Failure Analysis of LP turbine of an Aero-engine

Madan, M and Sujata, M and Raghavendra, K and Bhaumik, SK (2011) Failure Analysis of LP turbine of an Aero-engine. Project Report. National Aerospace Laboratories, Bangalore, India.

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There was an incidence to an aircraft wherein after flight, pilot noticed “Decelerate Right Engine” caption on MFWS along with longitudinal thud. Also, there was momentary display of “Right Engine Vibration” caption. Following this, the pilot switched off the RH engine and the aircraft landed with one engine (port). Boroscopic examination of the affected engine revealed damages in the turbine area, mostly concentrated in and around LP turbine. On strip examination, it was found that the LPTR blades, and a labyrinth with the LP turbine disc were severely damaged. A few of these damaged components, viz., (a) fractured and dislodged labyrinth support ring of LP turbine disc, (b) fractured and dislodged labyrinth, (c) parts of LPTR disc containing the fracture surface, (d) LPTR blades, and (e) lock and pin of LPTR blades retainer ring were sent to this laboratory for establishing the probable cause(s) of failure. Examination revealed that the damages to LPTR blades were mostly confined to the tip region except for one where the blade had fractured in the airfoil at one-third height from the blade root platform. Evidences suggested that the damages to the LPTR blades were caused due to tip rubbing with the turbine casing. Fractographic examination confirmed that the fracture in all the blades was by overload. None of the blades showed any signatures of progressive failure. The damages on the lock and pin of LPTR blades retainer ring were also found to be secondary in nature. A part of the labyrinth was retrieved from the damaged engine. It was found mangled and separated from the labyrinth support ring with the LP turbine disc. No pins were available with the labyrinth. The fractured ends of the labyrinth were severely damaged. Fractographic examination showed dimple rupture wherever the fracture features were preserved, indicating failure under overload. The labyrinth support ring got completely separated from the LP turbine disc. Again, only a part of this ring was available for examination. But, the entire mating fracture surface of the ring was available with the disc. Fracture surface analysis showed that separation of the labyrinth support ring from the disc was by overload fracture. The force acted on the support ring was in the radial direction from outside to inside. Further examination of the separated labyrinth support ring showed presence of cracks in four pin holes, located in a sector adjacent to each other. The cracks were located at the edge of the holes on the free surface. It was confirmed through scanning electron fractographic study that these cracks were progressive in nature and they had propagated by fatigue mechanism. From the laboratory investigation, it could not be established whether or not these cracks have bearing on the separation of the labyrinth support ring from LP turbine disc. The presence of fatigue cracks, however, does indicate some distress at the pin holes originating either from assembly or during engine run. Considering the overall damages to various components of the engine in and around LP turbine and the laboratory investigation, it appears that the failure in the labyrinth was due to some location specific problem(s). Analysis also suggests that the overload fracture of labyrinth support ring is indicative of sudden overload on this component during the engine run. Although it was not possible to establish as to what caused this sudden overload, it is believed that the detachment of labyrinth support ring was the first in the chain of events that led to engine failure. In view of this, it is suggested that various possibilities that could lead to sudden overload on the labyrinth support ring of the LP turbine disc be investigated. It also needs to be investigated whether or not there could be any common cause(s) which is responsible for generation of fatigue cracks at the pin holes as well as the separation of the labyrinth support ring from LP turbine disc. Laboratory test results showed that the materials used for fabrication of the components in question conform to specification in terms of chemical composition. No comments can be offered on the microstructure and hardness parameters because of the insufficient data made available to this laboratory. However, based on the overall understanding of the failure, any metallurgical abnormality leading to separation of the labyrinth support ring appears to be a remote possibility.

Item Type: Monograph (Project Report)
Uncontrolled Keywords: LP turbine;Labyrinth;Fatigue;Clearance;Assembly
Subjects: CHEMISTRY AND MATERIALS > Chemistry and Materials (General)
Depositing User: Users 11 not found.
Date Deposited: 06 Jul 2011 05:45
Last Modified: 06 Jul 2011 05:45

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