PSI - Issue 13

Khalid Eldwaib et al. / Procedia Structural Integrity 13 (2018) 444–449 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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It should be noted that in the cases where two cracks were propagated simultaneously (A, C 1 , C 2 , C 3 ) behaviour of the second crack (on the lower right cap) varied from case to case. For example, in case C 2 second crack met first (Figure 7) after 27 steps of propagation (cracks were propagated in steps of approximately 1mm in all five cases) and newly formed crack continued to grow in vertical spar wall, while in case A deformation of the spar prevented the growth of the second crack after 8 steps. All these findings prove that geometry of cross section significantly influences fatigue life of damaged structure; therefore, this matter clearly needs to be given more attention in the future.

Fig. 7 Two cracks met on the bottom cap (case C 2 )

5. Conclusions In aircraft design, cracks’ appearance on the wing parts is allowed (as defined in so-called fail-safe design philosophy ), but life until critical crack length must be evaluated with high confidence in order to prevent undesirable consequences. That’s not an easy job because in most cases cracks appear on parts with complex geometry subjected to variable loads during service and data on critical lengths are generally available only for simpler load/geometry configurations. This is reason why emphasis – in a research presented here – was on developing XFEM based computational method for successful crack growth life estimation. On the other hand, main goal was to obtain optimized size and shape (for given mass of the spar) that will significantly extend fatigue life. Five different shapes had been analysed (I-section, U-section and three variants of I-section with intermediate cap (flange)) and it was found that the optimum spar with the longest crack growth life and the lowest crack growth rate had I-section with intermediate cap shape (case C 3 ). Fatigue life of this spar was nearly 1,400,000 cycles, while in the next best case (C 2 ) life was significantly shorter (450,000 cycles). Typical I-section had the lowest fatigue life (250,000 cycles) and let’s not forget that this is spar shape used in more than 90% of the light airplanes nowadays. As the results of this research suggest, to increase fatigue life of light aircraft spar with intermediate cap should be used. It was proven that in the event of crack initiation at the bottom cap, where cracks usually appear, this cap will fail sooner or later, but the top cap, web and the intermediate cap will remain intact and spar could carry designed load much longer than any other configuration. References JSSG-2006, Joint Service Specification Guide, Aircraft Structures, Dept. of Defense, October 1988. Ajith, V. S., Paramasivam R., Vidhya, K. (2017). Study of optimal design of spar beam for the wing of an aircraft. International Journal of Engineering Development and Research, 5(3), 179-193. Jones, R., Pitt, S., & Peng, D. (2004). Structural optimisation for light weight durable structures. In Structural Integrity and Fracture International Conference (SIF'04) (pp. 171-178). Datta, D., and Deb, K. (2006). Design of optimum cross-sections for load-carrying members using multi-objective evolutionary algorithms. Int. J. of Systemics, Cybernetics and Informatics (IJSCI), 57-63. Girennavar M., et al. (2017). Design, Analysis and Testing of Wing Spar for Optimum Weight. International Journal of Research and Scientific Innovation (IJRSI), Volume IV, Issue VII, 104-112. Anđelić, N., and Milošević - Mitić, V. (2007). An approach to the optimization of thin -walled cantilever open section beams. Theoretical and applied mechanics, 34(4), 323-340. Eldwaib, K. A., Grbovic, A., Kastratovic, G., Radu, D., Sedmak, S. (2017). Fatigue life estimation of CCT specimen using XFEM. Structural Integrity and Life, 17(2), 151-156. Petrašinović, D., Rašuo, B., Petrašinović, N. (2012). Extended finite eleme nt method (XFEM) applied to aircraft duralumin spar fatigue life estimation. Technical Gazette, 19(3), 557-562. Schijve, J., Fatigue of Structures and Materials, 2nd edition, Springer (2008).