FAILURE ANALYSIS OF GAS TURBINE BLADE

• The failure of the stage I, II, III turbine rotor blades of an aircraft engine specifically the aero engine of a military transport aircraft is reviewed.

• This analysis is a journal paper published July 15th 2014.• The blades were made out of Ni-based super alloys of

different grades.• The aero engine had completed up to 80% of the assigned

life and several hundred hours since the last overhaul before the failure of the blades.

• The failure of these rotor blades was investigated by metallurgical analysis of the failed/damaged regions.

• Detailed Investigation included Visual examination of the blade surfaces, fractography, microstructural examination, chemical analysis and hardness measurement were carried out to identify the cause of the failure of the blades.

• MECHANISM OF FAILURE The rotor blades and other components of gas turbine suffer service induced damage which may be natural or accelerated. Blade failures can be grouped into; (a) Fatigue including both High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) (b) Creep/Stress rupture

There are generally three damage mechanisms affecting the life of turbine blades; (a) Mechanical damage through creep (b) Multi-axial fatigue (C) High temperature Corrosion.

FAILURE INVESTIGATIONSA set of four blades from all the three stages of a gas turbine were examined. The fracture as well as damaged portions of blades of different stages I,II and III were cut cleaned ultrasonically and examined using stereo and SEM for recording fracture features.

• Also, samples were cut from aerofoil region of blades of different stages, mounted in Bakelite, polished following standard metallographic practice and etched using a chemical reagent for microstructural examinations. Krolls agent was used for etching Stage I and II blade samples, while 5% nital for (electrolytic etching) stage III blade samples. (Fig. 2)

• Fig 1. Photographs of (a) fractured and (b) Damaged stage I blades Fig. 2 Photographs of damaged blades (a) II stage and (b) III stage blades.

• RESULTS• Chemical analysis of each blade was carried out using

instrumental analysis method. The elements present are shown below:

Table 1. Chemical composition of I stage blade (wt%).C Mo Nb Cr Al Ti W Co Ni

0.16 3.5 0.032 9.6 5 2.3 13 5.1 Balance

Table 2. Chemical composition of II stage blade (wt%).C Mo Nb Cr Al Ti W Co Ni

0.046 9.1 0.01 9.2 4.75 0.016 4.95 4.95 Balance

Table 3. Chemical composition of III stage blade (wt%).C Mo Nb Cr Al Ti W Co Ni

0.081 0.03 0.012 19.12 0.63 2.6 0.009 0.016 Balance

• Stereographic examinations of fractured surface revealed two distinct fracture regions as shown in Region I i.e., fracture surfaces near the leading and trailing edges reveal beach marks a characteristic of progressive fracture while Region II-the region between leading and trailing edge has rough and dull appearance.

Fig. 3 Fracture surfaces of I stage blade ‘A’ showing (a) leading and (b) trailing edges revealing beach marks

Fig. 4 Features in the damaged region of Stage I blade

Fig. 5 Stereographs at (a) low magnification and (b) high magnification showing features of severe plastic flow and fine surface cracks in the damaged region of II stage turbine blade.

Fig. 6 Stereographs at (a) low magnification and (b) high magnification showing features of severe plastic flow and rub marks in the damaged region of III stage turbine blade

• Fractographs reveal fracture surfaces at different regions of the stage I blade. Fig 7a shows beach marks as well as the damage of surface coating at the tip of the leading edge which could be the fracture initiating site.

Fig. 7 SEM micrographs of I stage blade ‘A’ revealing fracture features at different regions across the cross-section of aerofoil (a) leading edge region showing crack initiation site and beach marks (b) overload fracture region at trailing edge (c) overload fracture features and (d) oxide layer on fracture surface.

• Surface coating is found in some regions of blade A. The Presence of Nickel and Aluminium in the coating is revealed by EDX analysis and the coating used could be nickel aluminide.

SEM fractographs shows dimples-features of an overload fracture, and the presence of abrasion marks in the damaged region of stage I blade.

• Secondary electron images of damaged regions (dents/nicks) of stage II and III blades are shown below. The magnified view (b) shows dented region of stage II blade reveals deep cut into the blade and the presence of multiple surface cracks

Fig. 10 SEM images showing deep cuts and surface cracks in the dented region of stage II blade.

Fig 11. SEM images showing abrasion marks in the dented region of stage III blades.

Optical Micrographs revealing dendritic structure in the top portion of Stage I blade is shown, the grains are very coarse and essentially equi-axed. Similar observations were made in the middle and lower portion of the aerofoil section and the root of the blade. SEM micrographs reveal the presence of carbides in gamma matrix.

Fig 12. Optical micrograph of airfoil region of stage I blade showing dendritic structure.

EDAX spectra of matrix and carbide precipitates indicate the presence of elements such as Ni, Cr, Co, W, Ti, Al, and Mo in matrix while Ti, W and Mo in carbides. (This is Shown in Fig 14)

Further secondary electron micrograph examination revealed the absence of surface coating, but evidence of oxidation damage of the surface up to a depth of ~200μm was observed (Fig 15a). EDAX analysis of the surface (Fig 15b) reveals presence of Al, Cr and Ti oxides in this region.

Fig 13. Microstructure of stage I blade in the aerofoil region (a) low magnification image showing carbides in matrix and (b) magnified view showing cuboidal and partially rafted ’ structure

Fig 14. EDAX spectrum of (a) matrix and (b) carbides

Fig 15. SEM micrograph showing (a)oxidation damage close to surface of stage I blade and (b) EDAX analysis showing oxides of Cr, Al and Ti.

A detailed microstructural examination of a sample extracted from a region just below the fracture surface of stage I blade reveals that severe localized oxidation attack (i.e internal oxidation) of substrate in the regions where surface coating has got damaged during service. The attack has led to local dislodgement of surface coating thereby forming pits on the aerofoil surface.

Fig 16. BSE images showing (a) absence of coating, surface oxidation as well as severe localized oxidation attack and formation of pits caused by oxidation of stage I blade (b) optical micrographs showing dislodgements of surface coating near leading edge and (c) magnified view of encircled region in (a)

• Optical micrographs of stage II and III blades reveal equiaxed grains of phase. (Fig 17)

• Secondary electron images of Stage II blade reveal the presence of carbides along grain boundary as well as within the grains and cuboidal ’ precipitates are shown in Fig 18 (a) and (b) respectively.

Fig. 17. Optical Micrographs showing equi-axed grain structure in (a) Stage II and (b) Stage III blades.

Fig. 18. SEM images showing (a) carbides along gain boundary and within the grains and (b) cuboidal ’ in stage II blades.

• SEM images of stage III blade was captured as shown below and it can be seen that the microstructure comprises carbides along grain boundary as well as within grains.

Fig. 19. SEM image showing carbides along grain boundary and within grains in stage III blade.

Fig. 20. Schematic of various stages in dislodgement of surface coating (a) damaged surface coating (b) oxygen diffusion through damaged coating during service and formation of discrete oxides at coating/substrate interface at several locations (c) initiation of continuous oxide figure layer at the interface and dislodgement of surface coating and formation of pits and (d) dislodgement of surface coating and establishment of oxide layer on substrate surface.

• CONCLUSION• The existence of beach marks on stage I blade occurred by

fatigue. The dislodgement of surface coating provided partial repair of the blade but damage to the coating led to localized oxidation attack forming oxides at coating/substrate interface and pits. These surface pits facilitated the initiation of fatigue cracks during service and led to the fracture of several blades.

• The impact of broken pieces of stage I blade could have caused internal object damage to other blades of stage II and III in the form of deep cuts, dents and nicks on either of the edges.

• To prevent future occurrences of this damage, it is paramount that good coating be properly applied during overhauling

THE LIBERTY BELL

• The Liberty Bell is an internationally recognized symbol. In 1751, the speaker of the Pennsylvania Assembly, Isaac Norris ordered for the Bell from a Whitechapel Foundry in London, UK to commemorate the 50th Anniversary of the granting of William Penn’s Charter of Liberties for the State House.

• The inscription on the Bell was to read• “Proclaim Liberty through all the land unto all the

inhabitants thereof” (Lev. 25:10)

• The cost of the Bell including shipping and insurance was 150pounds 13 shillings 8pence, delivered at the End of September 1752.

• To utmost surprise of the populace, the Bell cracked the first time it was struck. The Bell cracked by a stroke of the Clapper as it was hung up to try the sound.

• The Bell was composed of cast Copper and Tin. John Pass and Charles Stow agreed to recast the Bell in time for the Charter’s 50th Anniversary.

• Pass and Stow observed that the metal was too brittle and augmented the Bell by addition of 10% copper – this affected the sound production.

• After adjusting the alloy chemistry and recasting the Bell twice, Pass and Stow produced a Bell with an acceptable tone. They were paid $295.25 and given free advertisement. Hence, their names on the shoulder of the Bell.

• When the Bell was re-casted, city officials gathered for a celebration. When the Bell was struck, it did not break, but the sound produced was not pleasant to the people. Mocked by the crowd, Pass and Stow took the Bell away and again re-casted it. This occurred in June 1753.

• Upon striking the Bell again, the sound was deemed satisfactory. • The final version—made of 70 percent copper, 25 percent tin and small

amounts of lead, zinc, arsenic, gold and silver—weighed around 2,080 pounds and measured 12 feet in circumference around the lip and 3 feet from lip to crown. On July 8, 1776, the bell was rung to celebrate the first public reading of the Declaration of Independence. After the British invasion of Philadelphia, the bell was hidden in a church until it could be safely returned to the State House. A popular icon of the new nation and its independence, it wasn’t called the “Liberty Bell” until the 1830s, when an abolitionist group adopted it as a symbol of their own cause.

• The Whitechapel Foundry claim and suggested that the Bell was either damaged in transit or was broken by an inexperienced Bell ringer, who incautiously sent the clapper flying agent against the rim rather than the body of the Bell.

• In 1975, Winterthur Museum conducted an analysis of the Bell and concluded that a series of errors made in the construction, reconstruction of the Bell resulting to a considerably higher level of Tin in the Liberty Bell. Suggesting that Whitechapel by using scraps with a high level of tin to begin the melt instead of pure copper.

• On second recasting, instead of adding pure tin to the Bell metal, Pass and Stow added cheap Pewter with a high lead content and incompletely mixed the new metal into the mold.

THANK YOU.

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