Chapter 8 Speckling and Testing with DIC at Microscales Kevin B. Connolly and W. Carter Ralph Abstract The full-scale performance of modern materials, such as composites and additively manufactured parts, is dependent on their characteristic substructure. An understanding of the material interactions within these repeating substructures can improve the understanding of the material’s bulk properties. Taking measurements within these substructures is made difficult by their small size and potentially non-homogenous nature. There are some techniques that already exist for making small-scale strain measurements, but these techniques can require extensive specimen preparation and expensive specialty equipment and can place significant limitations on the specimen and test design. Southern Research is working to develop a method to measure strain in these substructures using Digital Image Correlation (DIC). This approach relies on a welldesigned experimental setup to apply thermal loads and observe the resulting strains. The project design has moved through a number of iterations to address the various difficulties in capturing high magnification, high-resolution images with the correct quality for use with DIC. Keywords Strain measurement · Digital image correlation · Thermal strain · Optical microscopy · Surface preparation 8.1 Introduction Complex materials are becoming more common in both research and commercial sectors. Examples include the structures designed in modern micro-processors, variations in material properties associated with 3D printing methods, and material mismatches inherent to composite materials. The variations in these materials are highly localized making detailed analysis of the structures difficult. The details of these substructures can be useful when determining the performance of the full component and improving the design. Traditional strain measurements do not have a high-enough resolution to distinguish between different components of the structure. This project is a continuation of previous work [1] to investigate the possibility of using Digital Image Correlation (DIC) as a method for measuring strain on small structures. DIC uses a series of images to calculate the displacement and strain fields on a surface by tracking common features in the image and comparing them back to a reference image. Methods have existed for measuring small strain fields, but these techniques tend to require difficult specimen preparation, require expensive equipment, and place restrictions on the specimen size or test method. The versatility of DIC is the primary reason it is being investigated for this work. DIC does not place specific limitations on patterning designs, so specimen preparation tends to be inexpensive and quick. Capturing images for DIC processing can be done with any camera system. Finally, DIC can be performed in most environments, so there are fewer limitations on specimen size and testing fixtures. The purpose of this work is to measure the stiffness mismatch in the small scale structures previously mentioned. In the current test setup, the specimen is thermally loaded to create strain in the material. Two fields of view have been considered in this work, approximately 1.2 mm×1.0 mm and 0.4 mm×0.3 mm field of view. These fields of view were chosen so that components of microprocessors could be visible and distinct. As this project has progressed, technical problems have been addressed. This paper will report on the two issues that have given the most difficulty, specimen patterning, and noise in the strain measurements. K. B. Connolly ( ) · W. C. Ralph Southern Research Institute, Birmingham, AL, USA e-mail: kconnolly@southernresearch.org; cralph@southernresearch.org © The Society for Experimental Mechanics, Inc. 2021 M.-T. Lin et al. (eds.), Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59773-3_8 67
RkJQdWJsaXNoZXIy MTMzNzEzMQ==