Failure Analysis of Fatigue Failed M20 Class 8.8 Galvanized Steel Bolt

Abstract

An M20 Class 8.8 galvanized steel bolt (derrick bolt) was found to fail after five years of service in an offshore drilling station. A comprehensive failure analysis investigation was carried out to reveal the root cause of the detected failure. Visual inspection coupled with complete microstructural characterization as well as mechanical testing were performed in the investigation. The results showed that the failure was mainly caused by fatigue failure. The initiation of the fatigue crack was supposedly promoted by the existence of non-metallic inclusions that were present at both the surface and the interior of the microstructure. Furthermore, unbalanced preload forces resulted in bolt loosening which caused fretting wear of the galvanized layer thickness. The detected drop in the galvanized layer thickness by 42.20% at the primary origin of the fatigue crack along with the presence of non-metallic inclusions were thought to be the reason for the initiation of the fatigue crack. The crack was then found to propagate through a transitional complete cleavage to quasi-cleavage fracture as appeared from fracture surface topography studied by scanning electron microscopy (SEM). The final stage of the fatigue failure was found to be caused by ductile fracture of the overloaded zone.