Digital Image Correlation is an optical method to measure deformation on an object surface. The method tracks the gray value pattern in small neighborhoods called subsets (indicated in red in the figure) during deformation.
Two-dimensional Example
The two pictures below show a speckle pattern on an aluminum sample with two offset semi-circular cut-outs. The two pictures were taken from an animation with the left image taken from the beginning and the right picture taken from the end of the animation. Since the deformation is predominantly in-plane, a single camera can be used to measure the deformation.
 Small Deformation |  Large Deformation |
The pictures below show the horizontal strain measured by two-dimensional image correlation for the pictures shown above.
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Three-dimensional Example
The two speckle images below were taken simultaneously with the left and right camera of a stereo-system. The sample itself is a sheet of paper with a laser-printed speckle pattern mounted to plexiglas. There is a second layer of paper cut into a shape glued on top the first layer. Can you make out the shape?
 Left View |  Right View |
The plot below shows the shape of the second layer of paper measured using stereo-correlation. The thickness of the paper is approximately 0.004".
3-D Example Application: Aluminum Dog-bone Tensile Sample
The picture on the left shows the test setup for 3D image correlation measurements on an aluminum dog-bone sample. The Vic-3D measurement system is connected to the load ouput of the test frame controller and records load data synchronously with the images.
The close-up on the right shows the two cameras of the stereo system focused on the sample. The sample is illuminated using fiber-optic goose-neck style lights that can conveniently be adjusted to provide even illumination across the entire sample.
Speckle Images
The two images on the right show the speckle images of the dog-bone sample in the undeformed state recorded by the left and right camera of the stereo system, respectively.
The sample shown was 0.5" wide and 0.1" thick. The gage section was 5" in length.
The speckle pattern was applied by first coating the aluminum sample with a layer of white paint using a spray can. The black speckles were then applied by lightly over-spraying a black mist of paint.
Specimen Necking
The graph below shows the out-of-plane displacement (W) as a color-coded overlay on the sample just prior to failure. The sample showed the typical localized necking where ultimately failure occurred.
Stress-Strain Curve
The stress-strain curve for the sample is shown on the right. The measured strains were averaged over an area corresponding to a gage length of approximately 1/8".
Since Vic-3D calculates the Lagrangian strain tensor on the specimen surface, the transverse strain can be used to calculate the reduction in cross-sectional area of the sample using a volume conservation constraint. In the graph on the right, the actual stress-strain curve using the reduced cross-sectional area is shown in green.