Digital Image Correlation (DIC)

Digital Image Correlation (DIC) is a full-field optical method for determining the two-dimensional (single camera) or three dimensional (two or more cameras) displacement distributions from components or material coupons, and deriving the associated strain fields. Strain Solutions Ltd is pleased to offer DIC consultancy, test services and training using the Correlated Solutions Vic-3D system in association with EPT Ltd, the United Kingdom representative of Correlated Solutions.

How it works

Digital image correlation is exactly what it says on the tin: the capture of images using digital camera sensors, and the subsequent correlation of features within those images to track changes in displacement and strain. The method relies on photographing something which displays sufficient contrast such that features on the component can be tracked during a load or displacement event. Some engineering materials, for example micrographic material samples, have a sufficient native surface texture that they can be photographed without any surface preparation. However, the most common approach is to apply some form of speckle pattern (usually with paint or self-adhesive stickers) which creates a high contrast, random pattern on the surface of a specimen. The nominal pitch of the pattern is carefully selected to maximise the spatial resolution of the resulting DIC data.

DIC subset tracking

Tracking a typical subset in the speckle image

Equipment

Conventional 3D digital image correlation requires a pair of low noise, temporally stable cameras which can be rigidly mounted in relation to each other during system calibration and subsequent data capture. This is most easily achieved with a pair of high quality CCD machine vision cameras, bolted to a rigid beam (or 'stereo bar'), as shown below. The Strain Solutions DIC equipment can be powered from a laptop battery and is therefore a fully portable setup: a 100 metre optical fibre link can also be incorporated between the cameras and pc allowing for remote operation of the equipment.

Digital Image Correlation rig

Stereo camera setup using two machine vision CCD cameras and lightweight aluminium beam

3D data

An example stereo image pair is shown below for a simple cantilever beam calibration specimen (from the Inter Laboratory Study of the VANESSA project). The beam is clamped rigidly on the left hand side, and an upwards point load is applied to the free tip of the beam.

Left-hand DIC camera image Right-hand DIC camera image

Example stereo DIC image pair from a cantilever beam

After performing the image correlation between an unloaded reference pair of images and the above loaded image pair, the full-field map of out of plane displacement is available (shown below).

Out of plane displacement map

Out of plane displacement map, superimposed on the left-hand camera image

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