Failure Analysis of a Vehicle Engine Crankshaft
M. A. Alfares AElig; A. H. Falah AElig; A. H. Elkholy
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.
Keywords Engine crankshaft Nitriding Failure analysis Grinding Misalignment Hardness Microstructure
Introduction Crankshaft failures are usually due to either mechanical fatigue or improper engine operation. Mechanical fatigue failure may be attributed to one of these causes: shaft misalignment [1, 2], vibration caused by bearing assembly [3, 4], incorrect geometry at critical locations (e.g., journal fillet) that could result in high stress concentration [5], or a combination of more than one of these causes. Failures due to engine operation include: defective lubrication at journals [6, 7], high engine temperature [8], wrong bearing size [9], or overloading [10]. For the case investigated in this study, a six-cylinder diesel engine of a local public bus service company had run for almost 300,000 km when an engine overhaul was carried out as specified by the operation manual of the engine manufacturer. As indicated by the bus company, the crankshaft was disassembled and checked visually for defects and then ground to remove a thickness of 0.25 mm at the journals to restore surface smoothness. The engine was then assembled and the bus was put to normal service once again. The engine was running fine for the first few thousand kilometers when suddenly a crankshaft failure occurred. Some engines had as little as 15,000 km after assembly, while others accumulated slightly more mileage. The maximum mileage accumulated by the most durable engine was 100,000 km.
To determine the exact cause of damage, a study was undertaken in which a detailed metallurgical and fractographic investigation on the failed crankshaft was performed and the possible failure causes were assessed so that future precautionary measures might be adopted during the overhaul of similar engines.
Experimental Investigation The chemical composition of the failed crankshaft was analyzed by spectroscopy chemical analysis. The microstructure of the crankshaft material and its nitrided layer at the journals were observed by scanning electron microscopy (SEM). Macrohardness of crankshaft material and the surface hardness of the crankshaft nitrided layer were conducted to verify the manufacturer minimum hardness requirement at the nitrided layer surface of 52RC (540 HV). Microhardness profiles from the surface to the interior in different regions of journal, crankpin-web, and fillet were made to determine the depth of the nitrided layer. These depth profiles were confirmed by measuring the nitrogen content by energy-dispersive X-ray analysis (EDAX). The fractured surface was also examined visually and by SEM to determine the failure mechanism.
Results and Discussion The failed crankshaft is shown in Fig. 1. Fracture had extended between the fillet of the main journal, across the web to the crankpin fillet as seen in Fig. 2. The fracture plane at the fillet is nearly parallel to the crankshaft axis. Chemical Analysis Table 1 gives the chemical composition of the material from the failed crankshaft, compared with the specified chemical composition of the crankshaft material. It can be seen from Table 1 that the chemical composition of the materials for the failed crankshaft is considered to meet specifications, despite the slightly higher chromium content.
Surface Hardness Examination The surface hardness, HV, at all the crankpins and main journals as well as material macrohardness, HV, were measured for the crankshaft before and after grinding. It was found that the material hardness of the crankshaft before and after grinding was 420 HV, which exceeds the minimum requirement of 300 HV limit set by the manufacturer. The surface hardness, however, was 600 and 460 HV for the unground and ground crankshafts at the main journal, respectively. The crankpin of the unground crankshaft had a 670 HV hardness, whereas the ground crankshaft had a hardness of approximately 460 HV. Because the minimum limit of 540 HV was not satisfied in the crankpin of the ground crankshaft, it was concluded that improper grinding could have contributed to the failure. Hardened (Nitrided) Case Examination Nitrogen content, depth of nitrided layer, and micro-hardness of the crankshaft at the fracture surface were all examined. The topography of the nitrided surface was observed by SEM and is shown in Fig. 3. It was found that there was a zone in this layer that lacks a hardened case. This zone was in the fillet region of the crankpin on the failed web where fatigue striations were formed, and a long crack was also found to cross the nitrided layer as shown in Fig. 3. The microhardness profile from surface to the interior was performed at the fillet zone of the fractured crankshaft. Similar measurement was performed on an unmachined (unground) crankshaft. When the two measurements were compared, the results revealed that the unmachined crankshaft had a surface hardness of 675 HV (58.25 HRC) and 600 HV (54.5 HRC) at the crankpin and the main journal, respectively, which are both above the 52 HRC requirement set by the manufacturer. Nevertheless, the hardness at the crankpin fillet of the machined crankshaft was only 460 HV (45.8 HRC). This indicates that the hardness at the crankpin fillet was significantly below specifications. The microhardness gradient of both crankshafts at the crankpin fillet is shown in Fig.
以上是毕业论文文献综述，课题毕业论文、任务书、外文翻译、程序设计、图纸设计等资料可联系客服协助查找。
