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外文原文及翻譯
Failure analysis of a crankshaft of a helicopter engine
直升機(jī)發(fā)動機(jī)曲軸失效分析
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Contents lists available at ScienceDirect
Engineering Failure Analysis
journal homepage: www.elsevier.com/locate/engfailanal
EngineeringFailureAnalysis100(2019)49–59
Failure analysis of a crankshaft of a helicopter engine
V. Infantea,?, M. Freitasa,b, M. Fontea,c
a LAETA, IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
b ATLANTICA – University School of Management, Health, Technology and Engineering, Fábrica da Pólvora, 2730-036 Barcarena, Portugal
c Nautical School (ENIDH), 2770-058 Pa?o de Arcos, Portugal
A B S T R A C T
A detailed analysis of a crankshaft failure belonging to a helicopter engine is presented. The main objective of this work was to analyze the characteristics of the failure and determine the root cause of the failure of the crankshaft. In order to determine the causes of the crankshaft failure, a material analysis was performed, followed by a detailed observation of the failure mechanisms through macroscopic, microscopic and micro- structural examinations of the fracture surface. A preliminary observation of the fractured crankshaft indicates that this failure occurred by a fatigue process where the fracture surface shows obvious signs of cyclic propagation mechanisms. The existence of a large number of beachmarks indicates signi?cant crack growth characterized by the e?ect of successive starts and stops of the engine by the operating conditions. These beachmarks cover about two-thirds of the total area of the fracture surface and the uniform geometric pattern of the crack front, along the entire propagation zone, allows to conclude that the fatigue process occurred from a loading state consisting essentially of cyclic bending stresses between the crankweb and the main journal of the crankshaft. No original defect was observed either on the surface or inside the material that could be the source of the crack initiation and growth and subsequent ?nal fracture of the component. The analysis of the shell bearings applied to the main journal revealed a signi?cant damage, with fractured location lugs, that are believed to be at the origin of the crack initiation of the crankshaft.
1. Introduction
The crankshaft is a structural component and one of the critical components of an engine which converts the linear piston movement into rotary motion while the force connecting rod is transformed to torque. The dominant failure mode of engine crankshaft is fatigue since this type of component is subject to combined torsion stresses with rotating bending stresses [1,2]. The factors that can limit the fatigue strength of a crankshaft are the loading in service, the incorrect design of the component or materials, manufacture defects or poor lubrication of the rotating parts. The crankshaft failure can promote damage in other engine parts such as pistons, connecting rod, etc. [3]. The failure analysis of this kind of damage can contribute to avoid similar failures taking into account mitigation actions. Therefore, results of failure investigations of the engine crankshaft have been presented in several studies [1–13].
The work [6] presents results of the failure analysis of a truck diesel engine crankshaft where the failure mechanism of the
crankshaft was multiple-sources of fatigue and fracture. The authors concluded that the absence of the hardened case on the journal was mainly responsible for failure.
Lucjan Witek et al. [7] show an interesting study about the failure of a diesel engine crankshaft presenting a numerical analysis where large stresses were observed in the ?llet of the crankpin. The failure of the crankshaft was associated to high-cycle fatigue. Analysis of automobile crankshafts were presented in [8,9]. B. Kareem [8] found that torsional vibration and poor lubricity were the major factors responsible for failures and presented guides for selecting appropriate vehicular crankshafts. In [9] a probabilistic
and statistical analysis was presented to obtain the durability of the component.
? Corresponding author.
E-mail address: virginia.infante@tecnico.ulisboa.pt (V. Infante).
Availableonline26February2019
https://doi.org/10.1016/j.engfailanal.2019.01.072
Received 20 November 2018; Received in revised form 21 January 2019; Accepted 23 January 2019
1350-6307/?2019ElsevierLtd.Allrightsreserved.
EngineeringFailureAnalysis100(2019)49–59
V. Infante, et al.
Fig. 1. Helicopter engine.
Results of a crankshaft failure are presented by Wei Li et al. [10] where experimental methods and theoretical calculation are used to conclude that the fracture was due to high stress concentrations.
The fatigue and fracture behaviors of ductile cast iron of crankshafts were evaluated in article [11]. The fracture mechanisms were micro-cracks and secondary cracks (near nodular graphite).
The components, crankshaft and respective shell bearings analyzed in this study belong to a helicopter engine as shown in Fig. 1. The crankshaft failed between the main journal 3 and the crankweb and the two parts of the fractured crankshaft are shown in Fig. 2. The shell bearings that laid at the bedplate of the engine main journals are also shown in Fig. 3. These shell bearings were identi?ed with the numbers corresponding to the crankshaft main journals (Fig. 4) and to the right or left side in terms of the position of the crankcase in the helicopter (Fig. 5).
Fig. 6 shows the right side of the crankcase with the failed crankshaft fractured along the transversal web section starting at the main journal ?llet. n. 3.
2. Crankshaft failure
2.1. Engine
The helicopter engine (Fig. 1) is an air-cooled six-cylinder, horizontally opposed (boxer type) and a capacity of 200 HP. The right side of the crankcase is presented in Fig. 6, where it is possible to identify the part of the failed crankshaft from main journal 1 to main journal 3 where the failure is observed.
2.2. Crankshaft
The fracture surfaces of the crankshaft, presented in Fig. 7 were observed visually and by optical microscopy in order to char- acterize the fracture surfaces and to identify the initiation and propagation of the cracks that lead to component failure.
Fig. 8 shows one of the fracture surfaces corresponding to the side A of the crankshaft presented in Fig. 7, showing plane and ?at zones where fracture initiation is evident through fatigue cracks.
A more detailed observation of this fracture surface reveals that there is a propagation of a crack surface compatible with the occurrence of fracture mechanisms dependent of the loading cycles. The beachmarks on the fracture surface near the initiation zone are fatigue propagation characteristics [12].
Fig. 2. Crankshaft with integral fracture next to main journal 3.
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Fig. 3. Shell bearing n. 2, 3 and 4, Right and Left.
Fig. 4. Crankshaft components overview.
In the crack propagation zone, there is a lubrication hole, as shown in Fig. 8.
From the observation by optical microscopy of the fracture surface it is veri?ed that the fractured region presents a relatively regular surface with two di?erent zones of the fracture surface: the ?rst one, near the crack initiation and stress concentration indicating a crack initiation zone that propagates slowly along the surface causing fracture of the component and the second one where the propagation is fast until the ?nal fracture.
The presence of beachmarks was observed at the fracture surface, covering about 2/3 of the fractured section area (Fig. 9). However, the remaining area presents a distinct morphological pattern, characterized by a greater surface roughness caused by an overloading due to the smaller resistant area, which occurs suddenly and that precedes the ?nal instant of integral separation of the component. These beachmarks are well visible and numerous, Fig. 9, indicating a signi?cant fatigue crack growth at the crack tip due to the e?ect of successive starts and stops of the component imposed by the conditions of service. Another characteristic that can be seen from the observation of the beachmarks is their uniform progression pattern, with no signi?cant evidence of a change in the direction of the crack front by torsional e?ects.
A radial line on the crankshaft fracture surface is also visible under optical microscopy, as can be seen in Fig. 10. This radial line
Fig. 5. Crankshaft related parts.
Fig. 6. Crankcase and fractured crankshaft.
Fig. 7. General view of both sides of the crankshaft. a) side A; b) side B.
Fatigue fracture zone
Fig. 8. Fracture surface of the crankshaft (side A).
Fig. 9. Beach marks.
corresponds to the intersection of two planes in which two origins of cracks propagated independently until the ?nal fracture occurred. In Fig. 10 it is also possible to observe the existence of ratcheting marks located in the crack propagation zone. These ratcheting marks starts in the region of maximum e?ort, being associated with the high stresses veri?ed locally [13].
Fig. 11 is a magni?cation image of Fig. 10 in the initiation zone of the two fatigue cracks, crack 1 and crack 2, respectively in Fig. 11 a) and b).
Fig. 12 shows the edge between the fracture surface and the surface of the journal where no initial metallurgical defects were detected but where circumferential marks on the main journal surface are observed.
Fig. 13, shows a photograph of the crankshaft in-plane with the fracture surface, where the di?erent fatigue propagation planes are visible. It can be seen that the plane of propagation of the crack 2 intersects the crack 1 propagation plane, where the presence of some secondary cracking can be observed. It was also observed the presence of circumferential marks along the surface of the main
Fig. 10. Radial line and ratcheting phenomena and initiation zone of the fracture surface.
Fig. 11. Magni?cation of Fig. 14. a) Crack 1; b) Crack 2.
Fig. 12. Edge between the fracture surface and the surface of the component.
Fig. 13. Detail of crank surface.
journal and consistent with the direction of crack initiation and growth. These circumferential marks must have been caused by the contact of the shell bearing n. 3 with the lateral surface in the crankshaft concordance zone, in a movement of the shell bearing parallel to the axis of the crankshaft bearings.
3. Failure analysis
After the observation of the crankshaft fracture surface and the observed damage in the main journal n. 3 it was important to ?nd the mechanism that promoted the initiation of the fatigue cracks leading to the fracture of the crankshaft. During the detail ob- servation of the remaining components of the engine it was observed that the shell bearings did not present the geometries de?ned by the manufacture namely the con?gurations of the location lugs of the right shell bearings n. 3 and 4, as can be observed in Fig. 14. Therefore it was decided to analyze these components in detail.
Fig. 14 a), b) and c) show the left and right shell- bearings n. 4, 3 and 2, respectively corresponding to the respective main journals. It can be seen that on the right shells bearing n. 3 and n. 4 the location lugs that promote the connection between the shell bearings and the main journals are not visible and the location lug of right shell bearing n. 2 is damaged. The visual observation of the left shell bearings n. 2, 3 and 4 did not show any damage in the location lugs location.
The shell bearings were also observed by optical microscopy in order to ascertain the initial existence or not of the location lugs of the shell bearings 3 and 4 (right side).
Fig. 15 shows the location lug of the right shell bearing n. 2 where it is possible to observe its lateral wear. It is evident that if the crankshaft was to remain in operation for a longer time this location lug would eventually be worn out until fully removed.
A detailed observation of the zone adjacent to the location lug of the right shell bearing n. 2 allows detecting cracks in the location lug area (Fig. 16) which are probably due to the manufacturing process of this type of location lug.
Fig. 17 shows the location lug region of the right shell bearing n. 4, which shows the absence of location lug and a transverse
Fig. 14. Shells bearing a) n. 4 right and left; b) n. 3 right and left; c) n. 2 right and left.
Fig. 15. Right shell bearing n. 2. Location lug.
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Fig. 16. Right shell bearing n. 2. Zone of the location lug.
Fig. 17. Right shell bearing n. 4. Cracks located in the location lug zone.
Fig. 18. Right shell bearing n. 3. Zone of the location lug and the neckband.
cracking due to the parallel movement of the cover relative to the axis of the crankshaft main journal bearings. From the observation of Fig. 17 it is foreseen that at the time of the initial assembly, the shell bearing n. 4 had the location lug, and it is possible to conclude that the cause of the location lug wear is due to the axial friction movement of the shell bearing.
The right shell bearing n. 3 also presents cracks due to the existence of a location lug, although location lug is not visible in Fig. 18. Fig. 18 also shows the plastic deformation of the end of the shell bearing n. 3 with the formation of a neckband due to the movement parallel to the axis to which the shell bearing was subjected during of the helicopter engine operation. This evidence shows that the circumferential surface marks on the main journal (Fig. 13) were initiated by the contact of the shell bearing with this surface due to the axial movement of the shell bearing n. 3 against the surface of the crankweb and main journal promoting, not only the plastic deformation of the shell bearing (neckband) but also the appearance of wear marks in the support promoting the initiation of the two fatigue cracks. In Fig. 19 the neckband of the shell bearing can be observed in more detail. It is clear that the observed
Fig. 19. Right shell bearing n. 3. Neckband zone.
plastic deformation and wear of the shell bearing have contributed to the damage of the crankweb and main journal leading to crack initiation and ?nal failure of the crankshaft. Any type of wear phenomena on the left shells bearing was detected.
4. Conclusions
- Visual observation and optical microscopy of the fracture surface shows signs of cyclic propagation mechanisms and the oc- currence of a fatigue process at the web ?llet of crankpin n. 3;
- The existence of a large number of beachmarks indicates the plastic deformation of the material by the e?ect of successive starts and stops of the component imposed by the operating conditions. These beach marks cover about two-thirds of the total area of the fracture surface and the uniform geometric pattern of the front of the crack along the entire propagation zone, allow to conclude that the fatigue process arose from a loading state consisting essentially of cyclic bending stresses between the main journal and the crankweb;
- No defect was observed on the surface of the crankshaft main journal or inside the material that could be the source of the component cracking;
- It was visible the initiation of two distinct cracks that propagated independently;
- The circumferential marks caused by the contact of the right shell bearing n. 3 with the crank surface, show damages introduced by the right shell bearing n. 3 in the crankshaft surface area, which promoted the initiation of the two fatigue cracks on the crankweb surface; the contact of the shell bearing with the crankweb in the fractured zone was due to the wear and fracture of the location lugs of the right shell bearings.
Acknowledgements
This work was supported by "Funda??o para a Ciência e a Tecnologia" (FCT), through the Institute of Mechanical Engineering (IDMEC), under the Associated Laboratory for Energy, Transports and Aeronautics (LAETA), Project UID/EMS/50022/2019.
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