Metal sheet forming limit determination method and forming limit determination system, and computer program

A metal sheet forming limit determination system comprising a forming test machine, a strain measuring device, and a forming limit analysis device,

• wherein:
• the forming test machine is configured to perform press forming by, while clamping a test specimen of a metal sheet with an arbitrary lattice or a pattern for strain analysis displayed on the surface between a die and a blank holder, pressing a central portion of the test specimen with a tip end of a punch that is fitted into a forming test portion in a central hole of the die through a central hole of the blank holder, and comprises an image-capturing means for capturing an image of the surface of the test specimen during the press forming;
• that
• the strain measuring device comprises an image analysis unit that analyzes images of the surface of the test specimen obtained by the image-capturing means to acquire a strain distribution, a strain measurement unit that calculates strain from the obtained strain distribution, and a strain data accumulation unit that stores strain in time series from a start of the forming to occurrence of fracture to construct a strain database;
• and that
• the forming limit analysis device comprises
• a strain extraction unit that sets, on the test specimen, an evaluation point sequence for acquiring a strain distribution in the vicinity of a portion where fracture occurs, and extracts the strain at a position of the evaluation point sequence in an arbitrary forming step from the strain database of the strain data accumulation unit,
• a strain curve approximation unit that calculates from the extracted strain an approximation strain curve corresponding to the position of the evaluation point sequence,
• an approximation error analysis unit that determines, as an approximation error, a difference between the strain at the position of the evaluation point sequence based on the calculated approximation strain curve and strain at the position of the evaluation point sequence measured by analyzing the images, and
• a forming limit determination unit that determines a forming limit based on the obtained approximation error.