PSI - Issue 82

ScienceDirect Structural Integrity Procedia 00 (2026) 000–000 Structural Integrity Procedia 00 (2026) 000–000 Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect Available online at www.sciencedirect.com ScienceDirect

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Procedia Structural Integrity 82 (2026) 1–2

© 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Preface; fatigue; fracture; structural integrity; residual stresses 1. Preface The 8 th International Conference on Structural Integrity and Durability, ICSID 2025, was organized by the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, and the Croatian National Group of the European Structural Integrity Society (ESIS), in Dubrovnik in Croatia from September 16 to 19, 2025. ICSID 2025 was organized as the Hybrid Conference i.e, a combination of on-site and online participation to provide convenience for participants. The Conference was held in the Centre for Advanced Academic Studies (CAAS) of the University of Zagreb, in the city of Dubrovnik. The magnificent old building of CAAS is situated in the center of Dubrovnik on the Croatian Adriatic Coast, in the vicinity of the most prominent historical places of the Old Town (https://icsid2025.fsb.unizg.hr/). Online presentations were held using a commercial meeting platform. In total 88 on site and online presentations were given, including six plenary lectures. Eleven posters were also presented. 8th International Conference on Structural Integrity and Durability (ICSID2025) Preface Željko Božić a, *, Siegfried Schmauder b , Katarina Monkova c , Robert Basan d , Goran Vukelić e , Emanuele Vincenzo Arcieri f , Francesco Iacoviello g a University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Ivana Lučića 5, 10000 Zagreb, Croatia b University of Stuttgart, Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), Pfaffenwaldring 32, Stuttgart, Germany c Technical University of Kosice, Faculty of Manufacturing Technologies, Sturova 31, 080 01 Presov, Slovakia d University of Rijeka, Faculty of Engineering, Vukovarska 58, 51000 Rijeka, Croatia e University of Rijeka, Faculty of Maritime Studies, Studentska 2, 51000 Rijeka, Croatia f University of Bergamo, Department of Management, Information and Production Engineering, Viale Marconi 5, 24044, Dalmine (BG), Italy g Università di Cassino e del Lazio Meridionale, via G. DI Biasio 43, 03043, Cassino (FR), Italy © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Preface; fatigue; fracture; structural integrity; residual stresses 1. Preface The 8 th International Conference on Structural Integrity and Durability, ICSID 2025, was organized by the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, and the Croatian National Group of the European Structural Integrity Society (ESIS), in Dubrovnik in Croatia from September 16 to 19, 2025. ICSID 2025 was organized as the Hybrid Conference i.e, a combination of on-site and online participation to provide convenience for participants. The Conference was held in the Centre for Advanced Academic Studies (CAAS) of the University of Zagreb, in the city of Dubrovnik. The magnificent old building of CAAS is situated in the center of Dubrovnik on the Croatian Adriatic Coast, in the vicinity of the most prominent historical places of the Old Town (https://icsid2025.fsb.unizg.hr/). Online presentations were held using a commercial meeting platform. In total 88 on site and online presentations were given, including six plenary lectures. Eleven posters were also presented. 8th International Conference on Structural Integrity and Durability (ICSID2025) Preface Željko Božić a, *, Siegfried Schmauder b , Katarina Monkova c , Robert Basan d , Goran Vukelić e , Emanuele Vincenzo Arcieri f , Francesco Iacoviello g a University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Ivana Lučića 5, 10000 Zagreb, Croatia b University of Stuttgart, Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), Pfaffenwaldring 32, Stuttgart, Germany c Technical University of Kosice, Faculty of Manufacturing Technologies, Sturova 31, 080 01 Presov, Slovakia d University of Rijeka, Faculty of Engineering, Vukovarska 58, 51000 Rijeka, Croatia e University of Rijeka, Faculty of Maritime Studies, Studentska 2, 51000 Rijeka, Croatia f University of Bergamo, Department of Management, Information and Production Engineering, Viale Marconi 5, 24044, Dalmine (BG), Italy g Università di Cassino e del Lazio Meridionale, via G. DI Biasio 43, 03043, Cassino (FR), Italy

* Corresponding author. Tel.: +385 1 6168 536; fax: +385 1 6156 940. E-mail address: zeljko.bozic@fsb.unizg.hr * Corresponding author. Tel.: +385 1 6168 536; fax: +385 1 6156 940. E-mail address: zeljko.bozic@fsb.unizg.hr

2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers

2452-3216 © 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 10.1016/j.prostr.2026.04.001

Željko Božić et al. / Procedia Structural Integrity 82 (2026) 1– 2

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The objective of the ICSID 2025 Conference was to bring together scientists, researchers, and engineers from around the world to discuss how to analyze, predict and assess the fatigue and fracture of structural materials and components. The Conference provided a forum for discussion of contemporary and future trends in experimental, analytical, and numerical fracture mechanics, fatigue, failure analysis, structural integrity assessment, and other important issues in the field. A wide range of topics was covered such as: Failure investigation and analysis; Structural integrity assessment; Analytical Models; Advanced testing and evaluation techniques; Applications to components and structures; Non destructive evaluation (NDE); Fatigue and fracture simulation and testing at all length scales; Fracture and failure criteria; Multiscale materials modeling; Mixed-mode and multiaxial fatigue and fracture; Durability and life extension of structures and components; 3D-printed materials and structures; Models, criteria and methods in fracture mechanics; Finite element methods and their applications; Residual stress effects; Fatigue and fracture of weldments, welded components, joints and adhesives; Corrosion, environmentally enhanced degradation and cracking, corrosion fatigue; Fracture and damage of cementitious materials; Fatigue and fracture of polymers, elastomers, composites and biomaterials; and others. Prior to the ICSID 2025 Conference a two-day Summer School with the topic “Fatigue and fracture modelling and analysis” was organized for graduate students, researchers, and engineers from industry. Those participants who came to Dubrovnik, besides the excellent technical program, had the opportunity to enjoy their stay in Dubrovnik, one of the most famous Mediterranean cities, world celebrated symbol of historical heritage and beauty, which has found its place in the UNESCO World Heritage List. As the Guest Editors of this Conference Proceedings, we wish to thank all authors for their contributions. Guest Editors of the Procedia Structural Integrity ICSID 2025 Conference Proceedings: Željko Božić, University of Zagreb, Croatia Siegfried Schmauder, University of Stuttgart, Germany Katarina Monkova, Technical University in Kosice, Slovakia

Robert Basan, University of Rijeka, Croatia Goran Vukelić, University of Rijeka, Croatia Emanuele Vincenzo Arcieri, University of Bergamo – DIGIP, Italy Francesco Iacoviello, Università di Cassino e del Lazio Meridionale – DICeM, Italy

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Procedia Structural Integrity 82 (2026) 220–226

© 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Abstract Fracture of the turbine rotor thrust collar is a very rare failure. However, such a failure can occur due to some serious malfunction in the operation of the turbine. In this paper, the root cause of the rotor thrust collar fracture, which occurred in the history of a 35 MW steam turbine, is analyzed. The analysis was carried out based on the facts found and using numerical simulations in the Ansys software package. The cause of the fracture was determined to be high-cycle fatigue as a result of cyclic loading and a significant notch effect at the root of the shaft-collar transition radius. Cyclic loading was a consequence of the non-uniform loading of the collar over the surface, which was the result of incorrect sliding of the front bearing housing during the thermal expansion of the turbine casing in the start-up phase. The turbine rotor was repaired so that the cracked thrust collar was removed by machining, and the new collar was shrink fitted onto the rotor shaft. After the repair and the final assembly, the turbine was put into operation without any observed deficiencies. © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Fracture; Thrust Collar; Highcycle Fatigue; Turbine Rotor; FEM; Stress Analysis 1. Introduction The purpose of the turbine thrust bearing is to ensure an adjusted axial position of the rotor in relation to the stationary elements of the turbine. To achieve this, it must be able to withstand axial load due to steam pressure and blade reaction. The most common thrust bearing is a fluid film bearing with tilting pads. The pads are usually steel 8th International Conference on Structural Integrity and Durability (ICSID2025) Analysis of a steam turbine rotor thrust collar fracture: root causes and repair Marko Katinić a, *, Pejo Konjatić a a University of Slavonski Brod, Mechanical Engineering Faculty, Ulica 108. brigade ZNG 36, Slavonski Brod 35000, Croatia Abstract Fracture of the turbine rotor thrust collar is a very rare failure. However, such a failure can occur due to some serious malfunction in the operation of the turbine. In this paper, the root cause of the rotor thrust collar fracture, which occurred in the history of a 35 MW steam turbine, is analyzed. The analysis was carried out based on the facts found and using numerical simulations in the Ansys software package. The cause of the fracture was determined to be high-cycle fatigue as a result of cyclic loading and a significant notch effect at the root of the shaft-collar transition radius. Cyclic loading was a consequence of the non-uniform loading of the collar over the surface, which was the result of incorrect sliding of the front bearing housing during the thermal expansion of the turbine casing in the start-up phase. The turbine rotor was repaired so that the cracked thrust collar was removed by machining, and the new collar was shrink fitted onto the rotor shaft. After the repair and the final assembly, the turbine was put into operation without any observed deficiencies. © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Fracture; Thrust Collar; Highcycle Fatigue; Turbine Rotor; FEM; Stress Analysis 1. Introduction The purpose of the turbine thrust bearing is to ensure an adjusted axial position of the rotor in relation to the stationary elements of the turbine. To achieve this, it must be able to withstand axial load due to steam pressure and blade reaction. The most common thrust bearing is a fluid film bearing with tilting pads. The pads are usually steel 8th International Conference on Structural Integrity and Durability (ICSID2025) Analysis of a steam turbine rotor thrust collar fracture: root causes and repair Marko Katinić a, *, Pejo Konjatić a a University of Slavonski Brod, Mechanical Engineering Faculty, Ulica 108. brigade ZNG 36, Slavonski Brod 35000, Croatia

* Corresponding author. Tel.: +385-35-493-429. E-mail address: mkatinic@unisb.hr * Corresponding author. Tel.: +385-35-493-429. E-mail address: mkatinic@unisb.hr

2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers

2452-3216 © 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 10.1016/j.prostr.2026.04.034

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coated with white metal. The thrust collar, which is part of the turbine rotor, rests on tilting pads and transmits the axial load to the stationary elements of the turbine. The thrust collar is either an integral part of the rotor shaft or is shrink fitted onto the rotor shaft. A steam turbine can experience various malfunctions during its operation. Rotor failure is one of the statistically most common failures because it is under the influence of serious loads. Some rotor failures can cause catastrophic damage to the entire turbine and thus cause long unplanned downtimes (Barella et al., 2011; Trebuňa et al., 2017; Yadavar et al., 2016; Vinod et al., 2020; Katinić et al., 2019). Fatigue caused by cyclic load change is the most common cause of rotor failure. The result of fatigue is the initation and propagation of a crack or multiple cracks. Ultimately the development of a crack or cracks can lead to fracture of either the shaft or rotor disc or blade and catastrophic failure of the turbine. Fatigue, one of the primary mechanisms responsible for the failure of steam turbine rotors, occurs as a result of the material being subjected to typically high-cycle stresses that do not exceed its tensile strength. Subjecting a material to cyclic stresses can, over time, locally degrade the material and lead to the formation of microcracks. With each new stress cycle, the microcracks grow, coalesce, and eventually become macrocracks. When a macrocrack reaches its critical size, a sudden and rapid brittle fracture occurs. The role of fatigue in steam turbine failures has been investigated in several studies (Qiqi et al. (2023), Segure et al. (2017), Mazura et al. (2008), Weija et al. (2020) and Ramírez et al. (2023)). Studies have shown that fatigue can significantly reduce the life of critical turbine components such as the rotor. The importance of precise engineering analysis and prediction of the material behavior of turbine components under the action of cyclic stresses was emphasized, in order to undertake preventive and corrective maintenance activities in time. Accumulation of damage in materials due to long-term effects of cyclic stresses combined with other loads and phenomena such as material creep make components such as turbine rotors susceptible to failure. Specialized powerful computer software that performs finite element analysis (FEA) is used to model and simulate the behavior of mechanical systems under various loading conditions. FEA is an important tool that is also used to investigate the fatigue phenomena that occur in systems subjected to cyclic stresses. Fatigue life calculations for critical steam turbine components use FEA. FEA is often used to investigate and identify the causes of failures such as turbine rotor fractures. Engineers are enabled to use computer software to simulate the behavior of turbine components subjected to mechanical and thermal loads (Katinić et al. (2023)). In this way, potential weak points can be detected, failure locations predicted, and the time to failure can be predicted. If the turbine is properly designed and regularly maintained, fracture of the rotor thrust collar should not occur. In the available literature and open access sources, no cases of fracture of the rotor thrust collar have been recorded. In this paper, a case of thrust collar fracture that occurred in the distant past and has not yet been published in a scientific journal is analyzed. The rotor of a 35 MW single-case steam turbine generator experienced a thrust collar fracture during operation in 1998. The broken collar led to an extended turbine shutdown which resulted in power supply disruptions. The researchers conducted a thorough investigation to determine the fracture origin and establish preventive measures for future occurrences.

Nomenclature p d

contact pressure in a shrink-fitted joint (MPa) diametrical interference value (mm)

δ E r 1 r 2 r 3

Young's modulus (MPa) inner radius of the shaft (mm)

outer/inner radius of the shaft/collar hub (mm) outer radius of the collar hub (mm)

axial force (N)

F a

connection length (mm)

l

friction factor (-)

μ

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2. Short description of the collar fracture The scheduled shutdown of the turbine was in September 1998. The turbine was restarted in October 1998. After about 40 days of normal operation, the turbine had to be shut down due to a sudden increase in axial displacement of the rotor from 0.4 mm to 0.58 mm and increased noise in the front bearing housing. After cooling the turbine, the front bearing housing was opened and a sever fracture of the thrust collar was observed with the naked eye. The control of the thrust collar was carried out with a dye penetrant test, which confirmed the fracture of the collar (see Fig. 1). It is clearly visible that the fracture occurred at the radius of the shaft-collar transition along the entire circumference and on both sides of the collar.

Fig. 1. Fracture appearance as obtained by penetrant testing.

After removing the rotor from the turbine casing, an ultrasonic inspection was performed to determine the size and position of the observed crack. The largest estimated depth of the crack in the rotor shaft was 18 mm. 3. Root cause analysis of the collar fracture The thrust collar was an integral part of the turbine rotor shaft, as can be seen in Fig. 2 (Technical documentation (1998)). It can be seen that the shaft-collar transition radii were only 0.5 mm. As is well known, the transition radius represents a stress raiser and the smaller the radius, the higher the stress at the root of the radius.

Fig. 2. Cross-sectional drawing of the original thrust collar.

Numerical stress analysis was performed in the area of these shaft-collar transition radii. The analysis was performed using the finite element method (FEM) implemented in the specialized program Ansys 2023 R1. Due to the axial symmetry of the geometry and loading, only a quarter of the shaft with the collar was modeled. The material properties of 21CrMoV5-11 were associated with the geometry of the model. The geometry of the model was discretized with a coarse mesh of 10-node higher order 3D finite elements type SOLID187. In order to obtain the most

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accurate stresses and strains the mesh is refined in the area of transition radii. After performing a sensitivity analysis, the final mesh consists of 2976361 elements and 4231073 nodes. The axial force, which is a maximum of 300 kN, was converted into uniform pressure on the active surface of the thrust collar, which defined the loading of the model. The distribution of von Mises stresses is shown in the Fig. 3. The equivalent stress in the root of the transition radius is about 272 MPa, which is less than the material's yield strength of 550 MPa. In other words, assuming uniform static loading of the thrust collar, fracture is not expected to occur.

Fig. 3. Distribution of von Mises stresses obtained using FEM analysis.

However, in the case of cyclic loading and due to the notch effect at the radius, a crack could initiate at the root of the notch and propagate until it reaches its critical size, leading to rapid brittle fracture. Cyclic loading of the thrust collar can occur due to the non-uniform pressure distribution on the active surface of the collar in combination with the rotation of the turbine rotor. Upon reviewing the manually recorded turbine operation data, it was observed that the absolute expansion displacement of the turbine casing, prior to the incident described, was approximately 1 mm less than the normal value. Axial thermal expansion of the turbine casing is accommodated by the front bearing housing, which is designed to slide along the surface of the base plate. A lower measured absolute expansion indicates that the casing did not elongate sufficiently due to difficulty in sliding the bearing housing. Specifically, increased friction between the sliding surfaces limited the axial elongation of the casing, causing the bearing housing to tilt slightly in the vertical plane and partially lift at the rear. As a result, the rotor thrust collar rested unevenly on the thrust bearing pads, leading to non-uniform collar loading. Since the rotor is rotating, this uneven loading is cyclical, which can result in high-cycle fatigue failure. However, fatigue analysis of the thrust collar could not be performed because the S-N curve for steel 21CrMoV5 11 was not available, and the exact pressure distribution across the collar surface was not known. 4. Rotor and turbine repair The damaged thrust collar was removed from the shaft through machining. The shaft was machined to a final diameter of 185 mm over a length of 152 mm, measured from the shaft front end. A new thrust collar was then fitted onto this shaft journal, creating a shrink fit connection as can be seen in Fig. 4 (taken from internal technical documentation Petrokemija d.d. Kutina). The connection is further secured with a nut. The shrink fit connection between the thrust collar and the shaft must provide strong resistance to the longitudinal movement of the connected parts. Pressure acts on the contact surfaces of the collar and shaft, generating the frictional force needed to transmit the axial force of the turbine rotor. The maximum axial force on the thrust collar, which results from steam expansion through the turbine, is 300 kN. The selected minimum and maximum values for the diametrical interference in the shrink fit connection are 0.165 mm and 0.25 mm, respectively. The actual interference achieved was 0.2 mm. The pressure on the contact surface of the thrust collar-shaft connection is calculated using the following expression:

Marko Katinić et al. / Procedia Structural Integrity 82 (2026) 220 – 226 M. Katinić, P. Konjatić/ Structural Integrity Procedia 00 (2026) 000–000

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(

)(

)

( ! ! ! " " " " " " " " " " ! ! " ! ! # " ! ! " ! # )

!

!

(1)

#

=

$

The pressure p d for the achieved overlap of the connection is 76.89 MPa. The axial force F a that can be carried by the shrink fit connection is calculated using the following expression:

! # " ! " # $ ! µ = "

(2)

Assuming a friction factor of µ = 0.15, the axial force supported by the achieved overlap is 771 kN. Due to the rotation of the turbine rotor, the surface contact pressure pd decreases, and the bearing capacity of the axial force also decreases. However, since the maximum axial force that can occur is 300 kN, the shrink fit connection is considered safe for operation.

Fig. 4. New shrink-fitted thrust collar.

The contact pressure p d in the shrink fit joint generates radial and hoop stresses. The achieved overlap of the shaft thrust collar joint is usually between the minimum and maximum values. As previously mentioned, the achieved interference value is 0.2 mm and the stresses are calculated for this value. Table 1 shows the calculated hoop, radial and equivalent stress values for the shaft and collar diameters.

Table 1. Calculated stress values.

Inner shaft dia.

Outer shaft dia.

Inner collar dia.

Outer collar dia.

Hoop stress, MPa Radial stress, MPa

-201.46

-124.57 -76.89 108.87

102.46 -76.89 155.84

25.57

0

0

Equivalent stress, MPa

201.46

25.57

Along with the rotor repair, the sliding of the front bearing housing on the base plate surface was enhanced. Grooves were milled into the sliding surface of the bearing housing, into which graphite grease is injected, improving its anti friction properties (see Fig. 5). After the repair, the rotor was balanced and installed in the turbine. The turbine was commissioned at the end of June 1999. It has been operating with this rotor for over 25 years, accumulating more than 100,000 working hours. During overhauls, the rotor is regularly checked both dimensionally and using non-destructive methods.

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Fig. 5. Grooves in the surface of the bearing housing.

5. Conclusions Some malfunctions in steam turbine operation are extremely rare, but unfortunately, they do occur under certain circumstances. The sudden fracture of the thrust collar described is one such example. This type of failure, although uncommon, can have serious consequences if not detected and addressed promptly. In this particular case, the failure was caused by high-cycle fatigue, which developed as a result of inadequate sliding of the front bearing housing during the thermal expansion of the turbine casing in the start-up phase. This issue arose from a mismatch between the thermal expansion of the turbine casing and the movement of the bearing housing, which prevented smooth operation during the initial heating phase. The described case is a reminder of the complexity of mechanical interrelationships within steam turbines and the importance of proper design and careful monitoring of turbine operation. This is also an example of a successful turbine repair as a result of detailed engineering analysis, an in-depth understanding of the turbine's mechanical behavior and the application of a targeted, effective technical solution. By addressing the root cause of the thrust collar failure and implementing the necessary corrective actions, the turbine was restored and returned to full operational condition. The success of the repair and the robustness of the applied solutions are evidenced by the fact that the turbine with the redesigned rotor thrust collar has been operating for more than 25 years with an accumulated service life of more than 100,000 hours. The reliable and safe operation of the turbine over this long period is an indication that even a serious failure can be resolved efficiently and qualitatively, thereby extending the service life and improving the reliability of the turbine. Furthermore, the importance of preventive maintenance for the reliable and safe operation of steam turbines is also highlighted in this case. It is crucial to carry out preventive activities both during operation and during planned turbine downtimes. Inspections and testing of the rotor during each planned overhaul, which follow a carefully and precisely defined plan, are crucial for detecting any defects that may have arisen during turbine operation. Regular monitoring of turbine operation and parameter analysis, as well as dimensional and non-destructive inspections, help in the early detection of defects, thus preventing costly and damaging failures in the future. Acknowledgements The authors express their gratitude to the company Petrokemija d.d. Kutina for providing technical documentation and useful data. References

Barella, S., Bellogini, M., Boniardi, M., Cincera, S., 2011. Failure analysis of a steam turbine rotor. Engineering Failure Analysis 18(6), 1511-1519.

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Trebuňa, P., Pástor, M., Trebuňa, F., Šimčák, F., 2017. The Analysis of Failure Causes of The Rotor Shaft of Steam Turbines, Metalurgija 56(1-2), 233-236. Nikravesh, M. Y., Sharafi, M. M., 2016. Failure of a steam turbine rotor due to circumferential crack growth influenced by temperature and steady torsion, Engineering Failure Analysis 66, 296-311. Vinod, P. S., Prashanth, A. S., 2020. Failure Analysis of Steam Turbine Rotor Due to Low Flow Conditions, International Research Journal of Engineering and Technology (IRJET) 7(8). Katinić, M., Kozak, D., Gelo, I., Damjanović, D., 2019. Corrosion fatigue failure of steam turbine moving blades: A case study, Engineering Failure Analysis 106, 104-136. Qiqi, H,, Song, X., Hongmei, H., Fengtao, Hu., Hong, C. G., Wei, Hu., 2023. Fatigue fracture failure analysis of 12Cr12Mo steam turbine blade, Engineering Failure Analysis 150, 107356. Segura, J. A., Castro, L., Rosales, I., Rodriguez, J. A., Urquiza, G., Rodriguez, J. M., 2017. Diagnostic and failure analysis in blades of a 300 MW steam turbine, Engineering Failure Analysis 82, 631-641. Mazur, Z., Garcia-Illescas, R., Aguirre-Romano, J., Perez-Rodriguez, Norberto., 2008. Steam turbine blade failure analysis, Engineering Failure Analysis 15(1–2), 129-141. Wei, Y., Li, Y., Lai, J., Zhao, Q., Yang, L., Lin, Q., Wang, X., Pan, Z., Lin, Z., 2020. Analysis on corrosion fatigue cracking mechanism of 17 4PH blade of low pressure rotor of steam turbine, Engineering Failure Analysis 118, 104925. Ramírez, J. A. R., Mirafuentes, C., M., C., Garibay, M. A. Z., Castrejón, J. C. G., Anaya, L. G. G., 2023. Corrosion Fatigue Analysis in Power Steam Turbine Blade, Metals 13, 544. Katinić, M., Travica, D., Konjatić, P., Bošnjaković, M., 2023. Finite Element Analysis of a Shrink Fitted Disc-Shaft Rotating System, Technical Gazette 30(6), 1176-11783.

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Procedia Structural Integrity 82 (2026) 30–36

© 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Abstract This study focuses on the numerical simulation of concrete fracture in steel concrete composites under blast loading caused by the deflagration of nitromethane. The simulation employs 3D dynamic nonlinear finite element analysis. The analysis method is based on a novel poro-mechanical approach to analyze blast induced fracture in concrete considering gaseous kinetics through cracks. The experimental findings obtained on two specimens presented in previous research were utilized as a reference of verification and validation of the simulation. The study investigates the crack network development, the influence of pressurized gas, stress level in the stud dowels and in the steel girder and verifies its findings experimentally. The study shows that the numerical simulation successfully reproduces the kinetics of the experiments. Reduced dowel spacing causes high pore pressure rise due to a strong confinement against crack propagation in concrete media. Stud dowel configuration and cartridge count affect fracture behaviors. Both the stud dowels and steel girder remained undeformed, supporting their reuse. © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Explosion; steel concrete composites; fracture; simulation 1. Introduction As many surface and underground structures approach the end of their service life, efficient demolition methods are increasingly necessary. While blasting with explosives enables rapid demolition, it remains largely experience- Abstract This study focuses on the numerical simulation of concrete fracture in steel concrete composites under blast loading caused by the deflagration of nitromethane. The simulation employs 3D dynamic nonlinear finite element analysis. The analysis method is based on a novel poro-mechanical approach to analyze blast induced fracture in concrete considering gaseous kinetics through cracks. The experimental findings obtained on two specimens presented in previous research were utilized as a reference of verification and validation of the simulation. The study investigates the crack network development, the influence of pressurized gas, stress level in the stud dowels and in the steel girder and verifies its findings experimentally. The study shows that the numerical simulation successfully reproduces the kinetics of the experiments. Reduced dowel spacing causes high pore pressure rise due to a strong confinement against crack propagation in concrete media. Stud dowel configuration and cartridge count affect fracture behaviors. Both the stud dowels and steel girder remained undeformed, supporting their reuse. © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Explosion; steel concrete composites; fracture; simulation 1. Introduction As many surface and underground structures approach the end of their service life, efficient demolition methods are increasingly necessary. While blasting with explosives enables rapid demolition, it remains largely experience- Abstract This study focuses on the numerical simulation of concrete fracture in steel concrete composites under blast loading caused by the deflagration of nitromethane. The simulation employs 3D dynamic nonlinear finite element analysis. The analysis method is based on a novel poro-mechanical approach to analyze blast induced fracture in concrete considering gaseous kinetics through cracks. The experimental findings obtained on two specimens presented in previous research were utilized as a reference of verification and validation of the simulation. The study investigates the crack network development, the influence of pressurized gas, stress level in the stud dowels and in the steel girder and verifies its findings experimentally. The study shows that the numerical simulation successfully reproduces the kinetics of the experiments. Reduced dowel spacing causes high pore pressure rise due to a strong confinement against crack propagation in concrete media. Stud dowel configuration and cartridge count affect fracture behaviors. Both the stud dowels and steel girder remained undeformed, supporting their reuse. © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers Keywords: Explosion; steel concrete composites; fracture; simulation 1. Introduction As many surface and underground structures approach the end of their service life, efficient demolition methods are increasingly necessary. While blasting with explosives enables rapid demolition, it remains largely experience- 8th International Conference on Structural Integrity and Durability (ICSID2025) A numerical analysis of blast-induced dynamic fracture in steel concrete composite structures Addisu Bonger a, *, Akira Hosoda b , Koichi Maekawa c , Stanislav Zaˇzirej b a Sumitomo Mitsui Construction Co.,LTD., Research and Development Institute, 518-1 Komagi, Nagareyama 270-0132, Japan b Yokohama National University, Institute of Urban Innovation, 79-5 Tokiwadai, Hodogaya 240-8501, Japan c Yokohama National University, Institute of Multidisiplinary Sciences, 79-5 Tokiwadai, Hodogaya 240-8501, Japan 8th International Conference on Structural Integrity and Durability (ICSID2025) A numerical analysis of blast-induced dynamic fracture in steel concrete composite structures Addisu Bonger a, *, Akira Hosoda b , Koichi Maekawa c , Stanislav Zaˇzirej b a Sumitomo Mitsui Construction Co.,LTD., Research and Development Institute, 518-1 Komagi, Nagareyama 270-0132, Japan b Yokohama National University, Institute of Urban Innovation, 79-5 Tokiwadai, Hodogaya 240-8501, Japan c Yokohama National University, Institute of Multidisiplinary Sciences, 79-5 Tokiwadai, Hodogaya 240-8501, Japan 8th International Conference on Structural Integrity and Durability (ICSID2025) A numerical analysis of blast-induced dynamic fracture in steel concrete composite structures Addisu Bonger a, *, Akira Hosoda b , Koichi Maekawa c , Stanislav Zaˇzirej b a Sumitomo Mitsui Construction Co.,LTD., Research and Development Institute, 518-1 Komagi, Nagareyama 270-0132, Japan b Yokohama National University, Institute of Urban Innovation, 79-5 Tokiwadai, Hodogaya 240-8501, Japan c Yokohama National University, Institute of Multidisiplinary Sciences, 79-5 Tokiwadai, Hodogaya 240-8501, Japan ˇ

* Corresponding author. Tel.: +81-50-3085-3915; fax: +81-4-7140-5020. E-mail address: b-addisu@smcon.co.jp * Corresponding author. Tel.: +81-50-3085-3915; fax: +81-4-7140-5020. E-mail address: b-addisu@smcon.co.jp * Corresponding author. Tel.: +81-50-3085-3915; fax: +81-4-7140-5020. E-mail address: b-addisu@smcon.co.jp

2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 2452-3216 © 2026 The Authors. Copy from the contract: Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers

2452-3216 © 2026 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID organizers 10.1016/j.prostr.2026.04.006

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based and poses risks particularly for tightly reinforced structures due to overcharging and uncontrolled energy release. These risks limit its use in urban areas (Uenishi et al., 2023). To address this, an effective and controllable technique for precise demolition of concrete, which is known as Electric Discharge Impulse Crushing System (EDICS), has been developed and is being used in practice (Sasaki et al., 2011). The EDICS system consists of a power generator, control panel, electric discharge generator and a wire connected cartridge. Inside the cartridge, there is a thin metal wire and a liquid explosive material nitromethane ( 3 2 ). The electric discharge generator can provide a pulse of 3,000 V. After an electric impulse is discharged into the cartridge within the hundreds of microseconds, the thin metal wire is vaporized, and the deflagration of nitromethane’s phase change is initiated (Tanaka et al., 2020). Then, high pressure is generated by rapid volumetric expansion and applied to demolishing targets. Since the pressure generated during the deflagration is lower than that of detonation, the destructive effect on a structure is rather mild. That makes the whole process more controllable compared to high explosives. This study focuses on the numerical simulation of concrete fracture in steel-concrete composites under blast loading caused by the deflagration of nitromethane. The simulation employs 3D dynamic nonlinear finite element analysis. The experimental findings obtained on two specimens presented in previous research (Wada et al., 2021) were utilized as a reference of verification and validation of the simulation. The study investigates the crack network development, the influence of pressurized gas, stress level in the stud dowels and in the steel girder and verifies its findings experimentally. 2. Scheme of numerical analysis and modeling The 3D nonlinear finite element analysis was performed to simulate the blast-induced fracture in concrete. This study employs a multiscale poro-mechanical modeling approach to investigate the response of concrete structures under blast loading. 2.1. Multiscale poro-mechanical modeling The analysis method in this study is based on a novel poro-mechanical approach to analyze blast induced fracture in concrete considering gaseous kinetics (Zazirej et al., 2024). In this method, the original framework by Biot (1941) and Biot (1963) extended to compressible fluid media with nonconstant density that varies according to pore pressure. The analysis method targets three specific aspects: (1) concrete as a nonlinear porous material with initial porosity; (2) multidirectional crack interaction across three dimensions; and (3) accounting for phase changes between liquid and gas. The poro-mechanical approach to analyze blast induced fracture in concrete considering gaseous kinetics is an extension of a multiscale integrated system (Maekawa et al., 2008) to consider the three issues as stated above. These complexities motivate the application of a multi-phase poro-mechanical approach, known for its success in nonimpactive scenarios. a b

Stemming Cartridge

Cartridge Stemming

Steel rebars

Steel rebars

Stud dowel

Stud dowel

D L = 100 mm

D L = 200 mm

Cartridge

Cartridge

Steel girder

Steel girder

Fig. 1. Dimensions of two referenced specimen [unit: mm] (a) specimen A; (b) specimen B (Wada et al., 2021)).

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In the analysis method, the effective stress of concrete is determined based on the assumed strain of the solid skeleton (Maekawa et al., 2003). An elasto-plastic and fracturing model is applied to solid concrete, considering both long and short-term time dependency of the constitutive law, as well as the effect of 3D confinement. Stresses are computed using direct path integration of the straining of finite elements, which inherently accounts for the impact of strain rate. The impact action is closely related to the inertia of mass and may lead to the creation of 3D confinement around the explosive area. Then, the models apply to integrate strain rate and 3D confinement for explosive fracture analysis. After cracking, the analysis method applies for a multi-directional, non-orthogonal post-cracking model. This model incorporates compression-tension along cracks, shear transfer through cracking planes, and tension transfer by the local bond mechanism of reinforcement and concrete to construct the space-averaged constitutive model of cracked concrete. Additionally, to address extreme damage induced by significant shear and compression, the graveling of concrete is considered in this modeling approach (Yamanoi and Maekawa, 2022). 2.2. Specimens’ information The experimental findings obtained on two specimens presented in previous research (Wada et al., 2021) were utilized as a reference of verification and validation of the simulation. Fig. 1 illustrates the plane, side and front views of the two steel-concrete composite structures. The structure is composed of a reinforced concrete slab on top of a steel girder and stud dowels having a diameter of 22 mm and a height of 130 mm. In the slab of dimensions 900 mm ´ 300 mm ´ 200 mm, blast holes that can hold the cartridges (energy sources) containing the self-reactive liquid are drilled and covered by stemming material. The bars indicated in orange are steel rebars, and the blue and red sections in the figure are the stud dowels and cartridges producing electric discharge impulses (EDICS). The distinct dissimilarity between the two types of specimens, specimen A (Fig. 1 (a)) and specimen B (Fig. 1(b)), is the horizontal spatial distance ( D L ) between each stud dowel in the direction of the axis of the specimen. The distance D L is 200 mm for the specimen A and 100 mm for the specimen B. Accordingly, the cartridges set in the specimen B are double in number. a b

Stud dowel

Concrete

Cartridge

ΔL = 100 m

ΔL = 200 mm

girder

Stemming

.

Fig. 2. Finite element models (a) specimen A; (b) specimen B.

Table 1. Material properties of nitromethane.

3 )

t start (μs)

t peak (μs)

p (GPa)

ρ (g/cm

Specific figures of nitromethane

0

55

1.1

1.14

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Table 2. Material properties.

Concrete

Stemming

Young's modulus

GPa MPa MPa

34.2

0.08

Compressive strength

31

1

Tensile strength

2.6

0.1

Kg/m 3

Density

2300 10 -7 0.02

1800

10 -1

Permeability

(˗) (˗) (˗)

Porosity

0.4 2.0

Tension softening factor ( c parameter)

1.0

2.3. Simulation model Finite element model was discretized using hexahedral finite elements (1.4 cm mesh size) as shown in Fig. 2. The properties of nitromethane were defined by the time of initiation of deflagration t start and t peak when pressure reaches its maximum p peak and by the density ρ as shown in Table 1. Material properties were assigned to each element representing concrete and stemming as lined up in Table 2. The tension softening model for plain concrete is taken into account by c parameter (Maekawa et al., 2003) that is determined from fracture energy of concrete and the reference length of the finite element. Steel rebars, stud dowels and steel girders were defined by Young's modulus E = 210 GPa and yield strength f yt = 400 MPa. The upper steel plate (flange) of the girder is perfectly bonded to the concrete slab. 3. Simulation results Simulation results are evaluated based on crack propagation, gas pressure effects, and stress in the stud dowels and The final crack patterns immediately after the application of EDIC in the experimentally and computationally obtained are compared in Fig. 3a and Fig. 3b. Conical cup-shaped fractures connecting the cartridges and the heads of the stud dowels are formed. The development of principal strain trajectory nearly coincides with those of cup shaped fractures in the experiment (indicated by pink lines in Fig. 3(a)). In fact, the concrete located above the stud dowel heads can be removed as a single unit at construction site. The remaining concrete sections can then be easily removed manually. Following the nitromethane explosion, high pore pressure rapidly builds around the cartridge (Fig. 3c), with gas inflation initially confined. Cracking soon initiates, enabling gas to migrate through the cracked fracture planes. Driven by the pressure gradient, gas moves outward, inducing 3D ring-tension stresses that may further promote cracking. As gas permeates both concrete pores and cracks, it reaches crack fronts, increasing local pressure and aiding propagation. Once surface cracks form, the gas escapes into the atmosphere, assumed to be at ambient pressure in simulations. Gas primarily travels through micro-pores and cracks, as it cannot penetrate solid elements such as steel rebars and stud dowels. Consequently, the build-up of gas pressure may lead to deformation of the reinforcing bars and displacement of the surrounding concrete, promoting concrete crushing and crack propagation, as illustrated in Fig. 3d. A deformation of 6 mm in z-direction was recorded in the top reinforcing bar located at the center of the specimen, as illustrated in the graph in Fig. 3d. To assess the feasibility of reusing the steel girder and stud dowels, strain was evaluated at locations with maximum deformation from the simulation. As shown in Fig. 3e, the strain remained below the yield threshold, indicating no yielding occurred. This was further confirmed by experimental results (Wada et al., 2021), supporting the potential for reuse. girder, and are validated experimentally. 3.1. Simulation results for specimen A