Hi heongyu, only the limit curve in FLD is something that's certain and you can use to assess formability. And the best way to get the FLD limit curve is to use experiments to cover the the whole range of strain ratio for the particular steel grade and thickness in concern to generate the limit curve. The marginal zone is usually ("rule of thumb") set as 10% for steel and 8% for aluminum, and it varies from company to company. All other zones are based on user's experience/practice, and individual stampings (part dependent), and they vary from company to company. Hope this helps.
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