Failure Analysis of a Vehicle Engine Crankshaft

Abstract

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.

Truck Diesel Engine Crankshaft Failure Analysis

“…The case studied here clearly reveals that the fillet region was not hardened successfully by the inductionhardening operation and the initiation of fatigue was influenced by local surface conditions prevailing in the fillet region [4]. Surface rolling process was conducted on the fillet region, but the hard-rolling depth is relatively shallow and was not sufficient to prevent initiation of the fatigue in the fillet region.…”

Reliability analysis and prediction for time to failure distribution of an automobile crankshaft

“…The contributing factor for failure mechanism of the crankshaft is due of the bending and torsional stresses with the fluctuation of loads during the operational time period [1,2,3,5,6,32]. The optimal desired reliability life prediction (Fig.…”

“…Generally, there are two methods used in determining the appropriate model in determining the time to failure for the automobile crankshaft which were (1). Understanding of the physical nature of failure through the experimental analysis, (2). By analysing and predicting the reliability based physical nature of failure through a stochastic process.…”

“…By analysing and predicting the reliability based physical nature of failure through a stochastic process. It is known that the crankshaft is designed to last a lifetime with a significant safety limit [2][3]5] but its failure is still unavoidable due the variation of loading sequence during its operating condition. Therefore, the consequences of failure of the crankshaft over its operating period would cause a more severe failure towards the engine block and the other connecting subcomponents [6,22].…”

“…This is because the Weibull distribution has the capability to increase or decrease the failure time without changing the level of stress through the various properties of the shape, scale and location parameter that was further characterized mathematically into 2 modes: (1). Non-monotonic failure -observing the bath tub curve, (2). Monotonic failure; in predicting the failure rate [21,28].…”