2 2.3. Multi-Objective Evolutionary Algorithms 45 ANDgate (i) ORgate (ii) (a) (b) (c) Top/Intermediate Event Event symbols Gate symbols VoTgate (iii) k/N Basic Event (i) (ii) Example: Fault Tree for Container Seal Design 2/3 Sealing function fails Seal fails independently Metal-to-metal seal fails Common cause seal failure Basic cause seal failure Fused plug fails Compression seal failure Compression seal 1 fails Compression seal 2 fails Compression seal 3 fails Contamination tape fails TE G2 BE1 BE2 BE3 BE5 BE4 BE6 G4 BE7 G3 AND AND OR p1 p2 p3 p4 p5 p6 p7 Figure 2.2: Elements in an FT: (a) event symbols, (b) gate symbols, and (c) example of an FT, adapted from NASA, 2002. Appendix B.2 provides a detailed explanation of these concepts and demonstrates the application of NSGA-II in the automatic inference of FTs. 2.3.2 Crowding-Distance If the last non-dominated front obtained through NSGA-II does not completely fill the available slots for the new population, the Crowding-Distance is utilised to determine which individuals from the last front should advance to the next generation. This mechanism promotes diversity (Martí, Segredo, (nchez-Pi, et al., 2017). Appendix B.2.2 provides details on the application of Crowding-Distance in the automatic inference of FTs.
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