Improved Extended Digital Image Correlation Method And Its Application

Extended digital image correlation method is one novel test method in the sphereof experimental mechanics. This method has two outstanding advantages. Firstly, thesmoothing of the displacement field of the subset is not necessary. Secondly, thedisplacement field of the crack region can be directly measured. Nevertheless thetechnical link of this method is not mature and the related accuracy still remains to befurther enhanced. The major work of this paper deals with the improvement of thistechnical link, investigating those factors that affect the accuracy of measurement, andfinally extends the improved extended digital image correlation method to the actualtest of the mechanical properties of materials.The extended image correlation method may encounter the failure of convergencein the N-R iteration method. To address this problem, we put forward a scheme ofchoosing the N-R initial value and improve the interpolation scheme with points nearthe crack, leading to the avoidance of non-convergence and the advancement of theaccuracy of extended digital image correlation method. The relationship between theunit numbers of subset, the size of subset, the gray value distribution and the accuracyof the extended image correlation method is also discussed in this paper. Conclusionsare as follows: we can obtain an exact displacement field when the extended imagecorrelation method applies the scheme which takes the core area (central area of thesubset) from one subset as a unit; for the actual deformation that is in consistent withthe assumed displacement model, the selection of large-size subset and small-sizecore area can further advance the accuracy of the measured displacement field; smallspeckle size and big gray value contrast can also help enhance the accuracy.In this paper, the reliability of the improved extended image correlation methodused in the measurement of continuous deformation and slide discontinuousdeformation is verified by the simulated speckle image. Conclusions are as follows:when the improved extended digital image correlation method applies the schemewhich takes the core area from one subset as a unit, we can obtain a displacementfield with the magnitude of absolute error of about103pixel and a strain field with themagnitude of absolute error of about200με. We also use three-point bending testand the interface opening test in order to verify the reliability of this method. We apply this improved extended image correlation method to measure thefracture behavior of polycarbonate (PC). The elastic constants, the value of J-integraland the plane strain fracture toughness of polycarbonate are measured. Experimentalresults show that the improved extended image correlation method is a novelexperimental mechanical test method which has a wide application.