FAILURE ANALYSIS OF HIGH-PRESSURE TURBINE BLADES OF A TURBOFAN ENGINE

Abstract

The turbine blades, which are exposed to a harsh environment, are one of the most important and vital components of a gas turbine. During each engine operating cycle, gas turbine blades are subjected to very high levels of stress (> 450 MPa) and higher temperatures (>1400˚C). Under these adverse operating conditions failure of turbine blades can take place in the form of cracks and corrosion. An in-service turbofan engine is frequently encountering turbine blade failure problems of trailing edge (TE) cracks and corrosion/oxidation at the blade tip and leading edge (LE). No earlier research is available in open scientific literature to investigate the turbine blade failure of said engine. This research focuses on investigating the initiation of cracks, their propagation mechanism, and the analysis of corroded surfaces coupled with temperature distribution, which could pave the way for formulating mitigation strategies. The investigation is divided into two parts. Part I involves the experimental analysis including the characterization of near-pristine blade alloy and coating followed by analysis of cracked blades by coating analysis, fractography, microstructural analysis, phase analysis, and microhardness testing. The analysis of corroded surfaces is also included in part I. Part II includes the stress analysis of the turbine blade using the structural module of Ansys® software followed by its analytical validation. Chemical, microstructural, and mechanical characterization determined that blade material corresponded to the nickel-based superalloy of the second generation. The observed fractographic characters by stereomicroscope and scanning electron microscopy showed that the fracture of the blade was due to a fatigue mechanism. The fatigue phenomenon was initiated by the penetration of FCMAS particles inside the coating at the inner trailing edge of the suction side and progressed due to the cyclic stresses. Here the coating thickness is found to be less than 5µm, which got deteriorated due to the hot air rich in oxygen and impurities (sulfur, salts, sands, volcanic ash, etc) striking the turbine blades. Deposits of FCMAS particles and traces of sulphur are found all over the coating which in molten form penetrate inside the coating and attack the base material. The infiltration of molten FCMAS upon cooling causes stiffening of the coating, which leads to delamination, increased coating density, stress accumulation, and cracking of the coating surface and substrate. The presence of MC, M6C carbides at the coating substrate interface and Al2O3 deposits outside and inside coating is a contributing factor for enhanced crack propagation. The maximum stress of 237.87MPa occurs at the root airfoil interface while in the crack growth region, the stress is found to be 103MPa. This research concludes that the cracks at the turbine blade trailing edge are due to coating failure, infiltration of foreign particles, and microstructural and phase transformations. For the corroded samples primarily oxides of FCMAS with low concentration of Ni and Co oxides are found. At blade leading edge corrosion products additionally, Sulphur was also detected in low concentration.