PSI - Issue 4

Volume 4 • 2017

ISSN 2452-3216

ELSEVIER

ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria

Guest Editors: H ans -P eter G ä nser Stefano Beretta

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. Copyright © 2017. The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24 ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria Editorial Hans-Peter Gänser a,* , Stefano Beretta b, † a Materials Center Leoben Forschung GmbH, Roseggerstraße 12, 8700 Leoben, Austria b Politec ico di Milano, Dipartime to di Mecc nica, Via La Masa 1, 20156 Milano, Italy © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. Since its establishment in 2004, ESIS Technical Committee 24 “Integrity of Railway Structures” has regularly been conducting workshops to discuss ongoing work and recent research results. Continuing this tradition, the 2016 meeting in Leoben again brought to ether engineers and scientists from all ov r Europe and Asia. The slides of the about twenty presentations given are available online at http://esistc24.mecc.polimi.it. Extended versions of twelve of these lectures are published in this conference volume, giving a representative – albeit by no means complete – overview about current work and issues regarding the reliability of railway structures. The open access model of the Procedia Structural Integrity is expected to help in disseminating the committee’s activities as broadly as possible, and to foster discussion within the scientific and technical community beyond the “railway family”. Keeping this aim in mind, the topics covered in this volume span a broad range from probabilistic considerations regarding damage accumulation by S.-P. Zhu et al. over fatigue tests under VA and damage calculations in the project EURAXLES by M. Filippini et al. to an on-hands analysis of an axle fracture by Z. Odanovic. Adv ced xperiments and models for plasticity- and oxide debris-induced crack closure effects on fatigue crack growth are reported from the ongoing project EBFW3 (Eise bahnfa rwerke 3, Railway Running Gears 3) by J. Maierhofer et l. and D. Simunek et al. , whereas I. Černý r ports on advances in rotating bending axle testing. Ever-increasing attention is being paid to the role of residual stresses on the fatigue lifetime, reflected by two contributions from P. Hutař et al. and H.-J. Schindler, and to corrosion fatigue, with results from the project RAAI (Whole life Rail Axle Assessment and Improvement) reported by S. Beretta et al . R. Boehm et al. and M. Pavlovic et al. present recent advances in ultrasonic testing of axles, inextricably linked to the d rivation of inspection intervals for predi tive aintenance. ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria Editorial Hans-Peter Gänser a,* , Stefano Beretta b, † a Ma rials Center Le ben Forschung GmbH, Roseggerstr ße 12, 870 Leoben, Austria b Politecnico di Milano, Dipartimento di Meccanica, Via La Masa 1, 20156 Milano, Italy © 2017 The Autho s. Publ shed by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. Since its establishment in 2004, ESIS Technical Committee 24 “Integrity of Railway Structures” has regularly been conducting workshops to discuss ongoing work and recent research results. C ntinui g this tradition, the 2016 meeting i Leoben again brought together engineers nd scientists from all over Europe and Asia. The slides of the about twenty presentations given are avai able onlin at http://esistc24.mecc.polimi.it. Extended v rsions of tw lve of th se lect res are published in this confere ce volume, giving representativ – alb it by no means complete – overview abo t c rrent work and issu s regarding the reliability of railway structur s. The op n access model f the Procedia Structural Integrity is expected to help in disseminating the committee’s activities s broadly as possible, and to foster discussion within the scientific and technical community beyond the “railway family”. Keeping this aim in mind, the topics covered in this volume span a broad range from probabilistic considerations regarding damage accumulation by S.-P. Zhu et al. over fatigue tests under VA and damage calculations in the project EURAXLES by M. Filippini et al. to n on-hands analysis of an axle fracture by Z. Odanovic. Advanced experiments and models for plasticity- and oxi e debri -induc d crack closure effects on fatigue crack growth are eported from the ongoing proj ct EBFW3 (Eise bah f hrwerke 3, Railway Ru ning G ars 3) by J. Maierhofer et al. nd D. Simun k et al. , whereas I. Černý reports on advances in rotating b nding axl testing. Ever-increasing attention is being paid to the role of resi ual stresses on the fatigue lif time, reflected by two contributi ns from P. Hutař et al. and H.-J. Schindler, and to corrosion fatigue, with results from the project RAAI (Whole life R il Axle Assessment and Improv ment) reported by S. Beretta e al . R. Boehm et al. and M. Pavlo ic et al. present recent advances in ultrasonic testing of axles, inextricably linked to the derivation of inspection intervals for predictive maintenance. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016.

Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 Copyright  2017. The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24 10.1016/j.prostr.2017.07.011 * Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt 2452 3216 © 2017 Th Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. hp.gaens @mcl.at * Tel.: +433842-45922-41; fax: +433842-45922-5. E-mail address: stefano.beretta@polimi.it * Tel.: +433842-45922-32; fax: +433842-45922-5. E-mail address: hp.gaenser@mcl.at * Tel.: +433842-45922-41; fax: +433842-45922-5. E-mail address: stefano.beretta@polimi.it 32

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The determination of these intervals on the basis of probabilistic fracture mechanics is demonstrated for welded rails by S. Romano et al. , while the contribution by S. Kolitsch et al. discusses tolerance concepts for switches from a materials’ point of view . Finally, some insights into condition monitoring – one of the salient topics with a view towards condition-based maintenance – are given by U. Oßberger et al. Over the two days of the meeting, we have seen many interesting discussions. Our sincere thanks go to all participants and authors – it is them who make TC24 a success!

Hans-Peter Gänser and Stefano Beretta

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria Analysis of the railway freight car axle fracture Zoran Odanovic IMS Institute, Bulevar vojvode Misica 43, 11000 Belgrade, Serbia Railway axles are vital parts of passenger or freight railway car. Their failure may result in potentially disastrous consequences with possible human victims. Accordingly, railway axles are designed to be highly reliable, while the maintenance system requires periodically regular non-destructive inspection. However, due to complex exploitation conditions, complex stress state and multiple stress concentration, railway axles could experience fatigue failures. This study presents an attempt to clarify the causes of an axle fracture of the railway freight car for coal transport. Detailed analyses were conducted on the axle mechanical properties. Novel methodology for calculation of the plane strain fr cture toughness K Ic based on the measured values of the yield strength and impact energy from data of samples with U grove, is estimated. Failure analysis of fractured axle was performed. Macro and microstructure of the axle material is included in analysis. Performed analysis has shown that the axle failure was caused by inadequate maintenance and insufficient properties of the axle material in the railway axle critical cross sections. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. Keywords: railway axle, failure analysis, destructive testing, pitting 1. Introduction Railway axles are the most loaded parts of the railway vehicles. Complex and variable stress state, multiple stress concentration, inadeq ate maintenance and exploitation conditions, material-related errors and inadequate mechanical properties are the most common causes of failure – fracture of the railway axles, as shown in the literature Zerbst et al. (2013), Torabi et al. (2012), Meral et al. (2010). Corrosion process could form crack initials, which may reduce the fatigue strength of axles, as discussed in the literature Beretta et al. (2015) and Odanovic et al. (2015). ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria Analysis of the railway freight car axle fracture Zoran Odanovic IMS Institute, Bulevar vojvode Misica 43, 11000 Belgrade, Serbia Abstract Railway axles ar vital parts of passe ger or fre ght railway car. Their failure may result in potentially disastrous co s quences with possible human victims. Accordingly, railway axles ar designed to be highly reliable, while the maintenance sy em requires periodically regula n -destructive in pection. How ver, due to complex exploitation conditio s, co lex stress stat and multiple stress concentration, railway axles could experience fa igu failures. Thi study presen s an attempt to clarify the caus s of an axl fracture f the railway fre ght car for coal transport. Detailed analy es were conducted on the axle m chanical properties. Novel methodology for calculation of the plane strain fracture toughness K Ic based on the measured values of the yield strength and impact ene gy f om data of samples with U grove, is estimated. Failure analy is of fractured axl was performed. Macro and microstructure of the axle material is includ d in analysis. Performed analysis has shown that the axle failure was caused by inadequate maintenance and insufficient properties of the axle material in the railway axle critical cross sections. © 2017 The Authors. Publ shed by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. Keywords: railway axle, failure analysis, destructive testing, pitting 1. Introduction Railway axles a e the most loaded pa ts of the railway vehi les. C mplex nd variabl str ss state, multiple stress conc ntration, inad quat maintenan and exploitation conditions, material-related errors and inadequat m chanical prop rties are the most common causes of failure – fracture f the railway axles, as shown in the literature Zerbst t al. (2013), Torabi et al. (2012), Meral et al. (2010). Corrosion process could form crack initials, which may reduce the fatigue strength of axles, as discussed in the literature Beretta et al. (2015) and Odanovic et al. (2015). Copyright © 2017. The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. Abstract

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 Copyright  2017. The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24 10.1016/j.prostr.2017.07.009 * Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt 2452 3216 © 2017 Th Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. * Corresponding author. Tel.: +381-63-306-120; fax: +381-11-3692-772. E-mail address: zoran.odanovic@institutims.rs * Corresponding author. Tel.: +381-63-306-120; fax: +381-11-3692-772. E-mail address: zoran.odanovic@institutims.rs

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In this study the fractured axle of a railway freight car used for transportation of coal from the coal mine to the thermal power plant was analyzed. The railway axle was in the exploitation for the past 35 years. In order to clarify the cause of this failure, detailed analysis of the mechanical properties, macro and microstructure of the axle material was performed. This study is an attempt with the aim to improve control and maintenance of the axels in exploitation and to avoid future accidents.

2. Experimental procedures

Within the scope of this research in order to identify the cause of the axle fracture, following analyses and activities have been performed:  Visual examination.  Analysis of the chemical composition by Optical Emission Spectroscopy (OES) technique.  Mechanical properties. Testing was conducted in the longitudinal and transverse directions of the axle. Tensile and impact energy testing were performed according the standard EN 13261 and the hardness testing was conducted using the Brinell method. Samples for the testing were cut from the axle part next to the fracture location. Based on the results of the tensile and impact energy testing, a parameter of fracture mechanic - plane strain fracture toughness K Ic was estimated.  Metallographic investigations. Macrostructural examination was performed by macroscopic method using sulphur print (Baumann method). Microstructural investigations were conducted using Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray analysis (EDX).

3. Results and discussion

3.1. Visual examination

The damaged freight car for coal transportation from the mine to the thermal power plant has two axles. The failure has taken place on an industrial gauge used for coal transport, as shown in Fig. 1. Under exploitation conditions nominal axle load amounts to 200 kN, while railway car speed of motion is up to 70 km/h. The stopping and braking of railway car is done by brake shoes. Accordingly, under exploitation conditions, the axle is subjected to bending stress only. The axle fracture occurred in the cross-section of the rear axle between the roller bearing and the wheel on the transition radius. The fracture was not identified on the other end of the axle. The location of the axle fracture is shown in Fig. 2. Details of the solid railway axle with designated fracture cross-section and the testing zone is given in Fig. 3.

Fig. 1. Damaged railway freight car

Fig. 2. The railway wheel and axle with signed fractured surface

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Zona ispitivanja Axle journal and dimensions

Testing zone

Zona loma Fracture location

Fig. 3. Details of the tested railway axle, geometry with the dimensions in [mm], fracture location and tested zone

The other available data has shown that the axle was designed and manufactured 35 years ago, according to the requirements of the national standard SRPS P.F2.310:1968 (now replaced with similar standard SRPS EN 13261:2010). The investigated axle was manufactured as solid and coated axle. Available user’s data show that similar axle fractures were not evidenced before and the railway axle is regularly periodically inspected and overhauled.

3.2. Chemical composition

Ispitani radijus A

Ispitani radijus B

Chemical composition of the axle material was analyzed by the OES method. Analysis was done on the plate samples prepared according to the standard EN 13261 procedures. Results are presented in Table 1.

Table 1. Chemical composition of the axle material (in mass. %)

Chemical element

C

Si

S

P

Mn

Ni

Cr

Mo

V

Ti

W

Al

Fe

Tested sample

0.441 0.260 0.005 0.009 0.640 0.034 0.097 0.012 <0.003 <0.003 0.020 0.069 rest

Specified according to EN 13261:2010 standard

max. 0.40

max. 0.50

max. 0.020

max. 0.020

max. 1.20

max. 0.30

max. 0.30

max. 0.08

max. 0.06

-

-

-

rest

Requirements for chemical composition of steels for rail vehicles axles specified by standard EN 13261:2010 and national standard SRPS EN 13261:2010, are also presented in Table 1. Comparison of the results leads to the conclusion that chemical composition of the investigated axle material is in accordance with the standard requirements. Only the carbon content falls outside the specified limits.

3.3. Mechanical properties

Samples for the testing of mechanical properties were cut from the wheel seat of the axle next to the fracture location as presented in Fig. 3. Results for the tensile properties in longitudinal and transverse direction are presented in Table 2. A comparison between the tensile properties in longitudinal direction, yield strength - Re and tensile strength - Rm, and the requirements specified by standard EN 13261, has shown that the obtained values are substantially below the recommended standard values. The results from the transverse direction tests are also unsatisfying. Only the results of the elongation - A5 for longitudinal direction meet the corresponding standard requirements. Extreme difference for the elongation - A5 and contraction – Z values between the longitudinal and transverse direction are evidenced. Deviation of yield stress from a specified standard value is substantially higher compared to tensile strength deviation. Impact energy test results are given in Table 3. The results indicate that the axle material impact energy in longitudinal direction is approximately 30% lower than the standard values. In addition, impact energy in transverse direction is approximately 45% lower compared to the standard values.

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Table 2. Results of material tensile properties testing

Yield strength Re (MPa)

Tensile strength Rm (MPa)

Elongation A5 (%)

Contraction Z (%)

Test specimens position

Longitudinal Transverse

235 233

534 523

30.70 16.25

51.58 17.86

Specified by standard EN 13261 for longitudinal test specimens

min 320

550 - 650

min 22

/

Table 3. Impact energy results of the steel axle

Test specimens position

Test temperature T(°C)

Mean value KU 5/300 (J)

Standard EN 13261 KU 5/300 (J)

Longitudinal Transverse

+ 20 + 20

21.8

min. 30 J min. 25 J

11.33

Preparation of test pieces for the axle hardness testing was performed at the axle longitudinal cross-section. Hardness was measured at three cross-sections: on the axle surface, at mid-radius and in the center of the axle. At each location, hardness was measured at three measuring points. Test results obtained by the Brinell method are presented in Table 4. The highest values of hardness are on the axle surface and the lowest in the center. Measured values at each axle cross section were not significantly different.

Table 4. Measured material hardness through the cross section from surface to center of the axle Measured hardness (HBW) Mean value (HBW)

S: 146 – 148 – 149 M: 145 – 145 – 144 C: 143 – 145 - 143

145.3

Comment: S - Surface, M - Mid-radius, C - Centre In order to anticipate the ability of the tested steel to resist crack propagation, the fracture mechanics parameter - plane strain fracture toughness K Ic was estimated from the measured values of the yield strength and impact energy data. Estimation was based on the Barsom-Rolfe correlation model presented in literature Tauscher (1981): K Ic 2 =5 *CVN *  ys - 0.25 *  ys 2 (1) Where CVN represents Charpy V-notch impact energy in units [ft lbf],  ys represents yield strength in units [ksi], and K Ic was in units [ksi.in 0.5 ]. Equation (1) recalculated in SI units is as follows: K Ic 2 =0.619 * KV * R e - 0.00578 * R e 2 (2) Where KV is Charpy V-notch impact test data in units [J], Re is yield strength in units [MPa], and K Ic is in units [MPa.m 0.5 ]. Validity of Eq. (2) was checked for the similar steel C 45 in normalised and tempered condition, based on the data from the literature http://toscelikniksic.me (2016) and William (1973). For the input data for impact energy of KV=18 J and the yield strength of R e =275 MPa, calculated value for the plane strain fracture toughness was K Ic = 51.25 MPa.m 0.5 , while measured value for the same steel from literature William (1973), was K Ic =50 MPa.m 0.5 . These results confirmed the validity of Eq. (2). In performed investigation impact energy was measured on the samples with U grove (Table 3), and the relation between impact energies KV 2/300 in [J] and KU 2/300 in [J], was established as KV/KU = 0.44. Obtained relation is

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based on the measured results for the similar steel from literature Odanovic (2011). Equation (2) was then modified to: K Ic 2 =0.27236 * KU * R e - 0.00578 * R e 2 (3) Where KU is impact energy in [J] and R e is yield strength in [MPa]. Applying equation (3) using the values for impact energies and yield strength from standard EN 13261, and test values shown in Table 2 and 3, values of the strain fracture toughness K Ic were calculated and presented in Table 5.

Table 5. Calculated plane strain fracture toughness K Ic

Impact energy KU 5/300 (J)

Yield strength, Re (MPa)

Test specimens position Longitudinal Transverse Longitudinal Transverse

Plane strain fracture toughness K Ic (MPa.m

0. 5 )

min. 30 J min. 25 J

min. 320

Standard EN 13261 Measured

44.97 39.83 32.80 20.13

21.8

235 233

11.33

values

Table 5 shows that in the longitudinal direction values for strain fracture toughness K Ic were approximately 25% lower than the minimum impact energies and yield strength values required by standard EN 13261. The transverse direction values were approximately 50% lower than those required by the standard. Such low values of the strain fracture toughness K Ic , especially in transverse direction, indicate low resistivity to crack propagation of the crack initials from the axle surface. Summarizing the results, it is evident that the steel used to manufacture investigated axle is with respect to its chemical composition in accordance with the requirements of the corresponding standard. On the other hand, mechanical properties of applied material are below the standard requirements. This suggests that the investigated axle is susceptible to formation of the initiations for different kind of damages as a result of the exploitation conditions. Also, resistivity to propagation of the initial cracks is below the required resistivity. Low values of mechanical properties may be a result of insufficient reduction rate in hot rolling/forging process, or inadequate heat treatment process during the axle production. The fractured surface in a cross section of the railway axle is shown in Fig. 4. The following characteristic zones could be distinguished on the fractured surface. The first one labeled as A in Fig. 4 is the zone around the circumference of the fractured surface with tooth-like numerous initial cracks of different sizes. This zone covers about half of the radius of the axle. The size of these initial cracks ranges from 10 to 20 millimeters in depth, and from 2 to 20 millimeters in width. These cracks were probably initiated from the numerous initial parallel cracks on the outer surface of the critical axle radius, labeled as AR in Fig. 4. One can assume that these cracks were initiated by corrosion. Due to the high stress concentration they spread parallel to the axle cross section or they connected and integrated with parallel initial cracks of similar kind. In this process they formed, tooth-like initial cracks having a specific shape of ratchet. These marks show a presence of corrosion products. The following characteristic zones are labeled in Fig. 4 as zone B and C. Appearance of these zones is similar in morphology, but different in shape. It can be concluded that they were initiated at the ratchet marks and propagated by the fatigue mechanism. One can notice in these regions fatigue zones resulting from the slow fatigue crack propagation. These zones are highly oxidised which indicates long duration of axle exploitation. Zone B and C are separated by a radial crack. One can conclude based on their depth that these zones formed around the same period of exploitation, but in parallel cross sections of 3.4. Macro and microstructure analysis of the fractured railway axle In order to identify the cause of the axle fracture, macro-fractography was performed and the fractured surface of the axle journal was analysed. Also, segregation testing was conducted by macroscopic method using sulphur print - Baumann method, Schumann (1989). Results are presented in Figs. 4 and 5.

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the axle. The final axle fracture surface accounts for approx. 30 – 40% of the cross-sectional area and is shown as light-colored surface in Fig. 4 labeled as zone D. This type of the fracture could be defined as a ductile. Generally, it can be concluded that the crack initiations formed at the surface of the critical radius of the axle. The newly formed cracks then propagated by the fatigue mechanism, until the final axle fracture happened when the remaining ligament of the axle could not withstand the bending stress of the loaded axle. Macrostructure and presence of the defects and imperfections in the axle material was analyzed using sulphur print method (Baumann method). The axle cross section next to fractured surface was tested and the result is presented in Fig. 5. Only small-scale heterogeneities were observed in the central zone of the cross section. However, dark etching strings of inclusion between the marked white lines at the mid radius of the cross section were observed. By comparing Figs. 4 and 5, one can notice matching between zones labeled as A-B-C- D and A’ - B’ - C’ - D’. This observation could clarify and additionally explain possible mechanism of axle fracture. The initial cracks from the axle surface likely propagated through the cross section in the transversal direction of the axle by a different mechanism, as explained before, until reaching the strings of inclusions in zone between white lines in Fig. 5. Orientation of these inclusions are perpendicular to the direction of the cracks propagation, and one can assume that they represents a temporary barrier to the further crack propagation in the radial direction. Remaining axle ligament, labeled as zone D or D’ in Figs. 4 and 5, carried the load until the moment when the axle could not resist the bending stress any more leading to the final fracture of the axle.

AR

Fig. 4. Fracture surface with zones characterized by different types of fracture mechanism

Fig. 5. Macrostructure of the axle cross section obtained by sulphur print (Baumann method)

Fig. 6. Lamellar perlite and ferrite in the axle material microstructure (etched in 3 % Nital)

Microstructure was analyzed using LOM, SEM and EDX analysis at magnifications up to 500 times. Testing was performed on the samples extracted from the axle near the axle fracture as labeled in Fig. 3. Sample preparation was performed by the classical methods of grinding and polishing. Etching was done with 3% Nital solution. The axle material has a lamellar ferritic-perlitic microstructure, banded in the longitudinal direction as shown in Fig. 6. The content of non-metallic inclusions was determined by the comparison with reference charts, according to the standard ISO 4967. The presence of non-metallic inclusions of the A, B, C and D type was detected. Their sum for A, B and C type was not greater than specified by standard EN 13261:2003. But in the samples from locations near the axle surface and from axle center, inclusions of type C (silicate) and type D (globular oxide) were identified in the amounts above allowed specifications. Corrosion pits were observed in the material surface layer in the locations of the transition radius zone labeled as AR in Fig. 4, in the vicinity of the cross-section where the axle fracture occurred. Characteristic corrosion pits are presented in Fig. 7. Cross-sectional shape of analysed pits could be defined, according the standard ISO 11463:1995, as wide and shallow in Fig. 7a and elliptical in Fig. 7b. Size of registered corrosion pits ranged from approx. 25 to

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100 μm i n diameter and from 25 to 40 μm in depth. Corrosion pits were observed to connect with non-metallic inclusions displayed in Fig. 8. Qualitative EDX analysis was performed on non - metallic inclusions, and presented in Fig. 8 as spectrum 1 and compared to that of the matrix material presented as spectrum 2 . Results of the quantitative analysis of the non-metallic inclusion and the matrix material near the axle surface are presented in Table 5. Based on the chemical composition presented as Spectrum 1, presence of non-metallic inclusions of different types, such as (Fe,Mn)S, FeO, Al 2 O 3 , MnO, could be assumed. Analyzed inclusion is located at the crack initiation in axle surface continuing on corrosion pit and spreading through the soft ferrite. For comparison, results of the quantitative EDX analysis of the base material are also presented in Table 6. Fig. 8 could be used as an illustration for the primary mechanism of crack initiation and crack formation at corrosion pits. Formed cracks propagate through the soft ferrite in ferrite - perlite microstructure, supported with presence of the non-metallic inclusions in structure. These initial cracks, under the complex stress state and exploitation conditions, lead to formation of the tooth-like cracks in zone labeled as A in Fig. 4. Mechanism of the crack propagation until the final fracture was suggested previously.

a) wide, shallow shape b) elliptical shape Fig. 7. Corrosion pits in the axle material cross section at the transition radius zone AR from Fig. 4.

Specrum 1

Specrum 2

Fig. 8. Axle surface in a cross section (SEM) and EDX analyzed points (Spectrum 1 and Spectrum 2)

Table 6. Chemical composition of the analyzed points in Fig. 8. by EDX

Analysis of the inclusion located in the crack – Spectrum 1

Analysis of the base material – Spectrum 2

Element

Weight, %

Atomic, %

Element

Weight, %

Atomic, %

C K O K Al K S K Mn K Fe K Totals

37.69 15.41 0.22 1.81 2.27 42.60 100.00

63.14 19.38 0.16 1.14 0.83 15.35

C K O K

23.17 4.23 0.50 72.09 100.00

55.22 7.57 0.26 36.95

Mn K Fe K Totals

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4. Conclusion

Based on the investigation results, the following can be concluded. The chemical composition analysis indicate that the steel used for the fractured axle is in accordance with the standard requirements. Mechanical properties of applied material are below the requirements defined in standard EN 13261:2010. Low values of the strain fracture toughness K Ic , especially in transverse direction, indicate low resistivity to the propagation of the initiated cracks from the axle surface. Low values of the tested mechanical properties could be explained as a result of the inadequate reduction rate in hot rolling/forging process during the axle production, or inadequate heat treatment process after hot working. It can be concluded that corrosion was initiated from the damaged axle coat, and from below the coat to the whole surface of the critical radius thus creating conditions for pit formation. Crack initials are caused by corrosion pits at the surface of the critical radius of the axle. Due to the weakness of the material, these cracks propagated, forming tooth-like cracks and spreading by fatigue mechanism, until the axle could not withstand the stresses arising from exploitation conditions. These circumstances lead to the final fracture of the axle. In order to prevent similar failures in the future, it is necessary to improve the control of the corrosion protection and the inspection of the axle from the aspect of the initial cracks during the regular maintenance.

Acknowledgements

The author wish to express gratitude to Serbian Ministry of Education, Science and Technology development, for supporting this paper through Projects TR 35002 and ON 172005.

References

Zerbst, U., Beretta, S., Kohler, G., Lawton, A., et al., 2013. Safe life and damage tolerance aspects of railway axles - A review. Engineering Fracture Mechanics 98, 214-71 Meral, B., Necati, T., Rahmi, G., 2010. Realibility and fatigue life evaluationof railway axles, Journal of Mechanical Science and Technology, 24(3), 671-679. Torabi, A.,R., Heidary, K., M., 2012. Fatigue Crack Growth in a Solid Circular Shaft under Fully Reversed Rotating Bending, J. Fail. Anal. and Preven;12, 419-426 Beretta, S., Conte, L., A., Rudin, J., 2015. From atmospheric corrosive attack to crack propagation for A1N railway axles steel under fatigue, Eng. Failure analysis, 47, 252-264. Odanovic, Z., Ristivojevic, M., Milosevic-Mitic, V., 2015, Investigation into the causes of fracture in railway freight car axle, Eng. Failure analysis, 55, 169-181. Odanovic, Z., 2011. Testing report Nb. 421116-168. Belgrade, pp. 1-16. http://toscelikniksic.me/supplier/manufacturer/carbon/special/alloy/products/mills/grade/standard/C45.html], 2016. William, M., 1973. Plane strain fracture toughness (KIc), Data handbook for metals, Army materials and mechanics research center, AD-773 673, pp. 38 Tauscher, S., 1981. The correlation of fracture toughness with charpy v-notch impact test data, Technical report ARLCB-TR-81012, pp. 10 Standard SRPS EN 13261:2009+A1:2010, Institute for Standardization of Serbia, http://www.iss.rs Standard SRPS P.F2.310:1968, Institute for Standardization of Serbia, http://www.iss.rs Schumann, H., 1989. Metalografija, TMF, Beograd, 225-230.

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria Crack closure and retardation effects – experiments and modelling J. Maierhofer a, *, H.-P. Gänser a , R. Pippan b a Materials Center Leoben Forschung GmbH, Roseggerstraße 12, 8700 Leoben, Austria b Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, 8700 Leoben, Austria The design of cyclically loaded components is in many cases carried out on the basis of experiments on small scale laboratory specimens. In this approach, many effects such as load ratio, residual stresses and short crack growth are taken into account and described in a com utational rack g owth model. However, it can be seen that the prediction of the actual component lifetime with such models often is clearly too conservative. The reason for this behaviour can be found in occurring load sequence effects during operation, which are often not dealt with in the context of small-scale experiments. This paper attempts to examine such variations of applied load stresses as they may occur during operation. On the asis of cyclically loaded single edge bending (SEB) specimens, crack retardation effects are investigated in detail. It will be shown that residual stresses and overloads as well as extended operation times under small loads can lead to a significant extension of the lifetime of a component. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. Keywords: crack retardation, crack closure, residual stresses, overloads, small loads 1. Introduction For damage tolerant design of cyclically loaded components such as railway axles, a detailed knowledge of the crack propagation behavior is essential. Therefore a few years ago the project ‘Safe and economic operation of running gears’ (Eisenbahnfahrwerke 2, EBFW2) was realized (Lütkepohl et al. (2009), Luke et al. (2010) and ESIS TC24 Workshop "Integrity of Railway Structures", 24-25 October 2016, Leoben, Austria Crack closure and retardation effects – experiments and modelling J. Maierhofer a, *, H.-P. Gänser a , R. Pippan b a Materials Center Leoben Forsch ng GmbH, Roseggerstraß 12, 8700 Leoben, Austria b Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, 8700 Leoben, Austria Abstract The design of cyclically loaded components is in many cases carried out on the basis of experiments on small scale laboratory specimens. In this approach, many effects such as load ratio, residual stresses and short crack growth are taken into account and described in a computational crack growth model. However, it can be seen that the prediction of the actual component lifetime with such models often is clearly too conservative. The reason for this behaviour can be found in occurring load sequence effects during operation, which are often not dealt with in the context of small-scale experiments. This paper attempts to examine such variations of applied load stresses as they may occur during operation. On the basis of cyclically loaded single edge bending (SEB) specimens, crack retardation effects are investigated in detail. It will be shown that r sidual stresses and verload as well as xtended operation ti es under small loads can lead to a significant extension of the lifetime of a component. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. Keywords: crack retardation, crack closure, residual stresses, overloads, small loads 1. Introduction Fo damage tolerant des gn of cycli ally loaded components such as railway axles, a detailed knowledge of the crack propagation behavior is essential. Therefore a few years ago the project ‘Safe and economic operation of running gears’ (Eisenbahnfahrwerke 2, EBFW2) was realized (Lütkepohl et al. (2009), Luke et al. (2010) and Copyright © 2017. The Authors. Published by Elsevier B.V. Peer-review und responsibility of the Scientific Co mittee of ESIS TC24. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. Abstract

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 Copyright  2017. The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24 10.1016/j.prostr.2017.07.014 * Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt 2452 3216 © 2017 Th Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ESIS TC24. * Correspon ing author. Tel.: +433842-45922-41; fax: +433842-45922-5. E-mail address: juergen.maierhofer@mcl.at * Corresponding author. Tel.: +433842-45922-41; fax: +433842-45922-5. E-mail address: juergen.maierhofer@mcl.at

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Luke et al. (2011)). Within EBFW2 a computational method for determining the residual life and/or inspection intervals of railway axles by means of fatigue crack growth calculations was developed. Although even various residual stress states can be considered in this computational model, non-negligible differences between computation and tests on full-scale components have been observed, with the full-scale components consistently exhibiting longer lifetimes than predicted. The computational method was developed using fracture mechanics material parameters derived from laboratory specimens. Provided that there exists no size effect between laboratory specimens and full scale component tests, further effects or mechanism are expected to be responsible for the occurring differences. To clarify these differences, the project ‘Probabilistic fracture mechanics concept for the assessment of railway wheelsets’ (Eisenbahnfahrwerke 3, EBFW3) was started. The computational model developed in Lütkepohl et al. (2009) was based on the NASGRO fatigue crack growth equation. The NASGRO equation is able to describe the crack propagation rate for long cracks. Maierhofer et al. (2014a) modified the NASGRO equation slightly to consider also the behavior of short cracks. Also the growth of cracks emanating from deep sharp notches is not considered in the common NASGRO equation (Maierhofer et al. (2015)). This means that, considering the current state of knowledge (Maierhofer et al. (2014a, 2015)), the computational model will lead to even higher differences between prediction and full-scale tests. Hence, there must exist some additional mechanisms which are responsible for the deceleration of the crack propagation rate in full-scale tests in comparison to standard laboratory testing. Within the project EBFW3 the following main reasons for differences between constant load tests on small-scale fracture mechanics specimens and block program testing on full-scale test axles were found to be potentially responsible for crack retardation effects:

• Compressive residual stresses • Overloads • Small loads near the fatigue crack growth threshold

In the present contribution, the influence of these mechanisms on the fatigue crack propagation rate is investigated in detail.

Nomenclature a 0

notch depth d a /d N crack propagation rate ∆ a crack extension ∆ K

stress intensity factor range crack growth threshold at R =0

∆ K 0 ∆ K ox

stress intensity factor range ∆ K th,ox stress intensity factor range for building up an oxide layer K max maximum stress intensity factor during one load cycle K min minimum stress intensity factor during one load cycle K max,OL maximum stress intensity factor during an overload K ox model parameter for oxide induced retardation K res fictitious residual stress intensity factor due to overloads L OL model parameter for overload induced retardation L ox model parameter for oxide induced retardation N ox number of applied small load cycles m ox model parameter for oxide induced retardation Φ Gallagher’s retardation factor p OL model parameter for overload induced retardation p ox model parameter for oxide induced retardation q ox model parameter for oxide induced retardation r ox model parameter for oxide induced retardation