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Case study

Analysis of a failed rocker arm shaft of a passenger car engine

G.A. Nassef a, A. Elkhatib a, Mostafa Yakout b,*a Department of Production Engineering, Alexandria University, Alexandria 21544, Egypt b Department of Mechanical Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7 

1. Introduction

Suddenly during the start-up of a passenger car engine, a high abnormal noise accompanied by a jerky vibration of the

engine had been manifested. After dismantling, the local dealer service found that the rocker arm is broken near the middle

as shown in   Fig. 1(a). The fracture passes across the hole of one of the supporting bolts as shown in   Fig. 1(b). Visual

examination of the car engine showedthat the running distance of the engine just before failure was 40,626 km during which

regular services had been given to the engine as recommended by the manufacturer’s manual.One of the major causes of component failure is faulty manufacturing. This includes all effects that increase brittleness or

those inducing cracks and or stress raisers in the component. Improper heat treatment has been considered as major causes

of many failures in the literature. Examining the causes of the problem, we came across the following cases.

Torronen et al. [1]  examined the brittle fracture behaviour of a Cr-Mo-V alloyed pressure vessel steel after a variety of 

quenching and tempering treatments. They found the effective grain size of martensitic microstructure in the alloyed steel.

Lee et al. [2]  examined the failure of a rocker arm shaft for passenger car in the design stage and the robustness of its

boundary condition using orthogonal arrays and ANOVA. They found that a fatigue crack in rocker arm shaft was initiated at

Case Studies in Engineering Failure Analysis 5–6 (2016) 10–14

A R T I C L E I N F O

 Article history:

Received 30 July 2015

Received in revised form 15 December 2015

Accepted 4 January 2016

Available online 8 January 2016

Keywords:

Root cause analysis

Improper heat treatments

Failure of rocker arm shaft

Hardened steels

Material technology

A B S T R A C T

This paper investigates the failure of a rocker arm shaft of a passenger car. The shaft failedby brittle fracture across one of the four holes supporting the shaft into the cylinder head.

The running distance of the engine just before failure was 40,626 km. Visual examinations

of etched sections of the failed shaft and a new one revealed four distinct zones of darker

etching appearance. These zones correspond to the four locations where the rocker arms

fit the shaft.

Microscopic observations of the failed shaft revealed that the four dark-etching areas

are surface hardened zones of martensitic microstructure. Furthermore, scanning the

microstructure along the failed shaft showed that the heat treatment was so mistakenly

extended by excessive heating  so that the structure of the shaft near the supporting holes

contains considerablecontent of martensite phase. This conclusion has been confirmed by

the results of hardness measurements along the surface of the shaft.

Microscopic investigations of the failed shaft revealed the presence of microcracks

close to thesupporting holes. These cracks mayhave been induced in the shaft by the non-

uniform cooling during quenching in the course of heat treatment, or maybe nucleated byrepeated loading during service. This premature failure has occurred by the rapid crack

propagation because of the lower fracture toughness of the martensite.

 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

* Corresponding author. Tel.: +1 905 979 4509.

E-mail addresses:   mohamemy@mcmaster.ca, yakout_mostafa@yahoo.com (M. Yakout).

Contents lists available at ScienceDirect

Case Studies in Engineering Failure Analysis

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / c s e f a

http://dx.doi.org/10.1016/j.csefa.2016.01.001

2213-2902/ 2016 Publishedby Elsevier Ltd. This is anopen accessarticleunder the CCBY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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through hole and subsequently propagated along its sidewall. An extension of this work  [3]  shows that the failure stress

conditions of this kind of parts should be analysed before installation. They suggested FEA and SEM analysis to estimate the

stress conditions.

Muhammad et al. [4] investigated the failure of a diesel engine rocker arm and observed metal particles and scratches onthe crack area. Hence, they attributed the failure to a fatigue failure due to stress localisation.

Monlevade et al. [5] suggested that the untempered martensite leads to the appearance of microcracks that develop into

premature catastrophic failures due to its very high hardness. The failure may be avoided by considering the final heat

treatment process, the pre-heating prior to processing, and the proper assembly of the parts to avoid vibration or relative

movement that may cause friction between parts during use.

In their detailed paper, Ibrahim and Sayuti [6] studied the hardness, microstructure and cracking mechanism of hardened

and tempered AISI 1045 (CF 45 in DIN 17212-72 standrad). They concluded the proper heating and cooling conditions to

avoid cracking due to martensite formation. Another study [7] shows the effect of forming various grain size of austenite on

the martensite morphology, and consequently on the mechanical properties such as hardness and fatigue. It is concluded

that the privileges of controlling thetemperature and holding time of the heat treatment process lead to some enhancements

of martensite morphology.

Mateo et al. [8] studied the fatigue resistance of two different samples from austenitic stainless steel grade AISI 301 LN.

The first sample was annealed and the second sample was cold rolled. They observed different fatigue limits due to thetransformation of austenite to martensite. They hypothesised that the fatigue differences are attributed to the accumulation

of plastic deformation during the treatment which is different from process to another.

From this short review, it is clear that heat treatment, grain size, and the appearance of martensitic phase are main failure

causes of engine components.

2. Experimental work 

This paper presents the procedure used to investigate the failure of the rocker arm shaft. The shaft material was

investigated by chemical analysis. Microscopic investigations were applied to compare the microstructure of the failed shaft

with the microstructure of a new shaft. The shaft hardness was measured to investigate the failure cause.

 2.1. Characterisation of the shaft material

Chemical analysis of the shaft material gave the composition listed in Table 1. According to DIN 17212-72 standrad, CF 45

is the nearest grade to this steel. This steel belongs to steel grades suitable for surface heat treatments. The recommended

heat treatment condition of the DIN grade is given in Table 2. These conditions are general and may change according to the

application.

 2.2. Microscopic investigations

The etched sections of the new shaft and failed shaft were examined. The macro-etching of the new shaft showed areas of 

darker etching colours where rocker arms get into contact with the shaft as shown in  Fig. 2(a). It is observed that the darker

etching zones are equally spaced, regular and correspond to the areas of rocker arms contact. Conversely, the darker etching

zones of the failed shaft are wider and irregular and extend to the hole locations as shown in Fig. 2(b) and (c).

Observation of the microstructure of the failed shaft at the interface between the two different etching zones close to thehole, arrow location in Fig. 2(c), showed different microstructures at both sides of the interface as shown in  Fig. 3. The dark

[

Fig. 1. Photograph of the failed rocker arm shaft.

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etching zone is martensitic whereas the light etching one is pearlitic-ferritic. This had been confirmed by measurement of 

hardness in the two zones, which gave 240 and 460 HV respectively.

The microstructures of both the failed shaft and the new one at similar locations just beneath the hardened case are

shown in Fig. 4(a) and (b) respectively. The microstructures clearly showed that the grain size of the subsurface unhardened

material is coarser in the failed shaft than in the new one. This is attributed to the grain coarsening effect occurred by

excessive heating during surface treatments. Fig. 5 shows the etched cross sections of the failed shaft and the new one at

identical locations in one of the hardened zones. It is clearly evident that the case hardened layer in the new shaft is deeper

and far more uniform than in the failed one.

3. Hardness measurement

Measurement of hardness along both the failed shaft and the new one gave the hardness distribution shown in Fig. 6(a)and (b). The hardened zone extends to about 20 mm for the new shaft whereas it extends to about 28 mm for the failed shaft.

 Table 1

Chemical composition of shaft material (wt.%).

Element C Si Mn P S Cr Ni V Cu

wt.% 0.460 0.280 0.800 0.016 0.013 0.220 0.110 0.002 0.015

 Table 2

Heat treatment condition of DIN grade.

Hardening Quenching agent Tempering temperature

830–860 8C Water or oil 550–660 8C

[

Fig. 2. (a) Dark-etching zones at rocker arms seats, (b) comparison between dark-etching zones of the new shaft and the failed shaft and (c) dark-etching

zone of the failed shaft.

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Moreover, some of the four hardened zones in the failed shaft were mistakenly shifted so that the structure of the shaft near

the supporting holes contains considerable content of martensite phase.

4. Analysis of failure

The fracture surface shown in  Fig. 1(b) is one typical of brittle fracture. The location of failure at the region led theinvestigator to envisage the reasons for possible crack initiation in this area:

[

Fig. 3.  Microstructure of the shaft material at the interface between the two different etching zones.[

Fig. 4. Microstructure of: (a) the failed shaft and (b) the new shaft at similar locations beneath the hardened layer.[

Fig. 5. Etched cross section of: (a) the failed shaft and (b) the new shaft taken at one of the four hardened zones.

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1)  Measurement of hardness along the surface of the shaft revealed that the shaft is surface heat-treated at the four locations

where the rocker arms fit the shaft in service. This had been further confirmed by measurements on a new (unused) shaft

which was also investigated for comparison. However, the above results on the failed shaft revealed that the hardening

treatmenthad been mistakenly extended by excessive heating to affect the microstructure close to the hole. This had been

deduced from the higher hardness as well as from microstructural changes close to the hole.2)  Excessive heating had been also deduced from the considerable grain coarsening of the base metal of the failed shaft just

beneath the hardened case, as compared to the same location in the new reference shaft.

3)  Moreover etching polished cross section at the location of one of the rocker arms revealed irregular dark-etching surface

layer as shown in Fig. 2(c) indicating irregular heating or cooling during heat treatment of the failed shaft.

5. Conclusions

Excluding design reasons, failures in such cases are attributed to one of the following reasons:

1)  Consequential failure.

2)   Improper lubrication.

3)  Faulty manufacturing (including assembly).

Consequential failure is excluded because the shaft wasthe only failed componentin the car engine. Given that the engine

had not been dismantled before the incidence and that the engine was given the in-time recommended service, the second

cause of failure is improbable.

For all of the above reasons and given the results of the above investigation, such failure is attributed to  improper heat 

treatment  of the shaft during manufacturing. The failure mechanism is attributed to dynamic fatigue failure due to cyclic

crack propagation in the brittle zone of the arm. It is recommended to conduct a proper heat treatment to the whole body of 

the arm in order to prevent recurrent similar failures in the future.

 Acknowledgments

It is to acknowledge with gratitude the efforts of everyone who shared both practically and morally in the completion of 

this paper.

References

[1]   Torronen K, Kotilainen H, Nenonen P. A comparison of brittle fracture behaviourof variously tempered martensitic and bainitic structures of secondaryhardening Cr-Mo-V pressure vessel steel. In: 5th International Conference on Fracture (ICF 5); 1981.

[2]  Lee D-W, Lee S-J, Cho S-S, Joo W-S. Failure of rocker arm shaft for 4-cylinder SOHC engine. Eng Fail Anal 2005;12:405–12.[3]  Lee DW, Cho SS, Joo WS. An estimation of failure stress condition in rocker arm shaft through FEA and microscopic fractography. J Mech Sci Technol

2008;22:2056–61.[4]  Muhammad MM, Isa MC, Yati MSD, Bakar SRS, Noor IM. Failure analysis of a diesel engine rocker arm. Def S&T Tech Bull 2010;3(2):78–84 .[5]   Monlevade EF, Feitosa ME, Leite Junior PC, Bueno M. Fracture of cutting tools due to the formation of untempered martensite. Eng Fail Anal

2013;27:314–21.[6]  Ibrahim A, Sayuti M. Effect of heat treatment on hardness and microstructures of AISI 1045. Adv Mater Res 2015;1119:575–9.[7]   Prawoto Y, Jasmawati N, Sumeru K. Effect of prior austenite grain size on the morphology and mechanical properties of martensite in medium carbon

steel. J Mater Sci Technol 2012;28(5):461–6.[8]  Mateo A, Fargas G, Zapata A. Martensitic transformation during fatigue testing of an AISI 301LN stainless steel. In: IOP Conference Series: Materials

Science and Engineering, vol. 31, Paper #012010. 2012. p. 1–7.

[

Fig. 6.  Hardness distribution along identical portions of the two rocker arm shafts.

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