| Column is a very classical engineering configuration,widely used in biomedical,aerospace,civil engineering and other fields.It is of great significance to solve the problems in the full-field three-dimensional high-precision deformation measurement of the column surface to realize the full-surface deformation measurement of the specimen.Fortunately,the development of the digital image correlation(DIC)measurement method provides a good solution for high-precision full-field deformation measurement and has become an indispensable measurement method in the field of experimental mechanics currently.With the progress of science and technology,higher requirements for measurement techniques have been put forward.The single field of view test can no longer meet the measurement needs of some large structures or non-planar specimens with large curvature.How to flexibly use DIC method to extend the measured field of view with high-precision has become an urgent problem to be solved nowadays.The current study shows that the multi-camera DIC measurement method has great potential in solving the problem of high-precision measurement of three-dimensional deformation of column surface,but there are still many problems in the engineering application.Therefore,this paper presents an in-depth study of some key issues in the three-dimensional high-precision deformation measurement of column surface using the multi-camera digital image correlation measurement technique,covering the following aspects:(1)To ensure that the multi-camera system can work stably and synchronously,in-depth researches have been done in terms of the camera layout,hardware configuration and system control.By using the direct I/O modeand bypassing the system cache scheduling the data can be written directly to the solid-state disk array,achieving a high speed write of 4500 Mb/s data,which effectively improves the integration and operability of the multi-camera measurement system.Based on this storage mode,a multi-camera system consisting of a high-performance workstation and 18 5-megapixel industrial cameras is established,which to achieve stable synchronous acquisition at full-frame 50 Hz,and a specific system configuration given.(2)By analyzing the influence of camera pose on coordinate reconstruction,an optimization method of global external parameters calibration accuracy is proposed,which greatly improves the reconstruction accuracy of the control point coordinate system.The global external parameters can be modified by the multi-camera system based on calibration via the closed-loop system.However,the sub-systems of multi-camera system based on targets are mutually independent,which imposes higher requirements for the accuracy of global external parameters.The shooting poses reconstructed from the targets is optimized through the relationship between the polar constraint and the essential matrix,which improves the relative accuracy of the reconstruction.Further based on the influence of the number of shooting poses on the internal parameters of the camera,an optimized reconstructed pose is proposed,which makes the internal parameters of camera converge with very few number of images and improves the reconstruction efficiency.Based on the high-precision translation platform,a scale assignment method of control point coordinate system is proposed,which is free from the restriction of ruler machining accuracy and improves the absolute accuracy of the global external parameters coordinate system.(3)A multi-camera system accuracy evaluation of method is proposed based on the orthogonality of rotation matrix and the residuals of the control points after alignment registration,which can evaluate the accuracy of the multi-camera system in the system calibration stage and ensure the reliability of the system.Using the orthogonal property of the rotation matrix,the determinant of the rotation matrix without orthogonalization after coordinate transformation is taken to evaluate the calibration accuracy of the global external parameters,and then the global accuracy of the multi-camera system is evaluated based on the residuals of the control points after coordinate transformation.The error sources can be identified by the evalutation results,which helps to identify the problems and make adjustments in time before the test.(4)In order to make the physical meaning of the column surface deformation data clearer,the processing method of the 3D full-field deformation data of column surface is proposed based on orthogonal curvilinear coordinate system.Taking the full full-field column surface deformation measurement test of carrier rocket capsule as an example,the measured data is transferred to the column coordinate system for processing and analysis.It is necessary to extract the central axis of the column in Cartesian coordinate system to converting a column at any position in the cartesian coordinate system to an axis-symmetric form.Two methods of axis extraction are given by geometric method.In addition to transferring the geometric morphology data and displacement data into the column coordinate system,the strain results in the column coordinate system are also solved according to the geometric equation of the spatial axisymmetric problem.By expanding the column surface deformation field data into a twodimensional plane along the annulus,the full-field deformation data are made more intuitive and better positioned to facilitate the estimation of the diffusion angle of the concentrated force and the dimensions of the buckling region.In this paper,several key issues are investigated in terms of the requirement for full-field3 D high-precision deformation measurements on column surfaces.Firstly,the key configuration problems in the multi-camera system are solved,and a stable multi-camera system is established.Secondly,the optimization method and accuracy evaluation method of multi-camera system are proposed.Furthermore,the expression form of the column surface deformation data in the orthogonal curve coordinate system is given,which makes the physical meaning of the deformation data more clearer.The proposed system is validated by several practical tests,and the test results prove that the system is an effective means to achieve highprecision three-dimensional deformation measurement on column surface. |
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