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Failure analysis of a diesel engine cylinder head based on finite element method
Engineering Failure Analysis 34 (2013) 51–58
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a diesel engine cylinder head based on finite element method Qing Zhang, Zhengxing Zuo, Jinxiang Liu ⇑ School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 8 May 2012 Received in revised form 17 July 2013 Accepted 17 July 2013 Available online 31 July 2013 Keywords: Diesel engine Cylinder head Failure analysis Finite element method Stress concentration
a b s t r a c t Macro fatigue cracks are expected to occur in valve bridges of cylinder head when engine is operating in normal working condition. In order to determine the causes of these failures, stress analysis is carried out using finite element method with a concern of temperature dependency of material properties. Mechanical and thermal properties of material tested at high temperatures are applied to the finite element analysis. Furthermore, temperatures of the cylinder head in actual working condition are measured to validate the simulation results of finite element analysis. After that, stress computation is performed and the regions of stress concentration on the flame deck surface are obtained. The analysis results of stress show that the regions of stress concentration are in agreement with the actual failure regions of the cylinder head. When analyzing the failures on the flame deck surface of a cylinder head by evaluating stress concentration, temperature’s effect on mechanical strength of material should not be ignored. The methodology of failure analysis proposed in this paper is time-saving and also relatively accurate and predictive in actual engineering practice. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With the increasing demand of high power density in diesel engines, the operating temperature and peak firing pressure in combustion chamber increase significantly. As a result, the structural components in the combustion chamber have to endure severe mechanical and thermal loading [1]. Cylinder head is one of such heavy loaded combustion chamber components. The structure of cylinder head in modern high performance diesel engine is quite complicated in order to satisfy structural and performance demands of the engine. Several components are assembled in the cylinder head, such as valves, sparkplugs, and exhaust manifolds. Furthermore, there are water jacket, inlet and exhaust gas channels which enhance the complexity of the structure. The flame deck surface of the cylinder head, which has a direct contact with the high temperature gas, is divided into several pieces consisting of several valve bridges. High temperature gradient on the flame deck surface is produced due to the complicated structure. Maximum temperature and temperature gradient on the flame deck surface increase hugely with the rise of power density. Under the influence of structure and temperature, regions of stress concentration are formed on the flame deck surface. These regions of stress concentration mostly are the critical areas in the start-up and shut-down or large load-change cycles of the engine [2]. Previous studies have reported that the valve bridges are the failure regions of cylinder head [3,4]. Therefore, analysis of these regions is important for the understanding of failures on the flame deck surface, which is also critical for the optimized structure redesign of the cylinder head.
⇑ Corresponding author. Tel./fax: +86 10 6891 1392. E-mail address: [email protected] (J. Liu). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.07.023
52
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
Failures of components have been investigated a lot. The research methods can actually be categorized into three types. One is in the way of pure experiments. For instance, Yu and Xu analyzes the failure of a cylinder head in this experimentbased way [5]. In this method, microstructure and fracture surface in destroyed areas are analyzed with special attention by finding the crack initiation point and microstructure characters. Focus is put on the material characteristic. This method usually has no capacity of predicting the regions of failure which is important for structure redesign. The second method analyzes the failures by fatigue life prediction using numerical computation [6,7]. The main objective of this method is to predict the fatigue life of the component validated by fatigue tests of real components. Although the result of this method isintuitionistic, many tests of material properties, especially the fatigue tests which consume both much time and cost, have to be conducted in order to obtain an accurate result. Nevertheless, the predicted life of the component does not agree very well with the actual fatigue tests, just providing a probable range because of the scatter property of fatigue life. The third method analyzes failure by evaluating the regions of stress concentration using numerical computation [8,9]. In this method, timeconsuming fatigue test is not required. High efficiency and relative accuracy are the main advantage of this method. In this study, failure analysis of a cast iron cylinder head is performed using the third method mentioned above. Mechanical and thermal properties of material of the cylinder head are tested at high temperatures. The experimental data are utilized in the finite element analysis. Temperature distribution of the cylinder head is simulated by finite element method and validated by temperature measurement in the normal working condition. Stress analysis under thermal and mechanical loading is carried out with a concern of temperature dependency of material properties. Based on the results, the failures of the cylinder head are investigated in detail. 2. Failure description of the cylinder head The analyzed cylinder head is used in a heavy duty, four-stroke diesel engine, in which each cylinder is associated with one separate cylinder head. There are four valves on the flame deck surface, i.e. two exhaust valves and two inlet valves. Besides, in order to reduce temperature amplitude, some material in the valve bridges is removed which results in a largest depth of 2 mm in the center of the notches. The mentioned structures of the analyzed cylinder head can be seen in Fig. 1. Fig. 1 also gives the failure locations on the flame deck surface of the cylinder head, which occur after a working time of 50 h. Failure regions of A, B and C are detected where macro cracks can be seen. Failure regions of A and B are in the exhaust and exhaust-inlet valve bridge, respectively. Fig. 2 shows the zoom of these two regions. Failure morphology of A is similar with that of B. One large crack occurs almost perpendicular to the valve bridge, with one crack tip extending to the bound of the valve seat, and another tip extending to the center of the valve bridge. Furthermore, local thermal corroded region can be seen in the exhaust-inlet bridge as shown in Fig. 2, which is caused by excessive high temperature. Fig. 3 shows the failure region of C in magnified photograph. Region of C is characterized by some interlaced macro cracks which are different with the failure regions of A and B. It can be seen from Figs. 2 and 3 that the failure cracks on the flame deck surface all occur through or in the valve seats which indicates that these areas are critical regions. 3. Finite element analysis 3.1. Finite element model A finite element model of the cylinder head used in the numerical analysis is constructed, as shown in Fig. 4. The cylinder head is meshed using a eight-node hexahedral solid mesh, which is assigned the element type of C3D8I in stress analysis and
Exhaust
A
Exhaust
B
Inlet
C
Inlet
Fig. 1. Failure regions on the flame deck surface of the cylinder head.
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
53
B
A
Fig. 2. Zoom of failure regions of A and B in Fig. 1.
C
Fig. 3. Zoom of failure region of C in Fig. 1.
Fig. 4. Finite element model of the cylinder head.
DC3D8 in temperature analysis [10] in the commercial software ABAQUSÒ 6.10. The finite element model of the cylinder head has 743,957 elements and 819,020 nodes. The assembled finite element model, including cylinder head, cylinder bolts, cylinder sleeve, gasket and engine block, is constructed in order to precisely simulate the real contact and loading conditions
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Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
in the stress analysis. The engine block is meshed with four-node tetrahedral solid element, which supplies the real stiffness of the total model. The other components are meshed with hexahedron solid mesh. The total number of elements and nodes of the assembled finite element model are 908,511 and 1,090,708, respectively. Contact condition is assigned to the interfaces between components. A friction coefficient of 0.15 is defined to the contact surface between cylinder head and gasket [11].
3.2. Microstructure, mechanical and thermal properties of material of the cylinder head The analyzed diesel engine cylinder head is made of compacted graphite cast iron, with a chemical composition shown in Table 1. Fig. 5 shows the microstructure of the compacted graphite cast iron. It can be seen that the graphite shapes of the material are mainly vermiculate, mixed with a little of spheroidal graphite. The graphite shapes and chemical composition have a great influence on the material properties. Besides, material properties of cast iron depend on temperature greatly [12,13]. Considering the effect of temperature, mechanical and thermal properties of the compacted graphite cast iron are tested at high temperatures covering the range of loading temperatures of the cylinder head.
Table 1 Chemical composition (wt.%) of the compacted graphite cast iron used in the analyzed cylinder head. C
Si
Mn
P
S
Fe
3.36
2.32
0.86
0.051
0.018
Bal.
Fig. 5. Microstructure of the compacted graphite cast iron used in the analyzed cylinder head.
Fig. 6. The tested stress–strain curves of the compacted graphite cast iron at 25 °C, 250 °C, 400 °C and 550 °C.
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
55
Fig. 7. The tested thermal conductivity and thermal expansion coefficient of the compacted graphite cast iron.
Figs. 6 and 7 show the results of tensile and thermal tests, respectively. Fig. 6 shows that the mechanical strength of the material decreases tremendously with the increase of temperature, especially from 25 °C to 250 °C. The tensile strength in the temperature range of 250–400 °C does not decrease so fast. The tensile strength at 550 °C is about 268 MPa, which is almost half of that at 25 °C. Fig. 7 shows the dependence of thermal conductivity and thermal expansion coefficients on temperatures. The above experimental results are adopted in the finite element analysis. 3.3. Loading conditions In this paper, a static finite element computation, aiming to simulate the stress distribution of the cylinder head in the peak firing working condition, is accomplished. Loads applied to the cylinder head comprise mechanical and thermal loads. The mechanical loads mainly include pre-tightened bolt load of 130,000 N, and peak firing pressure of 18.73 MPa, respectively. There is no valve seat ring in the cylinder head. Thermal loads are more complicated compared with mechanical loads. There are two main thermal boundary conditions, i.e., the heat transfer from coolant in the water jacket and the high temperature gas generated from combustion process in the chamber to the cylinder head. Empirical boundary condition for thermal transfer is given in the finite element analysis. Furthermore, temperature test of the cylinder head in the actual working condition is conducted to validate temperature results of simulation, which is discussed in detail in the following section. 4. Results and discussion 4.1. Temperature distribution on the flame deck surface For the validation of the simulating temperature on the flame deck surface, temperature test in actual working condition is carried out. Fig. 8 shows the locations of the 15 measured points in the test. The measured points mainly are located in the valve bridges where temperature gradient is very high. Fig. 9 shows the simulating distribution of temperatures on the flame deck surface of the cylinder head. The highest temperature in the exhaust valve bridge is 497.8 °C, where the measured temperature is 496 °C. The exhaust valve bridge is the region with the highest temperature throughout the cylinder head. Fig. 10 shows the comparison between experimental and numerical temperatures on the flame deck surface. The numerical results is in good agreement with experimental data. Fig. 11 shows the temperature gradient in the exhaust and exhaustinlet bridges of the cylinder head. It is obvious that gradient on Path 2 is larger than that on Path 1. Besides, highest temperature exists in the center of each valve bridge. 4.2. Stress analysis Based on the obtained temperature distribution of the cylinder head, as discussed in Section 4.1, the stress simulation of the cylinder head is carried out, in which the temperature distribution is coupled with pre-tightened bolt load and peak firing pressure. Fig. 12 shows the von Mises stress distribution on the flame deck surface of the cylinder head. It can be seen that stress concentration mainly exists in the valve bridges. In the exhaust valve bridge, the highest von Mises stress is 281 MPa. The temperature range in this area is about 340–497.8 °C. Under this temperature, the mechanical strength of the material is 299–330 MPa. High temperature strongly reduces the mechanical strength of the material. The stress in this area is close to the mechanical strength of the material, which indicates that the exhaust valve bridge is a critical area and prone to be failed. The areas in the two exhaust-inlet valve bridges near to the exhaust valves are also the regions with high stress. Temperature and von Mises stress in these regions are about 240–325 °C and 289–303 MPa, respectively. The
56
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
Ex 3
2
1
10 9 8
In 11
12
13
4
14
5
15
6 Ex
7
In
Fig. 8. The measured points in the temperature test on the flame deck surface of the cylinder head.
Fig. 9. Distribution of temperatures (°C) on the flame deck surface of the cylinder head obtained from finite element analysis.
Fig. 10. Temperatures comparison between experimental and numerical results on the flame deck surface of the cylinder head.
mechanical strength of the material under this temperature range is about 342–380 MPa. These two regions are also risk areas which endure high thermal stress. The risk areas marked by A and B in Fig. 12 are in coincidence with the real failure regions of the cylinder head (see Fig. 2). The effect of high temperature on the failure can not be neglected. For the purpose of analyzing the stress in inlet valve bridge, Fig. 13 shows the von Mises stress distribution on the flame deck surface with a lower limit of 230 MPa. The maximum stress of 354 MPa occurs on the edge of inlet valve seat, where
57
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
500
2 o
Temperature ( C)
400 300
1 200
1 2
100 0
0
10
20
30
40
50
60
70
Distance along path (mm) Fig. 11. Temperature gradient in the exhaust and exhaust-inlet bridges of the cylinder head.
B A
Fig. 12. Von Mises stress (MPa) distribution on the flame deck surface of the cylinder head. Regions of A and B are the failure regions.
C
Fig. 13. Display of von Mises stress (MPa) distribution with a lower limit of 230 MPa on the flame deck surface of the cylinder head. Region of C is the failure region.
temperature is about 180 °C. Mechanical strength of the material in this temperature is 408 MPa. The maximum stress is close to the mechanical strength of the material under this temperature, which finally gives rise to the failure of this area. The region marked by C in Fig. 13 is in agreement with the real failure region of the cylinder head (see Fig. 3).
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Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
The regions of stress concentration obtained from finite element analysis are in agreement with actual failure regions. Because the exact value of stress distribution on the flame deck surface is difficult to measure in practice, the proposed methodology of analyzing the failures of a cylinder head, by analyzing von Mises stress which takes into account of temperature, is predictive and applicable. 5. Conclusions Failures occur on the flame deck surface of a cylinder head in normal working condition. Two macro cracks are perpendicular to the valve bridges. Another failure region is characterized by some interlaced macro cracks in the inlet valve seat. The failures are analyzed using finite element method. Tests of material mechanical and thermal properties indicate that temperature greatly affect the properties of the compacted graphite cast iron which is the material of the analyzed cylinder head. Simulation temperature distribution validated by test indicates that temperature distribution on the flame deck surface is inhomogeneous and the gradient in the exhaust valve bridge is the largest. The results of stress analysis indicate that the regions of stress concentration are in coincidence with the real failure regions. The maximum von Mises stress occurs on the edge of the inlet valve seat, which produces macro cracks. The other two failure regions are located in the exhaust and exhaust-inlet valve bridges. Compared with inlet valve bridge, the exhaust valve bridge has lower stress and higher temperature. The high temperature strongly affects the mechanical strength of the material. The failure analysis should not ignore the temperature effect. The methodology is feasible and low-cost in engineering practice. References [1] Thomas JJ, Verger L, Bignonnet A, Charkaluk E. Thermomechanical design in the automotive industry. Fatigue Fract Eng Mater Struct 2004;27:887–95. [2] Zieher F, Langmayr F, Jelatancev A, Wieser K. Thermal mechanical fatigue simulation of cast iron cylinder heads. SAE Technical Papers. SAE 2005-010796. [3] Barlas B, Massinon D, Meyer P, Cailletaud G, Guillot I, Morin G. A phenomenological model for fatigue life prediction of highly loaded cylinder heads. SAE Technical Papers. SAE 2006-01-0542. [4] Fontanesi S, Carpentiero D, Malaguti S, Giacopini M, Margini S. A new decoupled CFD and FEM methodology for the fatigue strength assessment of an engine head. SAE Technical Papers. SAE 2008-01-0972. [5] Xu XL, Yu ZW. Failure analysis of a diesel engine cylinder head. Eng Fail Anal 2006;13:1101–7. [6] Topaç MM, Günal H, Kuralay NS. Fatigue Failure prediction of a rear axle housing prototype by using finite element analysis. Eng Fail Anal 2009;16:1474–82. [7] Firat M. A computer simulation of four-point bending fatigue of a rear axle assembly. Eng Fail Anal 2011;18:2137–48. [8] Espadafor FJ, Villanueva JB, García MT. Analysis of a diesel generator crankshaft failure. Eng Fail Anal 2009;16:2333–41. [9] Becerra JA, Jimenez FJ, Torres M, Sanchez DT, Carvajal E. Failure analysis of reciprocating compressor crankshafts. Eng Fail Anal 2011;18:735–46. [10] ABAQUS User’s Manual, Version 6.10, ABAQUS, Inc.: 2010. [11] Trampert S, Gocmez T, Pischinger S. Thermomechanical fatigue life prediction of cylinder heads in combustion engines. J Eng Gas Turb Power 2008;130:771–80. [12] Seifert T, Riedel H. Mechanism-based thermomechanical fatigue life prediction of cast iron. Part I: Models. Int J Fatigue 2010;32:1358–67. [13] Seifert T, Maier G, Uihlein A, Lang KH, Riedel H. Mechanism-based thermomechanical fatigue life prediction of cast iron. Part II: Comparison of model predictions with experiments. Int J Fatigue 2010;32:1368–77.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a diesel engine cylinder head based on finite element method Qing Zhang, Zhengxing Zuo, Jinxiang Liu ⇑ School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 8 May 2012 Received in revised form 17 July 2013 Accepted 17 July 2013 Available online 31 July 2013 Keywords: Diesel engine Cylinder head Failure analysis Finite element method Stress concentration
a b s t r a c t Macro fatigue cracks are expected to occur in valve bridges of cylinder head when engine is operating in normal working condition. In order to determine the causes of these failures, stress analysis is carried out using finite element method with a concern of temperature dependency of material properties. Mechanical and thermal properties of material tested at high temperatures are applied to the finite element analysis. Furthermore, temperatures of the cylinder head in actual working condition are measured to validate the simulation results of finite element analysis. After that, stress computation is performed and the regions of stress concentration on the flame deck surface are obtained. The analysis results of stress show that the regions of stress concentration are in agreement with the actual failure regions of the cylinder head. When analyzing the failures on the flame deck surface of a cylinder head by evaluating stress concentration, temperature’s effect on mechanical strength of material should not be ignored. The methodology of failure analysis proposed in this paper is time-saving and also relatively accurate and predictive in actual engineering practice. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With the increasing demand of high power density in diesel engines, the operating temperature and peak firing pressure in combustion chamber increase significantly. As a result, the structural components in the combustion chamber have to endure severe mechanical and thermal loading [1]. Cylinder head is one of such heavy loaded combustion chamber components. The structure of cylinder head in modern high performance diesel engine is quite complicated in order to satisfy structural and performance demands of the engine. Several components are assembled in the cylinder head, such as valves, sparkplugs, and exhaust manifolds. Furthermore, there are water jacket, inlet and exhaust gas channels which enhance the complexity of the structure. The flame deck surface of the cylinder head, which has a direct contact with the high temperature gas, is divided into several pieces consisting of several valve bridges. High temperature gradient on the flame deck surface is produced due to the complicated structure. Maximum temperature and temperature gradient on the flame deck surface increase hugely with the rise of power density. Under the influence of structure and temperature, regions of stress concentration are formed on the flame deck surface. These regions of stress concentration mostly are the critical areas in the start-up and shut-down or large load-change cycles of the engine [2]. Previous studies have reported that the valve bridges are the failure regions of cylinder head [3,4]. Therefore, analysis of these regions is important for the understanding of failures on the flame deck surface, which is also critical for the optimized structure redesign of the cylinder head.
⇑ Corresponding author. Tel./fax: +86 10 6891 1392. E-mail address: [email protected] (J. Liu). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.07.023
52
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
Failures of components have been investigated a lot. The research methods can actually be categorized into three types. One is in the way of pure experiments. For instance, Yu and Xu analyzes the failure of a cylinder head in this experimentbased way [5]. In this method, microstructure and fracture surface in destroyed areas are analyzed with special attention by finding the crack initiation point and microstructure characters. Focus is put on the material characteristic. This method usually has no capacity of predicting the regions of failure which is important for structure redesign. The second method analyzes the failures by fatigue life prediction using numerical computation [6,7]. The main objective of this method is to predict the fatigue life of the component validated by fatigue tests of real components. Although the result of this method isintuitionistic, many tests of material properties, especially the fatigue tests which consume both much time and cost, have to be conducted in order to obtain an accurate result. Nevertheless, the predicted life of the component does not agree very well with the actual fatigue tests, just providing a probable range because of the scatter property of fatigue life. The third method analyzes failure by evaluating the regions of stress concentration using numerical computation [8,9]. In this method, timeconsuming fatigue test is not required. High efficiency and relative accuracy are the main advantage of this method. In this study, failure analysis of a cast iron cylinder head is performed using the third method mentioned above. Mechanical and thermal properties of material of the cylinder head are tested at high temperatures. The experimental data are utilized in the finite element analysis. Temperature distribution of the cylinder head is simulated by finite element method and validated by temperature measurement in the normal working condition. Stress analysis under thermal and mechanical loading is carried out with a concern of temperature dependency of material properties. Based on the results, the failures of the cylinder head are investigated in detail. 2. Failure description of the cylinder head The analyzed cylinder head is used in a heavy duty, four-stroke diesel engine, in which each cylinder is associated with one separate cylinder head. There are four valves on the flame deck surface, i.e. two exhaust valves and two inlet valves. Besides, in order to reduce temperature amplitude, some material in the valve bridges is removed which results in a largest depth of 2 mm in the center of the notches. The mentioned structures of the analyzed cylinder head can be seen in Fig. 1. Fig. 1 also gives the failure locations on the flame deck surface of the cylinder head, which occur after a working time of 50 h. Failure regions of A, B and C are detected where macro cracks can be seen. Failure regions of A and B are in the exhaust and exhaust-inlet valve bridge, respectively. Fig. 2 shows the zoom of these two regions. Failure morphology of A is similar with that of B. One large crack occurs almost perpendicular to the valve bridge, with one crack tip extending to the bound of the valve seat, and another tip extending to the center of the valve bridge. Furthermore, local thermal corroded region can be seen in the exhaust-inlet bridge as shown in Fig. 2, which is caused by excessive high temperature. Fig. 3 shows the failure region of C in magnified photograph. Region of C is characterized by some interlaced macro cracks which are different with the failure regions of A and B. It can be seen from Figs. 2 and 3 that the failure cracks on the flame deck surface all occur through or in the valve seats which indicates that these areas are critical regions. 3. Finite element analysis 3.1. Finite element model A finite element model of the cylinder head used in the numerical analysis is constructed, as shown in Fig. 4. The cylinder head is meshed using a eight-node hexahedral solid mesh, which is assigned the element type of C3D8I in stress analysis and
Exhaust
A
Exhaust
B
Inlet
C
Inlet
Fig. 1. Failure regions on the flame deck surface of the cylinder head.
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
53
B
A
Fig. 2. Zoom of failure regions of A and B in Fig. 1.
C
Fig. 3. Zoom of failure region of C in Fig. 1.
Fig. 4. Finite element model of the cylinder head.
DC3D8 in temperature analysis [10] in the commercial software ABAQUSÒ 6.10. The finite element model of the cylinder head has 743,957 elements and 819,020 nodes. The assembled finite element model, including cylinder head, cylinder bolts, cylinder sleeve, gasket and engine block, is constructed in order to precisely simulate the real contact and loading conditions
54
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
in the stress analysis. The engine block is meshed with four-node tetrahedral solid element, which supplies the real stiffness of the total model. The other components are meshed with hexahedron solid mesh. The total number of elements and nodes of the assembled finite element model are 908,511 and 1,090,708, respectively. Contact condition is assigned to the interfaces between components. A friction coefficient of 0.15 is defined to the contact surface between cylinder head and gasket [11].
3.2. Microstructure, mechanical and thermal properties of material of the cylinder head The analyzed diesel engine cylinder head is made of compacted graphite cast iron, with a chemical composition shown in Table 1. Fig. 5 shows the microstructure of the compacted graphite cast iron. It can be seen that the graphite shapes of the material are mainly vermiculate, mixed with a little of spheroidal graphite. The graphite shapes and chemical composition have a great influence on the material properties. Besides, material properties of cast iron depend on temperature greatly [12,13]. Considering the effect of temperature, mechanical and thermal properties of the compacted graphite cast iron are tested at high temperatures covering the range of loading temperatures of the cylinder head.
Table 1 Chemical composition (wt.%) of the compacted graphite cast iron used in the analyzed cylinder head. C
Si
Mn
P
S
Fe
3.36
2.32
0.86
0.051
0.018
Bal.
Fig. 5. Microstructure of the compacted graphite cast iron used in the analyzed cylinder head.
Fig. 6. The tested stress–strain curves of the compacted graphite cast iron at 25 °C, 250 °C, 400 °C and 550 °C.
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
55
Fig. 7. The tested thermal conductivity and thermal expansion coefficient of the compacted graphite cast iron.
Figs. 6 and 7 show the results of tensile and thermal tests, respectively. Fig. 6 shows that the mechanical strength of the material decreases tremendously with the increase of temperature, especially from 25 °C to 250 °C. The tensile strength in the temperature range of 250–400 °C does not decrease so fast. The tensile strength at 550 °C is about 268 MPa, which is almost half of that at 25 °C. Fig. 7 shows the dependence of thermal conductivity and thermal expansion coefficients on temperatures. The above experimental results are adopted in the finite element analysis. 3.3. Loading conditions In this paper, a static finite element computation, aiming to simulate the stress distribution of the cylinder head in the peak firing working condition, is accomplished. Loads applied to the cylinder head comprise mechanical and thermal loads. The mechanical loads mainly include pre-tightened bolt load of 130,000 N, and peak firing pressure of 18.73 MPa, respectively. There is no valve seat ring in the cylinder head. Thermal loads are more complicated compared with mechanical loads. There are two main thermal boundary conditions, i.e., the heat transfer from coolant in the water jacket and the high temperature gas generated from combustion process in the chamber to the cylinder head. Empirical boundary condition for thermal transfer is given in the finite element analysis. Furthermore, temperature test of the cylinder head in the actual working condition is conducted to validate temperature results of simulation, which is discussed in detail in the following section. 4. Results and discussion 4.1. Temperature distribution on the flame deck surface For the validation of the simulating temperature on the flame deck surface, temperature test in actual working condition is carried out. Fig. 8 shows the locations of the 15 measured points in the test. The measured points mainly are located in the valve bridges where temperature gradient is very high. Fig. 9 shows the simulating distribution of temperatures on the flame deck surface of the cylinder head. The highest temperature in the exhaust valve bridge is 497.8 °C, where the measured temperature is 496 °C. The exhaust valve bridge is the region with the highest temperature throughout the cylinder head. Fig. 10 shows the comparison between experimental and numerical temperatures on the flame deck surface. The numerical results is in good agreement with experimental data. Fig. 11 shows the temperature gradient in the exhaust and exhaustinlet bridges of the cylinder head. It is obvious that gradient on Path 2 is larger than that on Path 1. Besides, highest temperature exists in the center of each valve bridge. 4.2. Stress analysis Based on the obtained temperature distribution of the cylinder head, as discussed in Section 4.1, the stress simulation of the cylinder head is carried out, in which the temperature distribution is coupled with pre-tightened bolt load and peak firing pressure. Fig. 12 shows the von Mises stress distribution on the flame deck surface of the cylinder head. It can be seen that stress concentration mainly exists in the valve bridges. In the exhaust valve bridge, the highest von Mises stress is 281 MPa. The temperature range in this area is about 340–497.8 °C. Under this temperature, the mechanical strength of the material is 299–330 MPa. High temperature strongly reduces the mechanical strength of the material. The stress in this area is close to the mechanical strength of the material, which indicates that the exhaust valve bridge is a critical area and prone to be failed. The areas in the two exhaust-inlet valve bridges near to the exhaust valves are also the regions with high stress. Temperature and von Mises stress in these regions are about 240–325 °C and 289–303 MPa, respectively. The
56
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
Ex 3
2
1
10 9 8
In 11
12
13
4
14
5
15
6 Ex
7
In
Fig. 8. The measured points in the temperature test on the flame deck surface of the cylinder head.
Fig. 9. Distribution of temperatures (°C) on the flame deck surface of the cylinder head obtained from finite element analysis.
Fig. 10. Temperatures comparison between experimental and numerical results on the flame deck surface of the cylinder head.
mechanical strength of the material under this temperature range is about 342–380 MPa. These two regions are also risk areas which endure high thermal stress. The risk areas marked by A and B in Fig. 12 are in coincidence with the real failure regions of the cylinder head (see Fig. 2). The effect of high temperature on the failure can not be neglected. For the purpose of analyzing the stress in inlet valve bridge, Fig. 13 shows the von Mises stress distribution on the flame deck surface with a lower limit of 230 MPa. The maximum stress of 354 MPa occurs on the edge of inlet valve seat, where
57
Q. Zhang et al. / Engineering Failure Analysis 34 (2013) 51–58
500
2 o
Temperature ( C)
400 300
1 200
1 2
100 0
0
10
20
30
40
50
60
70
Distance along path (mm) Fig. 11. Temperature gradient in the exhaust and exhaust-inlet bridges of the cylinder head.
B A
Fig. 12. Von Mises stress (MPa) distribution on the flame deck surface of the cylinder head. Regions of A and B are the failure regions.
C
Fig. 13. Display of von Mises stress (MPa) distribution with a lower limit of 230 MPa on the flame deck surface of the cylinder head. Region of C is the failure region.
temperature is about 180 °C. Mechanical strength of the material in this temperature is 408 MPa. The maximum stress is close to the mechanical strength of the material under this temperature, which finally gives rise to the failure of this area. The region marked by C in Fig. 13 is in agreement with the real failure region of the cylinder head (see Fig. 3).
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The regions of stress concentration obtained from finite element analysis are in agreement with actual failure regions. Because the exact value of stress distribution on the flame deck surface is difficult to measure in practice, the proposed methodology of analyzing the failures of a cylinder head, by analyzing von Mises stress which takes into account of temperature, is predictive and applicable. 5. Conclusions Failures occur on the flame deck surface of a cylinder head in normal working condition. Two macro cracks are perpendicular to the valve bridges. Another failure region is characterized by some interlaced macro cracks in the inlet valve seat. The failures are analyzed using finite element method. Tests of material mechanical and thermal properties indicate that temperature greatly affect the properties of the compacted graphite cast iron which is the material of the analyzed cylinder head. Simulation temperature distribution validated by test indicates that temperature distribution on the flame deck surface is inhomogeneous and the gradient in the exhaust valve bridge is the largest. The results of stress analysis indicate that the regions of stress concentration are in coincidence with the real failure regions. The maximum von Mises stress occurs on the edge of the inlet valve seat, which produces macro cracks. The other two failure regions are located in the exhaust and exhaust-inlet valve bridges. Compared with inlet valve bridge, the exhaust valve bridge has lower stress and higher temperature. The high temperature strongly affects the mechanical strength of the material. The failure analysis should not ignore the temperature effect. The methodology is feasible and low-cost in engineering practice. References [1] Thomas JJ, Verger L, Bignonnet A, Charkaluk E. Thermomechanical design in the automotive industry. Fatigue Fract Eng Mater Struct 2004;27:887–95. [2] Zieher F, Langmayr F, Jelatancev A, Wieser K. Thermal mechanical fatigue simulation of cast iron cylinder heads. SAE Technical Papers. SAE 2005-010796. [3] Barlas B, Massinon D, Meyer P, Cailletaud G, Guillot I, Morin G. A phenomenological model for fatigue life prediction of highly loaded cylinder heads. SAE Technical Papers. SAE 2006-01-0542. [4] Fontanesi S, Carpentiero D, Malaguti S, Giacopini M, Margini S. A new decoupled CFD and FEM methodology for the fatigue strength assessment of an engine head. SAE Technical Papers. SAE 2008-01-0972. [5] Xu XL, Yu ZW. Failure analysis of a diesel engine cylinder head. Eng Fail Anal 2006;13:1101–7. [6] Topaç MM, Günal H, Kuralay NS. Fatigue Failure prediction of a rear axle housing prototype by using finite element analysis. Eng Fail Anal 2009;16:1474–82. [7] Firat M. A computer simulation of four-point bending fatigue of a rear axle assembly. Eng Fail Anal 2011;18:2137–48. [8] Espadafor FJ, Villanueva JB, García MT. Analysis of a diesel generator crankshaft failure. Eng Fail Anal 2009;16:2333–41. [9] Becerra JA, Jimenez FJ, Torres M, Sanchez DT, Carvajal E. Failure analysis of reciprocating compressor crankshafts. Eng Fail Anal 2011;18:735–46. [10] ABAQUS User’s Manual, Version 6.10, ABAQUS, Inc.: 2010. [11] Trampert S, Gocmez T, Pischinger S. Thermomechanical fatigue life prediction of cylinder heads in combustion engines. J Eng Gas Turb Power 2008;130:771–80. [12] Seifert T, Riedel H. Mechanism-based thermomechanical fatigue life prediction of cast iron. Part I: Models. Int J Fatigue 2010;32:1358–67. [13] Seifert T, Maier G, Uihlein A, Lang KH, Riedel H. Mechanism-based thermomechanical fatigue life prediction of cast iron. Part II: Comparison of model predictions with experiments. Int J Fatigue 2010;32:1368–77.