x + a2 y + a3 xy
y1 = a4 + a5 x + a6 y + a7 xy

Within the correlation algorithm the difference

å (Gt (xt ,yt ) - G(x ,y ))2 of these patterns is minimized.
By varying the illumination parameters ( g0 , g1) and theparameters of the affine transformation ( a0 ,....,a7 ) amatching accuracy of better than 0.01 pixel can beachieved.

Calibration
The quality of the measurement relies on exact knowledge of the intrinsic and extrinsic parameters of thesystem. The calibration is easily done by taking imagesof a calibration panel under different perspective views.
A bundle-adjustment algorithm calculates the intrinsic parameter (focal length, principal point, distortionparameter) for each camera and their respective orientation, as well as the extrinsic parameter (translationvector and rotation matrix).

Fracture mechanics
Crack growth. Live image (left) and pricipal strain (right).

APPLICATIONS
Digital Image Correlation offers characterization ofmaterial 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 material testing.
Digital Image Correlation is also ideal for fracture
mechanics investigation. The full-field measurement
delivers exact information about local and global strain
distribution, crack growth, and can be used for thedetermination of important fracture mechanics
parameters.
The software offers convenient data handling, reliable evaluation and extensive post-processing andanalysis capabilities (e.g. determination and visualisationof principal strain).

Component test
Deformation analysis on a CFRP (carbon fibre reinforced
plastics) structure. Live image (left) and out-of-plane
deformation (right).

www.dantecdynamics.com

andDeformation measurement
Calculating the transformation parameters for images
under different loading conditions, both the displacement
vector and deformation for each facet can be determined.

TP-CORR-05-001-EN

Displacement