Failure Analysis of a Truck Diesel Engine Crankshaft Made from Spheroidal Cast Iron

A diesel engine crankshaft fractured in service after 13,656 km of operation. The fracture took place on the sixth, the fifth, and the fourth crankpins and the fracture surfaces have a 45° inclination with respect to the axial of crankshaft. The cracks of the sixth and the fifth crankpin are across the oil hole and a complete fracture took place at the sixth crankpin which bore the maximum torque load. On the fourth crankpin, crack is only through the thin wall side of oil hole. The results indicate that fatigue fracture is the dominant failure mechanism of the crankshaft. It was observed that the fatigue cracks in the crankpins initiated at machining dents present on the wall of oil hole. The appearance of the machining dents on the wall of oil hole suggests improper machining and these dents supplied the stress concentration site that was mainly responsible for the fatigue fracture of crankshaft.     

Failure analysis of a crankshaft made from ductile cast iron

This paper describes the failure analysis of a diesel engine crankshaft used in a truck, which is made from ductile cast iron. The crankshaft was found to break into two pieces at the crankpin portion before completion of warranty period. The crankshaft was induction hardened. An evaluation of the failed crankshaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that fatigue is the dominant mechanism of failure of the crankshaft. It was observed that the fatigue cracks initiated from the fillet region of the crankpin-web. The absence of the hardened case in the fillet region and the presence of free graphite and nonspheroidal graphite in the microstructure of the crankshaft made fatigue strength decrease to lead to fatigue initiation and propagation in the weaker region and premature fracture.     

Analysis of a crankshaft fatigue failure

The reason of the crankshaft fracture of the air compressor has been analyzed through the chemical composition, mechanical properties, macroscopic feature, microscopic structure and theoretical calculation methods. The analysis results show that the crankshaft which has obvious fatigue crack belongs to fatigue fracture. The fatigue crack initiated from the fillet region of the lubrication hole because of the high bending stress concentration which is caused by both the small fillet and the misalignment of main journals. The crankshaft fatigue fracture was only attributed to the initiation and propagation of the fatigue cracks on the lubrication hole under cyclic bending and torsion. The high bending loading bending level is the root cause of the failure.     

Failure investigation of a truck diesel engine gear train consisting of crankshaft and camshaft gears

A failure investigation has been conducted on a diesel engine gear train consisting of a drive crankshaft and a driven camshaft gears that were used in a truck. The gears are made from a nitrided 42CrMo steel. Adjacent teeth fracture regions appeared on the gears after a service of 4.2 × 10⁴ km. Fractographic features indicate that multiple origins fatigue fracture was the dominant failure mechanism for the gear teeth. The crankshaft gear fracture first, followed by the camshaft gear. Low hardness in subsurface and core region of the nitrided crankshaft gear makes it difficult for the matrix to support the load by the engaged camshaft gear to lead to initiating the fatigue crack at the root fillet bearing the maximum tensile stress. The crankshaft gear is the component causing trouble for the failed gears train.     

Crankshaft failure analysis of a motor vehicle

A case study of a crankshaft catastrophic failure of a motor vehicle and its failure analysis is presented. The crankshaft suffered a mechanical seizure on the crankpin no. 2 after 3 years in service. It was repaired and after 30,000 km the vehicle had a damage again, with a catastrophic failure on the same crankpin. A transversal macrograph of the crankpin revealed that the crankpin was rectified and filled with a metal alloy for the same nominal diameter. Two fatigue cracks growing to the center of the crankpin where the final fracture occurred. The symmetric semi-elliptical crack front profile confirms the effect of a pure mode I under alternating bending. The catastrophic failure was a consequence of the inadequate repairing by a non-authorized manufacturer.     

Failure Analysis of a Vehicle Engine Crankshaft

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.     

Marine main engine crankshaft failure analysis: A case study

A case study of a catastrophic failure of a web marine crankshaft and a failure analysis under bending and torsion applied to crankshafts are presented. A microscopy (eye seen) observation showed that the crack initiation started on the fillet of the crankpin by rotary bending and the propagation was a combination of cyclic bending and steady torsion. The crack front profile approximately adopts a semi-elliptical shape with some distortion due to torsion and this study is supported by a previous research work already published by the authors. The number of cycles from crack initiation to final failure of this crankshaft was achieved by recording of the main engine operation on board, taking into account the beachmarks left on the fatigue crack surface. The cycles calculated by the linear elastic fracture mechanics approaches showed that the propagation was fast which means that the level of bending stress was relatively high when compared with total cycles of main engine in service. Microstructure defects or inclusion were not observed which can conclude that the failure was probably originated by an external cause and not due to an intrinsic latent defect. Possible effects of added torsional vibrations which induce stresses are also discussed. Some causes are analyzed and reported here but the origin of the fatigue fracture was not clearly determined.     

Failure mode analysis of a diesel motor crankshaft

A failure mode analysis of a diesel motor (110 kW) crankshaft from an automobile vehicle is presented. After 120,000 km in service, an abnormal vibration was detected which was increasing with the time. The diesel motor was first disassembled for determining the root cause, however without success. No defect was detected, but since a suspicion of damage was present, and being this failure recurrent in this type of diesel motor series, the crankshaft was disassembled again. Then the crankshaft was subjected to a simple vibration analysis and a preliminary indication of possible existence of a crack was concluded. The crankshaft was then replaced by a new one, and the old was subjected to a failure analysis for determining the root cause. A crack was found at the crankpin web-fillet and after a complete opening of the crack, the failure analysis showed that fatigue was the dominant failure mechanism. Observations were carried out by optical and Scanning Electronic Microscope. Material defects at the crack initiation zone were not found. The root cause of damage seems to be a misalignment of the main journals and a weakness of design close to the gear at the region where the crack was initiated. Therefore, probably a poor design and a deficient assembling of the crankshaft helical gear coupled to the main journal end was the first cause of the failure.     

曲轴弯曲疲劳试验裂纹研究与失效判定

通过对疲劳裂纹的产生、扩展机理研究和试验,结合疲劳试验数据分析了疲劳裂纹扩展与疲劳寿命的关系。试验结果及分析表明,大量经圆角滚压强化的球墨铸铁曲轴,疲劳试验运行107次在连杆颈与曲柄臂过度圆角产生微裂纹,该裂纹属于非扩展裂纹,继续试验时裂纹不扩展,试样在该载荷下具有无限疲劳寿命。     

Analysis of a diesel generator crankshaft failure

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. The rated 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.     

Fatigue Fracture of a Locomotive Diesel Engine Cardan-Shaft

A locomotive diesel engine cardan-shaft fractured when trial test for 360 h. Fractographic investigation reveals that the failure mechanism of cardan-shaft is multiple origins fatigue fracture. The crack origins are situated on the external circle surface of the transition fillet root of the splined portion, which acts as a stress concentrator. The macro-structure segregation and corresponding inhomogeneous microstructure are presented in cardan-shaft material. And the presence of more ferrite and abnormal Widmannstätten structure ferrite in core microstructure led to the low core hardness (below the minimum value required). Therefore, the estimated fatigue endurance limit of the cardan-shaft material is lower than the expected value by approximately 20%. Along with the stress concentrator in transition fillet, the fatigue crack initiated and propagated under the torsion load and the axial tensile load, ultimately leading to premature fatigue fracture of cardan-shaft. The defective forging and the heat treatment processes should be responsible for the occurrence of defective microstructure and the consequence of low core hardness.     

Photoelastic Stress Analysis of Crankpin Fillets of a Crankshaft

Journal of Failure Analysis and Prevention - The aim of this work was to investigate the stress analysis of crankpin fillets of a crankshaft. Fatigue failure in the crankpin fillet zone is one of...     

Fracture Failure Analysis of Ductile Cast Iron Crankshaft in a Vehicle Engine

The engine crankshaft of a vehicle suddenly fractured, as the vehicle was running normally on a highway. The engine crankshaft was made from ductile cast iron. The failure cause was analyzed by chemical and metallographic examination, evaluation of mechanical properties, determination of depth of the quenched layer, measurement of distance between the quenched layer and the web, observation on the fracture surface as well as value determination of the fillet radius. The results showed that the failure mechanism of the crankshaft was fatigue fracture resulting from co-effect of bending and twisting, and the crack originated from the subsurface shrinkage in the unquenched layer of the crankshaft journal. Several aspects of the crankshaft were not up to the technical standards, such as distance between the quenched layer and the web, chemical composition, hardness and microstructure of the quenched layer, yield strength, and impact toughness.     

240曲轴断裂分析

采用光镜、扫描电镜、透射电镜对240曲轴断裂原因进行了分析。轴颈圆角未强化和加工粗糙是导致曲轴断裂的主要原因；轴颈圆角处锻造纤维露头是疲劳的诱发因素。估算了曲轴的运行周次，提出了改进措施。     

Failure of a crankshaft of an aeroengine: A contribution for an accident investigation

The objective of this work is to determine the main cause of failure of a crankshaft from an ULM airplane aeroengine. The core of this paper is focused in the analysis of the damage mechanisms which were in the base of a catastrophic failure of the crankpin journal. Based on a preliminary observation of the fracture surface, there are clear evidences of a fatigue process as both beach and striation marks have been identified. The judicious characterization of the failing mechanism, including the identification of the crack initiation site and the assessment of the crack propagation rate, is of paramount importance to support the investigation of this aircraft accident. In this context, an exhaustive observation of the fracture surface by means of optical and electronic microscopic techniques, in parallel with microstructural examinations, were carried out as determinant information for the correct assessment of the contributive causes to this accident.     

Optimization of a crankshaft rolling process for durability

A crankshaft is often designed with a small fillet radius. The crankshaft fillet rolling process is one of the commonly adopted methods in engineering to improve fatigue life of the crankshaft. Compressive residual stresses on and below the fillet radius surface are induced through the fillet rolling operation. Consequently, fatigue life of the crankshaft is improved. An analytical technique is used to optimize the crankshaft rolling process to comply with a crankshaft design criterion for durability. A nonlinear finite element analysis is implemented to approximate the stress distributions induced by the crankshaft rolling process, and a crack modeling technique is developed to calculate the equivalent stress intensity factor ranges based on the combined residual and operational stress distributions along various crack growth planes. The threshold equivalent stress intensity factor range is obtained from previous staircase testing on crankshaft sections. The durability design criterion is met if the threshold equivalent stress intensity factor range exceeds the largest calculated equivalent stress intensity factor range. Due to the complexity of the modeling techniques in simulating the rolling process and calculating the equivalent stress intensity factors, a meta-model is generated based on the uniform design method for the choice of sample points and the quadratic polynomial fitting technique for a response surface generation. In the meta-model optimization process, rolling force, rolling angle, and fillet radius are the control factors, while the variations of the threshold equivalent stress intensity factor range, rolling force, rolling angle, and fillet radius are considered as the noise factors. By using the Hooke–Jeeves direct pattern search method and the Monte Carlo simulation technique, the optimal design is obtained for the highest reliability and the smallest coefficient of variation (COV).     

Berechnung der Schwingfestigkeit festgewalzter Kurbelwellen

Predicting the fatigue strength of fillet-rolled crankshafts Since three years Darmstadt University of Technology uses finite element method for simulation of fillet rolling process. Now, together with Daimler-Benz AG, a fracture mechanics based concept has been successfully applied predicting the fatigue strength of fillet-rolled crankshafts. For these parts conventional assessment of fatigue behaviour shows several disadvantages. The new concept reduces time and costs for development and design. It consists of three parts:

• calculation of residual stresses induced by fillet rolling and affected by crankshaft and roller geometry, rolling load and work hardening data of material
• simulation of residual stress redistribution due to cyclic load
• assessment of fatigue cracks starting from notch root and propagating under compressive residual stresses by means of linearelastic fracture mechanics.

     

Fatigue of fillet welded A515 steel

A study has been made of the fatigue behavior of fillet welded ASTM A515 steel. As-welded and stress-relieved skip fillet weld specimens were tested under pulsed tension and altering cyclic load to determine stress-life and crack propagation behavior. Crack initiation and propagation features were determined from sectioned surfaces. All fatigue cracks were semi-elliptic and initiated from weld end toes. The length/depth ratio was approximately constant during propagation. There was no consistent effect of tensile residual stress on fatigue life under pulsed tension but there was a detrimental effect under alternating loads. An equivalent crack model has been proposed to quantify the stress concentration effect at the crack initiation site based on the application of the Paris equation. The test results show that the equivalent crack can give a reasonable prediction of the fatigue life of a welded structure and is a potentially convenient tool in fatigue design.     

The failure analysis of the repeat geartooth breakage in a 40 MW steam turbine load gearbox and the butterfly in the carburized case

A repeat premature geartooth breakage occurred in a load gearbox of a 40 MW steam turbine. Fractographic examination indicated that the fatigue crack originated from the root fillet of the non-active flank and propagated to the active flank. The oil deposit on the fracture surface has been applied as an auxiliary fractographic method to distinguish the fatigue crack path. The root cause of the fatigue failure was the improper heat treatment, including the very coarse microstructures due to the direct quenching from the carburizing, plus the shallow case and the insufficient surface hardness by the post-quenching grounding. The solution was successful. Some butterfly characteristics in this case study contradicted to the previous theories. The butterflies initiated from neither non-metallic inclusion nor carbide. The butterfly cracks did not propagate into the martensitic matrix. A butterfly observed in the subsurface of the gear-root indicated that the butterfly formation was irrelevant to the Hertzian stress.     

FE‐Festwalzsimulation und Dauerfestigkeitsberechnung festgewalzter Bauteile mit Konzepten der Schwingbruchmechanik

FE‐Simulation of Fillet Rolling and Fatigue life Calculation based on Fracture Mechanics Concepts Fillet rolling is a method which significantly improves the fatigue strength of members. Residual stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. For fatigue strength prediction of fillet rolled notched members a fracture mechanics based concept is described. It consists of three parts: • Finite element simulation of the fillet rolling process to calculate the residual stresses • Simulation of residual stress redistribution due to cyclic load • Assessment of fatigue cracks starting from notch roots and propagating under compressive residual stresses by means of fracture mechanics.