Investigation of HP Turbine Blade Failure in a Military Turbofan Engine

Teardown examination

The conditions of the HP and LP compressor assemblies were checked and no abnormalities were found except for carbon deposits. Parts were found to be intact and balanced. Combustion chamber liner and casings were found to possess no cracks. The liner front end revealed the presence of carbon deposits around the atomizers. The HP nozzle guide vane platforms were found to be chipped-off at the tips. The vanes were eroded and contained cracks. One HP turbine rotor blade was found broken approximately at mid- airfoil height as shown in Figure 2. The broken piece of the blade airfoil could not be located. Whitish discoloration, indicative of blade erosion, was noticed on the aerofoil sections of other blades of the HP turbine an example of which is also shown in Figure 2. Impact damage was noticed on LP turbine nozzle guide vanes where trailing edge airfoil chipping had occurred in one of the LP turbine rotor blades as shown in Figure 3.The condition of fuel and oil systems were found to be satisfactory. Magnetic Chip Detectors (MCD) were also checked and found to be satisfactory. Oil sample from the engine was sent for laboratory examination.

Figure 2:

The failed HP Turbine blade (left) and whitish discoloration of the blades (right).

Figure 3:

Impact damage suffered by the NGV (left) and a damaged rotor blade (right) in the LP turbine.

Forensic examination

A systematic approach was followed to establish the root cause of the blade failure. The following checks were carried out to collate information about the engine history and the incident [10].

1. The history of the components and life consumed, maintenance and operational records etc. for the said engine were reviewed. These revealed no abnormalities.

2. The engine had logged nearly 90% of its time between overhaul (TBO) life.

3. Study of the engine sorties prior to this failure and analysis of the flight data recorder failed to reveal any engine operating parameter beyond the prescribed operating limits.

Forensic examination failed to reveal any abnormality that might be responsible to the HP blade failure.

Visual observations

A photograph of the broken HP turbine blade is shown in Figure 4. The close-up view of the fracture surface is shown in Figure 5. The blade had broken in the mid-aerofoil section. Two different views of the fractured blade are shown in Figure 6. Noticeable trailing edge deformation in the vicinity of the fracture surface is quite evident. This fractured blade is identified as “HPTB-1”.

Figure 4:

Fractured HP turbine blade HPTB-1.

Figure 5:

Close view of the fracture surface of HPTB-1.

Figure 6:

Two different views of the fractured blade HPTB-1. Deformation noticed in the trailing edge in the vicinity of the fracture surface.

Two different views of another HP Turbine blade, identified as “HPTB-2”, are shown in Figure 7. The HPTB-2 blade reveals the presence of a major trailing edge crack in addition to the trailing edge deformation similar to that observed in the case of HPTR-1 blade. Other smaller cracks were also observed in the deformed trailing edge region of this blade. Another HP turbine blade, identified as “HPTB-3” is shown in Figure 8. Cracks were observed in the leading edge region of this blade along with some coating erosion marks in the trailing edge region. Physical damage was also noticed on the surface of the blade HPTB-3 as shown in Figure 8. Photographs showing different views of a fourth HP turbine blade, identified as “HPTB-4” are presented in Figure 9. This blade appears to be in a better condition without significant damage such as cracking or severe coating erosion marks.

Figure 7:

Two different views of another HP Turbine blade identified as “HPTB-2”. Deformation in the blade material the trailing edge region and the presence of cracks in blade trailing edge.

Figure 8:

Cracks along the leading edge of the blade HPTB-3 (left) and physical damage in trailing edge region of HPTB-3 (right).

Figure 9:

Macrograph showing relatively undamaged blade HPTB-4 considered as the reference blade in the study.

Chemical analysis

The chemistry of the HP and LP turbine blades was carried out using spectrometric techniques. The chemistry of the samples conformed to the specifications of Nimonic 108 and Nimonic 115 respectively for the HP and LP turbine blades. The turbine vanes and platforms were also found to conform to their original material specifications.

Fractographic analysis

Severe discoloration of the fracture surface of blade HPTB-1 was observed. At the macroscopic level, the failure mode of the blade could not be identified. Hence, the fracture surface of the blade HPTB-1 was examined using Scanning Electron Microscopy (SEM). Thick high temperature oxides were present on the fracture surface of the blade and this was obscuring the view of the majority of the fracture features lying underneath. The fracture surface of the blade was cleaned repeatedly using a replica stripping technique followed by ultrasonic cleaning in an attempt to remove the oxide products. After repeated steps of stripping and cleaning, the fracture surface was again examined under the SEM. The SEM images of the fracture surface in the trailing edge region of the blade are shown in Figure 10(a) and 10(b) whereas an image in the leading edge of the blade is shown Figure 10(c) for comparison. The high magnification SEM analysis revealed the presence of a heavily oxidized intergranular fracture including extensive intergranular cracking in the trailing edge region of the blade as shown in Figures 11 and 12.

Figure 10:

(a) SEM images of fracture surface of the blade HPTB-1 captured towards its trailing edge region showing intergranular fracture features. Portion of the blade surface showing intergranular features is encircled. (b) SEM image of the fracture surface of the blade HPTB-1 captured towards its trailing edge. (c) SEM image of the fracture surface of the blade HPTB-1 captured towards its leading edge.

Figure 11:

High magnification SEM image showing the presence of intergranular fracture features on the fracture surface in HPTB-1 blade towards its trailing edge (Magnification = 1,500X).

Figure 12:

High magnification SEM image showing the presence of intergranular fracture features on the fracture surface of HPTB-1 blade in its trailing edge region (Magnification = 3,150X).

The intergranular fracture features were limited to the trailing edge region of the blade and this region is encircled in Figure 10(a). At locations beyond this encircled area, the fracture surface of the blade predominantly showed the presence of ductile dimples that are typical of an overload failure, Figure 13. The presence of ductile overload features indicates that the region corresponds to the final or high strain rate failure zone. These observations clearly indicate that the cracking in the HPTB-1 blade originated in the trailing edge region of the blade.

Figure 13:

High magnification SEM image showing ductile overload fracture features (dimples) in the final failure zone of the fracture surface of the blade “HPTB-1”.

Microstructural examination

Considering that the HPTB-2 blade also exhibited surface trailing edge erosion, cracking and deformation, similar to that observed in the case of the HPTB-1 blade fracture region (Figure 7), metallographic specimens from the deformed and cracked trailing edge regions of the HPTB-2 blade were thus prepared. Metallographic specimens from the HPTB-4 blade from the same airfoil location were also included for microscopic studies for comparison since this blade revealed the least amount of damage from a macroscopic perspective. All sample sections were mounted, polished and etched for microscopic examination [11].

The etched microstructures from the trailing edge regions of the blades HPTB-2 and HPTB-4 are presented in Figures 14 and 15, respectively. The metallographic analysis was carried out as per ASM Handbook [12]. In both cases, the aluminide coating in some regions of the trailing edge regions has completely eroded away or suffered spallation and the remnants of coating in other regions of the trailing edge contain heavily transformed coating diffusion zone. This is indicative of the inability of the coating to withstand the engine operating environment. In the case of HPTB-2 blade, intergranular oxide networks including significant intergranular cracks are also observed close to the trailing edge region where coating has suffered severe degradation, Figure 14. The etched microstructures of both HPTB-4 and HPTB-2 specimens also reveal severe gamma prime coarsening in the form of intragranular mottling, Figures 15 and 16, respectively. The gamma prime coarsening of this magnitude is indicative of severe aging of the substrate and its inability of the blade substrate material to withstand the operating temperatures that the HP turbine section is exposed to during service [13, 14].

Figure 14:

Microstructural features in the trailing edge region of HPTB-2 blade in etched condition at different magnifications.

Figure 15:

Microstructural features in the trailing edge region of the HPTB-4 blade in the etched condition at different magnifications.

Figure 16:

Microstructural features slightly inside the HPTB-2 blade trailing edge region showing coating degradation and intense transgranular mottling.

A low magnification montage of the microstructure of the HPTB-2 blade including its cooling cavity is shown in Figure 17. Signs of coating degradation and heavy internal cooling passage oxidation are clearly visible in this micrograph. More importantly, however, the presence of very large and wide intergranular cracks close to the blade external surface and internal cooling passage is indicative of oxidation damage playing a major role in the crack nucleation in this blade. In addition, the curling of the blade aerofoil in the trailing edge region, Figure 7, indicates that the blade substrate is not strong enough from a creep strength perspective to withstand the magnitude of temperatures and stresses that the HP blade is subjected to during service. This coupled with coating spallation and a heavily transformed coating-substrate diffusion zone clearly indicates that the chosen coating system for the blade is not adequate enough to provide the oxidation protection it is supposed to deliver.

Figure 17:

A low magnification montages of micrographs of the HPTB-2 blade material at 50X magnification showing wide intergranular cracks.