Done analysis of thermal mechanical coupling strength and fatigue life

In order to analyze the phenomenon of bolt preload when piston of low speed diesel engine is assembled and maximum explosion pressure and temperature during piston working impact on piston’s strength and fatigue life, Coupled analysis of mechanical stress and thermal stress on the piston of 5S60 low-speed diesel engine have been done, and the fatigue life of the piston on the alternating load condition was calculated. Firstly, the FEM-model which consists of 10-node tetrahedral meshes was built for the piston by using Hypermesh software with arranging different density of element quality which was guaranteed with the mesh parameters. Secondly, after setting the boundary conditions, the thermal stress, the mechanical stress and the coupling stress of the piston were calculated by using Abaqus software. Finally, the fatigue life of the piston on the alternating load condition was calculated by using nSoft software. The results indicate that the fatigue damage is easily occurred on the side of the surrounding area of the threaded holes, and that position should be made an especially consideration for design.

Analysis of thermal-mechanical coupling strength and fatigue life of piston of low speed diesel engine

Zhongming Liu 1, a, Cong Li 1, b and Xiaoqin Hou2, a

1College of Mechanical Engineering, Hangzhou Dianzi University, China 2Ningbo C.S.I Power & Machinery Group Co., Ltd, China

alzhm@zju.edu.cn, bLC_5267@126.com

Keywords: Piston, Thermal-mechanical coupling, Fatigue;

Abstract In order to analyze the phenomenon of bolt preload when piston of low speed diesel

engine is assembled and maximum explosion pressure and temperature during piston working impact on piston’s strength and fatigue life, Coupled analysis of mechanical stress and thermal stress on the piston of 5S60 low-speed diesel engine have been done, and the fatigue life of the piston on the alternating load condition was calculated Firstly, the FEM-model which consists of 10-node tetrahedral meshes was built for the piston by using Hypermesh software with arranging different density of element quality which was guaranteed with the mesh parameters Secondly, after setting the boundary conditions, the thermal stress, the mechanical stress and the coupling stress of the piston were calculated by using Abaqus software Finally, the fatigue life of the piston on the alternating load condition was calculated by using nSoft software The results indicate that the fatigue damage is easily occurred on the side of the surrounding area of the threaded holes, and that position should be made an especially consideration for design

Introduction

The piston’s working condition is very strict During High speed reciprocating motion, the piston should pass the entire engine driving force, so it withstands very high mechanical and thermal loads The piston bears various forces including the explosion pressure, the bolt’s pre-tightening force, the lateral force, the reciprocating inertia force, and, the temperature of each part of the piston is very uneven, so, the piston which bears great mechanical stress and thermal stress is likely to be cause fatigue damage

At present, the research method for the piston is the calculation of fatigue testing and simulation Because that the volume of the piston in this study is too big, the calculation of fatigue testing is limited, it is difficult to verify its structural life by using test, while the simulation is not subject to restrictions Therefore, the method used in this study is the simulation

Jingwei Tong who was from Tianjin University come to a conclusion that the piston’s deformation was mainly thermal deformation by carrying out 3-D finite element analysis of stress and deformation of piston under the action of thermal and mechanical load in 1997[1] Wenxiao Zhang analysed three-dimensional temperature field of marine diesel engine piston and three-dimensional stress and strain fields with finite element program, and carried out piston’s life prediction experiment and calculation in 2000[2] In 2002, Liyan Feng accomplished a coupled analysis of mechanical and thermal loads for piston of 8E160 type of diesel engine, calculated coupled stress field of piston, and drew a conclusion that stress problem do not become obstacles to further strengthen the modified diesel engine, the main obstacle is unfavorable to the normal operation of the piston with the increase of heat load which made thermal deformation of piston too big[3] In 2004, Jiong Li completed a coupled analysis of mechanical and thermal loads of piston from a gasoline in the cylinder expansion by using Three-dimensional finite element technique, calculated temperature field and coupled stress field of piston, identified the dangerous stress position, and provided a theoretical basis for the improvement and optimization of piston structure after the expansion cylinder[4] In 2007, Mengzhang Tao who was from Tongji University analyzed the piston with thermo-mechanical coupling method on the base of the thermal analysis results which All rights reserved No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans

obtained from conducting a 3D thermal analysis on piston by taking piston, piston pin and connecting rod as a package, calculated the stress field of the piston under thermal load and mechanical load, and made a comparison that stress field with the stress field only under mechanical load to analyze the effect of thermal load on the distribution of piston stress[5].In 2011, Junhong Zhang who was from Tianjin University analyzed stress and strain of piston under thermal load and mechanical load and coupled load with finite element method, came to a conclusion that the piston deformation is mainly thermal deformation in piston heat engine coupled analysis[6] Overall, analyzing the piston with thermo-mechanical coupling method is the main direction about the piston’s finite element calculation, in the domestic, many scholars has done related research also But the analysis reports for the piston of low speed marine diesel are not many Based on the piston of 5S60-type diesel engine, this paper presents a calculation method which can be used for calculating the fatigue life of this type piston, including analyzing the stress flied of the piston and then calculating the fatigue life of the piston

Finite element model of the piston

The piston’s FEM-model which consists of 10-node tetrahedral meshes is built by using Hypermesh software with arranging different density of element quality which is guaranteed with the mesh parameters, and is simplified for reducing the computational workload with calculation accuracy being not affected, shown in Fig 1 and Fig 2

Calculation of the piston’s temperature field

Third thermal boundary condition including ambient temperature and surface heat transfer coefficient is selected in the calculation of piston’s temperature field Temperature boundary condition of the piston top surface can be obtained for simulation working process of engine in the rated condition by using Boost software Temperature boundary condition of the piston’s side can be obtained for empirical formula and experimental test, and will be determined by repeated correction

The piston’s temperature field calculation by using Abaqus software after applying the thermal boundary is shown in Fig 3 The maximum temperature is 391.1℃ in the top surface of the piston, and the minimum temperature is121.2℃ in the surface of the ring groove of the first compression ring

Fig 3 The cutaway view of the piston’s temperature field

Calculation of the piston’s stress field

Thermal stress calculation of the piston It is assumed that the temperature of the piston in the

working process remain unchanged for the high-speed of the engine Based on the result of the piston’s temperature field, the thermal stress field can be obtained, shown in Fig 4

Fig 4 The cutaway view of the piston’s thermal stress field

Mechanical stress calculation of the piston This paper analyzes the piston’s mechanical stress

in the two conditions including preload condition and maximum explosion pressure condition It is known that each bolt’s preload is 238.797kN and that the gas’s maximum explosion pressure is 16MPa from the data provided by the manufacturer In the preload condition, the degree of freedom in the Y direction on the surface of the piston’s bottom is limited In the maximum explosion pressure condition, the piston cannot be move in the Y direction guaranteed by symmetrically adding ground spring at points under small stress on the piston After limiting the degrees of freedom in the X and Z directions on the surfaces of the ring grooves, the mechanical stress field can be obtained, shown in Fig 5 and Fig 6

Fig 5 The cutaway view of the piston’s mechanical stress field in the preload condition

Fig 6 The cutaway view of the piston’s mechanical stress field in the maximum

explosion condition

Thermal-mechanical coupling stress calculation of the piston Because that the piston does

not only bear the heat load generated by the top of the gas, but also bear mechanical load, it is meaningful to calculate the thermal-mechanical coupling stress Based on the analysis of mechanical stress, the thermal-mechanical coupling stress field can be obtained with boundary conditions unchanged in the maximum explosion pressure condition, shown in Fig 7

Fig 7 The piston’s thermal stress field

Analysis of the fatigue life of the piston

The piston bears explosion pressure and inertia force in actual working process, but the explosion is extremely bigger than the inertia force So, it is assumed that the piston only bears explosion pressure in the working process, and the fatigue life is calculated with the Goodman method

The piston’s fatigue life can be obtained for fatigue life calculation by using nSoft software, and its material is 42CrMn, the mechanical parameters have been shown in Table 1 Based on the finite element stress analysis result, the fatigue life of the piston is obtained, shown in Fig 8

Fig 8 The piston’s fatigue life

It can be seen from Fig 8 that the region near the threaded holes is serious affected by fatigue The region is concentrated in large temperature gradients and mechanical stress, and needs to be improved in order to enhance the fatigue strength The most dangerous 20 elements which are selected show in Fig 9

Fig 9 The piston dangerous element fatigue life

[3] L.Y.Feng, X.Y.Gao and H.M.Xia: Transaction of CSICE, Vol 20 (2002) No.05, p 441

[4] J.Li, S.W.Yang and Y.Zhang: Journal of North China Institute of Technology, Vol 25 (2004) No.05, p 319

[5] M.Z.Tao, Y.X.Liu and Y.S.Fu: Design & Manufacture of Diesel Engine, Vol 15 (2007) No.04, p 25

[6] J.H.Zhang, Z.P.He and G.C.Zhang: Transaction of CSICE, Vol 29 (2011) No.01, p 478

Analysis of Thermal-Mechanical Coupling Strength and Fatigue Life of Piston of Low Speed DieselEngine

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