Method for identifying necking limit strain of metal sheet

A method for identifying a necking limit strain of a metal sheet, the method identifying a relation between a necking limit strain, at which necking occurs at a sheet edge of a metal sheet by imparting tensile deformation, and a strain gradient, and the method comprising:

• a strain distribution measurement step of measuring, for two or more types of sheet specimens having a notch geometry for which a portion of a sheet edge is notched inward and having a different strain gradient in a tensile orthogonal direction that is orthogonal to a tensile direction when tensile deformation is imparted to a notch root of the notch geometry, distribution of strain of the notch root in the tensile orthogonal direction in a tensile deformation process;
• a strain increment ratio/strain gradient acquisition step of obtaining, for each of the sheet specimens, a strain increment ratio of the notch root in the tensile deformation process and a strain gradient in the tensile orthogonal direction, from the distribution of the strain in the tensile orthogonal direction measured at the strain distribution measurement step;
• a necking limit strain acquisition step of obtaining, for each of the sheet specimens, a necking limit strain that is a strain at a time of occurrence of necking in the notch root based on the strain increment ratio in the tensile deformation process obtained at the strain increment ratio/strain gradient acquisition step;
• and
• a necking limit strain identification step of identifying the necking limit strain as a function of the strain gradient from a relation between the necking limit strain obtained for each of the sheet specimens and the strain gradient at a time of the necking limit strain out of the strain gradients in the tensile deformation process obtained at the strain increment ratio/strain gradient acquisition step.