Abstract: To achieve limit strains measurement of metal sheet forming, a dynamic deformation measurement method is proposed and implemented based on digital image correlation. The applied key technologies are investigated. To solve the problem of large deformation and strain measurement in metal sheet forming, a fractionized matching algorithm based on deformation continuity of adjacent image sequences is proposed, a successful image matching results even in large deformation situation is provided. The algorithms for three-dimensional coordinates reconstruction and strain calculation are discussed separately. Based on section line of the measured strain field, a fitting algorithm for forming limit curve is proposed. Based on these algorithms and the Visual Studio 2010 platform, a forming limit strains measurement system is developed. With our self-developed image acquisition setup and a sheet metal forming machine, a steel cupping test is conducted to validate the performance of our method. Experimental results demonstrate that the proposed method is applicable to the measurement of 3D deformation and limit strain of the sheet metal forming. Compared with the traditional measurement methods, the method can be more comprehensive and more intuitive to achieve the strain measurements during the whole forming processes. It provides a good solution for forming limit prediction.

Key words: Digital image correlation, Limit strain, Sheet metal forming