Failure analysis

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    Undergraduate
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Failure analysis

Introduction

A passenger car carrying passengers develop mechanic problem on its way causing an accident on the highway, though nobody was injured the passengers was traumatized by the event. As the vehicle was moving, it developed a high abnormal noise with jerky vibration of the engine and the vehicle start losing control before hitting another vehicle and it stopped. The vehicle was towed to the local service dealer for repair. After being dismantled by the dealer he found that the vehicle rocker arm shaft was broken near the middle.

Background

The dealer found that the vehicle shaft failed by brittle fracture in one of the holes that support the shaft on the cylinder head. The total distance that the engine had covered before the failure was 40, 600 km, this distance was within the distance the car should cover before being given maintenance as recommended by the manufacture [1]. There are many causes of materials failures and the major one is faulty manufacturing, this include everything that increase material brittleness or those that can cause cracks. In literature, improper heat treatment is considered the main cause of failure in components. There are many cases of components failure that end up being tragic sometimes, this cases include. Lee et al [2] looked into what caused a rocker arm shaft failure of a vehicle at design stage by use of orthogonal arrays. They found that a fatigue crack in the shaft started at the hole and propagated through its walls. Muhammad et al examine a diesel engine rocker arm failure; during examination they found particles and scratches on crack part [3]. They recommended that the failure was caused by fatigue resulting from stress.

Discussion

To understand what caused the failure, the material was investigated using both chemical analysis and microscopic investigations were used to compare the structure of the new shaft and failed shaft, the hardness was also measured. Shaft is largely made up of stell; steel requires heat treatment of around 830 to 860 degrees centigrade to harden. Microscopic observation of both broken shaft and new shaft shows that the surface of the failed shaft is coarser as compared to the new one [1]. This coarse surface is caused by excessive heating during surface treatment. The hardness of the shaft was measured and it was found that it was surface heat-treated in all four locations where rocker arms fit the shaft. This was confirmed using the new shaft that was used for comparison. The analysis from the failed shaft shows that hardening treatment that was given to this shaft was mistakenly extended by too much heating that end up affecting microstructure near the hole [4]. This was deduced from the higher hardness and microstructural changes near the hole. Too much heating is also deduced on the shaft near rocker arms, this is shown by irregular dark etching surface showing that there was irregular heating and cooling of the shaft during heat treatment, this occurs during manufacturing.

Conclusion

Failure in materials normally occurs because of certain reasons. In our case when the car was dismantled by the service dealer he found that the rocker arm shaft was broken. The vehicle was in good shape because the engine had covered distance that is required before requiring service. The investigation to ascertain what caused the failure was carried out using both chemical and microscopic means. The shaft is made up of steel and this material needs heat treatment of up to 830 degrees centigrade to harden. Microscopic observation of both broken shaft and new shaft shows that the surface of the failed shaft is coarser as compared to the new one [1]. This coarse surface is caused by excessive heating during surface treatment. The hardness of the shaft was measured and it was found that it was surface heat-treated in all four locations where rocker arms fit the shaft.

Failures such as this are attributed to three things that are consequential, improper lubrication, and faulty manufacturing; in this case consequential unlikely because it is only the shaft that failed in the vehicle engine. Improper lubrication is also unlikely because the engine has not been dismantled before and the engine was given in-time recommended service. The results of the investigation show that the failure is attributed to improper heat treatment of the rocker arm shaft during manufacturing. The mechanism of the failure is associated with dynamic fatigue failure that is caused by cyclic crack propagation on the arm brittle zone. To prevent this problem from happening in future, it is recommended that proper heat treatment for the whole shaft should be carried out before being released to the market for use.

Bibliography

[1] G. A. Nassef, A. Elkhatib, and M. Yakout, «Analysis of a failed rocker arm shaft of a passenger car engine,» Case Studies in Engineering Failure Analysis, vol. s 5–6, pp. 10–14, Apr. 2016. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S221329021600002X. Accessed: May 13, 2016

[2] D. W. Lee, S. S. Cho, and W. S. Joo, «An estimation of failure stress condition in rocker arm shaft through FEA and microscopic fractography,» Journal of Mechanical Science and Technology, vol. 22, no. 11, pp. 2056–2061, Nov. 2008.

[3] Muhammad et al. Failure analysis of a diesel engine rocker arm. Defence S and T Technical Bulletin. 3, 78-84, 2010

[4] Lee, et al. «Failure of rocker arm shaft for 4-cylinder SOHC engine,» Engineering Failure Analysis, vol. 12, no. 3, pp. 405–412, Jun. 2005