Summary
Two compressor rotors of an aircraft engine sustained severe damage during operation. In both rotors, several blades got sheared off with some blades lifted from the dovetail portion of the drum.
Several mounting dovetails on the rotor drum had fractured. The severity of damage progressively decreased toward downstream.
Detailed fractographic studies revealed that the damages on both the rotors were due to SCC of the dovetail mountings.
Background
Two compressor rotors of an aircraft engine, identified as “A”
and “B,” suffered serious damage during operation. Detailed anal-ysis was carried out to find the cause of the damage.
Visual Examination of General Physical Features
Compressor Rotor A
It was observed in compressor rotor A that all the blades in the third and fourth stages had been sheared off, with some blades
lifted from the dovetail portion of the drum (Fig. CH21.1). Most of the fracture surfaces except those at the dovetail on the drum were obliterated by rubbing. The first-stage blades were largely intact with minor damages. The second- and fifth-stage blades on either side of the sheared blades had suffered extensive damages in the form of denting and tip bending. From the fifth stage on-ward, the severity of damage had progressively decreased toward the 10th stage, where it was only minor.
Several mounting dovetails on the rotor drum that had fractured in the third and fourth stages showed two distinct regions: a half-moon-shaped region typical of progressive failure and a region of rapid fracture with clearly delineated chevron markings (Fig.
CH21.2). The extent of the half-moon-shaped region varied among the fractured dovetails. The half-moon-shaped regions were con-siderably corroded and were dark brown in color compared with the light gray color of the rapid fracture region.
Compressor Rotor B
In this rotor, most of the blades on the first four stages had sheared off, with several blades lifted from the dovetail portion on the drum, particularly in the first two stages (Fig. CH21.3). The blades on the rest of the stages had suffered varying degrees of
3rdstage
4thstage
Fig. CH21.1 A view of the compressor rotor A showing the third- and
fourth-stage blades sheared at their roots Fig. CH21.2 Close-up view of one of the fractured dovetails in rotor A showing regions of slow crack growth and rapid fracture
DOI:10.1361/faes2005p115 www.asminternational.org
116 / Failure Analysis of Engineering Structures: Case Histories
damage, the severity of damage decreasing progressively toward the 10th stage. The roots of a few sheared blades were relatively intact, and their fracture surfaces showed features typical of over-load. On this rotor also, several mounting dovetails on the drum had fractured, particularly in the first two stages. The fractured dovetails exhibited the same two distinct regions that were ob-served on the broken dovetails in rotor A.
Testing Procedure and Results
Scanning Electron Fractography
A fractured dovetail containing the two regions was carefully cut from one of the rotors. After ultrasonic cleaning, the fracture
surface was observed in a SEM. The discolored half-moon-shaped region showed extensive intergranular fracture (Fig. CH21.4), and the region of fast fracture showed ductile/brittle mode of fracture.
Chemical Analysis
Semiquantitative analysis of the blade and the drum showed that they were made of the same material, i.e., high-chromium steel.
Metallography
Metallographic examination was carried out on samples taken from a blade and a dovetail region, after etching with a solution containing 2.5 g ferric chloride, 5 g picric acid, 2 ml hydrochloric acid, and 90 ml ethyl alcohol. Both the blade and the dovetail portion showed tempered martensitic structure, with carbides dis-tributed along the grain boundaries and in the interior of grains (Fig. CH21.5). The dovetail portion showed more carbides along the grain boundaries than the blade material. The dovetail material had a grain size between ASTM 5 and 6, while the blade had a finer grain size of ASTM 8. The dovetail portion showed a network of intergranular cracks (Fig. CH21.6).
Hardness
The blade as well as the dovetail material showed the same hardness of 310 HV.
Discussion
The presence of two distinct regions on the fracture surface of the broken dovetail clearly indicates delayed failure. The
discol-4thstage 3rdstage
2ndstage 1ststage
Fig. CH21.3 A view of compressor rotor B showing blades sheared in the first four stages
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Fig. CH21.4 SEM fractograph showing intergranular fracture in the slow-crack-growth region
25 m
Fig. CH21.5 Microstructure of dovetail material showing tempered mar-tensite with grain boundary carbide
ored half-moon-shaped regions represent slow, progressive crack growth, while the brighter regions with chevron marks represent rapid fracture. Intergranular fracture features on the half-moon-shaped region, as well as the network of branching intergranular cracks, is typical of SCC.
The way damages occurred on the rotors indicate that SCC had initiated and propagated simultaneously on several dovetail mountings on the drum (in the third and fourth stages on rotor A and in the first and second stages in rotor B). When one of the cracks had grown to critical dimensions, it would have led to frac-ture. Once the dovetail mounting cracks, it is easy for the blade mounted on its slot to get loose and fly off. Once a blade is de-tached, it can cause all the subsequent damages at the high speed of rotation.
In rotor A, the maximum secondary damage is seen in the sec-ond and fifth stages, which are adjacent to the third and fourth stages in which the blades are completely sheared. In this rotor, it is reasonable to suspect that a dovetail mounting on the second stage would have cracked first, leading to the loosening of a blade and the subsequent damages. In rotor B, it is probable that a dove-tail mounting in the first stage would have cracked initially, leading to all other damages.
Because the blade and the dovetail mountings are made of the same material, it is reasonable to expect SCC on both. But it is possible that, while the blades are subjected to stress only during rotation, the dovetail mountings can experience constant sustained stress arising from the mounting even while on ground and hence can become potential sites for SCC. This stress acting simulta-neously with the saline atmosphere while the aircraft is on the ground can cause SCC. Also, the damages are maximum in the first few stages on the suction side where the intensity of corrosion is expected to be maximum. It is possible that the finer grain size and less carbides in the grains in the blade material, compared with the dovetail material, can give it better stress-corrosion resis-tance.
Conclusion
The damages observed on both the rotors can be attributed to SCC of dovetail mountings.
Recommendations
The rotors should be subjected to fresh water compressor wash from time to time, and the engine intakes should be kept covered in coastal areas.
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Fig. CH21.6 Intergranular branching cracks in the dovetail region