Fig. 1. Mesh division in ROI

Fig. 2. Calculation for three types of methods. (a) Mesh without crack; (b) mesh of crack face; (c) mesh of crack tip

Fig. 3. Schematic diagrams of three kinds of smoothing calculation points. (a) Points far from crack; (b) points near crack face; (c) points near crack tip

Fig. 4. Simulated images with crack. (a) Reference image; (b) deformed image

Fig. 5. RMSE of displacement field U and strain field U_x versus element/subset size. (a) Displacement field U; (b) strain field U_x

Fig. 6. Comparison between theoretical solution curve and optimum solution curves of strain U_x obtained by different methods on lines. (a) Line 1; (b) line 2

Fig. 7. Relationship between RMSE of strain field U_x and Gaussian noise level for three methods

Fig. 8. Reference image and deformed image. (a) Reference image; (b) deformed image

Fig. 9. Track tips under an optical microscope. (a) 100×; (b) 200×

Fig. 10. Displacement field U and strain field U_x calculated by HELDIC method. (a) Displacement field U; (b) strain field U_x

Fig. 11. Comparison of strain field U_x at lines 1 and 2 obtained by three different methods