Extended digital image correlation(X-DIC) method is one novel test method in experimental mechanics.In this paper,the principle of the X-DIC method was introduced in detail.A selection scheme of the initial value of Newton iteration method was proposed when Newton iteration method was applied to solve the partial differential equations.This scheme could make the X-DIC method suitable for the large deformation measurement and avoid the non-convergence phenomenon effectively.The performance of the X-DIC method was verified by simulated images.Since the pixel point with the maximum absolute error occurred mainly at the corner or on the interface of the region of interest(ROI,region used for correlation calculation),measured deformation of the core area(area surrounding the center point of the ROI with a smaller size) was taken as the reliable measured value.The measurement accuracy of the X-DIC method could be improved greatly by using the core area.Combined with a long-distance microscope,the X-DIC method could be used in the deformation measurement of the micro-region.Zero deformation experiment was done to test the precision of the measurement system.Then,the X-DIC method was applied to measure the micro-region deformation of the specimen with a crack.Test value was proved to be in accordance with the actual deformation,showing that the X-DIC method is suitable for the research of microscale mechanical behavior of materials.
