Analysis of causes of a locomotive draw hook brittle fracture

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Procedia Structural Integrity 13 00 (2018) 2170–2173 Structural Integrity Procedia (2016) 000–000

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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity

Analysis of causes of a locomotive draw hook brittle fracture Analysis of causes of a locomotive draw hook brittle fracture

XV Portuguese Conference on Fracture, 2016, Paçoa de Arcos, Portugal a a PCF 2016, b 10-12 February a

Nový F. a, Jambor M. a, Trško L. b, Palček P. a, Bokůvka O. a Nový F. , Jambor M. , Trško L. , Palček P. , Bokůvka O.

University of Zilina, Department of Materials Engineering, Zilina, Slovakia Thermo-mechanical modeling of a Center, high pressure turbine blade of an University University of Zilina, Department of Materials Engineering, Zilina, Slovakia of Zilina, Research Zilina, Slovakia University of Zilina, Research Center, Zilina, Slovakia airplane gas turbine engine a a

b b

Abstract Abstract P. Brandãoa, V. Infanteb, A.M. Deusc* The traction force of locomotives is constantly increasing due to the pulling of heavier trains, while the material of the draw hooks a Department Mechanical Instituto Superior Universidade Lisboa, Av. Rovisco Pais,standards 1, 1049-001 Lisboa, The traction locomotives is constantly increasing due Técnico, toUIC the pulling of heavier trains, while the material of thedefining draw hooks remained theforce sameofofas 50 years Engineering, ago. The use of the current standards anddethe relevant national the Portugal remained the for same as 50 years ago. The use of of thethecurrent UIC standards and Despite the relevant national standards defining the methodology testing the mechanical properties hooks is very problematic. the fact that all material properties of b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, methodology for tested testingaccording the mechanical of therelated hooksPortugal is very problematic. the fact that all material properties of the draw hooks, to UICproperties 825 and other standards (In Czech Despite and Slovak republic TNŽ 28 2612 standard) cdraw hooks, tested according to UIC 825 and other related standards (In Czech and Slovak republic TNŽ 28 2612 standard) the CeFEMA, Department of Mechanical Instituto Superior Técnico, Lisboa, Av. Rovisco Pais, This 1, 1049-001 fully comply with the minimum valuesEngineering, set by the standards, in operation theUniversidade unexpected de failure occurs frequently. article Lisboa, deals Portugal fully complyofwith the fractures minimumofvalues set by the standards, in operation the unexpected occurs frequently. article deals with causes brittle locomotive draw hooks operated by Slovak Railways.failure An exemplary study usesThis a traction hook with causes of brittle fractures ofalocomotive hooks operated Slovak Railways. Anthe exemplary study a traction hook of a locomotive that broke when freight traindraw accelerated after theby stop at signal between two stations in uses the winter period at of a locomotive that broke when freight train accelerated after the stop signalfracture between in the winter at ambient air temperature of -2 °C. aThe performed analysis showed that theatbrittle of the the two drawstations hook was caused byperiod usage of Abstract ambient air temperature of -2the °C.coarsened The performed analysis showed that the brittle fracture material of the draw hook was byhook usagemet of the low-toughness steel with microstructure. All experimentally measured properties of caused a broken the limits low-toughness with theand coarsened experimentally measured material properties hook met the specifiedsteel in UIC 825 TNŽ 28microstructure. 2612 standardsAll as the material testing at temperatures below +20of°Ca broken is not required in their operation, modern aircraft 2612 engine components are subjected toat increasingly demanding operating conditions, theDuring limits specified in UIC and TNŽ the material temperatures +20of°C is notfractures required in those standards. This study825 shows that 28 the onlystandards technical assolution to thetesting long-term persistent below problem brittle of especially the high pressure turbinethat (HPT) blades. Such conditions cause parts topersistent undergo problem different types of time-dependent those standards. shows only technical solution thethese long-term locomotive draw This hooksstudy is to redefine theirtheacceptance conditions and to change the prescription for testing of of thebrittle impactfractures strength of of degradation, one of which creep. their A model using the finite element method was developed, to be able to predict locomotive draw hooks is draw to is redefine acceptance conditions and change the(FEM) prescription for testinginoforder the impact strength of the steels, from which the areFlight made. the creep behaviour of HPThooks blades. data records (FDR) for a specific aircraft, provided by a commercial aviation the steels, from which the draw hooks are made. company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model © needed 2018 The Published byaElsevier B.V. scrap was scanned, and its chemical composition and material properties were forAuthors. the FEM analysis, HPT © 2018 Published by Elsevier B.V.blade © obtained. 2018The TheAuthors. Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 The data that wasofgathered was fedorganizers. into the FEM model and different simulations were run, first with a simplified 3D Peer-review under responsibility the ECF22 organizers. Peer-review of to thebetter ECF22 organizers. rectangularunder blockresponsibility shape, in order establish the model, and then with the real 3D mesh obtained from the blade scrap. The Keywords: draw hookbehaviour failure, brittle fracture, analysis was observed, in particular at the trailing edge of the blade. Therefore such a overall expected in terms of cause displacement Keywords: draw failure, brittle cause analysis model can behook useful in the goalfracture, of predicting turbine blade life, given a set of FDR data. 2016 The Authors. Published by Elsevier B.V. 1. ©Introduction 1. Peer-review Introduction under responsibility of the Scientific Committee of PCF 2016.

Increasing of the railway transport, using the existing infrastructure, is accommodated introducing the heavier Keywords: amount High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. Increasing amount the railway using the existing infrastructure, is accommodated introducing the heavier trains, which traveloffaster, Sirongtransport, (2018). As a consequence, the requirements on the mechanical properties of the trains,hook which travel faster, Sirong increase. (2018). As a consequence, the requirements the mechanical properties of the draw assembly substantially Despite the aforementioned facts, theon properties of the draw hook used in draw hookare assembly substantially increase. Despite thestandard, aforementioned facts,introduced the properties of theThe draw hook used in Slovakia prescribed by the national TNŽ 28 2612 which was in 1979! question of the Slovakia are prescribed by theisnational TNŽ 28 2612 standard, which wasisintroduced 1979! The question the tensile mechanical properties not so problematic, much more important the impactintoughness and the wayofhow tensilevalues mechanical properties so problematic, much value more of important is the impact toughness the wayinhow these are prescribed in is thenot standard. The minimum the impact toughness at +20 °C,and prescribed the these values are prescribed in the standard. The minimum value of the impact toughness at +20 °C, prescribed in the 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.146

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TNŽ 28 2612 standard, is KU = 29 J. This is relatively low value, considering dynamic loading, to which the draw hook can be exposed during the operation, Holzman and Klesnil (1972), Skočovký et al. (2000). Even more unsettling is the fact, that the standard does not prescribe impact toughness values for lower temperatures than +20° C. During average winter in Central Europe, the temperature often drops to -20 °C and lower, thus the sufficient toughness properties at lower temperatures are essential for the reliable operation. In the presented paper, the cause analysis of the locomotive draw hook fracture is examined. The failure occurred when the locomotive accelerated with a heavy freight train in the winter, at the ambient temperature of -2 °C. The fracture exhibits fully brittle character, without the presence of any fatigue cracks or ductile fracture. 2. Experimental part The detailed analysis of the broken draw hook revealed fully brittle character (Fig. 1). There were no signs of the presence of any fatigue cracks or any region of the ductile fracture. Based on those findings, the material was found to be a cause of the examined failure. Considering material as a primary cause of failure, the next steps were focused on the characterization of the material and its behavior under different operation conditions.

a)

b) Fig. 1. Broken locomotive draw hook, a) overall view, b) view of the fracture surface.

To reveal the cause of the fatal failure, the mechanical properties of the draw hook were evaluated. Considering the brittle character of the fracture, the emphasis was set on the impact toughness and ductility. Primarily, the chemical composition of examined material was evaluated, using the Arc-spark OES. Three analyses were carried out and the average values of chemical composition are shown in Table 1. The tensile properties were evaluated using the standard tensile test according to EN 10002-1. The results are shown in Table. 2. Two specimens with the round cross section were used and the tests were carried out at ambient temperature of 23 ± 2 °C. The impact toughness tests were carried out using the standard Charpy testing device, with nominal energy of 300 J. Standard specimens were used, with the square cross-section (10 x 10 mm) with machined standardized U-type notch. To evaluate the transition behavior of the draw hook material and monitor the decrease of the impact toughness with decreasing temperature, the tests were carried out at several temperatures, in the range from -60 °C to +20 °C. The microstructure of the draw hook was examined using the light microcopy. Specimens were prepared by standard techniques for the light microscopy and etched by 1% Nital. Typical microstructure of the examined draw hook is shown in Fig. 2. Table 1 Chemical composition of the examined draw hook, steel grade 47 Mn Max. content of the alloying elements in wt.% C

Si

Mn

P

S

Cr

Cu

Mo

Ni

V

P+S

Standard TNŽ 28 2612

0.60

0.60

1.50

0.050

0.050

0.50

0.25

0.10

0.50

0.10

0.090

Examined draw hook

0.428

0.548

1.3

0.0156

0.0299

0.0628

0.0392

0.014

0.0197

0.0027

0.0455

Table 2 Mechanical properties of examined draw hook compared with the values prescribed in the TNŽ 28 2612 standard. UTS [MPa]

YS 0.2 offset [MPa]

Elongation [%]

Specimen No. 1 Specimen No. 2

725.3 719

418.1 405.1

17.2 14.25

Reduction of area [%] 46.9 48.3

standard TNŽ 28 2612

735-845

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a)

3

b)

Fig. 2 a) Microstructure of the examined draw hook – coarse grain pearlitic-ferritic microstructure. b) Proper fine grain microstructure of the same material (different draw hook).

3. Results and discussion The results of the chemical analysis (Table 1) revealed, that examined draw hook was manufactured from the 47Mn steel grade. The chemical composition of the draw hook meets the requirements for that material prescribed in the TNŽ 28 2612 standard, without any significant deviations from the prescribed chemical composition. One of the most important drawbacks of the TNŽ 28 2612 standard is that for the steel grade 47Mn it prescribes only the maximal amounts of chemical elements, what is not sufficient to cover all the requirements locomotive draw hooks material. The light microscopy observation revealed coarse pearlitic-ferritic microstructure (Fig. 2a), which is not desired for this application, as it could exhibit lower ductility, Gomes et al. (1997), Qiu et al. (2014). The appropriate microstructure of the same material (from different draw hook) is shown in the Fig. 2b. Note the significant difference in the average grain size. Microstructure also contains a higher amount of large MnS inclusions. The tensile mechanical properties obtained from the tensile tests are shown in Table. 2. Experimentally obtained values of the ultimate tensile strength do not meet requirements prescribed in the TNŽ 28 2612 standard, where the minimum required values of UTS are 735 MPa, while the UTS obtained from examined draw hook are 725 and 719 MPa. From the detailed observation an absence of any plastic deformation in the area of fracture is clearly evident and this suggest that the tensile overloading was not the cause of failure. Based on those facts, it can be stated that despite the finding that the UTS of examined draw hook does not meet requirements prescribed in the TNŽ 28 2612 standard, this was not the cause of the fatal failure. Charpy impact toughness test revealed insufficient toughness of the draw hook (Fig. 3). The TNŽ 28 2612 standard requires a minimal value of KU = 29 J at the ambient temperature of +20 °C. The average experimentally examined value was 15.5 J, what is much lower than the minimal allowed value. Even more dramatic results were obtained by testing at lower temperatures. At the temperature of 0 °C the average impact toughness drops to 6.5 J and at -20 °C to only 5 J. Both mentioned temperatures (0 °C and -20 °C) are nothing exceptional during the winter days in Central Europe.

Fig. 3 Results of the impact toughness tests for different temperatures.

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Fractography analysis (Fig. 4) revealed the brittle character of the fracture at all testing temperatures. Fig. 4a shows the mostly transcrystalline fracture with cleavage facets (specimen broken at +20 °C). At lower temperatures (Fig. 4b - 20 °C) more intercrystalline facets can be observed on the fracture surface.

Fig. 4 Fractography analysis of the impact toughness; specimens broken at +20 °C (a) and -20 °C (b).

4. Conclusions Based on carried out experiments and analysis, it could be concluded that the fracture of locomotive draw hook was caused by the insufficient impact toughness of the draw hook material. As it is usual in the similar cases, during the examination several factors causing brittle behavior were identified. • The results of the impact toughness tests revealed insufficient toughness over the whole range of testing temperatures. At the temperature of 0 °C (the draw hook failure occurred at ambient temperature of -2 °C) the impact toughness KU was only 6.5 J, what is not sufficient for any engineering application, when the dynamic loading is assumed. • The observed brittle behavior was caused by the low toughness of the material itself and together with no appropriate microstructure resulted in very low values of impact toughness. • The tensile tests revealed lower values of the UTS than prescribed in the TNŽ 28 2612 standard, but with regard to character of the fracture, this was not the cause of the examined failure. • The deeper analysis of the material requirements prescribed in the TNŽ 28 2612 standard revealed serious shortcomings in the testing methods and conditions in the standard itself. • To avoid similar failure in the future, it is necessary change the material used for the manufacturing of the locomotives draw hooks and to change the testing procedures prescribed in the relevant standard, as well. References Sirong Y. Principles of railway Location and Design. Academic Press, New York, 2018. Holzman M. Klesnil M. Brittle and fatigue fracture of materials and structures. SNTL Praha, (in Czech), 1972. Gomes M. et al. Effect of microstructural parameters on the mechanical properties of eutectoid rail steels. Materials Characterization, 39 (1), pp. 1-14, 1997. Qiu H., Hanamura T., Torizuka S. Influence of Grain size on the ductile fracture toughness of ferritic steel. ISIJ International, 54 (8), pp. 19581964, 2014. Skočovký P. et al. Structural materials. EDIS Žilina (In Slovak), 2000.