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Principle of Digital Image Correlation

Digital Image Correlation (often referred to as “DIC”) is an easy to use optical method which measures deformation on an object’s surface. The method tracks the changes in gray value pattern in small neighborhoods called subsets (indicated in red in the figure below) during deformation. Digital Image Correlation has repeatedly proven to be accurate when compared to valid FEA models. The commercially available VIC-2D and VIC-3D systems from Correlated Solutions utilize this advanced optical measurement technology.

Digital Image Correlation Book

The book “Image Correlation for Shape, Motion and Deformation Measurements: Basic Concepts,Theory and Applications” (seen below) was co-authored by the founders of Correlated Solutions.  The book is a collaboration of decades of research and development of 2D and 3D digital image correlation software, which have been implemented into our products.  The book can be purchased from many online retailers or directly from Correlated Solutions.
DIC Book

Digital Image Correlation Overview

Below are the reasons why its versatility, robustness, and ease of use make it the only choice when it comes to digital image correlation.

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.

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 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?

left-camera-CSI-logo

Left View
left-camera-CSI-logo

Right View

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.

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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.

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.

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