Further experimental verification of a proposed localized necking criterion

An approach to determine the forming limit diagram (FLD) using the concept of inflection point in the major strain rate curve was proposed earlier by the authors [1]. Once the strain history of the element (either grid or speckle) from the localized site is obtained, the localized necking occurs when an inflection point in the major strain rate curve, or, maximum point in the second order of derivative of major strain, (epsilon(1))(max), is reached. Experimental verification with various strain paths such as uniaxial tensile, biaxial stretching, and plane strain was carried out as confirmation of this criterion [1]. In this work, further validations are carried out with the following tests: (1) a roughened punch with sand paper over the dome surface for the baiaxial tension test, which produces a path far from dry condition and close to plane strain; (2) a punch stretching of a specimen with a central hole, which produces a path between the shear path and uniaxial tensile path; and (3) specimens with offset notches which lead to shear instability. The generation of FLD with proposed criterion demonstrates very little subjectivity. Also, this approach presents a definite numerical and physical meaning that explains the observation of typical events in major forming processes.