72 K. B. Connolly and W. C. Ralph of air due to heat, but were not present. Another cause for the increasing noise with temperature could be the temperature change of the microscope lens. It is possible that the increase in noise was due to the slight change in focus caused by changes in the lens length. Trying to insulate the lens does not affect the changes in the lens temperature. It is likely the clear path between the specimen and lens is a cause for the heating. One possible technique to address this problem could be active cooling of the lens using ice water to maintain a consistent temperature throughout the test. In addition to thermal causes for noise, there may also be problems stemming from the flatness of the acquired images. As the temperature increases, the specimen tends to move slightly as it expands. Exploration of the rigid body motion using the adjustable base was capable of generating distortions in the strain field. These distortions caused a lens that does not capture a completely flat image. This problem is most predominant at the edges of the field of view. Changing the objective lens for different magnifications does not seem to effect the distortion. By adding magnification at the rear of the lens assembly, much of the edge distortion can be reduced. However, this additional magnification reduces the maximum possible field of view the current system can achieve. 8.5 Conclusion Early testing illustrated that DIC could detect local differences in material properties. Attempts to validate DIC measurements found that system noise was too high for values to be considered reliable. The project has explored different options for generating a speckle pattern at the desired scale, but none worked as well as the natural pattern on the polished aluminum. Southern Research has not fully explored the techniques for applying speckle with carbon powder and SEM stamp pads. It is possible better results could be generated with additional experimentation. The project was able to measure the expansion of aluminum, but only using the natural surface not an applied speckle pattern. While these results were consistent with the known expansion of aluminum, there was significant noise in the system. The additional noise caused by the less than optimum speckle patterns made test results even less reliable. The apparatus discussed has shown potential but is limited by the small field of view being attempted. Future work will need to identify larger material structures that need to be examined or material with higher thermal expansion. With a larger field of view, there will be more options for applying speckle to specimens and should be less sensitive to changes in focus. With a larger field of view, it may also be possible to mechanically strain the specimen rather than relying on thermal loading. Acknowledgments Southern Research Institute. Cisco Systems, Inc. References 1. Connolly, K.B., Ralph, W.C.: Elevated Temperature Optical Microscopy DIC, Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Volume 3. Conference Proceedings of the Society for Experimental Mechanics Series (2019) 2. Jones, I., Iadicola, M.: A Good Practices Guide for Digital Image Correlation. International Digital Image Correlation Society (2018)
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