Drop on Demand Colloidal Suspension Inkjet Patterning for DIC

Digital image correlation (DIC) is a non-contact, optical, full-field displacement measurement technique used to map deformations on a body under an applied load by tracking surface features. It is a widely used method in experimental mechanics, owed in part to its ease of setup, and applicability across length scales and material systems. Most commonly, DIC surface patterns consist of contrasting black and white speckles applied by spray paint or an airbrush, which can lack control and consistency of the distribution of speckle sizes. In this paper, drop on demand (DOD) inkjet printing of colloidal suspensions is introduced as a means to apply a speckle pattern with precision on the micro- to mesoscale. A dot array and pseudo-random DOD pattern are applied to high-density polyethylene (HDPE) specimens, tested in compression following ASTM D695-15, and compared with a traditional airbrush speckle method. The results show that the DOD pseudo-random pattern provides a more consistent strain measurement than the other two methods. The array DOD and airbrush patterns introduce erroneous signals which can be explained on the basis of two different principles. A directional pattern, such as the array DOD, is known to introduce errors in the displacement vector identification. For the random airbrush pattern, a quantification of the speckle sizes revealed a large fraction of speckles that had a characteristic length scale of around 1 pixel, which is known to introduce bias errors, since the sampling of the speckle pattern does not satisfy the Nyquist-Shannon sampling theorem. These findings illustrate the utility of DOD speckling for certain DIC applications.