Fig. 1. Video extensometer principle based on 2D vision measurement.

Fig. 2. Process diagram of sub-planes: (a) calibration target with 6×22 point fixed to coincide with the specimen surface, (b) extract and save sub-pixel coordinates of every reference point from the target image, and (c) establish the mapping model between each image sub-plane and measurement plane.

Fig. 3. Schematic of coordinate location method in sub-plane.

Fig. 4. Effects of the lens distortion: (a) real chessboard image and image with lens distortion and (b) pixel deviations before and after image distortion.

Fig. 5. Relationship between sub-planes interval and mapping accuracy, obtained using lens with first distortion coefficient k1=4.1×10−8: (a) fixed interval deviation distribution in whole view range 400  mm×400  mm and (b) relationship between maximum deviation and sub-planes interval.

Fig. 6. Principle of the out-plane effects.

Fig. 7. Process of measurement experiment: (a) calibration target fixed on stretching machine, (b) image for calibration, (c) matrix of circular center, (d) displacement measurement using two methods, (e) 81 pixels × 81 pixels correlation subset, and (f) gauge length measurement.

Fig. 8. Comparative results for calibration accuracy using the two methods.