Strain Measurement for Sheet Metal Forming Based on Close Range Photogrammetry

To improve efficiency and automation of the 3D full-field surface strain measurement for sheet metal forming, a new grid strain measuring scheme was developed based on the close-range photogrammetry technology. A Local Canny Detector algorithm was proposed for grid nodes and coded targets detection. A 10-parameters nonlinear camera model and the bundle adjustment algorithm were used to optimize the calibration parameters. A multi-epipolar constraint method was employed for grid node matching. Finally, the surface strains were calculated according to the changes of the grid sizes. To evaluate the performance of the proposed scheme, a stamping forming experiment was conducted. Experimental results show that the scheme can provide a non-contact, intuitive and effective solution for strain measurement in sheet metal forming process.