Research On Measurement Of Mechanical Parameters And Consistency Analysis By Digital Image Correlation Method

With the development of science and technology in various fields,new materials emerge one after another,thus testing and obtaining their corresponding mechanical parameters are of great importance.Elastic modulus,Poisson’s ratio and other basic mechanical parameters of materials are obtained through testing.The correctness and consistency of their numerical values bring very important reference value for researchers to design and select materials.Among many materials,thin film material has been widely used in aerospace,electromagnetic and other fields.However,due to its thin and soft quality,traditional contact measurement methods such as sticking strain gauge and extensiometer are no longer applicable.Therefore,Non-contact measurement method--Digital Image Correlation(DIC)plays an important role in the measurement of such material parameters due to its advantages of simple optical path,easy adjustment of measurement field of view and low environmental requirements.Although the research of DIC method in parameter selection,speckle production,correlation function selection and other aspects has been quite in-depth,the research on the consistency of results in high-precision measurement is still insufficient.Therefore,in this paper,the consistency of digital image correlation measurement results was firstly studied,and 35 groups of polycarbonate four-point bending beams were bent,and the out-ofplane displacement measurement results were verified by electronic speckle interference(ESPI)measurement technology tracing to laser wavelength.The results show that under standard test conditions,when the camera resolution is 2048 × 2048 pixels,the strain test error of 3D-DIC is below50 με,and when the deformation is greater than 1000 με,the standard deviation is always below 0.05.In the study of displacement results,the displacement field of Y direction is stable and consistent.The metrological verification results show that under the condition of the specified digital speckle pattern,standard operation steps and calculation parameters,the cantilever beam is loaded by an electric displacement meter,and the measurement error of the 3D-DIC system does not exceed0.25 μm when the displacement range of the cantilever beam is 0～10 μm and the field of view is60 mm.The above results show that the accuracy of digital image correlation method in deformation test can be guaranteed by standardizing the operation steps and calculation methods,and the results also have good consistency.After the consistency and correctness of the measurement results by digital image correlation method were verified,the consistency of the film materials including space film structure and YBCO superconducting strip was tested by 3D-DIC system.As for the mechanical properties of spatial structure thin-film,the elastic modulus,fracture displacement and fracture load of rectangular thinfilm with 5:1 aspect ratio were obtained in several experiments.It was found that the standard deviations of the three experimental results were all below 0.05,which had a good consistency.At the same time,for the four rectangular films with the aspect ratio of 4:1,3:1,2.5:1 and 2:1,it was found that the specimens with the aspect ratio of 2:1 had the largest and most folds.The larger the aspect ratio was,the fewer folds and the fold amplitude also decreased.When the aspect ratio was greater than 4:1,there was almost no fold phenomenon.As the stretching continues,the number and area of folds produced by the film almost no longer change,but under the influence of tension,the deformation of the film will continue to increase,mainly manifested as the specimen gradually elongates,and the edge shrinks.When the load on the film exceeds the ultimate bearing capacity,it will fracture.The folds of dumbbell thin film samples gather at both ends of the specimen.When the ultimate load is reached,the fracture usually occurs at the place where the specimen size changes,which is usually the result of stress concentration.Finally,uniaxial tensile test was carried out on the strip processed by YBCO superconducting film.The standard deviation of the elastic modulus and poisson’s ratio were both less than 0.15 under multiple experiments,indicating that the test results were basically consistent.When the tensile load is greater than 0.25 k N,that is,it exceeds the elastic range,the specimen has plastic deformation.According to the full-field strain distribution cloud diagram of the direction of Y at the plastic stage,the plastic development in the middle part of the specimen is slower than that on the two sides,and the final warping occurs in the middle part of the specimen.The consistency of the measurement results of digital image correlation method get further verified in this article,and the advantage and value of DIC is demonstrated in the area of testing thinfilm’s mechanical parameters,which makes corresponding efforts to further expand the application field of DIC method.