Measurement Principles of DIC

Digital Image Correlation (DIC) is a full-field image analysis method, based on grey value digital images, that can determine the contour and the displacements of an object under load in three dimensions.

Due to rapid new developments in computer technology and high resolution digital cameras for static as well as dynamic applications, the applications for this measurement method has broadened and DIC techniques have proven to be a flexible and useful tool for deformation analysis.

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

image of dic correlation overview
Principle of DIC

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 field of view

PRINCIPLES

Using a stereoscopic sensor setup, each object point is focused on a specific pixel in the image plane of the respective sensor. Knowing the imaging parameter for each sensor (intrinsic parameter) and the orientation of the sensors with respect to each other (extrinsic parameter), the position of each object point in three dimensions can be calculated. Using a stochastic intensity pattern on the object surface, the position of each object point in the two images can be identified by applying a correlation algorithm.

Correlation

The correlation algorithm is based on the tracking of the grey value pattern G(x,y) in small local neighbourhood facets. Due to a loading of the object this pattern is transformed into

image of correlation algorithm equation

and

image of correlation algorithm equation

Within the correlation algorithm

image of correlation algorithm

the difference of these patterns is minimized.

By varying the illumination parameters

image of illumination parameters

and the parameters of the affine transformation

image of affine transformation

a matching accuracy of better than 0.01 pixel can be achieved.

Calibration

The quality of the measurement relies on exact knowledge of the intrinsic and extrinsic parameters of the system. The calibration is easily done by taking images of a calibration panel under different perspective views.

A bundle-adjustment algorithm calculates the intrinsic parameter (focal length, principal point, distortion parameter) for each camera and their respective orientation, as well as the extrinsic parameter (translation vector and rotation matrix).

Contour measurement

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

Deformation measurement

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

Strain calculation

Considering the curvature of the object, the strain can be calculated by the parameter of the affine transformation and by the gradients of the deformation.

APPLICATIONS

Material properties

DIC offers characterization of material 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.

image of live image, maximum principal strain chart, and principal strain
Live image (left), maximum principal strain (middle), principal strain (right)

Fracture mechanics

DIC is ideal for fracture mechanics investigation. The full-field measurement delivers exact information about local and global strain distribution and crack growth, and can be used for the determination of important fracture mechanics parameters.

image of live image and principal strain
Live image (left) and principal strain (right)

Component test

Deformation analysis on a CFRP (carbon fibre reinforced plastics) structure.

image of live image and out-of-plane deformation
Live image (left) and out-of-plane deformation (right)

The software offers convenient data handling, reliable evaluation, and extensive post-processing and analysis capabilities (such as determination and visualisation of principal strain).

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