Abstract
An image-processing-based automated grid method is investigated to determine the method's displacement and strain accuracy limits, and how these limits are influenced by the choice of camera-calibration models. A CCD camera and a PC-based frame grabber are used to record grid spot motion, then ordering and centroiding are used to identify each spot and calculate their individual displacements. The displacements are fitted with a moving biquadratic surface, and the strains are obtained by analytical differentiation of that surface. Camera-calibration models which are considered include various combinations of image-perspective transformation, image stretching, and elliptical-lens distortion. The strain and displacement accuracy are explored through rigid-body motion and uniaxial tension tests. In the process, sensitivity to in-plane and out-of-plane rigid-body translation, and extreme sensitivity to in-plane rigid-body rotation (for non-synchronized frame grabbers) are confirmed. It is found that under the best conditions the displacement accuracy is 015 pixels and that the strain accuracy is 120 microstrain. Finally, the automated grid method is used to investigate the strains developed in an aluminum perforated strip subjected to uniaxial tension.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Sevenhuijsen, P.J., “Photonics for Deformations,” Proc. 5th Int. Con Mech., Montreal (1984).
Sevenhuijsen, P.J., “An Idea for a Simple Laser Grid Method for Measurement of In-plane Deformations,” NRL-TR 74072L, Netherlands (April 1974).
Cardenas-Garcia, J.F. and Wu, M., “Further Development of Video Optical Diffractometry for Strain Measurement,” Proc. 1989 SEM Spring Conf. on Exp. Mech., 73–79 (1989).
Berger, J.R. and Kriz, R.D., “An Improved Optical Diffraction Strain Measurement System,” Proc. 1989 SEM Spring Conf. on Exp. Mech., 572–578 (1989).
Burger, C.P., Chona, R., Bowman, D.W., Lieder, R.M. and Parakh, Z.K., “High Temperature Failure of Materials,” Proc. 1990 SEM spring Conf. on Exp. Mech., 523–528 (1990).
Dennison, E.W. andStanton, R.H., “Ultra-Precise Star Tracking Using Charged Coupled Devices (CCDs),”Proc. of SPIE (252),54–63 (1980).
Fail, R.W. andTaylor, C.E., “An Application of Pattern Mapping to Plane Motion,”Experimental Mechanics,30 (4),404–410 (1990).
Sirkis, J.S. andTaylor, C.E., “Displacement Pattern Matching an Element Methods for Elastic-Plastic Stress Analysis,”Experimental Mechanics,30 (1),26–33 (1990).
Sirkis, J.S., “System Response to Automated Grid Methods,”Opt. Eng.,29 (2),1485–1493 (1990).
Choa, Y.J., Sutton, M.A., Peters, W.H. and Luo, P.F., “Measurement of Three-Dimensional Displacements and Deformations in Deformable Bodies Image Processing,” Proc. 1989 SEM Spring Conf. on Exp. Mech., 139–146 (1989).
Sevenhuisjen, P.J., “Two Simple Methods for Deformation Demonstration and Measurement,” Strain,17 (1), (1981).
Sirkis, J.S. and Lim, T.J., “Accuracy Limits for Automated Grid Methods,” Proc. 1990 SEM Spring Conf. on Exp. Mech., 498–505 (1990).
Tsai, R.Y., “An Efficient and Accurate Camera Calibration Technique for 3D Machine Vision,” Comp. Vis. and Pat. Rec., 364–374 (May 1986).
Gonzales, R.C. and Wintz, P., Digital Image Processing, Addison Wesley (1987).
Lancaster, P. andSalkauskas, K., Curve and Surface Fitting: Introduction, Academic Press, London (1986).
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Sirkis, J.S., Lim, T.J. Displacement and strain measurement with automated grid methods. Experimental Mechanics 31, 382–388 (1991). https://doi.org/10.1007/BF02325997
Received:
Revised:
Issue Date:
DOI: https://doi.org/10.1007/BF02325997