Chapter 4 On the In-Plane Displacement Measurement by 3D Digital Image Correlation Method Chi-Hung Hwang, Shou Hsueh Wang, and Wei-Chung Wang Abstract In this study, a three-step object was moved in-plan to different positions and determined the displacement by 3D DIC method. To monitor the out-of-plane displacement of the object during movement, a laser triangulation method was performed together with a typical 3D DIC. The laser triangulation setup consists of a laser source with a cylindrical lens mounted on the center of the camera-pair to generate a line pattern and then the discontinuous line patterns were then recorded by left- and right- cameras for step-height calculation. The measurement results reveal that there is displacement deviation introduced by viewing angle between camera and the object as the object is positioned at different locations. The experiment also shows that the deviation of 3D DIC determined in-plane displacement is small but the determined displacement is higher than the nominal displacement determined by a precision stage; in addition, out-of-plane displacements have been reported and cannot be ignored. In the final, a single camera model based on geometrical parameters of 3D DIC is proposed to correct the 3D DIC determined displacements; according to current results, the in-plane displacement of the 3-steps object can be tremendously reduced from 9.7% to 1.7%. Keywords 3D DIC • In-plane displacement • Laser triangulation method • Different depths • Step high 4.1 Introduction Digital image correlation (DIC) method becomes essential optical measurement method because DIC method is one of noncontact measurement methods which can be implemented for industrial and other on-site applications. The principle of DIC method is based on tracking artificial or natural surface random pattern among series captured images, therefore, two images from different status are needed for evaluating the change from one frame to the others, which means the associated displacement, strain and stress can be evaluated from two images captured or extracted from the recorded video stream. Moreover, the “image” is not limited to the traditional picture captured by the optical lens; for DIC method, the so called “image” has been successfully extended to two dimensional/ three dimensional spatial digital data which are obtained by SEM, TEM, AFM and XCT for micro-scale, nano-scale and volume object respectively. DIC methods can be classified into 2D DIC and 3D DIC method according to the optical system used for capturing optical images for analysis. There are possible errors could be introduced by 2D DIC method unless surfaces of objects under testing are flat, the camera is perpendicular aligned with respect to the specimen and in-plane displacement [1–3]. In general, 3D DIC which is based on the stereo-photogrammetry has been considered as the solution to eliminate errors introduced by out-of-plane displacement [1, 3–5] because the visual triangulation introduced by stereo-images can provide extra data to reconstruct the depth information of the test object. Different from previous studies, in this paper, a three-step aluminum block is manufactured as a test object which was moved in-plane with well aligned precious linear stage along x-axis to evaluate the 3D DIC measurement accuracy of in-plane displacement through depth-direction. By using the images obtained by the camera-pair setup, distance from camera-pair to the aluminum block and the DIC software, the measurement results showed that the in-plane displacement between the 3D DIC determined one is extremely large than the nominal in-plane displacement, moreover, out-of-plane displacement was also presented. Be aware of this new challenge, the measurement differences were discussed and then a model was proposed to correct the 3D DIC determined in-plane displacement. C.-H. Hwang ( ) ITRC, NARL, Hsinchu, Taiwan, 30073, Republic of China e-mail: chhwang@itrc.narl.org.tw S.H. Wang • W.-C. Wang Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan, 30013, Republic of China © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_4 19
RkJQdWJsaXNoZXIy MTMzNzEzMQ==