68 K. B. Connolly and W. C. Ralph 8.2 Apparatus The project apparatus consists of a microscope, specimen stage, heater, and super structure to hold components in alignment (see Fig. 8.1). The design of the test setup is covered by patent 10,379,333. The microscope currently being used is a 5 megapixel camera attached to telocentric lens. The objective lens on this system is interchangeable and in most cases results in a working distance of approximately 40 mm. The specimen stage consists of a two-axis adjustable base for in-plane motion, insulated supports, and a low thermal expansion stage that supports the specimen and heater. The low thermal expansion stage includes a hole to allow a clear optical path to the specimen. Holes in the stage also allow room for a thermocouple to lay under the specimen and measure temperature from the same surface that is being viewed for DIC measurements. The microscope lens is also instrumented with a thermocouple. The apparatus uses a simple resistance heater with holes to ensure that the air flow is sufficient to prevent air stagnation in the optical path to the microscope. The primary purpose of the superstructure is to reduce vibrations into system and ensure that the microscope is held perpendicular to the specimen stage. Testing is conducted by taking image sets at predetermined temperatures. As the temperature of the specimen reaches the appointed mark, the camera is triggered, and the temperature of the specimen and lens is recorded. After heating up to the desired temperature, the apparatus is allowed to cool naturally and additional images are taken for analysis. For most test runs, the specimen was heated to 170 ◦C before being allowed to cool. Once the test is concluded, the images sets are imported into DIC software for analysis. 8.3 Specimen Preparation DIC requires high contrast and sufficiently small features to track displacement of the surface properly. Creating an acceptable pattern for the desired fields of view can be difficult. The optimum size for contrast features is 3–5 pixels [2]. For the 1.2 mm by 1.0 mm field of view, this would require features about 2.5 μm in size. For the 0.4 mm by 0.3 mm field of view, this would require features about 0.8 μm in size. The most successful DIC pattern for this project has relied on the natural microstructure of materials. This worked reasonably well for aluminum and certain microprocessors in mid process, shown in Figs. 8.2 and 8.3, respectively. Unfortunately, most materials of interest do not produce an acceptable pattern when prepared for microscopy. The standard method for applying a DIC pattern is to sputter paint using a spray can or an airbrush for smaller specimens. Standard black and white airbrush paint did not produce enough contrast at high magnification. It was found that tempera 19 72 43 13 17 41 14 60 16 50 44 40 18 12 12a 12b 20 Fig. 8.1 Apparatus schematic from patent 10,379,333
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