Effect of manufacturing-induced microscopic surface defects on crack initiation and fatigue mechanisms in hot-dip galvanized steel

Abstract

When designing against fatigue, the surface condition of steel is of utmost importance. Since hot-dip galvanizing is a surface treatment primarily aimed at preventing corrosion, it is critical to assess its influence on the fatigue properties of steel. Given the challenges in producing large structural galvanized steel without imperfections, it is crucial to understand the influence of manufacturing-induced microscopic surface defects on crack initiation and fatigue mechanisms in hot-dip galvanized steel. In this study, to gain such insights, tension-tension fatigue testing was conducted on hot-dip galvanized steel, followed by a detailed analysis of the crack initiation sites to determine any correlation with manufacturing-induced microscopic surface defects. It was observed that under low-cycle fatigue, pre-existing surface defects did not appear to have influenced fatigue mechanisms. However, under high-cycle fatigue, crack initiation sites exhibited evidence of pre-existing manufacturing induced defects which were significantly smaller than the total crack initiation area. This indicates that the defects could not immediately penetrate the substrate but expanded within the layer and only penetrated the substrate after reaching a critical size. Therefore, it is discovered that while manufacturing-induced defects may be challenging to eradicate entirely, those below a threshold size may not immediately evolve into a stable fatigue crack. Hence, if manufacturing induced defects are kept below a critical size, they may not significantly influence the transition of stage I crack into a stable stage II crack.