Digital image correlation and tracking
This method is often used to measure full-field displacement and strains, and it is widely applied in many areas of science and engineering.
Digital image correlation (DIC) techniques have been increasing in popularity, especially in micro- and nano-scale mechanical testing applications due to their relative ease of implementation and use.
The concept of using cross-correlation to measure shifts in datasets has been known for a long time, and it has been applied to digital images since at least the early 1970s.
It is also fairly common to normalize the magnitudes to unity at this point, which results in a variation called phase correlation.
Thus, the correlation coefficient rij is a function of displacement components (u, v) and displacement gradients DIC has proven to be very effective at mapping deformation in macroscopic mechanical testing, where the application of specular markers (e.g. paint, toner powder) or surface finishes from machining and polishing provide the needed contrast to correlate images well.
First, vapor deposition at normal temperatures on semiconductor grade substrates results in mirror-finish quality films with RMS roughnesses that are typically on the order of several nanometers.
Typically this challenge can be circumvented by applying paint that results in a random speckle pattern on the surface, although the large and turbulent forces resulting from either spraying or applying paint to the surface of a free-standing thin film are too high and would break the specimens.
In addition, the sizes of individual paint particles are on the order of μms, while the film thickness is only several hundred nanometers, which would be analogous to supporting a large boulder on a thin sheet of paper.
Advances in pattern application and deposition at reduced length scales have exploited small-scale synthesis methods including nano-scale chemical surface restructuring and photolithography of computer-generated random specular patterns to produce suitable surface contrast for DIC.
The application of very fine powder particles that electrostatically adhere to the surface of the specimen and can be digitally tracked is one approach.
The candidate that worked most effectively was a silica powder designed for a high temperature adhesive compound (Aremco, inc.), which was applied through a plastic syringe.
A light blanket of powder would coat the gage section of the tensile sample and the larger particles could be blown away gently.
Sub-pixel tracking can be achieved by a number of correlation techniques, or by fitting to the known intensity profiles of particles.
[8] Digital Volume Correlation (DVC, and sometimes called Volumetric-DIC) extends the 2D-DIC algorithms into three dimensions to calculate the full-field 3D deformation from a pair of 3D images.
Similar to the other DIC techniques, the images must exhibit a distinct, high-contrast 3D "speckle pattern" to ensure accurate displacement measurement.
[10] DVC was first developed in 1999 to study the deformation of trabecular bone using X-ray computed tomography images.
[12] DVC is currently considered to be ideal in the research world for 3D quantification of local displacements, strains, and stress in biological specimens.
