Principle of Digital Image Correlation
Digital Image Correlation in print
Digital Image Correlation Overview
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.
The pictures below show the horizontal strain measured by two-dimensional image correlation for the pictures shown above.
The two speckle images below were taken simultaneously with the left and right camera of a stereo-system. The sample itself is a piece of glass with our company logo sticker adhered to the surface. The speckle pattern was applied using standard off-the-shelf flat white and black spray paint. Can you make out the shape?
The plot below shows the shape of the logo sticker measured with the VIC-3D System. The thickness of the logo sticker is approximately 0.003″ or 0.070mm.
3-D Example Application: Aluminum Dog-bone Tensile Sample
The first 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 left 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.
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.
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.
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.