Characteristic influence of thermo-mechanical stresses leading to premature failure of compressor turbine blades of short-haul aircraft fleet
Ngoret, Joshua Kimtai
PublisherUniversity of Botswana, www.ub.bw
MetadataShow full item record
Short-haul aircrafts play an integral role in air transport network, facilitating; passenger and cargo transport, security drills, ambulance services, disaster, wildlife and environment management, tourism amongst others. Unlike medium and long-hauls, these aircrafts are missioned to fly for under 3 hours and distances not exceeding 2000 km. As a result of short but, voluminous flight turnovers, changes in power settings, starts and stops; the compressor turbine (CT) blades of the engines are subjected to intense cyclic thermo-mechanical stresses. Compressor turbine blades are specially profiled aerodynamic engine components specially designed to extract energy from high-temperature, high-pressure gases produced by the combustors. Due to severity of their operational environment, the CT blades degrade over time and often catastrophically fail without warning. In this regard, monitoring of CT blade life is crucial to ensure their proper health in service. The current techniques employed to investigate the life of CT blades only inform of failure after exposure to service with scanty information on the numerous prematurely occurring failures. Consequently, accounting for CT blade life still poses a great challenge to air operators. This research therefore probed characteristic influence of thermo-mechanical stresses leading to premature failure of high pressure (HP), PT6A-114A engine CT blades from an assimilative approach. The prematurely retired HP, CT blades were collected from Vector Aerospace Kenya Limited after being in service for only 6378 creep-fatigue hours contrary to 10000 creep-fatigue hours preset by the manufacturer. The CT blade was modeled for thermo-mechanical degradation in an environment that mimics the operational conditions to determine service life and resulting damage using commercial ANSYS tools version 15.0. A detailed microstructural and metallographic characterization was then performed using x-ray florescence (XRF), x-ray diffraction (XRD) and energy dispersive spectroscope-scanning electron microscopy (EDS-SEM) on the protective coating and the substrate material. Mechanical testing was ultimately executed to ascertain the micro hardness and residual strength at time of retire from service. The modeling results revealed that the tip of the CT blade had been rigorously attacked by exposure to heat as compared to the airfoil and the base. Notwithstanding this, it was noted that the CT blade could have served for another 1.44% more of the time it was in service, being exploited in the transient regime. The XRF results affirmed the existence of the bulk constituent elements that matched the manufacturers’ specification. The XRD analyses enabled positive identification of the resultant compounds which constituted the protective coating and the substrate material. The EDS-SEM results established that the protective coating of the tips was more attacked compared to the airfoils and the bases. As such, the substrate material degraded from evolution of creep and fatigue. The pores at the bases of the CT blades were found not influence distribution of uniform cuboidal phase at the bases in comparison the rafted tips and airfoils. This confirmed that degradation of the substrate material occurred as a result of creep and fatigue and not from manufacturing defects. Though micro hardness testing indicated that the material was still of high strength, continued safe service was not warrantied. From this research, a robust assimilative approach was adopted for investigating life of CT blades enabling determination of instantaneous material status, magnitude of damage and remaining useful life (RUL). This work will assist air operators improve flight safety, enhance availability of aircrafts for operations while planning for maintenance will be made easier. Premature failure of the CT blades can further be averted by; adhering to the engine’s operation limits, avoiding long and overexploitation of the engine and sticking to preset flight environments. In a move to improve heat, oxidation and corrosion resistance of the protective coating, inclusion of rare earth element such as yttrium, cesium and lanthanum would be noble. The substrate material could similarly be improved by addition of refractory elements such as ruthenium, iridium and rhenium in the composition of Inconel 713LC.
- PhD Theses (FET)