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Pergamon Engineering Failure Analysis, Vol. 3, No. 3, pp. 203-210, 1996

Copyright 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved

1350-6307/96 $15.00 + 0.00

PII:S1350-6307(96)00011-8

AN A IR CRASH CASE STUDY

A, TAUQIR, I. SALAM, F. H. HASHMI and A. Q. KHAN

Metallurgy Division, Dr. A. Q. Khan Research Laboratories, G.P.O. Box 502, Rawalpindi, Pakistan

(Received 18 April 1996)

Abstract--This paper examines the causes of the catastrophic failure of a gas-turbine aero engine which led to an air crash. The failure was traced to the fatigue failure of a 1.5 mm diameter retaining ring made from AISI 304 stainless steel. Copyright 1996 Elsevier Science Ltd.

1. INTRODUCTION

An aircraft was involved in an accident 14 min after take-off. The initial investigations led to the suspicion that the failure had occurred in the engine compressor near to the third-stage rotor blades. The engine has completed less than 55 flying hours since its last overhaul, in which the blades and the retaining ring had been changed.

The engine had an axial flow compressor. The initial and final stages of the blades and the vanes of the compressor were manufactured from steel, whereas the intermediate stages were made from an AI alloy. The third-stage blades are considered critical as it is in that region that the material of the blade and the diameter of the rotor/casing changes. The blades are kept from sliding by a retaining ring (Fig. 1). The minimum permissible gap between the tip of the blade and the compressor casing is 1.4 ram. Typically, the gap, depending on the wear on the blades, is -2 mm.

The main wreckage caught fire after impact with the ground. A portion of the casing is shown in Fig. 2. Sixteen third-stage blades were detached from the engine before the plane crashed. These were found well away from the main wreckage and escaped the fire. The inside of the compressor casing portion which escaped the fire faced the ground, thus escaping deposits from its surroundings and products of the fire. Initial impact with the ground may, nevertheless, have resulted in the general scratching of the surface by sand.

2. SUSPECTED CAUSES OF FA ILURE

Initial investigations led to a few suspect areas. "Screw-thread"-like cut marks in a third-stage blade were noticed (Fig. 3). The third-stage retaining ring was found to be broken,

t Blade__

_ Blade _Breakage r~) \

~\ \ \ \ \ \~ ."~z ~.-.~ ~.:: z-a-~ ~.v. i k\\ \ \ \ \ \ \ \ \ '~.

"::-- .[

Disc stage

Missing portion Fig. 1. Sketch indicating the location of the retaining ring and its missing portion.

203

204 A. TAUQIR et al.

Fig. 2. General view of retrieved compressor casing portion. Locations subjected to detailed analysis are marked.

Fig. 3. Retrieved fracture surface of compressor blade.

and a 3-in.-long piece was missing. On various parts of the compressor casing, different scratches, grooves and dents were observed (Fig. 2). The damage was suspected as having been caused by one of the following:

(a) Damage from a foreign object to one of the third-stage blades. It may be mentioned here that 16 third-stage blades were recovered unburned.

An air crash case study 205

(b) Internal damage due to a loose screw. (c) Internal damage due to a broken piece of the third-stage retaining ring.

Detailed metallurgical investigations were conducted to establish the actual cause of the failure.

3. INVESTIGATION TECHIQUES AND RESULTS

Suspected components were subjected to visual examination and stereomicroscopy. Spectro- chemical analyses were conducted in order to determine the scratching on different components. These included elemental analysis of samples by an electron microprobe using energy-dispersive X-ray analysis, atomic absorption spectroscopy of the bulk material, and C/S analysis of combustion products of the specimens. Fractography of retrieved fractured surfaces helped to determine the possible mode of failure. Based on these preliminary investigations, the components which seemed critical were subsequently subjected to detailed metallurgical investigations.

3.1 Spectrochemical analysis of the components

Investigations were primarily concentrated on the compressor casing, third-stage blades, retaining ring and screws typically used in the affected region. The spectrochemical analyses of these parts were conducted and materials were identified [1]. The results are summarized in Table 1. The casing was made of Mg-Zn alloy ZK21A. The blades were forged from A1 alloy 2618 containing Cu, Ni, Mg and Fe. The retaining ring was made from stainless steel AISI 304; it contained Ni and Cr. Typical screws used in the region were machined from a low alloy steel of type SAE 3240 and were Cd-plated.

The components which were retrieved from the accident had marks, dents and burning deposits. These were subjected to a spectrochemical study. The analyses of different regions of the components retrieved from the wreckage are summarized in Tables 2-4.

3.1.1. Third-stage retaining ring. The retaining ring for the third-stage blades was retrieved, but a 3-in.-long piece was missing. The locations of the retaining ring and the missing part are

Table 1. Chemistry of compressor region parts

Composition (wt %)

Part Ni Cr Mg A1 Fe Others Designation

Casing -- -- Balance -- -- Zn = 2.4 + 0.2: traces of AI, ZK21A Mo and Si found

1.0 + 0.1 -- 1.1 + 0.3 Balance 0.9 + 0.2 Cu = 2.1 + 0.1 2618 forging 8.3 + 0.4 20.3 + 0.3 -- -- Balance C 0.06, Mn 0.7, Si 0.5, S 0.02: AISI 304

traces of Ca, Si, A1, Mg 1.4 + 0.1 1.0 + 0.2 -- -- Balance C 0.34, S 0.003, Si and Mn SAE 3240

present: surface is Cd-plated ( -25/~m)

Blade Retaining

ring Screw

Table 2. Chemistry of retrieved parts--third-stage retaining ring

Region analyzed Composition (wt %)

Deposits on fracture surface Mg 2-5, Cu 5-10, Si 3, A1 2: Sb, Pb, Zn detected Isolated spots on the fracture surface Rich in Ni, Cu, Zn, Pb and Sb A piece of retrieved wire was notched and fractured by The inclusion are rich in Ca and AI:* contains Si and S. the impact: the inclusions observed in cups on the freshly Minor quantities of Ti, Cu, Zn and Mg fractured surface are analyzed in a scanning electron microscope

*Ca, A1 and Mg are added in steels in minor quantities as grain refiners.


Table 3. Chemistry of retrieved parts--compressor casing

Sample # in Fig. 2 Region analyzed Composition

Sample #10 Groove in compressor casing--for first-stage blade Rich in Al, Si, Cr, Fe region

Region near the grooved patch for comparison Sample #11 Inside region of compressor was not smeared

General composition of features in surface Scratches/streaks

Sample #12 In between two scratches

Nothing except base metal A1 and Si Rich in Cr, AI and Si Rich in Cr, AI and Si Concentration of Cr lower than the

concentration inside the scratches Fine particles Rich in Ti

Table. 4. Chemistry of retrieved parts--a third-stage blade

Surface analyzed Region in Fig. 3 Composition

Surface of the blade near the cut Region A Rich in Cu, Zn, Si and contains Mg, Ca, C1, Fe and S Surface of the blade away from the cut Region B Nothing unusual except base metal The surface generated from cut Region C Rich in Mg, Si, Fe, Zn, Cu and Ni Inside surface of a fine cut Region D Rich in Mg, Si, C1, Ca, Zn, Cu, Fe, Ni

schematically shown in Fig. 1. The fractured surface was subjected to spectrochemical analysis: the results are summarized in Table 2. The high concentrations of Mg and Zn on the surface are probably due to the scratching of the part by the casing. The Cu concentration at some locations is quite high, exceeding 5%. The only critical component in the region containing Cu is the blade which is made of an A1-Cu alloy. Some isolated spots on the fracture surface were found to be rich in Ni, Cu, Zn, Pb and Sb. No prominent part of this region of the engine is made from an alloy composed of these elements. Similarly, rings retrieved from other used engines do not indicate the presence of these elements on the exposed surfaces. It seems that the deposition of particles rich in Cu, Zn, Pb and Sb were from a component located elsewhere, possibly in the electronic circuitry. It may be pointed out that the electrical connectors are typical components made of Cu-Zn-Pb alloys, and Sb is used as an alloying element in the soldering material [1].

The wire retrieved from the accident was tested in the laboratory, and the freshly fractured surface was analyzed for comparative purposes. The "cups" of the deformed regions contained Ca-A1-Mg-Si - r ich inclusions, probably silicates. At some of the locations, man- ganese sulphide precipitates were also detected.

Summarizing the results of the spectrochemical analyses, it was found that: (i) the ring material contained non-metallic inclusions, possibly silicates, and (ii) the piece retrieved from the accident portion of the ring was scratched by the casing and the blade material.

3.1.2. Different regions of compressor casing. Different samples were obtained from the retrieved unburned section of the compressor casing. The locations of these samples are shown in Fig. 2 and the results of the spectrochemical analysis are summarized in Table 3. The analysis of location #10, which had a groove, showed high concentrations of A1, Si, Cr and Fe. These elements were present in either the third-stage blades or the retaining ring. The region in the vicinity of the groove did not contain these elements, showing that the deposits in the groove were from the object which had hit it. Locations #11 and #12 also showed that the scratches were rich in A1 and Cr. It seems that a portion of the retaining ring already scratched by the blade material probably hit the casing and made the groove. The higher Si content could be from the sand which might have scratched the casing on impact with the ground.

3.1.3. A third-stage blade. A third-stage unburned blade was retrieved which had unusual "screw-thread-like-cuts" across the midrib region. The marks of the "cuts" are shown in Fig. 3. The cuts were both fine and coarse. The energy dispersive spectroscopy of the cut surfaces was performed to find the object which had hit it: the results are summarized in


Table 4. It was assumed that the elements which scratched the cut surfaces were retained better in the finer grooves. Inside the cut surface (region C), and especially inside the fine cuts (region D), high concentrations of Mg and Zn were present, while Ni and Fe were also detected. Since these elements were not present in the region away from the cuts, it was quite likely that the object which made the cuts was rich in them. The Mg and Zn had probably come from the casing material, while the Ni and Fe could be from the retaining ring. It seems that a broken piece of the retaining ring was first scratched by the casing material and then hit the blade, making the cuts.

4. FRACTOGRAPHY

The retrieved parts were subjected to fractography. Some of the parts had thick deposits, which were carefully removed using appropriate chemicals [2], and the fracture surfaces were studied under optical and scanning electron microscopes. For the purpose of a comparative study, specimens from similar components were freshly fractured in the laboratory and studied.

4.1. Fractography of retaining ring

The retrieved fracture surface of the retaining ring wire was studied using optical, stereo and scanning electron microscopes. The side view in Fig. 4(a) shows a small flat fracture surface which has then tilted to an angle of -30 . The flat region formed a length of -0.3 mm along the diameter of the wire, which was ~1.5 mm.

Detailed fractography was not possible without removing the thick deposits from the surface of the wire. After chemically cleaning the deposits, fractography was conducted in a scanning electron microscope. The fracture surface in the "flat" region was almost featureless and did not show any signs of ductile failure [Fig. 4(b)]. The typical "thumbnail"-shaped striations led to the edge from where the crack seemed to have initiated. The fracture surface in Fig. 4(b) shows deformation bands, indicating a gradual propagation of the crack front. The structure in Fig. 4(c) further shows that the bands on the fracture surface were related to longitudinal cracks, indicated by arrows (X), on the surface along the length of the wire. A closer look at the features in Fig. 4(c) reveals that the location of crack initiation matched one of the more prominent surface cracks well. These features are the subject of a detailed study and are discussed in Part II of this study [3].

4.2. Simulated laboratory experiments

To ascertain the mode of failure, specimens from the retrieved retaining ring were subjected to tensile, impact and fatigue loading. The specimen tested in uniaxial tension showed considerable "necking" and deformation before it failed [Fig. 5(a)]. Attempts to fracture the ring using impact loading failed, and only bending was registered. A notch was then introduced halfway across the diameter of the wire and tested under the impact load. Even this resulted in bending of the wire before it fractured [Fig. 5(b)]. The fracture surface showed ductile fracture. In the deformation "cups" fine and uniformly distributed Ca-AI - Mg-Si-rich non-metallic inclusions were observed. Neither of the two specimens resembled the retaining ring retrieved from the failed engine.

A piece of wire was then subjected to fatigue cycles in the tension-tension mode.* The fracture surface compared well with the surface of the ring retrieved from the accident. The side view showed a flat region formed due to the fatigue crack and a shear lip due to the final rupture [Fig. 6(a)]. The end view in Fig. 6(b) shows steps in the crack propagation similar to those in the retrieved wire. As observed in the retrieved ring, the steps in the crack propagation matched the surface cracks on the cylindrical surface of the wire [Fig. 6(b)].

*A constant stress of 16.2 ksi was applied and an oscillating sinusoidal stress of +16.2 ksi was superposed. The specimen failed after 105 cycles.


Fig. 4. Retrieved fracture surface of a third-stage retaining ring showing: (a) 0.3 mm flat region at one end, (b) "thumbnail" pattern of crack growth (marked Y) and (c) longitudinal cracks along the length of the wire (marked X).

Fig. 5. Fracture surfaces of retaining ring tested in laboratory under: (a) uniaxial, and (b) impact loading.


Fig. 6. (a) Laboratory-simulated fatigue testing of a third-stage retaining ring showing flat region at one end. (b) Fracture surface of retaining ring tested in fatigue, showing "thumbnail" pattern of crack growth (marked Y) and longitudinal cracks along the length of the wire (marked X).

4.3. Fractography of the third-stage blades

The blades retrieved from the accident included a number of blades fractured from the midrib region, while one had unusual "screw-thread"-like cuts in the airfoil region.

4.3.1. A blade with "screw-thread"-like cuts. The "screw-thread"-like cut marks on the blade in Fig. 3 were compared with the composition and pitch of a typical screw used in the compressor region. Neither the pitch nor the composition of the screw, or its coating (see Table 1), match the marks and the composition of the surfaces of cuts on the third-stage blade.

4.3.2. Three blades fractured from a region near the midrib area. Three third-stage compressor blades were recovered unburned. They experienced fracture near the midrib region. To compare the features of the fractured surface, specimens were fractured in the laboratory. The fractographs of the blades fractured during the accident and the blades fractured in the laboratory using impact loading at the fiat surface (pressure side) of the blade showed marked similarities. This result seemed to indicate that, during the accident, the blades failed on impact and so were not the primary cause of failure.


4.3.3. A blade recovered from the burned region. One blade recovered from the wreckage of the compressor region remained intact with the disc. The fracture surface indicated that the fracture proceeded due to an "overload". The fracture surface then burned. The blade did not show any signs of the primary cause of failure.

4.4. Compressor casing

A side of the compressor casing section, near location #11 in Fig. 2, was studied with a scanning electron microscope. The fracture surface shows a well-developed "chevron pattern". The direction of crack propagation is from the stage 3 turbine region to the stage 1 turbine region. The origin of the crack is not within location #11. The pattern indicated discontinuous growth. Multiple crack initiations developed in the main crack front. Typically, cracks grow in this way in moderately brittle materials [4].

5. DISCUSSION

The fractography of the retaining ring eliminated the possibility of its failure during/after the accident. A marked similarity was observed between the retaining ring specimen subjected to fatigue failure in the laboratory and the specimen retrieved from the accident. The retaining ring seemed to have failed due to a crack growth mechanism prior to the accident, which probably caused the subsequent damage resulting from the accident.

A probable sequence leading to the damage may be as follows. The retaining ring failed due to fatigue and got stuck between the casing and the blade. The ring was scratched by material from the blade. It subsequently came out and hit other regions of the casing, making grooves, e.g. at location #10. The grooves were generally rich in the blade material, while a deeper groove, showing a more severe cut in the material, was rich in Cr and Fe.

5.1. Probable cause of fatigue failure of the retaining ring

An important question is "Why did the ring fail by fatigue prior to its anticipated service life?" The metallurgical study of the retaining ring revealed that the material had an inhomogeneity along the wire axis. A detailed study of the material and the defects in it was carried out, and the results are presented in [3]. Elongated bands of non-homogeneously distributed defects probably resulted in surface cracks when under strain. A deeper crack seemed to become the initiation site for the fatigue crack.

6. SUMMARY

The retaining ring was made of stainless steel AISI 304. When subjected to a cyclic load, cracks formed along the axis of the wire and a deeper crack became the site of a fatigue crack initiation. The fatigue crack developed in the retaining ring at a location approximately 3 in. from one end. The crack grew with time up to a length of -0.3 mm in cross-section. The region of the wire with reduced cross-section finally failed by shearing. The 3-in.-long broken piece of the ring got stuck between a third-stage blade and the casing, and led to the blade failure which subsequently caused the accident.

REFERENCES

1. Metals Handbook (9th edn), Vols 1-3, ASM, Metals Park, OH (1989). 2. SEM/TEM Fractography Handbook, pp. 5-9, compiled by McDonnell Douglas Astronautics, Huntington Beach,

CA. Published by Metals and Ceramics Information Center, Battelle Columbus Laboratories, 505 King Avenue, Columbus, OH (1975).

3. A. Tauqir and I. Salam, Submitted to Engng Failure Analysis. 4. Metals Handbook (9th edn), Vol. 8, ASM, Metals, Park, OH (1989).

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