Abstract
Resistance to edge cracking is an important consideration in the development of advanced high-strength steel (AHSS) grades for automobile body-in-white components. Edge cracks may occur at the stamped edges of steel sheets as they undergo forming processes. In the laboratory, edge cracking resistance is often characterized using the hole-expansion ratio, which can be measured according to ISO 16630. In this test, a circular hole is first stamped in the centre of the square sample. The stamped edges are then formed upwards using a conical punch until through-thickness edge cracks appear. The hole-expansion ratio is thus the normalized increase in radius of the hole when edge cracking occurs. Recent work has shown that for hot-dip galvanized steels the hole-expansion ratio decreases with increasing time between stamping the test specimen and performing the test. This paper provides both theoretical and experimental evidence for the role of hydrogen in this phenomenon. Small bulk concentrations of hydrogen are invariably present in hot-dip galvanized steels due to the production process. This hydrogen accumulates over time in the bulk material adjacent to the stamped edge due to trapping by crystallographic defects generated because of high plastic deformation. This accumulation of hydrogen reduces the resistance of the material to localized cracking and is thereby reflected in the time-dependence of the hole-expansion ratio.