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FAILURE_ANALYSIS_OF_A_SHOCK_ABSORBER_PISTON_减震器活塞的失效模式分析.pdf

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FAILURE_ANALYSIS_OF_A_SHOCK_ABSORBER_PISTON_ 减震器 活塞 失效 模式 分析
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5 thInternational Advanced Technologies Symposium (IATS’09), May 13-15, 2009, Karabuk, Turkey FAILURE ANALYSIS OF A SHOCK ABSORBER PISTON PRODUCED BY POWDER METALLURGY PROCESSING Bekir YAL?IN a , ?smail UCUN b ve M. Re ?it USAL ca Süleyman Demirel University, Technical Education Faculty, Isparta, Turkey, bekir@tef.sdu.edu.tr bKocatepe University, Technical Education Faculty, Afyonkarahisar, Turkey, iucun@aku.edu.tr c Süleyman Demirel University, Technical Education Faculty, Isparta, Turkey, usalmr@tef.sdu.edu.tr Abstract In this study, the fracture behavior of a shock absorber piston produced by means of powder metallurgy method was investigated as experimental and numerical. In the experimental study, specimens prepared from piston materials were used in experiments such as scanning electron microscopy (SEM), hardness, tensile, microstructure. In the numerical analysis, a stress analysis was carried out by finite element method for the determination of stresses and displacements region on the piston. Keywords: Shock Absorber, Piston, Failure Analysis,Finite Element Method. 1. Introduction Powder metallurgy (P/M) technology is technique of the part producing the near-net shape. P/M components have been widely used in the automobile industry for gear, connecting roads, engine parts, self lubrication bear and shock absorber systems [1-4]. P/M industries have expanded more rapidly due to the recognition of the explicit advantages such as ease of component manufacture, cost/energy saving, optimal material utilization, rapid manufacturing and other factors [2, 6]. Most important advantage of the P/M method is the obtaining of required alloying composition. P/M technology is three basic steps in making a part: mixing elemental or alloying powder; pressing the powder mixture in a die, and sintering the part in a controlled atmosphere furnace to bond green particles [7]. To selection of the P/M technology, key design consideration must be recognized. These may be followed such as shape complexity, size, tolerance, properties, material system, quantity and cost [3]. Especially, this technology is selected to rapid manufacturing of small complex size parts. But, sharp corner and thin cross- section geometry may not be suitable for P/M process sometimes. Because, density, strength, amount of porosity are very important for this geometry. In this state, to improve properties P/M parts are needed the secondary operations such as steam treating, heat treatment, surface coating and pores filling [8]. Today, component subjected to cycle loading, and the mechanical properties has also become an important P/M steel characteristic. Reasons of the failure in a shock absorber system can be described as an incorrect piston rod installation, extremely torque values during installation, overload the vehicle and accident. For optimization of the mechanical parts, some properties were determined with animation and analysis programs during design stage. The failure analysis is a special field of study for materials, mechanical systems and mechanical engineering. Mechanical engineering studies on the possible failure location and types and amount of the existent stress levels using analysis programs for materials selection in mechanical system. A large number of papers concerning with failure analysis of the mechanical and industrial parts have been published [10-15]. In this study, failure analysis of a shock absorber piston used in the automotive industry was performed. The shock absorber system used in the study is illustrated in Fig 1. Yield and tensile strength, hardness and bending strength of the shock absorber piston material alloy were determined with experiments. Fractography studies on the fracture surface of the piston were performed by means of scanning electron microscopy (SEM). The stress analysis was conducted by finite element method. Fig. 1. Shock absorber system. 2. Experimental Procedure 2.1. Materials and Production Method Specimens were produced using P/M technique. The sintered powder compact was prepared using premix particles (Hoeganaes Corporation). Table 1 shows the characterization of the powders used in the alloy preparation. The starting powders were prepared with pre- alloying method and the mixture were blended for 20 minute at 20 rev./min. in the double-cone mixer. Secondly, elemental powders were compacted at room temperature inside tool steel by double-action pressing. Pressing loads was selected as 550 MPa according to literature [14]. After pressing, green specimens were sintered at 1150 o C peak temperature for 25 minute under heated argon atmosphere controlled. In the heating cycle, the compacts were kept at 850 o C for 20 minute in order to allow complete burning of the lubrication (Zn-S) and oxides. Sintering heat treatment cycle was complete at 40 minute. ? IATS’09, Karabük üniversitesi, Karabük, Türkiye Yal??n, B., Ucun, ?. ve Usal, M.R. Table 1. The characterization of the powders used in the alloy preparation and chemical analysis of the failure part (%wt) C MnS Cu Fe Zn-S Composition (%) 4 5 2 88.4 0.6 Average particle size ( μm) 25-40 25-40 25-40 25-40 - Morphology Pre-mix and sponge powders The specimens shape has two types as tensile test piece and shock absorber piston. Tensile test sample complied with ISO 2740 (Fig. 2) and the shock absorber piston were manufactured for using in the automobile. Shock absorber piston produced by P/M was showed in Figure 3. The bearing of the ring was formed with the turning process. The ring bearing has tiny cross-section. Generally, this tiny cross-section may be fractured under working condition. Fig. 2. Tensile test specimens produced with P/M. Fig. 3. Shock Absorber Piston Produced by Powder Metallurgy. 2.2. Mechanical and Metallurgic Properties Theoretical density was performed by Archimedes method. Average theoretical density ratio after sintering of all specimens was measured as % 90 according to steel. Porosity amount was identified as % 10. Test specimens and P/M piston density values were calculated to be 6.7 g/cm 3 . Elastic module and poisson ratio of the piston material were determined with tensile test are given in Fig. 2. Besides, other mechanical properties of P/M piston were determined by hardness, charpy and tensile test. Mechanical properties of P/M piston are given in Table 2. Table 2. Mechanical test results of the shock absorber piston Yield Strength (MPa) 160 Tensile Strength (MPa) 191.87 Elastic Module (GPa) 170 Total Strain (%) 6.1 Hardness HRC 29.8 Impact Energy with notch, (J) 5.66 Poisson ratio 0.27 Three different mechanical tests namely tensile, hardness and, impact energy tests are applied. An Instron and a Psd 300/150-1 test machines are used for the tensile and impact energy tests respectively. Fracture surface analysis is carried out with Jeol Model 6060 Scanning Electron Microscope (SEM). In addition, the hardness measurements are carried out by a MetTest-HT type computer integrated hardness tester. These values are given in Table 2. Elastic module of the porous materials (Ep) is calculated with a mathematical equation as: ( ) 3 / 2 21 . 1 1 p Eo Ep ? = (1) where, Eo is the elastic modulus of the bulk materials and p porosity function (15). Therefore, elastic module of the porous material because of the porosity function is lower than similar casting alloy. Fracture characteristic of the piston materials was determined by tensile tests and micro structural analysis. According to the tensile test, microstructure of piston materials was characterized to be brittle and ductile fracture type. Namely, piston material hasn’t ductile structure. Fractured surface of P/M iron based piston material was given in Fig. 4 and Fig. 5. Particles inner and bonding fracture was shown in Fig.4. and Fig. 5. Particles inner fracture may be determined to be not formation of the desirable particle bonding at some region. Fig 5 shows to be particles bonding formation is not desired level because of density variations caused from double acting pressing method. Excellent particle bonding formation at complex P/M parts is provided with four direction pressing method (Hot Isostatik Pressing). Especially, because of complex and thin structure which starting region of fracture, particles bonding fractures were occurred at these region. Yal??n, B. ve Ucun, ?. ve Usal, M.R. Fig. 4. SEM Photography of fracture surface (fracture of the particles) Fig. 5. SEM Photography of fracture surface (Fracture of particles bonding) 3. Stress analysis using finite element method The stress analysis was carried out for the determination of stress concentration level at the fracture region by finite element model. The finite element model was presented in Fig. 6. Mechanical properties of the model were determined by tensile experiments. ANSYS 9.0 commerical finite element packet program was used in the analyses. According to experiments, elastic module and poisson ratio was selected to be 170 GPa and 0.27. Solid 187 element type (3-D Node Tetrehedral Structural Solid type) was assigned for this model. The model consist 76907 elements and 116064 nodes. Symmetric boundary conditions are selected to model geometry. Loads acting on the model were applied to external surface of piston. All degree of freedoms of piston road bear surface (internal surface) is constrained. Fig. 6. Finite element model for piston As a result of the analysis, some interesting stress distributions were obtained. During the working of the absorber piston is affected by different forces. In the analysis, nine stress levels were carried out for different loading condition and determination of the most dangerous position. As can be seen, highest total displacements were occurred at region having tiny cross-section in the piston part (Fig. 7). Especially, relatively high shear stress concentrations and displacements are considered on the tip sections of the piston. This a critical state and it leads to the fracture. (a) (b) Fig. 7. Stress Analysis of the absorber piston (a) Total displacement distributions in the absorber piston, (b) Shear stress Yal??n, B., Ucun, ?. ve Usal, M.R. According to analyses, causes of the failure in the piston were determined to be maximum shear stress concentration in the fractured section. This phenomenon may be caused from density variations in this region. The density of vertical regions of the pressing action at P/M parts produced via double and single action pressing is lower than others region. In these complex P/M parts, four direction pressing method (Hot Isostatik Pressing) and powder injection molding technique preferred for the homogeneous density and mechanical properties. 4. Conclusion and Suggestions In this study, failure analyses of an absorber piston in the automobile shock absorber system are carried out. The analyzed piston is abruptly fractured at working condition. This failure is occurred as a sudden fracture at tiny cross- section, where this region has a highest stress concentration. As a result of the analyses, the main reason of the fracture is determined as very tiny cross-section and density variation due to incorrect selection of the pressing method. Hot isostatic pressing (HIP) method is preferred for pressing type for these complex parts. A variation of the mechanical property and tiny cross-section may lead to the piston fracture. Variations of porosity and mechanical properties can be minimized by HIP method. As a result; density and mechanical properties can be increased with increasing of pressing compression or each side pressing. Besides, tiny cross-section geometry can be optimized and designed according to finite element analyses. 5. Acknowledgements Authors thank to Maysan Mando A. ?. supported this paper. References [1] Cedergren, J., Melin, S., Lidstr?m, P.: Numerical investigation of powder metallurgy manufactured gear wheels subjected to fatigue loading. Powder Technology vol 160, pp. 161-169. (2005) [2] Al-Qureshi, H.A., Klein, G.A.N.: On the mechanics of cold die compaction for powder metallurgy. Journal of Materials Processing Technology vol 116, pp. 135-143. (2004) [3] Powder metallurgy and applications. ASM Handbook; Vol. 7, ASM International (1998) [4] Properties and selection: Nonferrous alloys and special-purpose materials, ASM Handbook Vol. 2, ASM International (1998) [5] Goto, R.: Powder metallurgy growth in the automotive market. Business Briefing: Global Automotive Manufacturing &Technology, pp.44 (2003) [6] Orban, R.L.: New research direction in powder metallurgy. Romanian Report in Physic, vol 56, pp. 505-516 (2004) [7] The process and its products. European Powder Metallurgy Association (EPMA), United Kingdom (2004) [8] German, R.M.: Powder metallurgy science. MPIF, USA, (1989) [9] http://www.maysanmando.com [10] Witec, L.: Failure analyses of turbine disc an aero engine. Engineering Failure Analyses, vol 13, pp. 9-17. (2006) [11] Yu, Z., Xu, X.: Failure analyses of an idler gear of diesel engine gearbox. Engineering Failure Analyses Vol 13, pp. 1092-1100. (2006) [12] Ossa, E.A.: Failure analyses of a civil aircraft landing gear. Engineering Failure Analyses Vol 13, pp. 1177-118. (2006) [13] Sudhakar, K.V., Parades, J.C.: Failure mechanism in motor bearing. Engineering Failure Analyses Vol 12, pp. 35-42. (2005) [14] Kalpakijan, S.: Manufacturing engineering and technology. Chapter 17, Processing of Metals, Ceramics, Glass and Super Conductors, Prantice Hall (2001) [15] Leyen, C., Peters, M.: Titanium and titanium alloys. Wiley VCH, Germany, (2003)
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