The Application Of Digital Image Correlation Method In Strain Measurement

Digital image correlation(DIC)is a kind of optical measurement method.It uses the method of image matching to analyze the speckle pattern before and after the deformation of the object surface,tracks the movement of the geometric points on the speckle image,and obtains the displacement field.DIC method has many successful application experience in plane issue,and has been applied more and more in relative engineering deformation measurement.This paper attempts to extend the DIC method to the three axis test of rock band confining pressure.For 3D problems,there is no mature technique for strain measurement based on DIC technology at present.In view of this situation,a new method is proposed to transform the cylindrical speckle image into two-dimensional speckle pattern image,and the mature two-dimensional speckle pattern recognition technique is used to solve the deformation measurement of rock three axis test.That is try to use the binocular vision testing principle of double cameras to measure the three-dimensional coordinates of the speckle on the surface of the cylindrical specimen before and after deformation.The speckle image of each time is expanded into two-dimensional speckle image,and the surface strain of the specimen is identified.Then the strain is mapped to the surface of the cylindrical specimen by the mapping method.In this paper,3D surface measurement,camera calibration,lens distortion correction method and image mapping are studied.At the same time,five important aspects of the two-dimensional DIC algorithm are comprehensively studied,including displacement characterization model,correlation coefficient,search method,sub pixel solution method and strain calculation method.The method based on displacement field to calculate the strain field,through the comparative analysis,we found that two-dimensional quadratic function do as fitting function was obviously more precise than that of a two-dimensional function.And also it can take the calculation speed and calculation precision into account.Based on the above work,this paper attempts to establish the relevant modules,using these modules for experimental research,and completed the camera calibration,lens distortion calibration and so on.In the later stage,the two-dimensional surface test was carried out,that is,on the uniaxial tensile specimen,the deformation in the tensile process was measured by extensometer and DIC method.The experimental results are in the correct trend,but the error is great.The follow-up work can be carried out in the direction of improving the measurement accuracy.