Engin Analysis of a Diesel Generator Crankshaft Failure

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Analysis of a diesel generator crankshaft failure

F. Jimnez Espadafor *, J. Becerra Villanueva, M. Torres Garca

Departamento Ingeniera Energtica, Escuela Superior de Ingenieros, Universidad de Sevilla, Camino de los Descubrimientos S/N, 41092 Sevilla, Spain

a r t i c l e i n f o

Article history:

Received 3 March 2009

Accepted 11 March 2009

Available online 20 March 2009

Keywords:

Crankshaft

Engine failure

Fatigue crack initiation

a b s t r a c t

This paper analyses a catastrophic crankshaft failure of a four-stroke 18 V diesel engine of a

power plant for electrical generation when running at a nominal speed of 1500 rpm. Therated power of the engine was 1.5 MW, and before failure it had accumulated 20,000 h

in service operating mainly at full load. The fracture occurred in the web between the

2nd journal and the 2nd crankpin. The mechanical properties of the crankshaft including

tensile properties and surface hardness (HV1) were evaluated. Fractographic studies show

that fatigue is the dominant mechanism of crankshaft failure, where the beach marks can

be clearly identified. A thin and very hard zone was discovered in the template surface

close to the fracture initiation point, which suggests that this was the origin of the fatigue

fracture. A finite element model of the crankshaft has predicted that the most heavily

loaded areas match the fractured zone.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Diesel plants for electrical power generation are especially sensible to outage events. In case of a crankshaft failure, the

cost of the reparation includes not only that of the crankshaft itself, but also the cost of other parts of the engine affected by

crankshaft failure (connecting rod, piston, cylinder, and bearings) and the lengthy time period required for repair, mainly

because of the crankshaft location inside the engine. Smith and Donovan conducted a study [1] for US Army Engineering

and Housing Support Centre (EHSC) which showed results of up to 2 MW from diesel engines. This study includes a detailed

classification of the parts involved in the failure in this power range and reveals that even though failures per year related to

the engine crankshaft were low, these resulted in a higher mean time to perform corrective maintenance. Similar conclusions

are shown in[2,3].

The most common cause of crankshaft failure is fatigue. In order for fatigue to take place, a cyclic tensile stress and a crack

initiation site are necessary. Diesel engines crankshafts in power plants run with harmonic torsion combined with cyclic

bending stress due to the radial loads of combustion chamber pressure transmitted from the pistons and connecting rods,

to which inertia loads from pistons and connecting rods have to be added. Although crankshafts are generally designed witha high safety margin in order to not exceed the fatigue strength of the material, the high cyclic loading and local stress con-

centrations allow cracks to grow even when fatigue strength does not exceed in average values. Pandey[4]analysed failures

in the crankshafts of 35 hp two-cylinder engines used in tractors, where the break plane was located between the main bear-

ing and the journal. The crack started in the crankpin web region in a plane of about 45with respect to the rotational axis,

showing a typical fatigue failure with beach marks. The stress related to the fatigue initiation was estimated at 175 MPa, far

below the tensile stress of the nodular cast iron of these crankshafts which is close to 680 MPa. Taylor et al. [5] developed

two fatigue experiments in a crankshaft of a four-cylinder engine made of spheroidal graphite cast iron, with a tensile

strength of 440 MPa: one torsional and the other flexural. The crankshafts underwent torsional and flexural cyclic loading

1350-6307/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2009.03.019

* Corresponding author. Tel.: +34 95 448 72 45; fax: +34 95 448 72 43.

E-mail addresses:[email protected],[email protected](F.J. Espadafor).

Engineering Failure Analysis 16 (2009) 23332341

Contents lists available at ScienceDirect

Engineering Failure Analysis

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until failure, and in both types of tests the same fracture angle of 45with respect to the rotational axis was observed. Yu

et al.[6]investigated the fracture of the web between the 2nd journal and the 2nd crankpin of the crankshaft of a four-cyl-

inder diesel engine of a truck plant. The failure occurred after 200 h in service and the fracture plane was inclined by about

45with respect to the shaft axis. The macroscopic view of the fracture surface indicated stable crack growth regions with

beach marks in the middle of it. Bhaumik et al. [7] studied the fracture of the crankshaft of a four-cylinder aircraft engine

made of steel forging of SAE 4340 grade and case hardened. The fracture had occurred along the webs at No. 2 and No. 3

journals after 1460 h in service and 262 h since the last overhaul. In both cases, the fracture took place along the web radius,

and transverse to the axis of the crankshaft. In journal 3 the fatigue crack had propagated to about 80% of the web cross sec-

tion before giving rise to the final overload fracture. In these cases it was possible to arrive at the fracture origin by tracing

back the beach marks, which were found to be in the web radius region. Other investigations related to crankshaft failures

produced similar results[8,9]: the fracture was due to fatigue initiated by cracks located at the web fillet radius and pro-

gressed to the journal inner radius up to the final overload fracture.

The present work is focused on a methodology that has allowed predicting the point of crack initiation at the surface of

the crankshaft, considering both torsional and bending loads, via the evaluation of the von Mises stress at the crankshaft sur-

face. It is based on the results of a dynamic lumped model developed jointly with a finite element model. The whole meth-

odology has been applied to a case study of a catastrophic failure of a diesel generator crankshaft. The names of the engine

manufacturers and power plant owners have been omitted to preserve the anonymity of the parts involved.

2. Crankshaft material and failure description

2.1. Engine description, crankshaft material and composition

The power generator is integrated by a four-stroke 16 V 60turbo diesel engine coupled to an electrical generator. This

belonged to a 10 MW diesel power plant on a Spanish island which was not connected to the national electrical grid. The

system operates at 1500 rpm with a maximum electrical power of 1500 kW. Opposed cylinders of the two engine banks

are linked to the same crankshaft journal. This results in an uneven firing angle between cylinders; cylinders coupled to

the same crankpin fire 60each and cylinders linked to a different crankpin fire 30each.

The engine is linked to the alternator through a flexible coupling. Between the coupling and the engine there is a flywheel.

At the free end there is a viscoelastic damper for detunning the natural frequency of the torsional system and for dissipating

the energy produced by torsional vibrations.

The crankshaft was made of low alloy steel forged as one, then machined and, finally, tempered. Chemical analysis of the

fractured crankshaft was carried out using a spectrometer, and is given in Table 1. Standard cylindrical tensile specimens

were machined from a crankpin portion far from the broken plane. Tensile properties are listed out in Table 2, and are within

the expected range for this application.

2.2. Crankshaft failure description

The crankshaft broke after 20,000 h of continuous operation next to maximum rating.

The fracture had occurred in the crank next to the alternator end, between the cranks where cylinders 1516 and 1413

are connected, seeFig. 1, and took place along the web radius, seeFig. 2. The fracture plane at the fillet was inclined by about

45with respect to the shaft axis. Careful examination revealed that the fracture surface had beach marks, with a progressive

crack that had propagated to about 70% of the cross section. These observations indicate a fatigue failure considered as being

a high cycle-low stress type. The initiation point can be observed from the evolution of the elliptical lines and is located on

the surface of the web radius, which is pointed out in Fig. 2. In the same figure the extension of the low speed growth of the

crack can be observed. So, the fracture surface commonly appears in a fatigue fracture, initiation point, low speed region with

beach marks and high speed fracture region [10].

2.3. Hardness and microstructure investigation

Two samples were taken from the crankshaft. One was cut vertically from the journal close to the fracture for the sake of

crack route observation (second journal). Another one was cut from a journal next to the cracked journal, but was not dam-

aged (third journal). Both samples are shown inFig. 2and are named R and L, respectively.

Fig. 3shows the specimen cut from part L (seeFig. 2), where the lubricating oil channel can be seen next to the results of

the hardness test (HV1) through the tempered zone (see the white arrow indicating hardness measurement direction). A

Table 1

Chemical composition of the fractured crankshaft (wt%).

C Si Cr Mn Mo Ni V S P

0.38 0.29 1.11 0.83 0.23 0.42 0.10 0.012 0.014

2334 F.J. Espadafor et al./ Engineering Failure Analysis 16 (2009) 23332341


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uniform tempered thickness of about 4 mm can be appreciated, with a smooth hardness evolution, from journal surface to

the more ductile nuclei.

The micrograph of Specimen L is shown in Fig. 4, where a matrix formed by ferrite and pearlite can be appreciated. The

image is compatible with an induction surface hardening treatment and does not reveal any anomaly.

Fig. 5shows the results of the hardness test (HV1) through the tempered zone of part R. The white arrow indicates the

hardness measurement direction. In this case the tempered thickness is not uniform, it presents low depth (about 1 mm) and

a very high hardness value (up to 670 HV1) as compared to part L. The hardness transition to the more ductile nuclei is also

quite sharp, giving an all together fragile behaviour to this zone.

Table 2

Tensile properties.

Yield strength,r0.2 (MPa) Tensile strength,rb (MPa) Elongation, d5 (%)

760 895 16

Fig. 1. Finite element model of the crankshaft.

Fig. 2. Fracture surface in the crankshaft.

F.J. Espadafor et al. / Engineering Failure Analysis 16 (2009) 23332341 2335


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The micrograph of specimen R is shown inFig. 6. Here a matrix formed by martensite can be seen next to surface cracks

that show indications of oxidation which are not recent.

Fig. 3. Specimen L cut and hardness distribution through tempered zone.

Fig. 4. Micrograph of specimen L.



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3. Estimation of maximum stress level and location in the healthy crankshaft

In order to analyse crankshaft failure more profoundly, a torsional lumped dynamic model of the system with 12 degrees

of freedom (DOF)[11]linked to a finite element (FE) model was developed.

3.1. Torsional lumped system model

The inertia of each DOF was evaluated directly from crankshaft dimensions, and material characteristics and torsional stiff-

ness of each crank were simulated with FE.Fig. 7shows the lumped system scheme next to the FE model, which has allowed

computation of the stiffness of each crank through a torsional static simulation. There are eight cranks, with two pistons linked

to each one. Positions A and B inFig. 7refer to different stiffnesses of these cranks, which are 11.9 MN m/rad for DOF A and

13.07 MN m/rad for DOF B. The alternator inertia is enormous (91 kg/m2), which is typical of a diesel engine generator, andthe inertia of the flywheel and torsional damper are 18.2 kg/m 2 and 5.82 kg/m2, respectively. The inertia of the cranks is be-

tween 1.06 kg/m2 and 1.22 kg/m2. The torsional damper is connected to the last crank through a fluid of high viscosity, in this

case, it is connected through silicon-based oil. The drag torque is proportional to the instantaneous angular velocity, and the

damping coefficient depends on the clearance between the external ring and the housing that is attached to the last DOF.

The main contribution to the torque on each crank is due to the pressure developed on the top of the piston of every cyl-

inder. This was simulated through the development of a combustion model[12]compatible with engine power at nominal

rating (1500 kW to 1500 rpm). Also mechanical losses produced by friction[13]were considered. The torsional lumped sys-

tem model has been formulated as:

JJalter:hC_hKh Mfric:h; _h;l;dim: Malternator Mindicated 1

whereJis theinertiamatrixassociatedto elements which rotate in the system (kg m2);Jalter the inertia matrix associated to recip-

rocating elements (kg m2); h theacceleration vector for each DOF(rad/s2); Kthe stiffness matrix (N m/rad);h theangulardisplace-

ment vector foreach degree of freedom (rad); Cthe damping matrix (N m s/rad);_h the velocity vector for each DOF (rad/s);l the

Fig. 5. Hardness distribution of specimen R through tempered zone.

Fig. 6. Micrograph of specimen R.



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torsional damper dynamicfluid viscosity (N s/m2); Mfric:themechanicallosses foreach DOF (N m); Malternatorthe alternator torque

(N m); andMindicatedis the torque for each DOF generated by the combustion chamber pressure at each cylinder (N m).

Solving the model (1) produces the instantaneous angular oscillation of each DOF from which torsional loads in the whole

crankshaft can be estimated.

3.2. Finite element model

A finite element (FE) model of the crankshaft was developed using MSC/Nastran for Windows, withFig. 1being the model

used for stress analyses. The model was meshed using a four-node tetraedric solid element, which is well suited to model

irregular meshes. The whole model has 62,990 elements and 17,172 nodes. Each journal is supported by its bearing which

allows free rotation. In order to consider the proper reaction when the forces applied to each crankpin are considered, journal

and bearing are linked through non-linear gap elements. These develop radial forces only when both surfaces are com-

pressed. The same type of element has been used to transmit the forces from the connecting rod big end to each crankpin.

3.3. Loading conditions

Two different types of loads were applied simultaneously to the crankshaft:

Torsional loads. These are derived from the lumped model described in 3.1. On each of the planes where the DOF are

defined, seeFig. 7b, the angular displacement produced by the dynamic lumped model has been imposed. Fig. 8 shows

Fig. 7. (a) Dynamic lumped model scheme of the crankshaft and (b) finite element model of a crank.



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the angular displacement on the eight cranks during one engine cycle. The oscillation amplitude is higher for the crank

closer to the engine generator and progressively diminishes till the last crank links to the damper.

Radial and tangential loads. The connecting rod big end transmits gas pressure from each cylinder to every crankpin as

forces distribute along the pin surface. These forces can be decomposed in tangential, which produces engine torque,

and radial, which produces bending on the crankshaft. Fig. 9shows radial and tangential forces during an engine cycle,

where the 0crank angle indicates the top dead centre of this cylinder. As can be observed, the maximum of the radial

and tangential force is not produced at the same crank angle.

3.4. Finite element analysis and results

Torsional loads and radial and tangential forces described in Section 3.3 were applied to the FE model shown in Fig. 1.

These loads applied simultaneously are an estimation of the dynamic loading of the whole crankshaft and, therefore, the

FE results are close to its stress behaviour under operation. Fig. 10a shows equivalent von Mises stress distribution provided

from the FE analysis. The maximum estimated stresses are between 260 MPa and 275 MPa; which is about 40% of the yield-

ing point of the material, the most loaded cranks being closer to the alternator end. InFig. 10b, crankshaft zones have been

highlighted where von Mises stresses are maximum for the four cranks closer to the generator side. It can be observed that

the cracked zone of the crankshaft, seeFig. 2, is located in the same part as shown in Fig. 10b.

4. Conclusions

The crankshaft fracture of a 1.5 MW diesel engine generator was due to fatigue. The fractured crank was the closest to the

generator end of the crankshaft. It was feasible to identify the crack initiation due to the elliptical beach marks, which were

located between the crankpin and the main journal. The fracture plane formed an angle of about 45 with respect to the

crankshaft axis. The fracture was investigated by material analysis and by a dedicated simulation model. Hardness and

microstructure investigation revealed that the failure area had a martensitic structure with very high hardness distributed

in very thin depth. Surface cracks were also observed. On the other hand, the healthy part of the crankshaft had a microstruc-

ture formed by ferrite and pearlite with appropriated hardness.

Fig. 8. Angular displacement for all the cranks during one engine cycle.

Fig. 9. Radial and tangential forces over crankpin for an engine cycle.



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The simulation model was composed of a dynamic lumped model linked to a finite element model. The analysis of the

simulation results revealed different zones where the maximum stresses are reached, which could be considered to have

the same failure probability, with the estimated stress level for fatigue initiation being in the range 260275 MPa. One of

these zones coincides with the zone where the fatigue failure originated and so it is well matched to the results of the mate-rial analyses. It can be concluded that the failure was produced in one of the most stressed areas, where the material had

anomalies. By using this simulation analysis, location of the failure can be predicted. This can be considered as a tool for

selecting crankshaft areas where especial quality control should be considered in order to prevent fatigue failures.

References

[1] Smith CA, Donovan MD. Reliability survey of 6001800 kW diesel and gas-turbine generating units. IEEE Trans Indus Appl 1990;26:74155.[2] Akhmedjanov FM. Reliability databases: state-of-the-art and perspectives. Roskilde, Denmark: Risk National Laboratory; 2001.[3] Energy and Environmental Analysis, Inc. Distributed generation operational reliability and availability database. Oak Ridge: Oak Ridge National

Laboratory, EEUU; 2004.[4] Pandey RK. Failure of diesel engine crankshafts. Eng Fail Anal 2003;10:16575.[5] Taylor D, Ciepalowiz AJ, Rogers P, Devlukia J. Prediction of fatigue failure in a crankshaft using the technique of crack modelling. Fatigue Fract Eng

Mater Struct 1997;20(1):1321.[6] Yu Z, Xu X. Failure analysis of a diesel engine crankshaft. Eng Fail Anal 2005;12:48795.

Fig. 10. Equivalent von Mises stress field for dynamic loading of the crankshaft: (a) complete stress field of the whole crankshaft and (b) stress field in zones

between 260 MPa and 275 MPa.



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[7] Baumik SK, Rangaraju R, Venkataswamy MA, Bhaskaran TA, Parameswara MA. Fatigue fracture of crankshaft of an aircraft engine. Eng Fail Anal2002;9:25563.

[8] Craighead IA, Gray TGF. Investigation of diesel generator shaft and bearing failures. Proc Inst Mech Eng Part-k 2004;218(3):1538.[9] Fonte M, Freitas M. Marine main engine crankshaft failure analysis: a case study. Eng Fail Anal 2009;16(6):19407.

[10] Lund RA, Sheybany S. Fatigue fracture appearance, Failure analysis and prevention. Metals handbook, 9th ed., Vol. 11. American Society for Metals;1996.

[11] Becerra JA. Metodologa para el estudio de las causas de rotura de cigeales en motores de combustin interna alternativos y compresoresalternativos. Aplicacin en un modelo de mantenimiento predictivo. PhD Dissertation, Universidad de Sevilla; 2007.

[12] Rahnejat H. Multi-body dynamics. UK: Professional Engineering Publishing; 1998.[13] Rezeka, SF, Henein NH. A new approach to evaluate instantaneous friction and its components in internal combustion engines. SAE paper no. 840179;

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Engin Analysis of a Diesel Generator Crankshaft Failure

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