Chapter 32 Pre-qualifying DIC Performance Based on Image MTF Correlation Coefficient Chi-Hung Hwang, Wei-Chung Wang, Yung-Hsiang Chen, Jia-He Chen, Yan-Ting Wu, Jheng-Yong Lyu, and Ya Hsi-Chiao Abstract Attempts on developing the method for qualifying digital image correlation (DIC) method results, especial the displacement field, is presented in this paper. By taking a series images from center and margin of imaging field, DIC method is implemented to calculate displacement fields with respect to center of imaging field as origin of movement. Concurrently, residual displacements are obtained by subtracting mean value from the obtained displacement fields. The residual displacements are considered to be errors and mainly introduced by the imaging system of DIC. In this paper, modulation transfer function (MTF) is adopted to relate the residual displacement, DIC quality, with imaging system performance. By investing the MTFs cross-correlation values of all positions with respect to the reference image, a potential qualitative method for inspecting the DIC results based on sub-image MTF cross-correlation is proposed in this paper. Keywords Digital image correlation • Modulation transfer function • Sub-image set • Cross-correlation • Field of view 32.1 Introduction Digital image correlation method (DIC) is one of the image based optical strain measurement methods, the resolution can easily reach sub-pixel level thank to the improvement of personal computer computing power. DIC is first proposed in early 1980s [1, 2]; and now based on optical setups, DIC can be categorized as 2D DIC, 3D DIC and V-DIC methods [3–7]. In principle, DIC can be applied to determine displacement and strain filed from two digitized data matrixes (the images); therefore, the applications are extended to various imaging systems such as optical instruments, scanning microscopes, electronic beam microscopic systems and many others which make the DIC become a powerful method for different scales [8–11]. DIC method has been widely used for variety deformation measurements, such as surface strain of amorphous glassy polymer, soft materials, and even the deformation of thin plates subject to explosive blast. While DIC is applied for displacement and strain field determination, imaging system must be calibrated. By calibration, the object-image coordinate relation can be obtained, and the affections of lens distortion and lens-sensor misalignment can also be minimized. Calibration procedures consist of taking a series of images of a target at giving position with various rotated angles; the calibration procedure can ensure DIC accuracy as the object is moved within the calibration region. However, in some DIC application scenarios, for example long-term displacement monitoring, the cross-correlations might be ill posed because large displacement might move objects out-of-depth-of-field (DOF) or to the margin of field-of-view (FOV) and the reference image and the second image set can be obtained from different viewing area, unpredictable DIC results are concluded. Considering Modulation transfer function (MTF) is always used to determine optical and imaging system performances and can be evaluated by random patterns [12–15]. Hwang et al. [16] improved the displacement standard derivation frommm to sub-mm level by fitting MTFs-coefficient curve at Nyquest frequency of the imaging system. It is important to develop a method for per-evaluating the reference image and second-image-set are mechanical related or not. Considering images C.-H. Hwang (*) • Y.-H. Chen ITRC, NARL, Hsinchu 30073, Taiwan ROC e-mail: chhwang@narlabs.org.tw W.-C. Wang • J.-H. Chen • Y.-T. Wu • J.-Y. Lyu • Y. Hsi-Chiao Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan ROC H. Jin et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06986-9_32, #The Society for Experimental Mechanics, Inc. 2015 287
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