DIC uses a contrasting, speckle pattern as the information source for the correlation algorithm. The performance of the correlation is, among other things, dependent on the quality of the speckle pattern. If there is no sufficiently identifiable natural pattern on the surface of the specimen, an artificial pattern must be applied. The fundamental assumption of DIC is that the motion and deformation of the (painted) pattern exactly represents that of the underlying material. There are four important elements that define an ideal speckle pattern; sufficient speckle size (pixels), high contrast (grey levels), fine speckle edge sharpness (contrast gradients) and good speckle density & distribution (coverage).
There are various patterning techniques available to create a speckle pattern, such as; stenciling, stamping and spray painting. What is important to the application of a speckle paint is that the coat is; thin (thickness), fine (speckle definition), not smudged or smeared and most importantly, does not degrade the material surface. Normal stamps tend to smudge the speckle and spray cans use solvents that can degrade the material and often require a fume cupboard. At Dantec Dynamics’, we have evaluated that there are two best types of speckle application techniques allowing these properties to be well achieved: airbrushes for smaller Field-of-Views (<20 cm) and stamp rollers for larger Field-of-Views (>20 cm).
For Fields-of-View (FoV) from 200 mm to 5000 mm, Dantec Dynamics’ has developed a speckle pattern kit consisting of: 3x hard-rubber, precision-made stamp rollers of optimized speckle pattern distribution and randomness, black ink, white paint, a foam roller and a paint tray. Each of stamp roller was developed for a predefined dot size through using a mathematical (boolean) algorithm for optimum (maximum) randomness. The stamp roller speckle kit is quick, easy and versatile to use for measurement applications with small or large Fields-of-View (FoV) and most importantly, gives reliable measurement results.
The real-time, speckle pattern quality evaluator (as of Istra4D 4.4.7) gives users the ability to quickly assess if the natural or generated speckle pattern is sufficient for the required correlation measurement accuracy of a test. This tool functions completely independent of the calibration or any DIC setup and only uses information based on the given facet or subset size. Users get clear visual information of the standard error resolution for a given facet or subset size to check if the expected accuracy is acceptable before the test is performed.
The Dantec SPG (Speckle Pattern Generator) is a useful software tool that can create & save images with an ideal, equal-distribution or strained, speckle pattern for a given resolution (pixel). Users can print-off the generated images on matte, white, self-adhesive, label printer paper (right) and stick the pattern directly to the object surface.