Typically, mechanical properties of materials are tested using a test machine which grips a dogbone specimen made of the material of interest and pulls the specimen apart. Based on the force used to displace the ends of the specimen, the mechanical properties of this material can be determined. Compliant materials, however, do not lend themselves to this method of testing due to stress concentrations tainting the data collected. To combat this, optical tests can be used to measure mechanical properties, in particular Digital Image Correlation, or DIC for short.
The DIC system is a set up of several cameras, which can measure mechanical properties of compliant objects. By taking the object of interest, using an airbrush to cover it in speckles, and then having the DIC system capture several photographs of the object while it is subjected to a force, the mechanical properties can be measured based on the deformation of the speckles.
The problem with operating the system is that there is no predetermined way to generate the ideal speckle patterns. An ideal pattern must have speckles that are about 3-5 pixels large, with a gray scale distribution of 50% pure black and 50% pure white. However, there are many different parameters within the different types of airbrush. For example, there are several kinds of airbrushes such as internal mixing siphon, external mixing siphon, and internal mixing gravity fed. Additionally, there are many sensitive parameters that drastically change the nature of the speckles such as incoming air pressure, paint to thinner ratio, the distance to target, and airbrush orifice opening.
In order to determine the ideal parameters, a pattern with every combination of parameters on a target, in this case a 2 by 1 inch foam board piece, was generated using an internal mixing gravity fed pen. Images of the pattern on the DIC system were taken to see if the pattern correlates. Then image processing software and various statistical tests were used to map out the gray scale pattern of each pattern. More than one set of parameters produced desirable speckles, however, many of the patterns were not useful. In order to save time and resources on future projects, it is important to find out which parameters produce the desired speckles.