537 Digital Image Correlation over a Range of Temperatures Using High Magnification Optical Microscopy

Full-field optical strain measurements as a function of temperature are difficult to perform at the micron scale. The thermal expansion mismatches result in stresses that can break interfaces and damage the parts. Digital image correlation is an ideal measurement technique for this type of application, but it requires surface features of a specific scale that can be tracked and a measurement surface that remains in focus throughout the range of temperatures. These requirements are not difficult at the centimeter scale, and have been demonstrated at the millimeter and nanometer scales, but have not been achieved at the micrometer scales that are ideal for electronic parts. The problems include the small depth of focus, movement of the specimen due to thermal expansion, damage to the microscope lens due to heating, optical distortions due to uneven air temperatures, and the application of surface features for tracking. A new test apparatus and test method has been developed to provide the conditions needed for high-quality digital image correlation measurements using a high-magnification optical microscope up to 1000x magnification over a range of several hundred degrees Celsius. In this paper, the apparatus and test methods are described, and test results are demonstrated.