An Outrun Competition of Corrosion Fatigue and Stress Corrosion Cracking on Crack Initiation in a Compressor Blade

First row rotating blades of four axial-flow compressors were prematurely fractured. Previous investigations showed that the site atmosphere contains corrosive compounds which lead to an increase in possibility of pitting on the blades. It was also revealed that the crack was originated from two corrosion pits. Thus, this work is conducted to ascertain which of fatigue or stress corrosion cracking (SCC) mechanisms was predominant in transition of pits into initial cracks. To this end, experimental, numerical and theoretical studies are considered. Scanning Electron Microscope (SEM) and fractography of the broken blade indicate that the pits join together and make one bigger pit under SCC mechanism which reduces the failure time. 3-D models of the pitting on the blade under existing forces are analyzed by ABAQUS software. The simulation results show the location of maximum stress concentration inside one of the pits which is compatible with the location of initial SCC crack. Theoretical analysis shows stress intensity range around the pits smaller than threshold stress intensity range; thus, cracks initiated and propagated at the mouth of the pits under SCC mechanism. Then the pits join together and make one equivalent pit, so that stress intensity range reaches to threshold stress intensity range and finally crack propagates under fatigue mechanism.