PSI - Issue 60

V. Venkatesh et al. / Procedia Structural Integrity 60 (2024) 372–381 V Venkatesh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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not the primary cause of failure in the present case, it appears possible that bearing loads on limited contact areas on the collar of the bearing housing increased the effective bending stress at these regions and thereby, aggravated the situation further in facilitating premature fatigue cracking. 5. Conclusions (a) The fatigue fracture of the non-rotating control rod was responsible for the loss of control of the helicopter and the subsequent accident. (b) The fatigue crack in the control rod originated at the transition fillet surface near the rod's eye-end and gradually propagated through approximately 85% of the rod's cross-section before ultimately experiencing a final overload failure. (c) Laboratory investigations identified two primary factors contributing to the premature initiation of the fatigue crack: (i) deep machining marks acting as stress raisers, and (ii) issues related to the fitment of bearings. 6. Recommendations Based on the findings from the laboratory investigation, the following recommendations are suggested. (i) Conduct a comprehensive review of stress analysis, considering the high-stressed fillet regions at the eye ends of the control rod. Additionally, investigate the effect of stress concentration on fatigue crack initiation. Based on the outcome of these studies, reassess the machining method to mitigate stress raisers and improve fatigue resistance. (ii) Address the issues related to the fitment of spherical bearings in the control rod assembly to ensure proper functioning and reduce stress-induced failures. (iii) As a short term measure, it is recommended to examine the surface condition of similar control rods in other flying helicopters for presence of crack(s). As a long time, preventive measure, it is recommended to subject the control rods to Non-Destructive Examination (NDE) every 200 hours of operation till the control rods are replaced with new ones with acceptable surface finish. Acknowledgements The work reported in this paper was financially supported by National Aerospace Laboratories, Council of Scientific and Industrial Research (CSIR), Bangalore, India vide Project No. M-1-298. References Bill, R.C., 1983. Fretting Wear and Fretting Fatigue — How Are They Related? Journal of Lubrication Technology 105, 230 – 238. Bhaumik, S.K., Sujata, M., Venkataswamy, M.A., 2008. Fatigue failure of aircraft components. Engineering Failure Analysis 15, 675 – 694. Chao, J., 2019. Fretting-fatigue induced failure of a connecting rod. Engineering Failure Analysis 96, 186 – 201. Dahlman, P., Fredrik, G., Michael, J., 2004. The influence of rake angle, cutting feed and cutting depth on residual stresses in hard turning. Journal of Materials Processing Technology. Findlay, S.J., Harrison, N.D., 2002. Why aircraft fail. Materials Today 5, 18 – 25. Gao, T., Sun, Z., Xue, H., Bayraktar, E., Qin, Z., Li, B., Zhang, H., 2020. Effect of Turning on the Surface Integrity and Fatigue Life of a TC11 Alloy in Very High Cycle Fatigue Regime. Metals 10, 1507. Javidi, A., Rieger, U., Eichlseder, W., 2008. The effect of machining on the surface integrity and fatigue life. International Journal of Fatigue 30, 2050 – 2055. Liu, G., Huang, C., Zhao, B., Wang, W., Sun, S., 2021. Effect of Machined Surface Integrity on Fatigue Performance of Metal Workpiece: A Review. Chin. J. Mech. Eng. 34, 118. Novovic, D., Dewes, R.C., Aspinwall, D.K., Voice, W., Bowen, P., 2004. The effect of machined topography and integrity on fatigue life. International Journal of Machine Tools and Manufacture 44, 125 – 134. Pujatti, M., Suhadolc, M., Piculin, D., 2014. Fretting-initiated Fatigue in Large Bore Engines Connecting Rods. Procedia Engineering 74, 356 – 359. Sasahara, H., 2005. The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel. International Journal of Machine Tools and Manufacture 45, 131 – 136. Arola, D., Williams, C., 2002. Estimating the fatigue stress concentration factor of machined surfaces. International Journal of Fatigue 24, 923 – 930.