Fatigue life evaluation of a bus charging door bracket

Abstract

This study presents a comprehensive evaluation of the fatigue performance of two different bracket designs used in the charging door mechanism of an M3-class electric bus-one made from Aluminum 6061-T6 and the other from ST52 structural steel. Both brackets were subjected to constant-amplitude cyclic loads derived from real service data. Fatigue life predictions were conducted using finite element analysis (FEA) and two stress-based approaches: the Soderberg criterion and the Smith-Watson-Topper (SWT) method. FEA of the Aluminum 6061-T6 bracket revealed von Mises stress levels ranging between 170–204 MPa, which approach the material’s yield strength of 208 MPa. The Soderberg result (1.717>1) did not meet the safe design criterion, which was further validated by the occurrence of fracture during field application. The fatigue life predicted by the SWT method was 9.81 × 10^9 cycles. The alternative bracket design made from ST52 steel yielded more favorable results with lower stress levels (100–115 MPa). For this design, the Soderberg value was found to be 0.485 (< 1), indicating a structurally safe configuration. Additionally, a fatigue life of 3.87×10^11 cycles was obtained using the SWT method, and approximately 10^6 cycles according to FEA-consistent with the infinite life threshold reported in the literature. The findings highlight the critical influence of material selection and structural geometry on fatigue performance. The ST52 steel bracket demonstrated superior performance in terms of both safety and durability, providing a methodological basis for fatigue-resistant design in electric bus components.

Keywords

• Fatigue Life
• Finite Element Analysis (FEA)
• SmithWatson-Topper Method
• Soderberg Approach

Bir otobüs şarj kapaği braketinin yorulma ömrü değerlendirmesi

Öz

Bu çalışma, M3 sınıfı bir elektrikli otobüsün şarj kapağı mekanizmasında kullanılan iki farklı braket tasarımının biri Alüminyum 6061-T6, diğeri ST52 yapısal çeliğinden üretilmiş yorulma dayanımı açısından kapsamlı bir değerlendirmesini sunmaktadır. Her iki braket, gerçek servis verilerinden elde edilen sabit genlikli çevrimsel yüklere maruz bırakılmıştır. Yorulma ömrü tahminleri, sonlu elemanlar analizi (FEA) ve iki farklı yöntem olan gerilme temelli Soderberg kriteri ile gerilme temelli Smith-Watson-Topper (SWT) yöntemi kullanılarak gerçekleştirilmiştir. Alüminyum 6061-T6 braketi için yapılan FEA, 170–204 MPa aralığında von Mises gerilmeleri ortaya koymuş; bu değerler malzemenin 208 MPa’lık akma sınırına oldukça yakındır. Soderberg sonucu (1.717>1) güvenli tasarım kriterini karşılamamış ve bu durum saha uygulamasında meydana gelen kırılmayla da doğrulanmıştır. SWT yöntemiyle elde edilen yorulma ömrü ise 9.81 × 10^9 çevrim olarak hesaplanmıştır. ST52 çeliğinden imal edilen alternatif braket tasarımı ise daha düşük gerilme seviyeleri (100–115 MPa) ile daha avantajlı sonuçlar vermiştir. Bu tasarım için Soderberg değeri 0.485<1 olarak bulunmuş ve güvenli bir yapı olduğunu göstermiştir. Ayrıca SWT yöntemiyle 3.87×10^11 çevrimlik yorulma ömrü elde edilmiş, FEA analizine göre ise yaklaşık 10^6 çevrim sonucuna ulaşılmış ve literatürdeki sonsuz ömür eşiğiyle uyum sağlanmıştır. Elde edilen sonuçlar, malzeme seçimi ve yapısal geometrinin yorulma performansı üzerindeki kritik etkisini ortaya koymaktadır. ST52 çeliğinden üretilen braket hem güvenlik hem de dayanıklılık açısından üstün performans sergilemiş ve elektrikli otobüs bileşenlerinde yorulma dayanımına yönelik tasarım sürecine katkı sağlayacak metodolojik bir temel sunmuştur.

Anahtar Kelimeler

• Yorulma Ömrü
• Sonlu Elemanlar Analizi (FEA)
• Smith-Watson-Topper Yöntemi
• Soderberg Yaklaşımı

References

1. [1] Eryılmaz E, Pekbey Y, Küçük Ö. "Hızlandırılmış yorulma testleri ile motor braketlerinin analiz metodolojisinin geliştirilmesi". 8. Otomotiv Teknolojileri Kongresi, Bursa, Türkiye, 1-7 Mayıs 2016.
2. [2] Dong Z, Wang W, Dai S, Zheng J, Feng Y. "Study on structure optimization and vibration fatigue damage of wire bracket for rail vehicles". Engineering Failure Analysis, 156, 107732, 2024.
3. [3] Liu K, Wu X, Chi M, Wen Z. "Research on vibration fatigue of sensor bracket in a metro bogie based on the pseudo excitation method". Engineering Failure Analysis, 145, 107046, 2023.
4. [4] Barros L de O, Hansen FA, Neves RS, Ferreira GV, Morales LLD, Malcher L. "Fatigue life estimate of metallic chain links of mooring systems assuming out of plane bending: From constant amplitude to random loading". Ocean Engineering, 288, 116139, 2023.
5. [5] Chin CH, Rahim AAA, Abdullah S, Singh SSK, Md Nor N. "Acceptability of the effective strain damage model for fatigue life assessment considering the load sequence effect for automotive coil spring". Engineering Failure Analysis, 126, 105462, 2021.
6. [6] Susmel L. "Notches, nominal stresses, fatigue strength reduction factors and constant/variable amplitude multiaxial fatigue loading". International Journal of Fatigue, 162, 106941, 2022.
7. [7] Irving S, Ferguson-Smith F, Hu XZ, Liu Y. "Comparative fatigue assessment of soft toe and nested bracket welded aluminum structures". Engineering Failure Analysis, 12, 679-690, 2005.
8. [8] Wei X, Ren Z, Yang G. "Investigation into fatigue failure of cable brackets and vibration suppression methods". Engineering Failure Analysis, 165, 108782, 2024.

9. [9] Thillikkani S, Nataraj M, Luther King MF. "Failure analysis of shackle bracket in Airbus suspension under dynamic loading conditions". Engineering Failure Analysis, 120, 105087, 2021.
10. [10] Brusa E, Sesana R, Ossola E. "Numerical modeling and testing of mechanical behavior of AM titanium alloy bracket for aerospace applications". Procedia Structural Integrity, 5, 753-760, 2017.
11. [11] Liu X, Ma M. "Cumulative fatigue damage theories for metals: Review and prospects". Journal of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, 1(1), 1-12, 2023.
12. [12] Yang X, Liu X, Wang J. "Random fatigue life analysis of carbon fiber-reinforced plastic for automotive drive shaft with environmental temperature data". Journal of Mechanical Engineering Science, 45(2), 123-135, 2023.
13. [13] Hao R, Wen Z, Xin H, Lin W. "Fatigue life prediction of notched details using SWT model and LEFM-based approach". Journal of Structural Engineering, 67(4), 455-468, 2023.
14. [14] U.S. Department of Defense. “MIL-HDBK-5H: Metallic materials and elements for aerospace vehicle structures”. Washington D.C., USA, Military Handbook, 5H, 1998.
15. [15] Usta T, Çiçek A, Uçak N, Çelik V, Ağca S. “Performance evaluation of S700MC and ST52 steels in terms of mechanical properties and fatigue life”. Metal 2016 Conference, Brno, Czech Republic, 25–27 May 2016.
16. [16] Braun M, Chen T, Shen J, Dancette S, Requena G, Menou E, Vayre B, Doumalin P, Boller E. "Fatigue crack initiation and propagation in plain and notched PBF-LB/M, WAAM, and wrought 316L stainless steel specimens". Materials & Design, 244, 113122, 2024.
17. [17] Zhao Z, Hu X, Guo Z. "Effect of notches on fatigue crack initiation and early propagation behaviors of a Ni-based superalloy at elevated temperatures". Metals, 13(4), 384, 2023.
18. [18] Childs P R N. "Clarification on classical fatigue design for biaxial stress fields". Journal of Applied Mechanics and Materials, 8(6), 125, 2024.
19. [19] Henriques B, Carvalho M, Tavares SMO, de Castro PMST. "A comparison of safety factor values for Soderberg and DIN 743 fatigue analyses". U. Porto Journal of Engineering, 7(2), 11–21, 2021.
20. [20] Zhang J, Li Y, Wang Y, Li H. "Fretting fatigue experiment and finite element analysis for dovetail structures". Applied Sciences, 11(21), 9913, 2021.
21. [21] Fatchurrohman N, Chia S T. "Performance of hybrid nano-micro reinforced Mg metal matrix composites brake calliper: simulation approach". IOP Conference Series: Materials Science and Engineering, 257, 012060, 2017.
22. [22] Chitransh C S, Saxena S. "Mesh convergence study for the assessment of fatigue life of the Al-MMC HPDC brake drum component". International Journal of All Research Education and Scientific Methods, 11(8), 2010–2013, 2023.
23. [23] Zhou M, Yin H, Zhang T, Zhang H, Liu G. “The fatigue life analysis of the battery bracket”. International Conference on Advances in Energy and Environmental Science, Paris, France, 30–31 July 2015.
24. [24] Johansson M. "Fatigue analysis of engine brackets subjected to road induced loads". DiVA Portal, 61(1), 1–56, 2016.
25. [25] COMSOL. "Bracket Fatigue evaluation". COMSOL Application Library, 18(3), 1–10, 2014.
26. [26] Ince A, Glinka G. "A modification of Morrow and Smith-Watson-Topper mean stress correction models". Fatigue & Fracture of Engineering Materials & Structures, 34(10), 805-814, 2011.
27. [27] Łagoda T, Vantadori S, Głowacka K, Kurek M, Kluger K. “Using the Smith-Watson-Topper parameter and its modifications to calculate the fatigue life of metals: the state-of-the-art”. Materials, 15(10), 3481, 2022.
28. [28] Kamala M, Rahman MM. "Advances in fatigue life modeling: A review". Renewable and Sustainable Energy Reviews, 82, 940-949, 2018.
29. [29] Karakaş Ö, Tüzün N. "State of the art review of the application of strain energy density in design against fatigue of welded joints". Pamukkale University Journal of Engineering Sciences, 25(4), 462-467, 2019.
30. [30] Browell R, Hancq A. “Calculating and Displaying Fatigue Results Using the ANSYS Fatigue Module”. ANSYS Inc., Canonsburg, PA, USA, Technical Report, 2006.
31. [31] Kınacı BF, Botsalı H, Ozarpa C, Esen İ, Ahlatçı H. "Investigation of fatigue life of draw hook equipment used in freight wagon: Miscellaneous result". Engineering Failure Analysis, 155, 107757, 2024.
32. [32] Silveira de Santiago R, Possamai TS, Donati DCX, Martins GS de M, Oba R, Tascheck BL, Paiva KV de, Goes Oliveira JL. "Structural behavior and mechanical fatigue of plate and shell heat exchangers through finite element analysis". International Journal of Pressure Vessels and Piping, 210, 105252, 2024.
33. [33] Zhang Z, Huang C, Xu Z, Yang J, Long S, Tan C, Wan M, Liu D, Ji S, Zeng W. “Influence of notch root radius on high cycle fatigue properties and fatigue crack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure”. Journal of Materials Research and Technology, 2023, 24, 6293–6311.
34. [34] Shigley JE., Mischke CR, Budynas RG. Mechanical Engineering Design. 7th ed. New York, USA, McGraw-Hill, 2004.
35. [35] SAE. “Fatigue Design Handbook (SAE J1099)”. Society of Automotive Engineers, Warrendale, PA, USA, 1996.Suresh S. Fatigue of Materials. 2nd ed. Cambridge, Cambridge University Press, 1998.
36. [36] Şirin ŞY. "Sıcak daldırma galvanizleme işleminin ıslah edilmiş ve edilmemiş AISI 4340 çeliğinin yorulma dayanımına etkisi". Pamukkale University Journal of Engineering Sciences, 24(4), 626-634, 2018.