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

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