Digital Image Correlation Method Based On Single Camera And Its Applications

In the modern photomechanics,digital image correlation(DIC)method is widely used in experimental mechanics and other interdisciplinary research fields due to their non-contact full-field measurement and good environmental adaptability.At present,the three-dimensional digital image correlation(3D-DIC)method used in the field of industrial inspection is still mainly based on dual-camera imaging system.When measuring objects in a narrow area,there is a problem that the baseline length between two cameras is insufficient and the system cannot be placed;and when applying to dynamic measurement,there is a problem of synchronization between two cameras.Aiming at the limitation of traditional dual-camera system,this paper proposes a 3D-DIC measurement method based on a single camera and applies it to the non-destructive inspection of pipelines and the low-velocity impact of honeycomb sandwich panels.The major achievements of this dissertation are as follows:1.Combine a single camera with two bi-prisms to form the dual bi-prism stereo camera(DBSC)measurement system.By studying the imaging principle,the optical model of the 3D-DIC based on single camera is established,and the adjustment range of the field-of-view(FOV)is given.In addition,based on the measurement results of traditional dual-camera system,the experiments of the profiles reconstruction and rigid body displacement show that the maximum relative error of the two methods is less than 1%,which verifies the accuracy of the proposed method.2.The 3D-DIC based on single camera is applied to the non-destructive detection of the pipelines,and the defect morphology on the surface of the pipeline is reconstructed.The deepest surface deformation measured accounted for 16.5% of the pipeline diameter.At the same time,the coaxial light source is used for illumination to detect defects inside the pipeline with a length of 750 mm and a diameter of 40 mm.According to the measurement,the defect of the weld in the pipeline has a crescent shape,which is about 86 mm from the bottom,and the length is about 48.2mm,and the maximum width is about 4.9mm.In addition,a method based on strain measurement is used to determine the location of the micro-crack in the pipeline,and the direction of the crack in the pipeline is predicted according to the normal strain distribution in the x direction of the inner surface under different loads.3.In the low-velocity impact experiments of the honeycomb sandwich panels,the variation of the energy absorption rate with the impact energy and the number of impacts is analyzed by studying the energy changes after single and multiple impacts at different energy levels.After the impacts,using the proposed method to reconstruct the 3D profiles of the damaged specimens,and obtain the damage parameters of the honeycomb sandwich panels under single and multiple impacts of different energies.And then,the damage of the honeycomb sandwich panels is characterized by the damaged depth,and the energy absorption process is analyzed.In the experiments of compression after impact(CAI),this method is used to measure the full-field strain distribution on the two surfaces of the specimens.The results show that the stress concentration at the impact point eventually leads to the compression failure of the damaged specimens.After experimenting on all specimens,three main compression failure forms are obtained: bending failure,local bending and adhesive failure,and adhesive delamination.4.The proposed method is also used to measure the transient damage of the non-impacted surfaces of the honeycomb sandwich panels under the low-velocity impact.By collecting dynamic images of the non-impacted surfaces during impact process and combining digital image correlation calculations,it is obtained that the out-of-plane displacement shows a non-linear increasing trend with time.Combined with the displacement cloud diagrams of two times impacts of 3kg weight,it is further obtained that the out-of-plane displacement is centered on the impact point and spreads elliptically to the surrounding.In addition,three identical moments are selected in the two times impacts,and the spatial distribution of out-of-plane displacement of the impact point is detected.Meanwhile,the damaged width of about 10 mm is measured near the impact point.Finally,by comparing the changes of the out-of-plane displacement during the second impact,the influence of the impact on the deformation of the specimen after the penetration of the specimen is analyzed.In this paper,a 3D-DIC method consisting of a single camera and two bi-prisms and its application are studied deeply.By establishing a physical model,principle analysis and experimental research,the DBSC system with high measurement accuracy is obtained,it applies in terms of non-destructive testing and low-velocity impact of composite materials.It provides a reliable theoretical basis and experimental methods for engineering applications,and it has good application prospects.