Keywords: calibration, digital image correlation, image distortion, measurement technique.

The measurement and analysis of the strain distribution is very important in civil and mechanical engineering, especially for the stress concentration problem in plasticity and fracture mechanics. The traditional measurement techniques can be divided into two types: contact-type and non-contact-type. The digital image correlation [1] is a new developed non-contact-type measurement technique. Because of the fast development of digital camera recently, the instrument cost of this method has dropped very rapidly but the precision raise contrarily. The advantage of this technique is that the whole displacement and strain field can be analyzed and the sample will not be disturbed. Therefore it can also be used to verify results of numerical analysis.

With the help of the rapid increment of the resolution of the digital camera and the calculation capability of computer, photogrammetry based on the digital image correlation is widely applied in different research fields [2,3,4,5,6,7]. However, the optical dispersion gives rise to the image distortion, when a 2D image is recorded from a 3D surface. Figure 1 shows the pictures taken by digital camera with different zooms. The image of Figure 1a shows an obvious distortion. The distortion can be caused by the lack of parallax of the CCD or CMOS of the camera with the specimen plan, the refraction of light through the lens. This image distortion can contribute an additional error when calculating the strain tensor. The main purpose of the work is to provide a calibration method in order to reduce the error caused by the image distortion and to raise the precision at the calculation of strain.

In this work the bi-quadratic and bi-cubic interpolation functions are used to establish the relationship between the real coordinate and the distorted image coordinate systems. The results show that the calibration has almost no influence on improving the precision of calculated strain, when the displacement is small. In the case of large displacement, the deviation of strain and displacement can be reduced with the help of the bi-cubic calibration model.

1

T.C. Chu, W.F. Ranson, M.A. Sutton and W.H. Peters, "Application of Digital-Image-Correlation Techniques to Experimental Mechanics", Experimental Mechanics, 25(3), 232-244, 1985. doi:10.1007/BF02325092

2

G. Vendroux and W.G. Knauss, "Submicron Deformation Field Measurements: Part 2. Improved Digital Image Correlation", Experimental Mechanics, 38(2), 86-92, 1998. doi:10.1007/BF02321649

3

D. Raffard, P. Ienny and J.-P. Henry, "Displacement and Strain Fields at a Stone/Mortar Interface by Digital Image Processing", Journal of Testing and Evaluation. 29, 115-122, 2001. doi:10.1520/JTE12237J

4

J.C. Kuo, S. Zaefferer, Z. Zhao, M. Winning and D. Raabe, "Deformation Behavior of Aluminum Bicrystals", Advanced Engineering Materials, 5, 563-566, 2003. doi:10.1002/adem.200300372

5

S. Zaefferer, J.C. Kuo, Z. Zhao, M. Winning and D. Raabe, "On the influence of the grain boundary misorientation on the plastic deformation of aluminum bicrystals", Acta Materialia, 51, 4719-4735, 2003. doi:10.1016/S1359-6454(03)00259-3

6

Shih-Heng Tung, Jui-Chao Kuo and Ming-Hsiang Shih, "Strain Distribution Analysis Using Digital-Image-Correlation Techniques", 18th KKCNN Symposium on Civil Engineering, Taiwan, 213-218, December 19-21, 2005.

7

J.C. Kuo, S.H. Tung, M.H. Shih and D. Chen, "Application of Digital-Image-Correlation Techniques to crystal plasticity", Proceedings of the Conference on Computer Applications in Civil & Hydraulic Engineering, Tainan, Taiwan, 2005. (in Chinese)

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