FEA BASED DURABILITY USING STRAIN-LIFE MODELS FOR DIFFERENT MEDIUM CARBON STEEL AS FABRICATION MATERIALS FOR AN AUTOMOTIVE COMPONENT

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1 Interntionl Journl o Mechnicl nd Mterils Engineering (IJMME), Vol. 4 (2009), No. 2, FEA BASED DURABILITY USING STRAIN-LIFE MODELS FOR DIFFERENT MEDIUM CARBON STEEL AS FABRICATION MATERIALS FOR AN AUTOMOTIVE COMPONENT N. A. Al-Asdy 1, S. Abdullh 1, A. K. Ariin 1, S. M. Beden 1 nd M. M. Rhmn 2 1 Deprtment o Mechnicl nd Mterils Engineering, Universiti Kebngsn Mlysi, UKM Bngi, Selngor 2 Deprtment o Mechnicl Engineering, Universiti Mlysi Phng, Locked Bg 12, Kuntn, Phng Emil: nwr2007@gmil.com ABSTRACT There hs been strong trend towrds the doption o optimum mterils nd components in utomotive industry. Automotive designers hve wide rnge o mterils nd processes to select rom. Ftigue ssessment o SAE1541_362_QT nd SAE1045_390_QT steels or utomotive component prticulrly or lower suspension rm, hve been numericlly studied using inite element nlysis under rndom loding conditions. Both Morrow nd Smith-Wtson-Topper (SWT) strin-lie models hve been used or live estimtion. These two models were oten been used in the tigue lie ssessment or metllic components. The results o this pper re bsed on nlyticl investigtions, including detiled inite element strin nlysis s well s tigue nd optimistion studies. Comprison o predicted lives indictes tht the SWTbsed estimtion nlysis method gve longer lie thn the Morrow estimtion. In ddition, steel or bricte the lower suspension rm gve higher durbility compred to SAE1541_362_QT steel. Such results provide urther credit to the ppropritely o using SWT s prediction model, s well s, SAE1045_390_QT steel s briction mteril o the utomobile lower suspension rm. Keywords: Ftigue lie ssessment; Finite element; medium crbon steel; Lower suspension rm; Vrible mplitude loding. INTRODUCTION Engineers hve recognized or over 150 yers tht metls cn il in tigue. Modern tigue nlysis cme to lie in the 1980s when in-vehicle lod mesurement becme vilble with nlysis sotwre nd low-cost computers. In utomotive design, durbility evlution o components bsed on experimentl ssessments is timeconsuming nd expensive, so nlyticl pproches tht include limited number o component veriiction tests hve gined more ttention (Kim et l., 2003). The nlyticl pproch combined with limited number o component testing reduces design cycle time due to reduced testing, llows inexpensive evlution o chnges in geometry, mteril, loding nd mnucturing process through perormnce simultion, nd inlly provides evlution techniques or product optimiztion nd ilure nlysis ( Ali nd Mehrdd, 2002). The inite element method (FEM) hs become powerul tool or the numericl solution o wide rnge o engineering problems (Clough, 1960). Results re importnt in clculting nd veriying se prt lietimes. In the pst, durbility nlysis ws lrgely the province o reserch. In clssic structurl nlyses, ilure predictions re solely bsed on the mteril strength or the yield strength. Crbon cn be considered s the only lloy element in Plin crbon steel. Crbon being powerul lloying gent cn give vriety o strength nd hrdness by vrying its composition in the steel. It is in this regrd tht crbon steel cn be clssiied s low, medium nd high crbon steel. Lindberg (1977) discussed bout the crbon steel with crbon content between 0.3 nd 0.6% is termed medium crbon steel. While those with lower nd higher re respectively clssiied s mild nd high crbon steel. In study by Devluki nd Brgmnn (1997) conducted the tigue ssessment o suspension rm using the deterministic nd probbilistic pproches. The strength reduction eect due to the surce roughness ws ccounted or by representing the surce s collection o notches nd mking use o Neuber s rule. It ws concluded tht the residul stress demonstrted more pronounced eect under constnt mplitude loding s compred to vrible mplitude loding. The cumultive dmge potentil under the vrible mplitude loding sequences o the long durtion on simple specimen dt nd strin-lie method ws conservtive by ctor o two. A work rom Hib et l. (2002) hs been seen s inite element nlysis (FEA) ppliction in tigue lie estimtion. They estimted the tigue lives o metllic mteril in both time nd requency domin methods under FEA. Comprison between severl pproches to tigue lie prediction using rel utomotive engineering cse study hs lso been perormed. In ddition, the study ws tking into ccount the optimistion bsed on tigue lie which requires ccurte reltive distribution rther thn exct vlues. 141

2 Another work by Hib et l. (2003) introduced new The pper investigtes the eects o dierent ssessment strtegies on the predicted tigue lie o lower suspension rm, the properties o which re modiied to generte dierent degrees o interction between the rm nturl requencies nd the requency rnge o the pplied orcing unctions. The results o this investigtion were used to derive new orm o structurl optimiztion lgorithm which is more robust nd eicient. In dierent sitution, Kim et l. (2003) nlyzed the hydroorming process o n utomobile lower rm using inite element progrm o HydroFORM-3D in order to ccomplish its proper design nd the process control. This work showed tht the FEM progrm o HydroFORM-3D provided vluble inormtion regrding to the orming process nd ws lso drmticlly improved the potentil o the hydroorming process. Though the computer-ided design pproch ws proposed in this study, the designer cn improve the design eiciency, s well s to void expensive nd time-consuming tril-nd-error nd extensive process design experience. Then Ftemi nd Zoroui (2004) did n experimentl nd nlyticl work using FEA, the durbility ssessment nd lso n optimiztion nlysis. They developed methodologies tht cn be pplied to wide rnge o utomotive nd other components. Some o the indings re the FEA simultion or cyclic lodings which is importnt or tigue dmge nlysis. The lie prediction bsed on locl pproches, i.e. the Morrow s men stress prmeter provided better predicted tigue lives thn the Smith-Wtson-Topper s (SWT) men stress prmeter. In cse study by Xinjie (2005), he investigted the cyclic strin low cycle tigue nd cyclic stress rtcheting ilure o crbon steel 45 with quenched nd tempered tretment. The tests or this cyclic strin low cycle tigue with or without men strins were crried out in order to investigte the eect o the men strin on low cycle tigue behvior. The evlution eqution o tigue dmge ws then proposed bsed on the symmetric cyclic strin LCF testing results, nd the eqution ws used to evlute the eect o the tigue dmge on the rtcheting ilure under dierent cyclic stressing. In work by Zoroui nd Ftemi (2006), the tigue behvior o vehicle suspension components (orged steel nd cst luminium steering knuckles) were investigted under constnt-mplitude lod-controlled tigue tests. Three o the inite element models o the knuckles were nlysed using liner nd non-liner methods. The nominl stress, the locl stress, nd the locl strin lie prediction pproches were then employed nd compred to the experimentl results in order to evlute the ccurcy nd vlidity o these pproches. It ws observed tht mong the contemporry lie prediction procedures used in the utomotive industry, the locl strin pproch using liner elstic FEA results in structurl optimiztion lgorithm bsed on tigue lie. conjunction with Neuber-corrected stresses were resonble. In ddition, it hs been ound tht the results were close to those obtined on the bsis o non-liner elstic-plstic FEA. The im o this pper is to exmine the eect o two medium crbon steel mterils on the durbility o n utomotive lower suspension rm using inite element nlysis. It hs been obtined tht SAE1045_390_QTsteel hs the priority to consider s the briction mteril or the utomotive lower suspension rm. LITERATURE BACKGROUND The strin-lie method ws pplied or estimting the tigue lie o the lower suspension rm becuse or suspensions prts it is importnt to predict crck initition in order to void tigue ilure by removing the prt rom service t the pproprite time. In the sme time, most o the time to ilure consists o crck initition (Dowling, 1999), thus, conservtive pproch is to denote the component s iled when crck hs initited. The tigue lie is perormed using the strin-lie pproch, Morrow (1968) nd Smith et l., (1970) or two kinds o medium crbon steel which hve been chosen s bricted mterils or the lower suspension rm becuse their bility to used s orging utomotive components. Ftigue lie prediction represents one o the ppliction or FE. Among the plstic strin models, the Coin- Mnson, Morrow, SWT re widely used. Ech o these models is n empiricl reltionship between cycles-toilure, nd nlyticlly, numericlly or experimentlly determined plstic strin rnge per cycle. The three models re described in the ollowings: The irst strin-lie model is the Coin-Mnson reltionship (Lindberg, 1977), σ b = ( 2Ν ) + ε ( 2N ) c (1) Ε ε where E is the mteril modulus o elsticity, is true strin mplitude, 2N is the number o reversls to ilure, ε σ is tigue strength coeicient, b is ε is tigue ductility tigue strength exponent, coeicient nd c is tigue ductility exponent. Bsed on the proposl by Morrow [12], the reltion o the totl strin mplitude ( ε ) nd the tigue lie in reversls to ilure (2N ) cn be expressed s 142

3 σ σ b = ( 2N ) + ε ( 2N ) c (2) m ε 1 E σ where σ m is the men stress. Another strin-lie men stress correction model ws suggested by Smith et l. (1970), or oten clled the SWT prmeter. This reltionship ws bsed on strin-lie test dt which ws obtined t vrious men stresses. Thus, the SWT expression is mthemticlly deined s σ mx 2 2b b + c ( σ ) ( 2N ) + σ ε E( 2N ) ε E = (3) where σ mx is the mximum tensile stress or the prticulr cycle. This eqution is bsed on the ssumption tht or dierent combintions o strin mplitude, ε, nd men stress, σ mx, the product σ mx ε remins constnt or given lie. METHODOLOGY 1- Mteril Speciiction In order to clssiy the lower suspension rm mteril speciiction, nlyzing the chemicl composition o the steel smple is done. Bsed on tble 1, the steel cn be clssiied s medium crbon steel, since both AISI nd SAE clssiied steel whose crbon content rnges between %, mngnese content rnges between % to be medium crbon steel (Lindberg, 19770) which represents the bricted mteril or the 2000 cc Sedn lower suspension rm. The mesured vlues hve been get using INCA Energy system. Three smples were cut rom the lower suspension rm using cutter. The smples were subsequently ground with successive SiC ppers (grit ) nd then polished with polishing cloth nd Alomin solution o grin size 6µm then 1µm. Tble 1: Chemicl composition o the steel Element C Mg Si V Cr Ni Mn Fe Mesured wt% 2- Finite element nlysis Bl. 2-1 Geometricl nd inite element model A geometric model o lower suspension rm or 2 Liter engine Sedn cr is considered in this study, nd this component is presented in Figure 1. The inite element pproch is used or modeling nd simulting. Threedimensionl lower suspension rm model geometry is drwn using CATIA sotwre, s shown in Figure 2. Fig. 1: A geometric model o lower suspension rm The uto tetrhedrl meshing pproch is highly utomted technique or meshing solid regions o the geometry. It crtes mesh o tetrhedrl elements or ny closed solid including boundry representtion solid. Tetrhedrl meshing produces high qulity meshing or boundry representtion solids model imported rom the most CAD systems. The TET10 mesh cn give more ccurte solution since the 10 nodes tetrhedrl (TET10) element is used or the nlysis with the doption o qudrtic order interpoltion unction. There re three min prts in the lower suspension rm which their behviour hs been considered in the FE boundry conditions, bll joint, pivot 1 nd pivot 2. The FEM (Figure 2) hs boundry conditions s ollowed Fig. 2. Finite element model o the lower suspension rm distributed lod hs been pplied on the inner surce o pivot 1. Pivot 2 considers s rigid section with rottion round x-xis rom the side o the vehicle body. In the sme time, rigid hs been considered on the bll joint with trnsltions in x nd y direction while rottion round x, y, nd z-xis to represent the brking nd cornering lods. There re no ccelertion lods s inputs, due to collecting dt during driving o the cr t constnt velocity. 2-2 Loding inormtion The lod history which hs been used or this nlysis ws obtined rom the rel utomotive lower suspension rm, which ws driven over country rod. The requency smpling, s or this cse is 500 Hz. This s vlue ws chosen in order to collect wide rnge o rod dt (Oh, 2001; Stephens et l., 1997). The dt ws mesured using tigue dt cquisition system 143

4 (Figure 3) t cr velocity o 25 km/h, nd, recorded dt in orm o strin time histories. Fig. 3: The setup o tigue dt cquisition system dt collection. A strin guge ws ixed in n exct position on the lower suspension rm s shown in igure 4. Finite element nlysis ws perormed in order to clssiy the criticl res in order to get cler ide to choose the positions re or ixing strin guge during the experimentl test to get the strin history dt, s shown in Figure 5. strin nlysis nd MSC Ftigue or the tigue nlysis. Liner inite element model o the lower suspension rm ws nlysed. In ct, the locl strin, or crck initition pproch to tigue lie estimtion requires ccurte vlues o locl elstic-plstic stresses nd strins, especilly t ree surces. In principl, these cn be obtined through non-liner FEA, but this pproch is usully imprcticl or lengthy nd complex lod histories. For this reson elstic-plstic stresses nd strins re commonly estimted using combintion o liner elstic FEA nd notch correction procedure (elstic-plstic correction) (Heyes et l., 1995). The inite element nlysis (FEA) hs been perormed using SAE1541_362_QT steel nd SAE1045_390_QT steel s these steels used or bricte the utomobile lower suspension rm. The strin vlue rom the nlyticl simultion ws within the rnge o the collected dt during driving o the cr on the rod. This ws due to ccurte boundry conditions nd sttic lod in the pre-processing stge. For the tigue nlysis stge nd ter pplying the rod dt s tigue lod, the tigue lie results re obtined s shown in tble 2. It shows the tigue component lie results using Morrow nd SWT strinlie methods or the two medium crbon steels, coded SAE1541_362_QT nd SAE1045_390_QT. The tigue lie contour or this cse is shown in igure 6. For the purpose o this FEA simultion, the polished surce inish hs been used. Fig. 4: Strin guge loction on the lower suspension rm Fig. 6: Model tigue lie contour Fig. 5: Experimentl loding history or the lower suspension rm. RESULTS AND DISCUSSION Ftigue lie ssessment or the lower suspension rm ws crried out using CATIA sotwre or model genertion, MSC Ptrn or pre-processing, MSC Nstrn or the 144 From the Morrow model results o tble 2, longer lie cn be obtined or the steel type o SAE1541_362_QT compred to SAE1045_390_QT while the opposite results cn be obtined when used SWT strin lie model. It is diicult to ctegoriclly select one procedure in preerence to the other. However, or loding sequences which re predominntly tensile in nture the Smith Wtson Topper pproch is more conservtive nd is, thereore, recommended. In the cse where the loding is predominntly compressive, prticulrly or wholly compressive cycles, the Morrow correction cn be used to provide more relistic lie estimtes.

5 Tble 2: Ftigue component lie using Morrow nd SWT or SAE1541 nd SAE1045_390_QT steel Mteril Ftigue lie (Cycles) Morrow SWT SAE1541_362_QT SAE1045_390_QT The behviour study o these two mteril type is shown in igure 7. showing tht these two mterils behve under cyclic loding conditions. It lso shows on how their behviour with respect to one nother. SAE1045_390_QT is obviously higher strength steel with its yield point well bove tht o SAE1541_362_QT. sme time, the trnsition point deines the dierence between high cycle tigue which represented by the right side becuse elstic events dominte plstic events nd low cycle tigue which represented by the let side becuse plstic events dominte elstic events. Figure 9 shows the number o cycles to ilure under rel rod loding or SAE1541_362_QT nd SAE1045_390_QT steel. A comprtive or these two strin lie curves cn be perormed. They cross ech other nd thereore exhibit dierent lie behvior depending on the strin level. So it is impossible to know rom the plot which would perorm better in order to consider one o them s briction mteril or the lower suspension rm, especilly higher durbility is the gol which is looking or. () Fig. 7: Cyclic stress-strin curves or SAE1541_362_QT nd SAE1045_390_QT steel Figure 8 shows Morrow lie plot or SAE1541_362_QT nd SAE1045_390_QT steel which cn be ully chrcterized by knowing our mteril prmeters s shown in the eqution 2 o the strin-lie plot. In the Fig. 9. Morrow lie plot or medium crbon steel SAE1541_362_QT nd SAE1045_390_QT 145 (b) Fig. 8. Morrow lie plot or () SAE1541_362_QT (b) SAE1045_390_QT steel CONCLUSIONS FEA hs been used in durbility comprison o the lower suspension rm using two kinds o medium crbon steel. Ftigue lie component using Morrow model gve higher durbility or SAE1541_362_QT steel while SWT gve higher durbility or SAE145 steel. The SWT model cn be considered s the tigue

6 lie prediction model or the cse study, especilly, the rod loding sequence re predominntly tensile. In ddition, SAE1045_390_QT shows better behvior thn SAE1541_362_QT under cyclic loding which will give the priority or SAE1045_390_QT to be the briction mteril or the utomotive lower suspension rm due to its high durbility. REFERENCES Ali, F., Mehrdd, Z Ftigue Perormnce Evlution o Forged Versus Competing Process Technologies: A Comprtive Study, 24th Forging Industry Technicl Conerence, Ohio, pp Clough, R.W., Erly history o the inite element method rom the view point o pioneer. Interntionl journl or Numericl Methods in Engineering, WILEY inter Science, Volume 60, No.1, pp Devluki, J. nd H. Brgmnn, Ftigue ssessment o n utomotive suspension component using deterministic nd probbilistic pproches. Proceedings o the 2 nd Interntionl Symposium o Ftigue Design, Dowling, Mechnicl Behviour o Mterils: Engineering Methods or Deormtion, Frcture nd Ftigue, 2nd ed., Prentice Hll, New Jersey. Ftemi, A. nd M. Zoroui, Ftigue perormnce evlution o orged competing mnucturing process technologies: A comprtive nlyticl nd experimentl study. Forging Industry Eductionl nd Reserch Foundtion (FIERF) nd Americn Iron nd Steel Institute (AISI). Hib, M., D. Brton, P. Brooks nd M. Levesley, Review o lie ssessment techniques pplied to dynmiclly loded utomotive components. Computers nd Structures, Elsevier Science, Volume 80, No.5, pp Hib, M., D. Brton, P. Brooks, nd M. Levesley, The development o n optimistion lgorithm bsed on tigue lie. Interntionl Journl Ftigue, Elsevier Science, Volume 25, No. 4, pp Heyes, P., J. Dkin nd C. StJohn, The ssessment nd use o liner sttic FE stress nlyses or durbility clcultion, SAE technicl pper, , Kim, J., Lei, L. nd kng B Preorm design in hydroorming o utomobile lower rm by FEM. Journl o Mterils Processing Technology, ElsevierScience, Volume138, No.1, pp Lindberg. R.A Processes nd Mterils o Mnucture, 2 nd ed. Allyn nd Bcon, Boston, MD, USA. Morrow, J Ftigue Design Hndbook, Advnces in Engineering, Society o Automotive Engineers, Wrrendle, P., section 3.2, Volume 4, pp Oh, C.S Appliction o wvelet trnsorm in tigue history editing, Interntionl Journl o tigue, Elsevier Science, Volume 23, No.3, pp Smith, K.N., Wtson, P. nd Topper, T. H A stress-strin unction or the tigue o metls, Journl o Mterils, Elsevier Science, Volume 5, No. 4, pp Stephens, R.I., Dindinger, P.M. nd Gunger, J.E Ftigue dmge editing or ccelerted durbility testing using strin rnge nd SWT prmeter criteri. Interntionl Journl o tigue, Volume 19, No.8, pp Xinjie Y., Low cycle tigue nd cyclic stress rtcheting ilure behvior o crbon steel 45 under unixil cyclic loding. Interntionl Journl Ftigue, Elsevier Science, Volume 27, No.9, pp Zoroui, M. nd A. Ftemi, Experimentl durbility ssessment nd lie prediction o vehicle suspension components: cse study o steering knuckles. Journl o Automobile Engineering, Proessionl Engineering Publishing, Volume 220, No.11, pp