Digital speckle based strain measurement system for forming limit diagram prediction

•A strain measurement system for forming limit diagram prediction is developed.•A fractionized matching algorithm for large deformation measurement is proposed.•A spline model is specially used to calculate the strain of the sheet metal in deep drawing.•The effectiveness of our system is demonstrated by evaluation experiment.•A feasible solution for investigating forming limit diagram of metal sheet is discussed.

A digital speckle based strain measurement system is developed to investigate the forming limit diagram (FLD) of sheet metal. A stochastic speckle pattern is sprayed on the surface of flat sheet metal before forming. A series of images are recorded by two cameras during the forming process. The strain field is then calculated for FLD determination, which involves the following key issues. First, to solve the problem of large deformation and strain measurement in sheet metal forming, a fractionized matching algorithm using deformation continuity of adjacent images is proposed. Second, an algorithm for three-dimensional (3-D) strain calculation is discussed, and a spline model is specially used to calculate the strain of the sheet metal in deep drawing. Third, a limit strain determination algorithm is proposed for forming limit curve (FLC) calculation by creating sections in the measured strain fields. Finally, a forming limit strain measurement system (also called Xi'an Jiaotong University Forming Limit Curve measurement system, XJTUFLC) is developed on the Visual Studio 2010 platform. With our self-developed image acquisition instrument and sheet metal bulging setup, FLD determination tests are conducted to validate the performance of the system. And the measured FLD is compared with the traditional experimental FLD as well as with that predicted by the empirical model. From the analysis of the experimental results, it can be concluded that the proposed system is feasible to measure the full-field strain during sheet metal forming processes and provides a better solution for forming limit diagram prediction.