Research On Ultra-high Temperature Deformation Measurement Method Based On Digital Image Correlation With Laser Speckle

With the development of technology in the field of aerospace and national defense,new challenges are brought to the deformation measurement of materials and components in complex environments such as ultra-high temperature.The service temperature of hot end components such as supersonic vehicle and solid motor is even over 2000?.Whether these components can work continuously and steadily directly affects the normal operation of the whole system Therefore,it is particularly important to characterize and evaluate the mechanical properties of materials and components under ultra-high temperature environment.It is urgent to study new methods and devices for testing the mechanical properties of materials and components under ultra-high temperature environment.However,in the traditional contact deformation measurement method,it is difficult for the components to withstand a high temperature environment above 2000?.Therefore,it is extremely important to develop anon-contact ultra-high temperature deformation measurement method.Digital Image Correlation(DIC)method is a widely used non-contact deformation measurement method.It has many advantages,such as non-contact,full-field measurement,high accuracy and strong environmental adaptability,and has obvious advantages and application value in deformation measurement under ultra-high temperature environment.However,in the ultra-high temperature environment,the non-contact deformation measurement method based on DIC method still has some problems,such as high temperature speckle preparation,speckle image quality evaluation,serious speckle pattern decorrelation,high temperature background radiation,heat flow disturbance and so on,which affect the accuracy of deformation measurement.In this paper,the theoretical basis issues affecting the measurement accuracy are studied to improve the deformation measurement accuracy of DIC method in the ultra-high temperature environment.The specific contents are summarized as follows:(1)For the problem that the traditional single speckle image quality evaluation index is not suitable for the quality evaluation of laser speckle image,a global evaluation method of laser speckle image quality based on multi-factor fusion is proposed.In view of the influence of internal and external factors on the quality of laser speckle pattern,the influence of external factors such as aperture,laser power,high temperature,surface roughness of material and reflectivity of light on the characteristics of laser speckle pattern is studied.The influence rules of different factors on the quality of laser speckle pattern are obtained,which provides a theoretical basis for the selection of parameters in speckle pattern acquisition.A multi-factor fusion index that comprehensively considers the gray distribution of the speckle pattern the contrast of the speckle pattern,and the shape of the speckle particles is proposed,which overcomes the limitation that the traditional single factor evaluation index is not applicable to the quality evaluation of laser speckle patterns.The experimental results show that the multi-factor fusion index is more suitable for the quality e-valuation of laser speckle patterns.(2)For the problem of low measurement accuracy of speckle patterns with serious de-correlation,anew initial guess method of digital image correlation based on the IIC-GN algorithm for deformation measurement of speckle patterns with serious de-correlation.In view of the fact that the initial value estimation points of speckle patterns with serious de-correlation are prone to search errors in the search process,which makes the IC-GN(Inverse Compositional Gauss-Newton)algorithm fall into local optimum,IIC-GN(Inproved Inverse Compositional Gauss-Newton)algorithm can use the calculation points with high correlation coefficient to establish a model to estimate the calculation points with low correlation coefficient,which reduces the search error or search erroneous calculation points in the process of digital image correlation method search.The simulated speckle patterns with serious de-correlation were analyzed by IIC-GN algorithm and the error between the calculated strain and the theoretical strain was within 0.02%.Also,the tensile test of the rigid thermal insulating material are carried out,and the error between the elastic modulus calculated by the IIC-GN algorithm and that measured by the contact extensometer are 2.2%,while that calculated by IC-GN algorithm is 6.5%.(3)For the problem of noise in displacement field caused by heat flow disturbance,a high temperature strain measurement method by combining digital image correlation of laser speckle and improved RAN SAC smoothing algorithmIn view of the problem of high temperature speckle preparation,laser speckle is proposed as the characteristic speckle in high temperature environment,which solves the problems of easy falling off deformation and discoloration of speckle in complex environment such as high temperature.Because of the influence of heat flow disturbance,the displacement field calculated by DIC method contains a lot of noise,IRANSAC(Inverse Compositional Gauss-Newton)algorithm processes the noisy displacement field by smoothing noise reduction,and then calculates the strain information by using the displacement field after noise reduction,which improves the accuracy of the non-contact deformation measurement method in high temperature environment.The tensile test of Carbon-Carbon(C/C)composites under inert atmospheric environment at 2000? is studied.The results show that the strain curve calculated by IR-DIC method is basically consistent with that measured by contact extensometer,and is better than that calculated by LS-DIC method.Compared with the least square method,the average deviation between the strain curve Calculated by IR-DIC method and the strain curve measured by contact extensometer is reduced by 13.94%.(4)For the influence of ultra-high temperature background radiation on speckle pattern quality,a non-contact deformation measurement method based on active laser illumination and background radiation suppressing method with multi-step filtering is proposedIn view of the influence of strong background radiation on the quality of speckle pattern in ultra-high temperature environment,active laser illumination improves the intensity difference between speckle intensity and background radiation in order to obtain high quality speckle pattern.The multi-step filtering makes use of the characteristics that the laser is coherent and polarized,the linear polarizer only permits visible light with the same vibration direction as its polarization direction,and the narrow-band filter only permits visible light in the bandwidth,thus effectively suppressing the influence of strong background radiation on the quality of speckle pattern.Tensile tests of C/C composite are carried out at ultra-high temperature of 2200?,2400?,2600? and 2800?.The experimental results showe that high quality laser speckle images are obtained and the deviations between the elastic moduli obtained by the proposed method and by the high temperature contact extensometer are all less than 7%.In this paper,the research work of high accuracy deformation measurement based on DIC method in ultra-high temperature environment provides a new idea and method for deformation measurement of materials and components in ultra-high temperature environment.