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Failure mode analysis of a 1.9 turbo diesel engine crankshaft
Engineering Failure Analysis 101 (2019) 394–406
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure mode analysis of a 1.9 turbo diesel engine crankshaft ⁎
J. Mateusa, V. Anesa, , I. Galvãoa,c, L. Reisb
T
a
Instituto Superior de Engenharia de Lisboa, Polytechnic Institute of Lisbon, Rua Conselheiro Emídio Navarro, 1, 1959-007 Lisbon, Portugal IDMEC, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal c CEMMPRE, Department of Mechanical Engineering, University of Coimbra, Pinhal de Marrocos, 3030-788 Coimbra, Portugal b
A R T IC LE I N F O
ABS TRA CT
Keywords: Failure mode analysis Automotive crankshaft failures Critical plane Crack initiation plane Case study
This paper reports a failure mode analysis of a 1900 cm3 turbo diesel engine of a well-known commercial brand. The engine is a supercharged diesel engine with turbocharger, producing a maximum power of 81 kW; it was produced in 1999 and collapsed at 120,000 km without warning. A fracture occurred at the crankpin n°1 of the crankshaft in the region of web-fillet. Crankshafts are mechanical power transmission components with complex geometries and experience multiaxial stress states in main journals and crankpins. The objective of this work is to determine the root cause that led to the crankshaft collapse. A fractographic, metallographic, and numeric analysis were performed to understand the crankshaft failure mode and its mechanical mechanisms. Results show that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible processes, namely, a notch in the crack initiation spot, or the crankshaft misalignment.
1. Introduction The study of failure modes in mechanical components has a particular interest to insurers, road accident experts and mechanical designers [1]. In most cases, experts are required to identify the root causes of mechanical failures to explain their mechanisms. The outcomes from these studies are used to stipulate and clarify responsibilities. Moreover, mechanical designers use these outcomes to improve the fatigue strength of mechanical components in order to reduce the rate of failures occurrence. Automotive industry has shown great interest on crankshafts failures since the very beginning of automobile mass production. Nowadays, these failures continues to occur despite design improvements and high standards in quality control developed by automotive producers. Understand how this type of components fail is of utmost importance to deal with insurers and automakers. Typically, a crankshaft failure leads to the engine loss with high costs for the owners. Also human lives may be jeopardized in automotive accidents as a results of these failures, therefore, crankshafts have been defined as critical safety components. Moreover, these failures have a negative impact in the manufacturer's brand image which creates concern about sales. An example of this concern is the recall of 42 million vehicles to replace the defective Takata airbags made by 19 automakers with an overall cost of 15 billion of US dollars. In literature it can be found a diversity of failure analysis of engines components. In most cases these components are engine pistons [2], connecting rods and crankshafts [3], but crankshafts are the most studied components [4]. The typical crankshaft failure results from a fatigue process which occurs at most loaded crankshaft regions [5]. Heat treatments must be applied in these regions to increase the component fatigue strength which is promoted by the hardness increase, in some cases a defective heat treatment leads
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Corresponding author. E-mail addresses: [email protected] (V. Anes), [email protected] (L. Reis).
https://doi.org/10.1016/j.engfailanal.2019.04.004 Received 29 November 2018; Received in revised form 28 March 2019; Accepted 1 April 2019 Available online 02 April 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 101 (2019) 394–406
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to a crankshaft failure [6]. Other common reason is the crankshaft misalignment or defective assembling which create unpredicted additional loadings reducing the component fatigue strength [7]. Fatigue has an important role in crankshafts failure modes, therefore fatigue crack initiation, propagation and final ruptures as well as multiaxial stress states are research topics that help to improve the design and production of crankshafts [8]. In this paper it is studied the root causes that lead to the collapse of a 1.9 turbo diesel engine crankshaft. The study performs a fractographic, metallographic, and numeric analysis to understand the crankshaft failure mode and its mechanical mechanisms. 2. Theoretical In internal combustion engines, the peak pressure in the combustion chamber and the average pressure produced during the engine cycle are the main parameters that can be used to estimate the crankshaft loads [6,9]. The peak pressure in a thermal loading cycle only contributes to 20% of the total engine torque. On the other hand, in a supercharging thermodynamic cycle, the average pressure of the combustion cycle contributes with the remaining percentage i.e. about 80% of the total engine torque [10]. Critical regions in the crankshaft are defined as the main journal fillets and crankpins web-fillets. Any region of the crankshaft with a sudden change of geometry or diameter, will always have regions with stress concentrations which potentiate fatigue failure [8]. Determining effective loads operating in internal combustions engines is crucial to effectively develop and improve concepts and designs of engine components i.e. the determination of mechanical stresses in service is an important requirement for dimensioning components, selecting manufacturing process, determine production costs, estimate life expectation, and improve engine reliability. The loading conditions of a crankshaft are quite complex. The forces transmitted to the crankshaft by the connecting rod creates three-dimensional cyclic stress states as a function of the crankshaft rotating angle. To overcome this complexity, the cylindrical internal pressure that causes the crankshaft to move and produces the motor torque is often used to determine a static force at the moment of highest bending stress in the crankshaft which occurs at the maximum combustion pressure, about 5–10° after TDC. Fatigue fracture is considered the most severe type of fracture in machines, occurring without excessive overload, under normal operating conditions. It is the result of cumulative fatigue damage and rupture typically occurs at large number of loading cycles. In the next section, critical plane models are described. These models are typically used in industry to estimate fatigue strength under multiaxial stress/strain states, moreover they also estimate the crack initiation plane which help to understand the predominant type of loading during the fatigue process [11]. 2.1. Critical plane models To estimate the critical plane orientation and multiaxial fatigue life it is often used critical plane models, where the multiaxial stress tensor is projected in several planes in order to determine the one which maximizes the inherent damage parameter, i.e. the critical plane. These damage parameters are then associated with the material coffin-mason equation to estimate fatigue life. 2.1.1. Fatemi-Socie criterion Fatemi-Socie [12] proposed a model which predicts that the critical plane is the plane orientation θ with the maximum F-S damage parameter, Eq. (1),
σn,max ⎞ ⎤ ⎡ Δγ ⎛ ⎢ 2 ⎜1 + kF − S σ ⎟ ⎥ y ⎠⎦ ⎝ ⎣ max
(1)
θ
where Δγ/2 is the maximum shear strain amplitude on a given θ plane, σn, max is the maximum normal stress on that plane, σy is the material monotonic yield strength and k is a material constant, k = 1.0 in this case. 2.1.2. SWT criterion Smith, Watson and Topper [13] proposed a model in which the fatigue crack plane is the plane orientation θ with maximum normal stress (the maximum principal stress), Eq. (2),
max(σn ) θ
Δε1 2
(2)
where σn is the normal stress on a plane θ, and Δε1 is the principal strain range on that same plane. 2.1.3. Liu criterion Liu [14] proposed an energy method to estimate fatigue life based on virtual strain energy (VSE). This model comprises two parameters for two different modes of fatigue cracks, namely, a tensile failure mode (Mode I) represented by ΔWI, and a shear failure mode (Mode II) represented by ΔWII. Failure is expected to occur on the plane θ that has the maximum VSE quantity. According to Mode I fracture, the parameter ΔWI is as shown in Eq. (3):
ΔWI = max(Δσn Δεn ) + Δτ Δγ
(3)
θ
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Fig. 1. Crankshaft analyzed in the study.
For Mode II fracture, the parameter ΔWII is as shown in Eq. (4):
ΔWII = Δσn Δεn + max(Δτ Δγ )
(4)
θ
where Δτ and Δγ are the shear stress range and shear strain range, Δσn and Δεn are the normal stress range and normal strain range, respectively. 3. Case study 3.1. Failure description In this case study a fractured crankshaft from a well-known automotive brand is analyzed. The engine has 1896 cm3 of displacement, being a supercharged diesel engine with turbocharger, producing a maximum power of 110 horse-power (81 kW) at 4150 rpm. It is a four stroke and four cylinder engine with a 79.5 mm bore and a 95.5 mm stroke, the stroke and compression ratio is 0.83:1 and 19.5:1, respectively. The maximum torque is 235 N.m at 1900 rpm and the maximum combustion pressure is 165 bar. It was produced in 1999 and collapsed at 120,000 km without warning. The crankshaft has 5 main bearing journals, with 4 connecting rod journals, each one having 2 counterweights, equivalent to a total of 8 equilibrating mass components. Fig. 1 shows the crankshaft analyzed in this study, being fractured. It has a total mass of 8.271 kg. The fracture occurred at the crankpin n° 1 (fracture spot identified in Fig. 1) in the region of the web-fillet which is adjacent to the first plate of the crankshaft. Superficially, any possible traces of corrosion on both fractures, as well as, any defects in the material were not found. To the best of the author's knowledge, there is no records of any accidents that could affected the engine integrity. Also the access to maintenance records was not possible. There is no information about potential damage on the bearings coupled in journals as well as the condition of the engine flywheel. All journals have good surface condition, and it was not observed any unusual wear. 3.2. Fracture morphology and results Fig. 2 identifies a typical fatigue crack surface where three distinct zones resulting from crack initiation and propagation processes can be found. The “O” depicted in Fig. 2 a) identifies the fracture initiation spot, “P” identifies the fatigue crack propagation region and “S” identifies the final rupture region. The fatigue crack growth region is about 80%, having a final fracture region of small size, which means that the fatigue crack growth rate was very low, as well as the bending stress amplitude. At crack initiation spot, it can be observed some ratchet marks which results of local high stress concentration at the web-fillet. Crack initiations start in different planes, but very close among them, that origin one main crack only. This is justified with the presence of several micro crack initiation spots found in the radial periphery of the journal, at different points, close to each other, see Fig. 2 b), which is a typically pattern found in stress concentration spots. Fig. 3 shows the fracture surface morphology, where beach marks are visible. It is also possible to observe a cleavage kind of crack propagation with radial propagation fracture. The distance between beach marks suggests a slow progression of fatigue crack where the overload applied in the crankshaft caused an increase of propagation rate and surface roughness as well. An interesting observation can be found in the beach marks semi-elliptical pattern which do not cover the entire crack opening front during propagation. Therefore, it seems that the ratio of axial stresses to shear stresses changed during the fatigue damage process. The final rupture region shows a very rough surface, feature explained by a much faster crack growth rate compared to the crack propagation region. Fig. 4 shows the crankshaft fatigue crack surfaces. Fig. 4 a) shows the left cracked part and Fig. 4 b) shows the right one. In Fig. 4 a) it is identified the crack initiation angle with two white perpendicular lines. The crack initiation angle measured in the crack 396
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Fig. 2. (a) perpendicular to the fatigue surface view; (b) crack initiation, fatigue crack growth, and final fracture (fast fracture).
Fig. 3. Analysis of the fracture surface of the crankshaft.
surface was 0° degrees considering the referential depicted in Fig. 4 a). In Fig. (s) 4 a) and b) white arrows identifies the same crack initiation spot, which are in agreement with each other, this indicates that the surface morphology was not affected by any fall, or any other process different from the failure mode itself. In this region, identified by the white arrows, one can see a polished surface which may resulted for the interaction between the two surfaces during the crack initiation and propagation process. In the other regions one can see a granulated morphology typically found in crack propagation without surface interference.
3.3. Metallography A crankshaft sample near the fracture surface was subjected to standard metallographic practices. The metallographic samples were etched with Nital 2% to reveal the microstructure of the material. Macro and micrographs were registered using an optical microscope. Fig. 5 shows a macrograph, Fig. 5 (a), and micrographs in Figs. 5 (b) and (c) of the transverse cross-section of the crankpin. From the Fig. 5 (a) it can be observed that a microstructural difference exists between the inner and outer side of the crankshaft crosssection. In fact, a well-defined layer, with a thickness ranging between 2 and 2.5 mm, is observed at the periphery of the cross-section. 397
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Fig. 4. Crankshaft fatigue crack, a) crack initiation spot identification and crack initiation orientation, b) crack spot identification on the homologous fractured part.
Fig. 5. Macro and micrographs of the crankpin cross-section: (a) Macrograph; (b) Micrograph of the interior; (c) Micrograph of the treated layer.
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Fig. 6. Data collected and simulated of the analyzed motor [15].
This layer resulted from the heat treatment conducted in the material to increase its surface hardness. So, while an equilibrium microstructure, consisting of perlite (dark etching phase) and ferrite (white etching phase), is observed in the interior of the component (Fig. 5b), martensite is observed in the heat-treated layer (Fig. 5c). The formation of martensite is associated with an increase in hardness, but also in the material brittleness. Therefore, it can be concluded that a heat treatment was performed to increase the crankshaft mechanical strength. 3.4. Crankshaft loads In order to get the local stresses on the crack initiation spot previously identified in situ, it was investigated the engine thermodynamic characteristics strictly related to the mechanical loads of the engine crankshaft under service. Based on the angular displacement diagrams as a function of the crankshaft 4-stroke cycle, it was intended to estimate the cylindrical internal pressure as a function of the crankshaft angular displacement over 720°, corresponding to a complete engine cycle. The main objective was to obtain the loads experienced by the crankshaft journals in order to perform a finite element analysis. Variables such as cylinder bore, piston stroke, rod length and piston weight were considered to perform the engine thermal modeling, some additional operational considerations such as oil viscosity and friction between cylinder surfaces were also considered. Fig. 6 shows the data obtained for the engine under study. The left (1 red) column simulates the engine regime for maximum rotation (5100 rpm) while the right column (2 blue) simulates for the maximum torque produced. Fig. 7 depicts the estimated cylindrical internal pressure using dashed lines and the cylindrical chamber temperature represented by solid lines. Both solid and dashed curves represented in Fig. 7 are related to only one cylinder. Considering these estimates, it was possible to analyse the crankshaft loadings by applying a variable pressure in all angular crankshaft displacements during the engine loading cycle, this allowed to identify the most loaded angular position and their respective loads. It should be noted that for a 4-stroke diesel engine only the compression and expansion phases of the loading cycle significantly load the crankshaft journal. 3.5. Finite element model A finite element analysis based on the solid crankshaft modeling was performed to primarily identify the regions with lower safety factors and then to identify the crankshaft regions of maximum stresses. In order to perform the simulation, it was firstly necessary to make the crankshaft solid modeling. All dimensions and geometric shapes were measured from the crankshaft using manual tools and appropriate software. Fig. 8 shows the crankshaft isometric view performed using the commercial software Solidworks. Next, the crankshaft geometry was imported to the commercial software “ANSYS Workbench” to perform a stepped-static analysis 399
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Fig. 7. Simulated temperature and pressure curve for the analyzed motor [15].
Fig. 8. Solid modeling of the crankshaft (isometric view).
using the pressure variation according the crank angle described in Fig. 7. A mesh convergence study was performed to set the mesh refinement in order to get results independent from the mesh size. The mesh was created manually in regions of geometric discontinuities, contour zones and details of the journals as show in Fig. 9. The overall crankshaft dimensions in x, y and z are respectively 405.12 [mm], 225.56 [mm] and 130.50 [mm], see referential in Fig. 9 a). The crankshaft overall dimensions are moderately high; therefore, the intent numeric simulation requires high computation resources, in this sense it was performed a mesh study to get the best results. Tetrahedral elements were selected to perform the mesh. These elements are suitable for complex geometries like the crankshaft geometry considered in this study. The finite element model comprises a total of 826,290 nodes and 502,073 elements, obtaining a medium/high mesh quality in a proportion of 0.70, where 1 is intended for a fully refined mesh, while 0 for a mesh without any element/refinement. The simulation comprises 72 boundary conditions according to the crank angle. The objective was to obtain the crankshaft stresses at each angular period of 10°, along the two complete rotations, equivalent to rotating a total of 720° degrees. These boundary conditions were obtained using the pressure/temperature curve depicted in Fig. 7, were the cylindrical internal 400
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Fig. 9. Mesh applied to the model: a) complete view; b) zoom to crank n°1.
pressure along with the motor cycle is set as function of the angular offset of the corresponding crankshaft journal. A 1900 rpm rotational speed was considered in simulations and calculated the corresponding time period for each step of 10° in the crankshaft complete rotation. Subsequently, the pressure was applied through a direct contact coupling with the connecting rod. No clearances or friction were considered in the couplings of the connecting rods. The motor order of ignition considered in simulations was set as 1–3–4-2. It was assumed that during intake and exhaust phase the pressure in the combustion chamber, therefore, the crankshaft stresses were simulated considering higher loading phases (compression and expansion). Constraints were assigned to the region of power transmission coupling, where displacements in x, y, and z directions were fully constrained. This boundary conditions aims to simulate the moment of greatest load in crankshaft which occurs during the time period where the vehicle starts to move. Cylindrical supports were assigned to all the crankshaft main journals, modeling the tightening constriction of the half shells presented in the crankshaft with the engine block, only allowing the tangential, permitting axial rotation of the crankshaft. This constraint is defined by “cylindrical supports 1” as depicted in Fig. 10. The remaining cylindrical support were defined with the same restriction imposed on the main journals but applied in the axial coupling region of the crankshaft, as show in Fig. 10. The
Fig. 10. Boundary conditions considered in the model. 401
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Fig. 11. Simulated total deformation on the crankshaft.
temperature was considered as constant, with the average temperature reached in the simulated motor cycle (563 °C). 3.6. Numeric results Fig. 11 shows the total deformation considering the tree directional deformation components. The moment of greatest deformation amplitude occurs when piston n°1 is in the expansion phase, being the crankshaft rotated 20° after AFTDC of piston n°1, as shown in Figs. 11 and 12. One can conclude that the region of greater deformation coincides with the crack initiation spot identified in the previous fractugrafic analysis. It should be noted that the simulated deformation reduces as the crank is closer to the power transmission. A secondary analysis allowed to conclude that the horizontal loading component strongly contributes to the total deformation on the maximum total deformation spot identified in Fig. 11. Finally, it was observed in this simulations that the cracnkshaft rotation speed has a negligible influence in the material deformation, however, the von Mises equivalent stress shows some dependence of the crackshaft speed expecially in stress concentration spots. Fig. 13 shows the normal and shear stress evolution, for the crack initiation spot previously identified by fracture analysis. These values where obtained through the implemented FEM model considering the working regime described in previous section. The maximum values obtained for normal stresses were 31 MPa, 17 MPa, and 6 MPa for x, y and z directions, respectively. Fig. 13 b) shows the maximum values found for shear stresses, 8.13 MPa, 3.71 MPa, 2.11 MPa for XZ, YZ, and XY, respectively. These values already have into account stress concentrations due to geometry discontinuities. The x axis is aligned with the crankshaft longitudinal axis, the y axis is vertical, and the z axis is horizontal. Fig. 14 shows the von Mises equivalent stress evolution for the stresses shown in Fig. 13. The von Mises equivalent stress was computed considering Eq. (5) were the six stress components are considered.
σ=
(σxx − σyy )2 + (σyy − σzz )2 + (σzz − σxx )2 2
2 2 2 ) + 3⋅(τxy + τ yz + τzx
(5)
The von Mises equivalent stress maximum value was 27 MPa at crack initiation spot which is a value very low considering the typical fatigue strength of crankshaft materials, as shown in Fig. 15. Fig. 15 shows the AISI4340 fatigue strength variation according
Fig. 12. Evolution of total deformation along the motor operating cycle: curve green – maximum amplitude; blue curve - average amplitude; red curve - minimum amplitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 402
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Fig. 13. Crankshaft load at crack initiation spot, a) normal stresses b) shear stresses.
Fig. 14. von Mises equivalent stress evolution at crack initiation spot.
to the heat treatments and surface treatments typically used to improve fatigue strength of crankshaft materials. As a rough analysis, and assuming that the AISI4340 fatigue behaviour is similar to the materials normally used in crankshafts, we correlate the maximum von Mises equivalent stress shown in Fig. 14 with the fatigue limits shown in Fig. 15. Table 1 shows the results of this correlation. The fatigue safety factors computed were 19, 23, and 31 for normal heat treatment, heat treatment plus shotpeening, and heat treat plus nitriding, respectively. Assuming that cyclic loadings bellow fatigue limit does not lead to fatigue failure, one can consider that for the assumptions considered the crankshaft failure does not resulted from the loading regime. To reinforce this idea, the stress level considered to analyse fatigue strength should be lower than the one considered in static loading conditions i.e. static stress concentration factors are higher than fatigue stress concentration factors due to the material cyclic plasticity. Therefore, the fatigue safety factors shown in Table 1 should be higher.
3.7. Crack initiation plane estimates and fatigue life analysis Fig. 16 shows the critical plane estimates for the critical plane directions. Fig. 16 a) shows the estimates given by the Fatemi-Socie 403
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Fig. 15. Typical SN curves for crankshafts made of AISI 4340 material according to treatment [16]. Table 1 Fatigue safety factor.
Fatigue s.f.
Normal heat treat
Heat treat plus shotpeening
Heat treatment plus nitriding
19
23
31
model. These estimates are very far from the measured crack initiation angle, i. e. ± 48° against 0° measured in the crankshaft crack surface. The crankshaft load at crack initiation plane has a strong component in x direction (normal stresses), see Fig. 13. This reinforces the idea that shear-based models like Fatemi-Socie and Liu II do not have a good performance in critical plane estimates under predominant normal stress loadings. On the other hand, axial based models typically have good estimates for this type of loadings. The results shown in Table 2 and Fig. (s) 16 b) and c) confirm this statement, where the difference between estimates and crack initiation plane measure is within the error margin, therefore it can be considered that SWT and LIU I estimates are in accordance with the crack initiation plane measure on the fatigue crack surface. Therefore, these two models are recommended to estimate crack initiation planes in crankshafts. The stress level in the crack initiation plane has a minimum fatigue safety factor of 19, for the assumption considered in this analysis, which is a huge safety margin. This margin is enough to waive the assumptions considered, therefore the crankshaft failure does not resulted from a poor mechanical design. Based on the aforementioned observations and fractography analysis one can concluded that the crankshaft failure occurred from a fatigue process resulted from high levels of normal stresses at crack initiation spot. This could result from two reasons. First, a notch created during production, assembly, or due to a material impurity could locally increase the normal stresses and reduce the crankshaft fatigue strength. In this case, it was not found any impurities in the crack initiation spot, this could be due to the fatigue crack closing process, where the interference between crack surfaces erased any trace of local impurities. The second possible reason for the normal stress increase is the crankshaft misalignment which increase locally stress levels. To verify this hypothesis, it is necessary analyse the engine block which was not possible during this study. It can be stated that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible processes, namely, a notch in the crack initiation spot, or the crankshaft misalignment. 4. Conclusion In this study it was analyzed a crankshaft failure mode of a 1900 cm3 turbo diesel engine. The crankshaft collapsed at 120 k kilometres without warning; the maintenance program performed is unknown but by inspection the crankshaft journals do not show 404
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Fig. 16. Critical plane estimates for the crack initiation plane. a) Fatemi-Socie damage parameter, b) SWT damage parameter, c) Liu I damage parameter, Liu II damage parameter.
Table 2 Critical plane estimates at crack initiation spot.
Angle
F. Socie
SWT
Liu1
Liu2
Measured
± 48
−4
−4
± 49
0
any unusual wear or superficial grooves, which detract any maintenance fault as reason for the crankshaft collapse. A descriptive fractographic analysis was performed to describe the failure mode process. It was concluded that the crankshaft collapsed due to fatigue crack nucleation and propagation in crankpin n°1. In order to investigate the stress levels in the crack initiation spot, a finite element model was created, and local stresses were acquired in the crack initiation spot. An estimate for a fatigue safety factor was performed considering the fatigue properties of a typical material used in automotive crankshafts. Moreover, it was performed an estimate for the direction of crack initiation plane using typical critical plane models. The finite element analysis show that the maximum total displacement occurs in the crack initiation spot identified in the fractographic analysis, however the stress levels found in this region are very low, with a fatigue safety factor around 19, which lead to conclude that the crankshaft does not collapsed due to a defective design. The results for the critical plane estimates show that the SWT and LIU I models predict very well the crack initiation plane, indicating that normal stresses are predominant during the fatigue process as a result of the bending load 405
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component. Results show that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible causes, namely, a notch in the crack initiation spot, or the crankshaft misalignment. As future works a SEM/TEM study can be carried out to analyse the chemical composition of the crankshaft material as well as a bearing integrity analysis. Acknowledgements The authors are very grateful to the reviewers for their insightful, constructive comments and suggestions which were very helpful in improving the quality of the paper. This work was supported by FCT, through IDMEC, under LAETA project UID/EMS/50022/2019. The second author gratefully acknowledges financial support from FCT - Fundação para Ciência e Tecnologia (Portuguese Foundation for Science and Technology), for the Ph.D. Grant PD/BD/52344/2013. References [1] Cedomir Duboka, Forensic evidence in road accidents caused by vehicle's mechanical failures, Automotive Engineering for Improved Safety, 2013 Belgrade, Serbia. [2] Z. Yu, X. Xu, H. Ding, Failure analysis of a diesel engine piston-pin, Eng. Fail. Anal. 14 (2007) 110–117. [3] A.M. Heyes, Automotive component failures, Eng. Fail. Anal. 5 (1998) 129–141. [4] M. Fonte, V. Anes, P. Duarte, L. Reis, M. Freitas, Crankshaft failure analysis of a boxer diesel motor, Eng. Fail. Anal. 56 (2015) 109–115. [5] O. Asi, Failure analysis of a crankshaft made from ductile cast iron, Eng. Fail. Anal. 13 (2006) 1260–1267. [6] R.K. Pandey, Failure of diesel-engine crankshafts, Eng. Fail. Anal. 10 (2003) 165–175. [7] F.S. Silva, Analysis of a vehicle crankshaft failure, Eng. Fail. Anal. 10 (2003) 605–616. [8] A.S.M. Handbook, Volume 12: Fractography, 55 ASM International Materials Park, 1987. [9] J. Haapakoski, Medium-speed four-stroke diesel engine cylinder pressure effect on component dimensioning, Master's thesis, University of Oulu, 2016, pp. 2–4. [10] C. Bell, Maximum boost, Cambridge, BentleyPublishers, USA, 1997. [11] D.F. Socie, G.B. Marquis, Multiaxial Fatigue, Society of Automotive Engineers Warrendale, PA, 2000. [12] D.F. Socie, Multiaxial fatigue damage models, J. Eng. Mater. Technol. 109 (1987) 293–298. [13] P. Watson, T.H. Topper, Fatigue-damage evaluation for mild steel incorporating mean stress and overload effects, Exp. Mech. 12 (1972) 11–17. [14] K.C. Liu, A method based on virtual strain-energy parameters for multiaxial fatigue life prediction, Advances in Multiaxial Fatigue, ASTM International, 1993. [15] Kozakewycz, Engine Thermodynamics, Black Art Dynamics, http://blackartdynamics.com/Engine/EngineThermodynamics.php, (2015) , Accessed date: 29 November 2018. [16] J. Kane, Contemporary crankshaft design, Race Engine Technol. Mag. 33 (2009) 1–14.
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure mode analysis of a 1.9 turbo diesel engine crankshaft ⁎
J. Mateusa, V. Anesa, , I. Galvãoa,c, L. Reisb
T
a
Instituto Superior de Engenharia de Lisboa, Polytechnic Institute of Lisbon, Rua Conselheiro Emídio Navarro, 1, 1959-007 Lisbon, Portugal IDMEC, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal c CEMMPRE, Department of Mechanical Engineering, University of Coimbra, Pinhal de Marrocos, 3030-788 Coimbra, Portugal b
A R T IC LE I N F O
ABS TRA CT
Keywords: Failure mode analysis Automotive crankshaft failures Critical plane Crack initiation plane Case study
This paper reports a failure mode analysis of a 1900 cm3 turbo diesel engine of a well-known commercial brand. The engine is a supercharged diesel engine with turbocharger, producing a maximum power of 81 kW; it was produced in 1999 and collapsed at 120,000 km without warning. A fracture occurred at the crankpin n°1 of the crankshaft in the region of web-fillet. Crankshafts are mechanical power transmission components with complex geometries and experience multiaxial stress states in main journals and crankpins. The objective of this work is to determine the root cause that led to the crankshaft collapse. A fractographic, metallographic, and numeric analysis were performed to understand the crankshaft failure mode and its mechanical mechanisms. Results show that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible processes, namely, a notch in the crack initiation spot, or the crankshaft misalignment.
1. Introduction The study of failure modes in mechanical components has a particular interest to insurers, road accident experts and mechanical designers [1]. In most cases, experts are required to identify the root causes of mechanical failures to explain their mechanisms. The outcomes from these studies are used to stipulate and clarify responsibilities. Moreover, mechanical designers use these outcomes to improve the fatigue strength of mechanical components in order to reduce the rate of failures occurrence. Automotive industry has shown great interest on crankshafts failures since the very beginning of automobile mass production. Nowadays, these failures continues to occur despite design improvements and high standards in quality control developed by automotive producers. Understand how this type of components fail is of utmost importance to deal with insurers and automakers. Typically, a crankshaft failure leads to the engine loss with high costs for the owners. Also human lives may be jeopardized in automotive accidents as a results of these failures, therefore, crankshafts have been defined as critical safety components. Moreover, these failures have a negative impact in the manufacturer's brand image which creates concern about sales. An example of this concern is the recall of 42 million vehicles to replace the defective Takata airbags made by 19 automakers with an overall cost of 15 billion of US dollars. In literature it can be found a diversity of failure analysis of engines components. In most cases these components are engine pistons [2], connecting rods and crankshafts [3], but crankshafts are the most studied components [4]. The typical crankshaft failure results from a fatigue process which occurs at most loaded crankshaft regions [5]. Heat treatments must be applied in these regions to increase the component fatigue strength which is promoted by the hardness increase, in some cases a defective heat treatment leads
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Corresponding author. E-mail addresses: [email protected] (V. Anes), [email protected] (L. Reis).
https://doi.org/10.1016/j.engfailanal.2019.04.004 Received 29 November 2018; Received in revised form 28 March 2019; Accepted 1 April 2019 Available online 02 April 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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to a crankshaft failure [6]. Other common reason is the crankshaft misalignment or defective assembling which create unpredicted additional loadings reducing the component fatigue strength [7]. Fatigue has an important role in crankshafts failure modes, therefore fatigue crack initiation, propagation and final ruptures as well as multiaxial stress states are research topics that help to improve the design and production of crankshafts [8]. In this paper it is studied the root causes that lead to the collapse of a 1.9 turbo diesel engine crankshaft. The study performs a fractographic, metallographic, and numeric analysis to understand the crankshaft failure mode and its mechanical mechanisms. 2. Theoretical In internal combustion engines, the peak pressure in the combustion chamber and the average pressure produced during the engine cycle are the main parameters that can be used to estimate the crankshaft loads [6,9]. The peak pressure in a thermal loading cycle only contributes to 20% of the total engine torque. On the other hand, in a supercharging thermodynamic cycle, the average pressure of the combustion cycle contributes with the remaining percentage i.e. about 80% of the total engine torque [10]. Critical regions in the crankshaft are defined as the main journal fillets and crankpins web-fillets. Any region of the crankshaft with a sudden change of geometry or diameter, will always have regions with stress concentrations which potentiate fatigue failure [8]. Determining effective loads operating in internal combustions engines is crucial to effectively develop and improve concepts and designs of engine components i.e. the determination of mechanical stresses in service is an important requirement for dimensioning components, selecting manufacturing process, determine production costs, estimate life expectation, and improve engine reliability. The loading conditions of a crankshaft are quite complex. The forces transmitted to the crankshaft by the connecting rod creates three-dimensional cyclic stress states as a function of the crankshaft rotating angle. To overcome this complexity, the cylindrical internal pressure that causes the crankshaft to move and produces the motor torque is often used to determine a static force at the moment of highest bending stress in the crankshaft which occurs at the maximum combustion pressure, about 5–10° after TDC. Fatigue fracture is considered the most severe type of fracture in machines, occurring without excessive overload, under normal operating conditions. It is the result of cumulative fatigue damage and rupture typically occurs at large number of loading cycles. In the next section, critical plane models are described. These models are typically used in industry to estimate fatigue strength under multiaxial stress/strain states, moreover they also estimate the crack initiation plane which help to understand the predominant type of loading during the fatigue process [11]. 2.1. Critical plane models To estimate the critical plane orientation and multiaxial fatigue life it is often used critical plane models, where the multiaxial stress tensor is projected in several planes in order to determine the one which maximizes the inherent damage parameter, i.e. the critical plane. These damage parameters are then associated with the material coffin-mason equation to estimate fatigue life. 2.1.1. Fatemi-Socie criterion Fatemi-Socie [12] proposed a model which predicts that the critical plane is the plane orientation θ with the maximum F-S damage parameter, Eq. (1),
σn,max ⎞ ⎤ ⎡ Δγ ⎛ ⎢ 2 ⎜1 + kF − S σ ⎟ ⎥ y ⎠⎦ ⎝ ⎣ max
(1)
θ
where Δγ/2 is the maximum shear strain amplitude on a given θ plane, σn, max is the maximum normal stress on that plane, σy is the material monotonic yield strength and k is a material constant, k = 1.0 in this case. 2.1.2. SWT criterion Smith, Watson and Topper [13] proposed a model in which the fatigue crack plane is the plane orientation θ with maximum normal stress (the maximum principal stress), Eq. (2),
max(σn ) θ
Δε1 2
(2)
where σn is the normal stress on a plane θ, and Δε1 is the principal strain range on that same plane. 2.1.3. Liu criterion Liu [14] proposed an energy method to estimate fatigue life based on virtual strain energy (VSE). This model comprises two parameters for two different modes of fatigue cracks, namely, a tensile failure mode (Mode I) represented by ΔWI, and a shear failure mode (Mode II) represented by ΔWII. Failure is expected to occur on the plane θ that has the maximum VSE quantity. According to Mode I fracture, the parameter ΔWI is as shown in Eq. (3):
ΔWI = max(Δσn Δεn ) + Δτ Δγ
(3)
θ
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Fig. 1. Crankshaft analyzed in the study.
For Mode II fracture, the parameter ΔWII is as shown in Eq. (4):
ΔWII = Δσn Δεn + max(Δτ Δγ )
(4)
θ
where Δτ and Δγ are the shear stress range and shear strain range, Δσn and Δεn are the normal stress range and normal strain range, respectively. 3. Case study 3.1. Failure description In this case study a fractured crankshaft from a well-known automotive brand is analyzed. The engine has 1896 cm3 of displacement, being a supercharged diesel engine with turbocharger, producing a maximum power of 110 horse-power (81 kW) at 4150 rpm. It is a four stroke and four cylinder engine with a 79.5 mm bore and a 95.5 mm stroke, the stroke and compression ratio is 0.83:1 and 19.5:1, respectively. The maximum torque is 235 N.m at 1900 rpm and the maximum combustion pressure is 165 bar. It was produced in 1999 and collapsed at 120,000 km without warning. The crankshaft has 5 main bearing journals, with 4 connecting rod journals, each one having 2 counterweights, equivalent to a total of 8 equilibrating mass components. Fig. 1 shows the crankshaft analyzed in this study, being fractured. It has a total mass of 8.271 kg. The fracture occurred at the crankpin n° 1 (fracture spot identified in Fig. 1) in the region of the web-fillet which is adjacent to the first plate of the crankshaft. Superficially, any possible traces of corrosion on both fractures, as well as, any defects in the material were not found. To the best of the author's knowledge, there is no records of any accidents that could affected the engine integrity. Also the access to maintenance records was not possible. There is no information about potential damage on the bearings coupled in journals as well as the condition of the engine flywheel. All journals have good surface condition, and it was not observed any unusual wear. 3.2. Fracture morphology and results Fig. 2 identifies a typical fatigue crack surface where three distinct zones resulting from crack initiation and propagation processes can be found. The “O” depicted in Fig. 2 a) identifies the fracture initiation spot, “P” identifies the fatigue crack propagation region and “S” identifies the final rupture region. The fatigue crack growth region is about 80%, having a final fracture region of small size, which means that the fatigue crack growth rate was very low, as well as the bending stress amplitude. At crack initiation spot, it can be observed some ratchet marks which results of local high stress concentration at the web-fillet. Crack initiations start in different planes, but very close among them, that origin one main crack only. This is justified with the presence of several micro crack initiation spots found in the radial periphery of the journal, at different points, close to each other, see Fig. 2 b), which is a typically pattern found in stress concentration spots. Fig. 3 shows the fracture surface morphology, where beach marks are visible. It is also possible to observe a cleavage kind of crack propagation with radial propagation fracture. The distance between beach marks suggests a slow progression of fatigue crack where the overload applied in the crankshaft caused an increase of propagation rate and surface roughness as well. An interesting observation can be found in the beach marks semi-elliptical pattern which do not cover the entire crack opening front during propagation. Therefore, it seems that the ratio of axial stresses to shear stresses changed during the fatigue damage process. The final rupture region shows a very rough surface, feature explained by a much faster crack growth rate compared to the crack propagation region. Fig. 4 shows the crankshaft fatigue crack surfaces. Fig. 4 a) shows the left cracked part and Fig. 4 b) shows the right one. In Fig. 4 a) it is identified the crack initiation angle with two white perpendicular lines. The crack initiation angle measured in the crack 396
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Fig. 2. (a) perpendicular to the fatigue surface view; (b) crack initiation, fatigue crack growth, and final fracture (fast fracture).
Fig. 3. Analysis of the fracture surface of the crankshaft.
surface was 0° degrees considering the referential depicted in Fig. 4 a). In Fig. (s) 4 a) and b) white arrows identifies the same crack initiation spot, which are in agreement with each other, this indicates that the surface morphology was not affected by any fall, or any other process different from the failure mode itself. In this region, identified by the white arrows, one can see a polished surface which may resulted for the interaction between the two surfaces during the crack initiation and propagation process. In the other regions one can see a granulated morphology typically found in crack propagation without surface interference.
3.3. Metallography A crankshaft sample near the fracture surface was subjected to standard metallographic practices. The metallographic samples were etched with Nital 2% to reveal the microstructure of the material. Macro and micrographs were registered using an optical microscope. Fig. 5 shows a macrograph, Fig. 5 (a), and micrographs in Figs. 5 (b) and (c) of the transverse cross-section of the crankpin. From the Fig. 5 (a) it can be observed that a microstructural difference exists between the inner and outer side of the crankshaft crosssection. In fact, a well-defined layer, with a thickness ranging between 2 and 2.5 mm, is observed at the periphery of the cross-section. 397
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Fig. 4. Crankshaft fatigue crack, a) crack initiation spot identification and crack initiation orientation, b) crack spot identification on the homologous fractured part.
Fig. 5. Macro and micrographs of the crankpin cross-section: (a) Macrograph; (b) Micrograph of the interior; (c) Micrograph of the treated layer.
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Fig. 6. Data collected and simulated of the analyzed motor [15].
This layer resulted from the heat treatment conducted in the material to increase its surface hardness. So, while an equilibrium microstructure, consisting of perlite (dark etching phase) and ferrite (white etching phase), is observed in the interior of the component (Fig. 5b), martensite is observed in the heat-treated layer (Fig. 5c). The formation of martensite is associated with an increase in hardness, but also in the material brittleness. Therefore, it can be concluded that a heat treatment was performed to increase the crankshaft mechanical strength. 3.4. Crankshaft loads In order to get the local stresses on the crack initiation spot previously identified in situ, it was investigated the engine thermodynamic characteristics strictly related to the mechanical loads of the engine crankshaft under service. Based on the angular displacement diagrams as a function of the crankshaft 4-stroke cycle, it was intended to estimate the cylindrical internal pressure as a function of the crankshaft angular displacement over 720°, corresponding to a complete engine cycle. The main objective was to obtain the loads experienced by the crankshaft journals in order to perform a finite element analysis. Variables such as cylinder bore, piston stroke, rod length and piston weight were considered to perform the engine thermal modeling, some additional operational considerations such as oil viscosity and friction between cylinder surfaces were also considered. Fig. 6 shows the data obtained for the engine under study. The left (1 red) column simulates the engine regime for maximum rotation (5100 rpm) while the right column (2 blue) simulates for the maximum torque produced. Fig. 7 depicts the estimated cylindrical internal pressure using dashed lines and the cylindrical chamber temperature represented by solid lines. Both solid and dashed curves represented in Fig. 7 are related to only one cylinder. Considering these estimates, it was possible to analyse the crankshaft loadings by applying a variable pressure in all angular crankshaft displacements during the engine loading cycle, this allowed to identify the most loaded angular position and their respective loads. It should be noted that for a 4-stroke diesel engine only the compression and expansion phases of the loading cycle significantly load the crankshaft journal. 3.5. Finite element model A finite element analysis based on the solid crankshaft modeling was performed to primarily identify the regions with lower safety factors and then to identify the crankshaft regions of maximum stresses. In order to perform the simulation, it was firstly necessary to make the crankshaft solid modeling. All dimensions and geometric shapes were measured from the crankshaft using manual tools and appropriate software. Fig. 8 shows the crankshaft isometric view performed using the commercial software Solidworks. Next, the crankshaft geometry was imported to the commercial software “ANSYS Workbench” to perform a stepped-static analysis 399
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Fig. 7. Simulated temperature and pressure curve for the analyzed motor [15].
Fig. 8. Solid modeling of the crankshaft (isometric view).
using the pressure variation according the crank angle described in Fig. 7. A mesh convergence study was performed to set the mesh refinement in order to get results independent from the mesh size. The mesh was created manually in regions of geometric discontinuities, contour zones and details of the journals as show in Fig. 9. The overall crankshaft dimensions in x, y and z are respectively 405.12 [mm], 225.56 [mm] and 130.50 [mm], see referential in Fig. 9 a). The crankshaft overall dimensions are moderately high; therefore, the intent numeric simulation requires high computation resources, in this sense it was performed a mesh study to get the best results. Tetrahedral elements were selected to perform the mesh. These elements are suitable for complex geometries like the crankshaft geometry considered in this study. The finite element model comprises a total of 826,290 nodes and 502,073 elements, obtaining a medium/high mesh quality in a proportion of 0.70, where 1 is intended for a fully refined mesh, while 0 for a mesh without any element/refinement. The simulation comprises 72 boundary conditions according to the crank angle. The objective was to obtain the crankshaft stresses at each angular period of 10°, along the two complete rotations, equivalent to rotating a total of 720° degrees. These boundary conditions were obtained using the pressure/temperature curve depicted in Fig. 7, were the cylindrical internal 400
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Fig. 9. Mesh applied to the model: a) complete view; b) zoom to crank n°1.
pressure along with the motor cycle is set as function of the angular offset of the corresponding crankshaft journal. A 1900 rpm rotational speed was considered in simulations and calculated the corresponding time period for each step of 10° in the crankshaft complete rotation. Subsequently, the pressure was applied through a direct contact coupling with the connecting rod. No clearances or friction were considered in the couplings of the connecting rods. The motor order of ignition considered in simulations was set as 1–3–4-2. It was assumed that during intake and exhaust phase the pressure in the combustion chamber, therefore, the crankshaft stresses were simulated considering higher loading phases (compression and expansion). Constraints were assigned to the region of power transmission coupling, where displacements in x, y, and z directions were fully constrained. This boundary conditions aims to simulate the moment of greatest load in crankshaft which occurs during the time period where the vehicle starts to move. Cylindrical supports were assigned to all the crankshaft main journals, modeling the tightening constriction of the half shells presented in the crankshaft with the engine block, only allowing the tangential, permitting axial rotation of the crankshaft. This constraint is defined by “cylindrical supports 1” as depicted in Fig. 10. The remaining cylindrical support were defined with the same restriction imposed on the main journals but applied in the axial coupling region of the crankshaft, as show in Fig. 10. The
Fig. 10. Boundary conditions considered in the model. 401
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Fig. 11. Simulated total deformation on the crankshaft.
temperature was considered as constant, with the average temperature reached in the simulated motor cycle (563 °C). 3.6. Numeric results Fig. 11 shows the total deformation considering the tree directional deformation components. The moment of greatest deformation amplitude occurs when piston n°1 is in the expansion phase, being the crankshaft rotated 20° after AFTDC of piston n°1, as shown in Figs. 11 and 12. One can conclude that the region of greater deformation coincides with the crack initiation spot identified in the previous fractugrafic analysis. It should be noted that the simulated deformation reduces as the crank is closer to the power transmission. A secondary analysis allowed to conclude that the horizontal loading component strongly contributes to the total deformation on the maximum total deformation spot identified in Fig. 11. Finally, it was observed in this simulations that the cracnkshaft rotation speed has a negligible influence in the material deformation, however, the von Mises equivalent stress shows some dependence of the crackshaft speed expecially in stress concentration spots. Fig. 13 shows the normal and shear stress evolution, for the crack initiation spot previously identified by fracture analysis. These values where obtained through the implemented FEM model considering the working regime described in previous section. The maximum values obtained for normal stresses were 31 MPa, 17 MPa, and 6 MPa for x, y and z directions, respectively. Fig. 13 b) shows the maximum values found for shear stresses, 8.13 MPa, 3.71 MPa, 2.11 MPa for XZ, YZ, and XY, respectively. These values already have into account stress concentrations due to geometry discontinuities. The x axis is aligned with the crankshaft longitudinal axis, the y axis is vertical, and the z axis is horizontal. Fig. 14 shows the von Mises equivalent stress evolution for the stresses shown in Fig. 13. The von Mises equivalent stress was computed considering Eq. (5) were the six stress components are considered.
σ=
(σxx − σyy )2 + (σyy − σzz )2 + (σzz − σxx )2 2
2 2 2 ) + 3⋅(τxy + τ yz + τzx
(5)
The von Mises equivalent stress maximum value was 27 MPa at crack initiation spot which is a value very low considering the typical fatigue strength of crankshaft materials, as shown in Fig. 15. Fig. 15 shows the AISI4340 fatigue strength variation according
Fig. 12. Evolution of total deformation along the motor operating cycle: curve green – maximum amplitude; blue curve - average amplitude; red curve - minimum amplitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 402
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Fig. 13. Crankshaft load at crack initiation spot, a) normal stresses b) shear stresses.
Fig. 14. von Mises equivalent stress evolution at crack initiation spot.
to the heat treatments and surface treatments typically used to improve fatigue strength of crankshaft materials. As a rough analysis, and assuming that the AISI4340 fatigue behaviour is similar to the materials normally used in crankshafts, we correlate the maximum von Mises equivalent stress shown in Fig. 14 with the fatigue limits shown in Fig. 15. Table 1 shows the results of this correlation. The fatigue safety factors computed were 19, 23, and 31 for normal heat treatment, heat treatment plus shotpeening, and heat treat plus nitriding, respectively. Assuming that cyclic loadings bellow fatigue limit does not lead to fatigue failure, one can consider that for the assumptions considered the crankshaft failure does not resulted from the loading regime. To reinforce this idea, the stress level considered to analyse fatigue strength should be lower than the one considered in static loading conditions i.e. static stress concentration factors are higher than fatigue stress concentration factors due to the material cyclic plasticity. Therefore, the fatigue safety factors shown in Table 1 should be higher.
3.7. Crack initiation plane estimates and fatigue life analysis Fig. 16 shows the critical plane estimates for the critical plane directions. Fig. 16 a) shows the estimates given by the Fatemi-Socie 403
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Fig. 15. Typical SN curves for crankshafts made of AISI 4340 material according to treatment [16]. Table 1 Fatigue safety factor.
Fatigue s.f.
Normal heat treat
Heat treat plus shotpeening
Heat treatment plus nitriding
19
23
31
model. These estimates are very far from the measured crack initiation angle, i. e. ± 48° against 0° measured in the crankshaft crack surface. The crankshaft load at crack initiation plane has a strong component in x direction (normal stresses), see Fig. 13. This reinforces the idea that shear-based models like Fatemi-Socie and Liu II do not have a good performance in critical plane estimates under predominant normal stress loadings. On the other hand, axial based models typically have good estimates for this type of loadings. The results shown in Table 2 and Fig. (s) 16 b) and c) confirm this statement, where the difference between estimates and crack initiation plane measure is within the error margin, therefore it can be considered that SWT and LIU I estimates are in accordance with the crack initiation plane measure on the fatigue crack surface. Therefore, these two models are recommended to estimate crack initiation planes in crankshafts. The stress level in the crack initiation plane has a minimum fatigue safety factor of 19, for the assumption considered in this analysis, which is a huge safety margin. This margin is enough to waive the assumptions considered, therefore the crankshaft failure does not resulted from a poor mechanical design. Based on the aforementioned observations and fractography analysis one can concluded that the crankshaft failure occurred from a fatigue process resulted from high levels of normal stresses at crack initiation spot. This could result from two reasons. First, a notch created during production, assembly, or due to a material impurity could locally increase the normal stresses and reduce the crankshaft fatigue strength. In this case, it was not found any impurities in the crack initiation spot, this could be due to the fatigue crack closing process, where the interference between crack surfaces erased any trace of local impurities. The second possible reason for the normal stress increase is the crankshaft misalignment which increase locally stress levels. To verify this hypothesis, it is necessary analyse the engine block which was not possible during this study. It can be stated that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible processes, namely, a notch in the crack initiation spot, or the crankshaft misalignment. 4. Conclusion In this study it was analyzed a crankshaft failure mode of a 1900 cm3 turbo diesel engine. The crankshaft collapsed at 120 k kilometres without warning; the maintenance program performed is unknown but by inspection the crankshaft journals do not show 404
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Fig. 16. Critical plane estimates for the crack initiation plane. a) Fatemi-Socie damage parameter, b) SWT damage parameter, c) Liu I damage parameter, Liu II damage parameter.
Table 2 Critical plane estimates at crack initiation spot.
Angle
F. Socie
SWT
Liu1
Liu2
Measured
± 48
−4
−4
± 49
0
any unusual wear or superficial grooves, which detract any maintenance fault as reason for the crankshaft collapse. A descriptive fractographic analysis was performed to describe the failure mode process. It was concluded that the crankshaft collapsed due to fatigue crack nucleation and propagation in crankpin n°1. In order to investigate the stress levels in the crack initiation spot, a finite element model was created, and local stresses were acquired in the crack initiation spot. An estimate for a fatigue safety factor was performed considering the fatigue properties of a typical material used in automotive crankshafts. Moreover, it was performed an estimate for the direction of crack initiation plane using typical critical plane models. The finite element analysis show that the maximum total displacement occurs in the crack initiation spot identified in the fractographic analysis, however the stress levels found in this region are very low, with a fatigue safety factor around 19, which lead to conclude that the crankshaft does not collapsed due to a defective design. The results for the critical plane estimates show that the SWT and LIU I models predict very well the crack initiation plane, indicating that normal stresses are predominant during the fatigue process as a result of the bending load 405
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component. Results show that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible causes, namely, a notch in the crack initiation spot, or the crankshaft misalignment. As future works a SEM/TEM study can be carried out to analyse the chemical composition of the crankshaft material as well as a bearing integrity analysis. Acknowledgements The authors are very grateful to the reviewers for their insightful, constructive comments and suggestions which were very helpful in improving the quality of the paper. This work was supported by FCT, through IDMEC, under LAETA project UID/EMS/50022/2019. The second author gratefully acknowledges financial support from FCT - Fundação para Ciência e Tecnologia (Portuguese Foundation for Science and Technology), for the Ph.D. Grant PD/BD/52344/2013. References [1] Cedomir Duboka, Forensic evidence in road accidents caused by vehicle's mechanical failures, Automotive Engineering for Improved Safety, 2013 Belgrade, Serbia. [2] Z. Yu, X. Xu, H. Ding, Failure analysis of a diesel engine piston-pin, Eng. Fail. Anal. 14 (2007) 110–117. [3] A.M. Heyes, Automotive component failures, Eng. Fail. Anal. 5 (1998) 129–141. [4] M. Fonte, V. Anes, P. Duarte, L. Reis, M. Freitas, Crankshaft failure analysis of a boxer diesel motor, Eng. Fail. Anal. 56 (2015) 109–115. [5] O. Asi, Failure analysis of a crankshaft made from ductile cast iron, Eng. Fail. Anal. 13 (2006) 1260–1267. [6] R.K. Pandey, Failure of diesel-engine crankshafts, Eng. Fail. Anal. 10 (2003) 165–175. [7] F.S. Silva, Analysis of a vehicle crankshaft failure, Eng. Fail. Anal. 10 (2003) 605–616. [8] A.S.M. Handbook, Volume 12: Fractography, 55 ASM International Materials Park, 1987. [9] J. Haapakoski, Medium-speed four-stroke diesel engine cylinder pressure effect on component dimensioning, Master's thesis, University of Oulu, 2016, pp. 2–4. [10] C. Bell, Maximum boost, Cambridge, BentleyPublishers, USA, 1997. [11] D.F. Socie, G.B. Marquis, Multiaxial Fatigue, Society of Automotive Engineers Warrendale, PA, 2000. [12] D.F. Socie, Multiaxial fatigue damage models, J. Eng. Mater. Technol. 109 (1987) 293–298. [13] P. Watson, T.H. Topper, Fatigue-damage evaluation for mild steel incorporating mean stress and overload effects, Exp. Mech. 12 (1972) 11–17. [14] K.C. Liu, A method based on virtual strain-energy parameters for multiaxial fatigue life prediction, Advances in Multiaxial Fatigue, ASTM International, 1993. [15] Kozakewycz, Engine Thermodynamics, Black Art Dynamics, http://blackartdynamics.com/Engine/EngineThermodynamics.php, (2015) , Accessed date: 29 November 2018. [16] J. Kane, Contemporary crankshaft design, Race Engine Technol. Mag. 33 (2009) 1–14.
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