Measurement of curved surface deformation by computer vision

• Main Authors: Chen, Jian Nan, 陳建男
• Other Authors: Luo P. F.
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/01052059292105838367

碩士 === 中華工學院 === 航空太空工程研究所 === 85 === Using the advantage of reconstructing the surface of a bodyt, the stereo vision is used to measure the three-dimensional deformation field of a curved surface and study the strain. The specimens used in the experiment are round tension specimens made of 2024 aluminum alloy. A displacement controlled loading was applied to the specimen until the specimen was subjected to a strain of 1.5%. Under the world coordinate X- Y-Z erected by the stereo vision,the position coordinate of each point on the surface of the undeformed round tension specimen was determined. By the use of nonlinear optimization technique, both the position of the centerline and the radius of the cylinder were obtained. Using coordinate transformation, the position coordinate determined relative to the world coordinate system can be expressed in the cylindrical coordinate. By measuring the position coordinate of each point on the surface of the deformed round tension specimen, the displacement fields u,v and w were determined relative to the world coordinate system and converted through coordinate transformations, to the displacement fields Ur, UTheta and Uz expressed in the cylindrical coordinate. Using least square fit of a polynomial, the displacements UTheta and Uz were smoothed in the direction of Theta and z to determine the strain fields ?zz, ???and ??z which were expressed in the cylindrical coordinates. Results indicate that the measured mean value of ?zz by the stereo vision is in a good agreement with the value measured by the Instron extensometer and the strain gage. The mean value of ??z approaches zero as expected. In the experiment, it is hard to determine ???since there is a difficultly in eliminating the effect of rigid body motion on the displacement Ur which is enclosed in the terms of ???. Experimental results indicate that the stereo vision system may erect a cylindrical coordinate system to measure the deformation field of a cylindrical surface and analyze the strain field. Hence, the analysis method established in the paper may be applied to study the deformation probelms such as piping under pressure, and fracture problems such as piping with cracks.