Elsevier

Engineering Failure Analysis

Volume 92, October 2018, Pages 84-94
Engineering Failure Analysis

Failure analysis of a truck diesel engine crankshaft

https://doi.org/10.1016/j.engfailanal.2018.05.007Get rights and content

Highlights

  • The crankshaft used in truck engine fractured during the service.

  • Zigzag cracking morphology were presented on the journal surface.

  • Fracture surface exhibited a complicated ratchet or star-shaped pattern.

  • The failure mechanism of the crankshaft was multiple-sources fatigue fracture.

  • The absence of the hardened case on the journal was mainly responsible for failure.

Abstract

A truck engine crankshaft fractured during the service. The fracture occurred on the crankshaft big-end on which the timing-gear and the flywheel flange were coupled and the fracture location was just situated at the assembling gap between them. The short cracks inclined at about 45° to the shaft axis initiated from the surface of the crankshaft journal at the timing-gear side and mainly extended to the timing-gear side, leading to a zigzag cracking morphology on the journal surface. A complicated ratchet or star-shaped pattern of fracture typical of multiple fatigue cracks occurred on the fracture surface. The journal surface was locally induction-hardened. The surface hardness and the effective case depth on the hardened journal at the flywheel flange side corresponded to the specification. At the timing-gear side the surface hardness on the hardened journal was much lower than the specified lower limit and a low hardness region of 0.4 mm occurred on the most-surface of the hardened journal within which the hardness values were lower than the specified lower limit. The low surface hardness on the induction-hardened journal made fatigue resistance of the crankshaft decrease to lead to initiation and propagation of fatigue cracks in the weaker region. The assembling gap at which the fracture occurred was structure stress concentration site of the assembly constituted of the crankshaft, the timing-gear and the flywheel flange, equivalent to the deep notch. The excessive tightening of the timing-gear on the journal surface also contributed for the increasing of stress concentration. The fatigue crack origins were easy to initiate due to large stress concentration.

Introduction

Diesel engine crankshaft run with a steady torsion combined with a rotatory bending stress [1,2] and the fatigue is the dominant failure mode [3,4]. Poor design, deficient assembly, shaft misalignment, wrong geometry and improper heat treatment process mostly contribute for the fatigue failure of crankshaft [[5], [6], [7]]. The losses due to crankshaft damage include not only crankshaft itself, but also other engine parts affected by the crankshaft failure. Therefore, the failure analysis plays an important role in avoiding recurrence of similar failure and improvement of design, manufacturing techniques, and so on.

It was reported that a truck stalled abruptly while in normal motion and the engine still did not started when trying several times. After disassembling the oil-pan, the phenomena of lacking oil, drawing cylinder and locking bearing-bush were not found. When lifting engine, it was found that the crankshaft big-end fractured. The truck had worked for 28,094 km before failure. The fractured crankshaft was made of S38MnSiV steel. The journal surface is specified to be locally induction-quenched. The effective case depth is specified as 1.2−2.8 mm and the case width as 15 mm. The core hardness and the surface hardness on the hardened region are, respectively, specified as HV30 250−290 and ≥ HRC 50.

In the present case, the fracture surface of the failed crankshaft showed a very complicated ratchet pattern, very different from the fractographic morphologies of most failed crankshafts in previous work [[1], [2], [3], [4], [5], [6], [7], [8]]. The present work is focus on analyzing probable failure causes of the crankshaft.

Section snippets

Experiment methods

The chemical composition of the failed crankshaft material was analyzed by spectroscopy chemical analysis. The microstructure in various regions was observed by optical microscope (OPM) and scanning electron microscope (SEM). Microhardness profiles from the surface to the interior in various regions were made by Vickers system with a load of 1000 g to determine the effective case depth. The criterion for determining the depth described in the technical specification is the depth of material

Observation results

The as-received crankshaft assembly is shown in Fig. 1. At the crankshaft big-end, the timing-gear and the flywheel flange were coupled to the crankshaft journal. Visual inspection revealed that the fracture location was just located at the assembling gap on the journal between the timing-gear and the flywheel flange (marked in Fig. 1). The fracture was basically perpendicular to the shaft axis (seen in left inset of Fig. 1) and zigzag crack morphology was presented on the journal surface (seen

Failure causes analysis

Fractography revealed that the fracture initiated from the journal surface of assembling gap between the timing-gear and the flywheel flange. The initial short cracks had the angles of ±45° to the shaft axis, and then zigzag cracking morphology was presented on the journal surface. The fracture surface of failed crankshaft exhibited a very complicated star-shaped or rachet pattern with large fluctuation. Beach marks and fatigue striations indicative of fatigue failure were observed on the

Conclusions

1. The failed crankshaft was made of S38MnSiV steel. No metallographic and forging defects were found in crack origins and the matrix material. The surface hardness and the effective case depth on the induction-hardened journal at the flywheel flange side correspond to the specification. The surface hardness of the induction-hardened journal is much lower than the specified lower limit value and a low hardness region of 0.4 mm occurred on the induction-hardened journal at the timing-gear side.

References (21)

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