PSI - Issue 5
Miloslav Kepka et al. / Procedia Structural Integrity 5 (2017) 1409–1416 Miloslav Kepka et al. / Structural Integrity Procedia 00 (2017) 000 – 000
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5. Conclusions
Fatigue failures often occur in highly-stressed structural details of machines during service. In the present case, they were macroscopic fatigue cracks in a welded joint in a transport vehicle. Usually, an immediate response is imperative. Theoretical guidelines for dealing with such situations are often known but sometimes the necessary input data is not. Macrofractographic and sometimes metallographic examinations are usually available to identify the cause of the failure, and sometimes rapid testing or calculation of in-service loads on the structure can be conducted. By contrast, laboratory determination of fatigue characteristics of a particular structural detail tends to be time-consuming and costly (or downright impossible due to technical reasons), which is why qualified estimates have to be used. This paper describes a real-life case of this kind. This case study offers several universally-applicable conclusions. a) If an assessment of a critical structural detail is carried out, albeit approximately, and the risk of fatigue failure is considered, future in-service defects can be prevented. b) Preliminary designs and fatigue (fatigue life) assessments of critical structural details can often be derived from valid theories and proven procedures and standards. c) Fatigue life calculations (assessments) should be interpreted considering a reasonable (specified, prescribed) reliability level, particularly in those cases where some input data are estimated values. d) Considerable improvement in the accuracy of calculations (assessments) of service fatigue life of structural details can only be achieved by their intensive fatigue testing and measurement of in-service loads. The problem whose ex-post evaluation is presented in this case study involved in-service fatigue cracks which were eventually eliminated by a combination of measures related to both loading and fatigue strength of the critical structural detail in question. A minor adjustment to the kinematics and elasticity of the attachment of radius rods between the body and undercarriage greatly reduced the internal forces and the resulting stress in the critical structural detail (see the recommendations that follow from section 4). Furthermore, better workmanship of the welded joints, such as grinding of the weld toe, certainly contributed to higher endurance of the joint as well. In this case, the code BS 7608:1993 specifies a shift of the design fatigue curves by 30% to higher permitted stress values. Naturally, this raises the predicted fatigue life as well. According to the standard, the increase is 2.2-fold. Immediate steps taken on the basis of calculations and the findings interpreted in this paper enabled to continue trouble- free operation of Škoda trolleybuses in San Francisco.
Acknowledgements
The present contribution has been prepared under project LO1502 Development of the Regional Technological Institute under the auspices of the National Sustainability Programme I of the Ministry of Education of the Czech Republic aimed to support research, experimental development and innovation.
References
Kepka, M., Rehor, P., 2015. Methodology of Experimental Research into Operating Strength and Fatigue Life of Bus and Trolleybus Bodywork. International Journal of Vehicle Design, 1992; 13 (3): 242-250. Kepka, M., Spirk, S., 2015. Tests and computer simulations of electric buses. Proceedings M2D2015 - 6th International Conference on Mechanics and Materials in Design, Porta Delgada/Azores/Portugal, 26-30 July 2015. Kepka, M., Kepka Jr., M., Chvojan, J., Vaclavik, J., 2016. Structure service life assessment under combined loading using probability approach. Frattura ed Integrità Strutturale, 38 (2016) 82-91 BS 7608:1993. Fatigue design and assessment of steel structures. Hobbacher, A.F., 2016. Recommendations for Fatigue design of welded joints and components. Second Edition IIW225915 (ex XIII246013/ XV144013), Publisher: Springer ISBN: 978-3-319-23756 5 Margetin, M., Durka, R., Chmelko, V., 2016. Multiaxial fatigue criterion based on parameters from torsion and axial S-N curve. Frattura ed Integrita Strutturale. 37 (2016) 146-152
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