MULTIAXIAL FATIGUE OF A RAILWAY WHEEL STEEL

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1 Journal of Enginring Scinc and Tchnology Vol. 10, No. 9 (2015) School of Enginring, Taylor s Univrsity MULTIAXIAL FATIGUE OF A RAILWAY WHEEL STEEL CHIN-SUNG CHUNG 1, HO-KYUNG KIM 2, * 1 R&D Cntr, Flow Mastr Co. Soul, Kora, Dpt. of Automotiv Enginring, Soul National Univrsity of Scinc and Tchnology, Kora *Corrsponding Author: kimhk@soultch.ac.kr Abstract Uniaxial and biaxial torsional fatigu spcimns wr xtractd from a railway whl stl. Th fatigu tsts wr prformd with th strss ratio of R= -1 by using uniaxial and biaxial torsional fatigu tst spcimns at room tmpratur in air. Th ultimat and yild strngths of th stl wr valuatd. Th uniaxial fatigu limit was MPa, which corrsponds to 67% of th ultimat tnsil strngth. Th ratio of / was Appropriat paramtrs to prdict th fatigu lif of th stl undr multiaxial strss stats wr rviwd. Kywords: Fatigu strngth, Multiaxial fatigu, Railway whl stl, SWT fatigu paramtr. 1. Introduction Railway whls ar subjctd to mchanical and thrmal loads, and as train spds and wights ar incrasd, ths loads incras. With th incras of train spds and axl loads, rolling contact fatigu of railway whls has bcom an important issu with rspct to failur. Minor fatigu damag in th whl can rduc th rid comfort and incras th maintain cost for a vhicl. It also incrass th vhicl track intraction forcs, rsulting in potntial for drailmnt and rduction of th intgrity of th vhicl. Thrfor, it is highly important to valuat th fatigu proprtis of th whl stl. Studis on th fatigu strngth of whls hav also bn limitd [1-4]. For instanc, Brnasconi t al. [1] usd spcimns of uniaxial and biaxial torsional fatigu takn from th rim of whl stl to valuat th multiaxial fatigu strngth of th whl stl undr combind out-of-phas altrnating torsion and pulsating comprssiv axial loads which is similar to that obsrvd undr th 1215

2 1216 C.-S. Chung and H.-K. Kim Nomnclaturs E Elong. FP G N f YS UTS Elastic modulus Elongation Smith-Watson-Toppr multiaxial fatigu paramtr Shar modulus Fatigu liftim Yild strngth Ultimat tnsil strngth Grk Symbols f Tru fractur strain Maximum principal strain Minimum principal strain Diffrnc btwn th maximum and minimum normal strain Uniaxial fatigu limit q Von Miss quivalnt strss max Maximum strss on th maximum principal strain plan 1 Maximum principal strss Torsional fatigu limit contact ara in th whl. Ahlstӧrm and Karlsson t al. [2] valuatd th low cycl fatigu bhaviors of th whl matrial undr dvlopmnt. Thy proposd a hat tratmnt for th forging and austnit procsss to nhanc th fatigu strngth and mchanical proprtis. Okagata t al. [3] prformd fatigu tsts on th actual siz whl. Thy rportd fatigu strngth, safty cofficint and dsign load of th whl plat. Mor rcntly, Wagnr t al. [4] invstigatd th fatigu strain bhaviors of th whl stls at a vry high cycl ovr 10 7 and in trms of microstructural proprtis. Thy rportd that fatigu bhavior changs from cyclic softning to cyclic hardning at 5% of lif and dislocation dnsity incrasd continuously up to 85% of lif. For th railway whl, multiaxial strss occurs du to th contact load btwn th whl stl and rail. Howvr, most studis on th fatigu strngth of th railway whl dal with th uniaxial load. And, thr a littl numbr of studis on fatigu strngth of whl trads which is critical to crack initiation. Th purpos of this study is to provid information about th fatigu strngth of railway whl stl for furthr dvlopmnt towards its saf opration. In this study, spcimns of uniaxial and biaxial torsion wr xtractd from railway whl stl for fatigu and tnsil tsts. From ths tsts, th fatigu limits undr uniaxial and torsional loading wr valuatd. Thn appropriat paramtrs to prdict th fatigu lif undr multiaxial strss stats wr rviwd. Th rsults obtaind will provid a usful guidlin for manufactur and dsign nginrs to valuat fatigu strngth of th whl stl. 2. Exprimntal Mthod Th whl stl s chmical composition is shown in Tabl 1. Th fatigu spcimns for th uniaxial tnsion and biaxial torsion wr xtractd from th whl trad, as

3 Multiaxial Fatigu of a Railway Whl Stl 1217 shown in Fig. 1. Th hardnss tst was prformd from th trad towards th dpth s dirction to find that th hardnss was H RB up to 50 mm, indicating that it had mor or lss th sam hardnss up to that dpth. Thrfor, spcimns at th trad wr xtractd until it was 50 mm dp. Th configuration and dimnsion of th uniaxial and biaxial torsional spcimns ar as shown in Fig. 2. Th fatigu tsts wr prformd with th strss ratio of R= -1 by using uniaxial and biaxial torsional fatigu tst spcimns at room tmpratur in air. Th tnsil tsts and uniaxial fatigu tsts wr prformd by using a tst dvic by Instron Inc. with a capacity of 10 tons at 15Hz, whras th biaxial torsional tsts wr prformd by using a torsional tsting dvic by JT TOHSI Inc. Tabl 1. Chmical Composition of Railway Whl Stl (wt.%). C Mn Si P S Cu F Rm. Fig. 1. Position and Orintation of Spcimns Extractd from Whl. (a) (b) Fig. 2. Spcimn Configuration for (a) Uniaxial and (b) Torsional Fatigu Spcimns.

4 1218 C.-S. Chung and H.-K. Kim 3. Exprimntal Rsults and Discussion 3.1. Tnsil tsts Tnsil tsts wr prformd by using th spcimns xtractd from th whl trad. Curvs in Fig. 3 illustrat th nginring strain and tru strain against nginring strss and tru strss. Th nginring strss-strain curv indicats that th maximum tnsil strngth and yild strngth ar MPa and 626.7MPa, rspctivly, and th longation is 40.4%. Manwhil, th tru strss-strain curv shows that th maximum strngth is MPa and th longation is 33.9%. Mchanical proprtis of th railway whl stl ar summarizd in Tabl 2. Hur t al. [5] rportd that th tnsil strngth and longation of a railway whl stl ar btwn 836 MPa and 919 MPa and btwn 18% and 23%, rspctivly. Kwon t al. [6] also rportd that th tnsil strngth and yild strngth of whl stl for high-spd railway ar 1067 MPa and 616 MPa, rspctivly, and th longation is 16.2%. Th railway stl whl studid in this study had a similar tnsil strngth and yild strngth to thos of th high-spd railway, whil its longation was about 2.5 tims suprior to that of th high-spd railway. UTS Tabl 2. Mchanical Proprtis of th Railway Whl Stl. YS Elong. (%) σ f ε f (%) σ τ Fig. 3. Enginring and Tru Strss-Strain Curvs of Railway Whl Stl Fatigu strngth Figur 4 shows th rlationship btwn th maximum principal strss amplitud and th fatigu lif from uniaxial and biaxial torsional fatigu tsts. According to Fig. 4, th uniaxial and biaxial torsional fatigu limits wr MPa and 265.0

5 Multiaxial Fatigu of a Railway Whl Stl 1219 MPa, rspctivly. This figur shows that th uniaxial spcimn had a longr fatigu lif than th biaxial torsional spcimn undr th sam maximum principal strss. Th fatigu limit found in th uniaxial fatigu tst was 67% of th tnsil strngth, which is rlativly high, compard to about 50% of typical stls. Th ratio of th uniaxial fatigu limit against th biaxial torsional fatigu strngth / was 0.63, and this valu is almost th sam as 0.6, which is th valu for typical ductil matrials. For brittl matrials, th / ratio is known to b about 0.8 or highr, typically [1]. Brnasconi t al. [1] rportd that som ductil whl matrial has a highr / ratio than 0.9, and analyzd that this was attributabl to inclusions, such as CaS and MnS in th stl whl. Th fact that th prsnt whl stl has a high tnsil strngth and a / ratio of 0.63 with a fractur longation of 40.4% indicats that inclusions, lik CaS and MnS, do not xist in larg quantitis at th whl stl. Fig. 4. Strss Amplitud against Numbr of Cycls for Uniaxial and Biaxial Torsional Fatigu Tsts Analysis of multiaxial fatigu strngth paramtrs This study valuatd th maximum principal strss, quivalnt strss, maximum principal strain and quivalnt strain as paramtrs to prdict th fatigu lif undr multiaxial strss stats. Figur 4 shows th maximum principal strss amplitud to rlat th lif of th uniaxial and biaxial torsional fatigu xprimnt. As a rsult, this figur found that th fatigu lif undr diffrnt strss stats did not agr with ach othr. In othr words, th maximum principal strss was not an adquat paramtr to prdict th fatigu lif undr uniaxial and biaxial stats. In gnral, von Miss quivalnt strss σ q is applid as on of th strss paramtrs to prdict th fatigu lif. For torsional biaxial strss, σ q is qual to

6 1220 C.-S. Chung and H.-K. Kim 3, whras for uniaxial strss, q 1. Figur 5 shows th fatigu liftim for uniaxial and torsional fatigu tsts using th von Miss quivalnt strss amplitud. Th rsults in Fig. 5 suggst that quivalnt strss, just lik th maximum principal strss, is not an adquat paramtr ithr to prdict th fatigu lif in uniaxial and biaxial stats with diffrnt strss conditions from ach othr for th whl stl. Th quivalnt strain, anothr paramtr for th strain, was applid to prdict th fatigu lif undr th multiaxial strss stats. To dtrmin th quivalnt strain for uniaxial and biaxial torsional fatigu tsts, E was usd for q q 1 / th uniaxial tst, and / 2, 0, 3 / 2G(1 ) wr usd for th biaxial torsional tst. Figur 7 rvals that th quivalnt strain is not an adquat paramtr to prdict th lif. Fig. 5. Von Miss Equivalnt Strss Amplitud against th Numbr of Cycls for Uniaxial and Torsional Fatigu Tsts. Th Smith-Watson-Toppr multiaxial fatigu paramtr (FP) [7] is on of th paramtrs that can b applid to prdict th fatigu lif undr multiaxial strss stats. This modl considrs that dpnding upon strain amplitud, matrial typ, and stat of strss, matrials gnrally form ithr shar or tnsil cracks. For cracks that grow in plans of high tnsil strain. This can b xprssd as FP max (1) 2 whr is th diffrnc btwn th maximum and minimum normal strain to th plan xprincd during th cycl and max is th maximum strss on th maximum principal strain plan. Figur 7 illustrats th uniaxial and biaxial

7 Multiaxial Fatigu of a Railway Whl Stl 1221 torsional fatigu liftims with th SWT fatigu paramtr. Figur 7 also suggsts that th SWT fatigu paramtr has a corrlation cofficint of R = 0.89 and is an adquat paramtr to prdict th uniaxial and biaxial fatigu liftims. It can b concludd from Figs. 5, 6 and 7 that th SWT fatigu paramtr is mor appropriat paramtrs than th quivalnt strain and quivalnt strss to prdict lif for uniaxial and biaxial fatigu tsts. Fig. 6. Equivalnt Strain Amplitud against Numbr of Cycls for Uniaxial and Biaxial Torsional Fatigu Tsts. Fig. 7. SWT Paramtr against Numbr of Cycls for Uniaxial and Biaxial Torsional Fatigu Tsts.

8 1222 C.-S. Chung and H.-K. Kim 3.4. Analysis of fatigu fractur surfac Figur 8(a) to (f) show fatigu fractur surfacs for high cycl fatigu liftim aftr uniaxial and biaxial torsional fatigu tsts. Th obsrvation of fatigu fractur surfacs found no inclusions, lik CaS and MnS, whr th fatigu failur initiatd. This finding rconfirms th fact that th typical / ratio of th railway whl stl of this study is 0.63, indicating that thr ar fw inclusions lik CaS and MnS. Figurs 8 (a), (b) and (c) show th fractur surfac at th uniaxial spcimn ( σ/2 = 421 MPa, N f = cycls), and Fig. 8(b) suggsts that th surfac was fracturd almost vrtically to th axial load angl, and is visibly vry smooth. Fatigu crack initiatd at 4:30 angl of th imag at th cntr, and consquntly, crack propagatd in to th rmaining ara. Figur 8(a) shows a typical final fracturd surfac with a clavag fractur mod. Figurs 8(d), () and (f) show th fractur surfacs at th biaxial torsional spcimn ( τ/2 = 300 MPa, N f = cycls), and Fig. 8() indicats that th surfac was fracturd on a 40 dgr slop against th axial load angl. Fatigu crack initiation occurrd on th outsid of th cylindrical spcimn. It also shows that th final fractur took plac at th cntr of th spcimn. For th fracturd surfac whr fatigu cracks propagatd, th surfacs ar smard du to friction btwn th fracturd surfacs in contact with ach othr at torsional fatigu, as shown in Fig. 8(f). Figur 8(d) shows a typical final fracturd surfac with a combination of a visibl clavag fractur mod and a ductil fractur mod. (a) ( b) (c) (d) () (f) Fig. 8. Fatigu Fractur Surfacs of (a), (b) and (c) Uniaxial Spcimn at σ/2 = 421 MPa and (d), () and (f) Torsion Spcimn at τ/2 = 300 MPa.

9 Multiaxial Fatigu of a Railway Whl Stl Conclusions This study prformd fatigu tsts on uniaxial and biaxial torsional spcimns of a railway whl stl. Th tsting rsults ar as follows: Th tnsil tsts found that th ultimat tnsil strngth and yild strngth wr MPa and MPa, rspctivly, and th longation was 40.4%. Th uniaxial fatigu limit was MPa, or 67% of th tnsil strngth, and /, which is th ratio of th uniaxial fatigu limit against th biaxial torsional fatigu limit, was 0.63, which is th valu for typical ductil matrials. Th study found that th SWT fatigu paramtr is mor adquat paramtrs to prdict th fatigu lif for uniaxial and biaxial fatigu tsts than th quivalnt strain or quivalnt strss undr multiaxial strss stats. Acknowldgmnt This study was financially supportd by Soul National Univrsity of Scinc & Tchnology. Rfrncs 1. Brnasconi, A.; Filippini, M.; Foltti, S.; and Vaudo, D. (2006). Multiaxial fatigu of a railway whl stl undr non-proportional loading. Intrnational Journal of Fatigu, 28(5-6), Ahlstrὂm, J.; and Karlsson, B. (2009). Modifid railway whl stls: Production and valuation of mchanical proprtis with mphasis on lowcycl fatigu bhavior. Mtallurgical and Matrials Transactions A, 40(7) Okagata, Y.; Kiriyama, K.; and Kato, T. (2007). Fatigu strngth valuation of th Japans railway whl. Fatigu & Fractur of Enginring Matrials & Structurs, 30(4), Wagnr, V.; Stark, P.; Krschr, E.; and Eiflr, D. (2011). Cyclic dformation bhavior of railway whl stls in th vry high cycl fatigu (VHCF) rgim. Intrnational Journal of Fatigu, 33(1), Hur, H.M.; and Kwon, S.J. (2004). An analysis of matrial tst rsults for rolling-stock whl. Procdings of Koran Socity of Railway, Kwon, S.-J.; So, J.-W.; L, D.-H.; and Ham, Y.-S. (2010). Fractur mchanics charactristics of whl and axl for high spd train. Journal of th Koran Socity for Prcision Enginring, 27(8), Bannantin, J.A.; and Comr, J.J.; Handrock, J.K. (1990). Fundamntals of mtal fatigu analysis. Prntic Hall Inc.