Method And Application Of Whole Field Deformation Measurement Based On Topological Similarity

The measurement of two-dimensional displacement field is the most basic task of experimental mechanics,and it is also one of the focuses and hot spots in the research of experimental methods of material mechanics.As a non-contact whole process deformation measurement method,digital image correlation method is widely used in the whole field deformation measurement of structures or materials.However,this method needs to spray speckles on the surface of the object to be tested.For the object that is not suitable or can not spray speckles on the surface,the digital image correlation method is no longer applicable.For the whole field displacement measurement of this kind of object to be tested,a new solution is urgently needed.For this reason,this paper proposes a whole field deformation measurement method based on a small number of gray-scale markers,which is called topological similarity method,and uses a small number of markers instead of speckle to estimate the whole field displacement.The main research contents and conclusions of this paper are as follows:?1?The basic principle and algorithm flow of topological similarity method,which is a full field displacement estimation algorithm based on the feature vector of the marker points,uses the feature vector to encode the relative position of the adjacent marker points.The feature vectors based on random neighborhood topological arrangement are used to match the mark points in the reference image and the deformed image one by one.The sub-pixel central coordinates of the matched mark points are calculated by the radial symmetry method,and the relative displacement between the matched mark points is calculated.Then the whole field displacement is obtained by biharmonic spline interpolation.Finally,the displacement anomaly is detected by the median detection.The value is eliminated to estimate the displacement of the whole field accurately.?2?The error analysis of topological similarity displacement estimation method is carried out from two aspects of location error and interpolation error.The results show that the maximum location error and interpolation error in the calculation process of this method are 10^-3pixels and 0.027 pixels,which meet the requirements of optical measurement method for measurement error.The recommended parameters of marker radius and density are determined by computer simulation test.The recommended radius of marker is 6 pixels,and the recommended density is 1 point/64×64 square pixels.?3?The topological similarity method is used to measure the full field displacement of a beam with equal strength in the three-point bending experiment,and the results are compared with the results measured by digital image correlation method,which verifies the effectiveness of the topological similarity method in measuring the full field displacement.The comparison results further show that compared with the digital image correlation method with fixed subset size of 23×23 pixels and step size of 5 pixels,the topological phase The CPU computing time of similarity method is reduced by72%.?4?The elastic modulus and Poisson's ratio of agar were measured by topological similarity method and uniaxial compression test.The results showed that the elastic modulus and Poisson's ratio of agar were 0.787 MPa and 0.855 respectively.The above research and experimental results show that the topological similarity method combines the radial symmetry center positioning method,the feature vector matching based on topological arrangement and the two-dimensional biharmonic spline interpolation to achieve the sub-pixel level accuracy of the full field displacement measurement,which is suitable for the full field displacement estimation of structures or materials that cannot be sprayed with speckle,and has important engineering practical significance.