Variability of sheet formability and formability testing

Abstract

Testing of sheet metal formability has been a long-standing challenge because variability in test procedures and testing machines can mask material variations. In spite of the importance of such testing for manufacturing, few standards exist. The Ohio State University formability test (OSUFT) has shown promising improvements in scatter and speed compared with the limiting dome height (LDH) test, but few laboratory-to-laboratory scatter data have been available. The scatter in formability testing was investigated using several testing protocols with three ferritic stainless steel alloys: Type 409, Type 439, and a new 18Cr–Cb alloy. The sensitivity of the OSUFT results to punch speed, lubrication, specimen width, hold-down force, and sheet thickness was measured and procedures were defined to maximize the test effectiveness. Tensile and OSUFT testing were carried out independently at three laboratories using more than 45 coils of material. Laboratory-to-laboratory scatter of 0.4–2.2% of measured formability is less than that for tensile testing and much less than that reported for LDH testing. The scatter among laboratories for the OSUFT is approximately equal to test-to-test scatter at one laboratory. Coil-to-coil variations are typically greater than the observed test-to-test scatter, particularly in the transverse direction.