Mechanical Properties Testing Technology Based On Three-dimensional Microscopic Digital Image Correlation(DIC)

With the continuous development of material technology and precision manufacturing industry,the structure of various devices tends to be miniaturized and the mechanical properties of materials of devices directly determine the reliability of structural design.It is of great significance to measure the mechanical properties of materials in millimeter and micrometer.Traditional testing methods are usually contact methods which are restricted by factors such as contact interference,small measurement range,single measurement point,low testing efficiency and cannot meet the full-field requirements of millimeter and micrometer specimen.As an important branch of experimental mechanics,testing technology of modern optical mechanical property is widely used in the field of mechanical property measurement due to its advantages of non-contact,full field measurement,high sensitivity and non-destructive.Among them,the three-dimensional(3D)microscopic digital image correlation(DIC)mechanical properties testing technology has the advantages of high resolution,low test environment requirements,and full-field measurement.It can perform non-contact high-precision fullfield measurement of mechanical parameters of materials in millimeter and micrometer.However,there are some problems and challenges in the application of three-dimensional microscopic image related mechanical property testing technology.In this thesis,several key issues are studied in depth and the main achievements are as follows:(1)Aiming at the problem of millimeter and micrometer level mechanical properties testing of materials in engineering applications,a three-dimensional microscopic image correlation(DIC)testing technology based on stereo light microscope is studied.The three-dimensional microscopic digital image correlation technology is used to measure the deformation of the specimen during the direct stretching.Furthermore,we can invert the material mechanical properties of millimeter and micrometer scale specimen.(2)The 3D microscopic imaging system based on SLM has the characteristics of small depth of field,complex optical path,large distortion and dispersion and it is difficult for traditional calibration methods to accurately obtain imaging model parameters.This thesis proposes a method based on non-coplanar feature points and the weighted radial constraint calibration method improves the calibration accuracy of the imaging model parameters.In this method,different weight values were given to the feature points and the weighted objective function was established by using the radial constraint relationship between the object point and the phase point.)The optimal solution of imaging model parameters is obtained by iterative computation and the reprojection error of the calibration result is less than 0.2pixel.The reprojection error of the characteristic points is used to construct the distortion deviation surface of the microscopic imaging system in the u and v directions of distortion and deviation surface to replace the traditional nonlinear distortion model.Finally,the validity of the proposed calibration method is verified by the shape and displacement experiments.(3)This thesis analyzes the influence of DIC method with different order shape functions on the calculation accuracy through numerical experiments of rigid body translation,rotation,uniform and non-uniform deformation that simulate the speckle pattern.According to the characteristics of large local deformation in the process of uniaxial biaxial stretching of the specimen,the DIC method of combining shape function to approximate the deformation of the sub-region is used to calculate the full-field deformation,which effectively avoids the phenomenon of local under-matching or overmatching caused by the deformation of a single shape function representing the sub-region.Matching phenomenon.On the basis of limit constraints,using the parallax surface to determine the estimated matching points of the points to be calculated can reduce the search range of matching points.(4)Aiming at the problem of clamping and micro-displacement loading of small-scale specimens,a micro-stretching machine based on worm and worm reverse ball screw transmission power is designed.The worm gear multi-stage reduction mechanism provides a great reduction ratio,which makes the s Tab stretching speed of the microstretching machine reach 0.07 ?m/s.The entire stretching process of the specimen in the test can be obtained and it is convenient to calculate the full-field deformation and strain of the specimen by DIC method.The transmission mode of the reverse ball screw can implement the true in-situ stretching of the specimen and it can effectively avoid the phenomenon of root fracture when one end of the specimen is fixed and the other end is stretched.Aiming at the mismatched points in the matching of corresponding points,a loop closure test method is used to eliminate gross error data(5)The software interface of DIC of testing system based on 3D microscopic images was written by QT platform.Through this interface,functions such as image and data acquisition,stretching machine control,system calibration,microscopic image correlation calculation,three-dimensional coordinate calculation and display,and inversion of material elastic modulus,Poisson's ratio and other mechanical properties parameters can be implemented.The built system is used to test the standard steel ball diameter,standard step height and in-situ tensile test of pure copper specimen.These data are compared with the measurement results of the commercial software DANTEC Q400 and the results of the experiments also verified that the test system can meet the test requirements of the mechanical properties of millimeter and micrometer scale specimen of materials.