Displacement Measurement Method Based On Digital Image Correlation

The speckle image used in digital image correlation method is the only source data for DIC method.The speckle distribution and imaging quality directly affect the measurement accuracy of the method.The relationship between speckle texture and measurement accuracy was analyzed to produce an effective evaluation methods.Furthermore,the influence of lens distortion on measurement accuracy was analyzed through simulation experiments.The effective distortion correction method was verified to improve the measurement accuracy.Firstly,the relationship between speckle patterns with different texture distribution and measurement results is analyzed.The gray level co-occurrence matrix was used to evaluate the speckle.The method was verified by simulated displacement experiments using the speckle images with different texture distribution,different brightness and uneven illumination.Secondly,the influence of lens distortion on the measurement results was analyzed.By comparing the measurement results of undistorted speckle pattern with the patterns added different kinds of distortion,it was found that the influence of tangential distortion is greater than that of second-order radial distortion.When the two kinds of distortion combine,the measurement accuracy was better than the case where only single distortion exits.However,there is still a significant error relative to the case of no distortion.Thus,the distortion correction models which are suitable for practice application was studied.Finally,the experimental device which was capable of displacement control and image acquisition was built up.The reliability of the device was verified by repetitive experiments.The evaluation method was further verified by testing the horizontal displacement of speckle images under different exposure times.It was verified that the problem of metal surface reflection and uneven illumination could be solved to a certain extent by adding polarizer on the lens.In this case,the measurement accuracy was guaranteed.According to the analysis of the distortion model in this paper,the distortion mainly occurs in the area far away from the image center.Thus,the speckle image is divided into three parts in the vertical direction for the DIC displacement measurement experiment.The result analysis are carried out for the upper,middle and lower parts respectively.According to the designed experiments,the proposed evaluation method for speckle pattern was verified.Furthermore,the influence of lens distortion on the experimental results was analyzed.The corresponding experimental data results were demonstrated.