Don’t preclude DIC

APRIL 2017

Digital Image Correlation (DIC) is a non-contact optical technique for the analysis of surface displacement fields of a specimen during deformation/loading. This information can be used for subsequent analysis of the surface strain and has the potential to allow for continual in-service monitoring of components.

The measurement of surface deformation on materials and structures subjected to loading is an important tool in solid mechanics.  Strain gauges are typically utilised to quantify such measurements, however, various non-contact optical methods, including speckle interferometry and DIC, have been developed and are being applied successfully to measure surface strains in various structures and components. 

DIC compares a sequence of consecutive images of the sample’s surface, taken during loading.  During post processing pixels in these images are discretized into subsets of NxN pixels.  Relative movement of the subsets is then analysed and tracked, using a correlation algorithm, to obtain deformation vector fields based on the initial ‘zero’ load condition.  From these deformation fields it is possible to identify the surface strain fields associated with the particular loading condition.  As the individual subsets of pixels are identified based on the grey scale intensity of adjacent pixels, it is important to ensure the grey scale in these groups of images is unique/random and that there is sufficient contrast.  Typically this is achieved by etching or spraying the surface with a speckle pattern.  In some cases where the surface texturing of the component is suitable, such preparation is not required.  Diffuse lighting from LED sources is often used to ensure suitable contrast and limit shadows and reflections.

Commercial DIC packages are capable of sub-pixel resolution from which deformation can be measured to one part per million.  These packages are multi-purpose in terms of the types of images which can be correlated.  Typical image sources include standard digital cameras, electron microscopes and high-speed video.  

DIC can be performed in both 2D and 3D, with the latter requiring multiple cameras to obtain a topographical map of the component being analysed.  3D techniques are typically required where the area of interest is non-planar or where significant out of plane displacement is expected.

Besides being used for surface strain measurements in various materials (including composites and bone) DIC has also be used:

  • To track the relative movement/displacement of components.
  • In thermal analyses such as the measurement of the distortion of electronic components during operation.
  • To map dynamic vibration-induced movement.
  • In advanced applications that include the measurement of crack opening displacement for fracture toughness testing.

Although DIC can theoretically be used for dynamic strain measurement, it is currently best suited for quasi static applications as the algorithms are resource heavy and need sufficient time to process.  However, the ease with which many DIC systems are set up enables the real-time monitoring of strain in a multitude of different components.  Permanently affixed tracking cameras can continually transmit images for strain analysis, enabling the real-time monitoring of many components including bolts, girders and even wind turbine blades.  By tracking strain changes within such components, maintenance intervals as well as the lifespan of the components can be gauged, thereby reducing the risk of these components failing in service.

Published in Technical Tips by Origen Engineering Solutions on 1 April 2017