High Order Time Domain DIC Algorithm And Application

Digital image correlation method is a simple,practical and efficient modern optical measurement mechanics method for measuring full-field displacement and strain.This method has many advantages,such as simple optical path,good adaptability to environment,wide measurement range and high degree of automation,and has been widely used in experimental mechanics and other related scientific research fields.The current digital image correlation method still has some deficiencies in solving accuracy under extreme test conditions such as illumination intensity,air humidity,atmospheric instability,dynamic load and impact load applied in the experiment,which cannot meet the actual requirements of the corresponding test.In order to make the digital image correlation method can collect from extreme test under the condition of the low quality of speckle image resolution and high precision of deformation field,based on the nonlinear optical flow equation of higher order temporal DIC algorithm was presented,and through simulated speckle field analysis the precision of the algorithm,the algorithm is applied to the rock Brazilian disc fracturing and fracture experiments.The research results are as follows:(1)A high-order time-domain DIC algorithm based on nonlinear optical flow equation was proposed to solve the problem of high-precision deformation field analysis of low-quality images.In this algorithm,the optical flow equation between the pixels of speckle pattern before and after deformation is established by nonlinear function,and the time continuity is established by integral.The iterative solution of deformation parameters is obtained by using the least square iterative algorithm,and its validity is verified.High-order time-domain DIC algorithm can effectively inhibit the low quality image of environmental noise impact on the accuracy of measurement,to solve practical test under the condition of environment light intensity change,imaging system instability and loading process caused by the change in the contrast effect speckle image quality,the traditional algorithms can’t from low quality problem of image resolution and high precision deformation field.(2)The test accuracy of the traditional DIC method and the new high-order time-domain DIC method was compared and analyzed through the Brazilian disc splitting test.The solution of the speckle image and the analysis of the deformation field show that the displacement field solved by the high-order DIC algorithm is smoothing,and the strain measured by the high-order DIC algorithm is more stable.By comparing the theoretical value of strain solution with the calculation error obtained by the two methods in the experiment,the calculation error of the traditional DIC algorithm is three times that of the high-order time-domain DIC algorithm.(3)The new high-order time-domain DIC algorithm was used to carry out the fracture experiments of Ⅰ type and Ⅰ-Ⅱ type cracks in rock.In the experiment,the speckle field at the crack tip of the Ⅰ type and the Ⅰ-Ⅱ type was observed by CCD camera,and the high-precision displacement field was analyzed by using the high-order time-domain DIC algorithm.Based on the displacement field data of the crack tip,the stress intensity factors of the I type and the Ⅰ-Ⅱtype were calculated.The evolution characteristics of deformation field and stress intensity factor at the crack tip of Ⅰ type and Ⅰ-Ⅱ type composite cracks during crack initiation,stable propagation(subcritical propagation)and instable propagation in rock are studied..In this paper,a high-order time-domain DIC algorithm based on nonlinear optical flow equation is proposed.The time-domain DIC algorithm theory is developed,and the DIC technology is explored to analyze high-precision deformation fields from low-quality speckle images in extreme test environments.The high order time-domain DIC algorithm is used to study the deformation and fracture characteristics of rock,the initiation and propagation characteristics of rock type I and Ⅰ-Ⅱ type composite cracks,which provides a new DIC testing technique for rock mechanics experiments.There are 64 figures,33 tables and 62 references in this paper.