QMT Features: September 2010
Power to the pixel!
The National Physical Laboratory (NPL) is unlocking the potential  of digital image correlation as an innovative new measurement technique with a broad range of applications.

Digital Image Correlation (DIC) is an innovative non-contact optical technique for measuring surface deformation including strain and displacement that is now being used extensively in experimental mechanics for a diverse range of applications.
DIC was first conceived and developed in the early 1980s, and has improved in accuracy since.

New software, coupled with readily available high-resolution cameras, has turned it into a promising technique for measuring the deformation characteristics of a range of materials, components and structures. Recent innovations in software and systems at NPL have seen new understanding of the potential of the technology leading to new applications, both in and out of the lab.

DIC works by comparing digital photographs of a component or testpiece at different stages of deformation. By tracking blocks of pixels, users can measure surface displacement and build up full field 2D and 3D deformation vector fields and strain maps.  For DIC to work effectively, the pixel blocks need to be random and unique with a range of contrast and intensity levels. It requires no special lighting and in many cases the natural surface of the structure or component has sufficient image texture for DIC to work without the need for any special surface preparation.
Software techniques have been developed to obtain sub-pixel resolutions and allow efficient execution of the algorithms.

Images can be obtained from a wide variety of sources including conventional CCD or consumer digital cameras, high-speed video, macroscopes, and microscopes, including scanning electron and atomic force microscopes. Using the most modern digital cameras, which can have 60 Megapixels means movements down to one millionth of the field of view, can be successfully measured. The DIC correlation process is not restricted to optical images and can also be applied to other datasets such as surface roughness maps, and complex 3D MRI and X-ray tomography datasets.

Why Digital Image Correlation?
DIC has several advantages over alternative conventional NDT methods and some of the other optical techniques such as laser shearography and speckle interferometery, which are generally more expensive and more difficult to use outside the laboratory. For large civil engineering structures, such as bridges and buildings and power generation infrastructure manual inspection techniques are often still used. This leads to inspections that can be influenced by subjectivity particularly when operatives are tired.  By capturing accurately positioned and aligned images, comparisons can be made between surveys and differences readily identified, whether these are due to surface change, deformation or crack opening.
What are the applications?

DIC is a simple to implement, and provides unambiguous results and hence is cost effective, leading to a huge range of potential applications. It has been used at NPL to examine a diverse range of material specimens including examining the evolution and uniformity of strain in materials testing, crack tip and crack propagation studies, detecting damage development in composites, structural deflections, high temperature strain mapping and dynamic vibrational analysis.
NPL has worked with a range of partners, including Airbus, AWE, Stresscraft and British Energy, to implement a DIC solution appropriate for the measurement of residual stress from incremental hole drilling in small structures. Other projects have included the application of DIC for measuring thermal expansion and distortion of electronic components, measuring the mechanical properties of nuclear graphite, 3D shape measurement on air bags, damage development in silk print screens and strain development during the processing of chocolate.

NPL’s DIC expertise is currently being engaged as part of IMPACT (Innovative Materials, Design and Monitoring of Power Plants to Accommodate Carbon Capture), a major carbon abatement project that aims to help carbon intensive industries reduce their CO2 emissions. The aim is to develop in-situ monitoring of power plant components to increase service life and help reduce emissions.
The technique has been used for monitoring displacements in rail and road bridges and for measuring crack opening in civil engineering components particularly in the nuclear industry.

Where next?
The current focus of the NPL DIC work is to provide a practical technique that can be used in a wide range of applications in the civil engineering and nuclear industries. Initial feasibility studies indicate the technique has great potential in these areas.

However, there are still some significant challenges with making accurate measurements with DIC, particularly with measurements outside the lab, because of the effects of the environment and due to the potential changes in surface conditions due to weathering and oxidation, temperature effects, component geometry issues, and the difficulty of access leading to non-optimal views of the surface.

To solve these problems NPL is running a UK government project to develop DIC applications with particular focus on precise -positioning of the camera equipment, and assessing the effect on the accuracy and reliability of the measurement. For the very large images generated from high resolution cameras, NPL has written its own software, which runs on the NPL Grid of desk top machines, enabling calculations to be sped up by several orders of magnitude.
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