Digital Image Correlation Technology

Due to rapid new developments in high resolution digital cameras for static as well as dynamic applications (along with advances in computer technology), applications for the Digital Image Correlation (DIC) measurement method have broadened. DIC techniques have proven to be a flexible and useful tool for deformation analysis. The dynamic range of DIC is wide, with the capability to measure both very large and very small deformations.

The proposed optical method uses a mathematical correlation analysis to examine digital image data taken while samples are involved in mechanical tests. By taking digital photographs, DIC operates by recording a number of images of the surface of the specimen, each of which corresponds to different mechanical states (a reference state and several deformed states). This matching of subsets between the two images happens by computer-automated recognition. From this information a change in the shape of an object can be determined in ways not previously possible and without direct contact with, or damage to, the object.

To apply this method, the material under test needs to be prepared by the application of a random dot pattern (a speckle pattern). The dots can be printed on samples by various means when possible, or else sprayed on the surface with water-soluble aerosol paint (which can be removed without harming the object if needed). Sometimes a speckle pattern can be imparted by flicking paint from the bristles of a toothbrush. It is important to note that in IPI’s research project no objects of significant historical or financial value will be used, though actual historical materials will be tested.

Using computer software the differences between patterns can be calculated by correlating all the pixels of the reference image and any deformed image. From this a strain distribution map can be created which will show how much the object has changed.Extension to 3D surface measurement is available by using two cameras (called stereo-correlation) and this method will be used in IPI’s research.

An example of the technique in two-dimensions can be seen in the images below. In Figure A, a speckle pattern exists on a sheet of material under test. After increasing the RH at a fixed temperature we see deformation of the surface in Figure B. Comparing A to B it is possible to determine the strain on the material from the movement of the dots. In this case ‘strain’ refers to the degree of movement or deformation. When the RH is then returned to that found in the initial condition, it is possible to determine if the elastic limit of the material has been exceeded by the RH change. If so, the two dot images would not match because permanent deformation would have occurred. Furthermore, other deformations such as small cracks can be seen as shown in Figure C. This analysis, when seen in three dimensions, enables further insight into behaviors such as warping and cockling, etc.

One of the advantages of DIC is that it can be used to efficiently study the behavior of a large number of samples all at once. In IPI’s research project both small squares of material (paper, vellum, photos, etc.) and composite structures, like bound volumes of various age and composition, will be examined. Automated computer algorithms available from the supplier will enable a matrix of approximately one hundred 10 x 10 centimeter flat samples to be analyzed simultaneously at a resolution of approximately 10 microns. Samples will include parchment, vellum, leather, paper, and textiles, and particular attention will be applied to physical damage such as cockling paper, flaking ink, warped covers on books, and cracked emulsion on photographs and prints. In other experiments, books on shelves and books in boxes will be used. From the flat sample experiments, useful information can be gained about component materials that ultimately shape the behavior of complex composites in which they play a role.