Research On The Key Technology Of Digital Image Correlation

Digital Image Correlation(DIC)is a non-contact full-field optical measurement method based on digital image processing and numerical analysis.It was first proposed and developed in the 1980 s.Compared with other popular full-field optical measurement methods(such as photo elasticity,laser interferometry,fringe projection profilometry,etc.),DIC has some advantages,for example the white light can be used as the illumination,and the simultaneous measurement of full field displacement,profile,deformation and strain can be achieved by this method.Due to these advantages,DIC has been widely used in the fields of basic manufacturing,aerospace,military etc.3D digital image correlation(3D-DIC),which plays an important role in the industrial field,generally uses two cameras to acquire images of different angles of view of the object,and The combination of traditional DIC method and binocular vision enables 3D measurement.The existing 3D-DIC technology has the following problems:(1)the decorrelation problem,the de-correlation is the phenomenon of matching failure that occurs when the image sub-facet is incorrectly matched,and commonly the de-correlation may be caused by the defocusing camera,or the image is over or under exposed;(2)The problem of insufficient spatial resolution,when measuring a large field of view,the resolution of the image is usually positively correlated with the resolution and accuracy of the measurement,and the resolution of the camera may be subject to cost or even diffraction effects.The limitation is that it is difficult to ensure the accuracy to meet the measurement requirements while measuring the large field of view;(3)the whole field measurement problem of the object with complex shape or large curvature,the measurement can be carried out only in the area covered by the both angle of views simultaneously,while the acquired image is limited by the viewing angle,the commonly used dual camera 3D-DIC is difficult to complete the full surface image acquisition of the object with large curvature or complex topography.This paper gives a detailed description of the basic theory of 3D-DIC,and introduces the multi-camera technology of computer vision into the digital image correlation method to solve the problems above.The main contents include:(1)Combining multi-camera technology with digital image correlation,providing images of additional viewing angles through multi-camera technology as redundant matching data to reduce the possibility of de-correlation and eliminate matching errors to some extent.A set of experiments were performed to verify the reliability of the method.(2)Using the method of projecting multi-camera acquired image data onto the same plane,also called image correction method,to unify the images of a non-redundant large field of view images in a global coordinate system,thereby avoiding the use of data stitching.A surround measurement experiment was performed to verify the feasibility of the method.(3)Three different 3D-DIC systems based on a single color camera are proposed,which can acquire two and three views of the object image for correlation calculation,and use the three systems for the measurement of displacement,shape and strain.The ability has been experimentally verified.(4)Extending the application of 3D-DIC method to the field of automobile safety testing,using high-speed digital image correlation method to carry out pedestrian head safety test in human-car crash,and discussing the feasibility and prospect of multi-camera system for such research.