PSI - Issue 77

Tianyu Wang et al. / Procedia Structural Integrity 77 (2026) 512–520 Wang et al./ Structural Integrity Procedia 00 (2026) 000 – 000 The surface peak, representing the highest load-bearing capacity, corresponds to the [+75°/−75°/+45°/−45°] layup, achieving an ultimate torque 138.46 kN·m. The existence of multiple peaks explains why gradient-based optimisation methods often fail to find global optima — the algorithm becomes trapped in the nearest local maximum. This multi-modal nature of the design space underscores the value of comprehensive exploration over local search methods. The diagrams reveal extensive ‘ high-altitude plateaus ’ , regions where performance remains within 5% of optimal across angle variations of ±5° . These plateaus are invaluable for practical design, as they identify configurations robust to manufacturing tolerances. For instance, the region surrounding [70 – 80°/40 – 50°] maintains capacity above 120 kN·m, providing a 10° × 10° window of acceptable angles. Certain regions exhibit precipitous performance drops with small angle changes, particularly near [30°/30°] configurations. These ‘ cliff ’ regions should be avoided in practice as minor manufacturing deviations could result in significant capacity reductions. The optimal configuration [+75°/−75°/+45°/−45°] embodies the design principles identified through parametric studies. This arrangement strategically positions high-angle plies internally to resist pressure-induced hoop stresses whilst placing medium-angle plies externally to handle the torsional shear. Note that, the physical rationale extends beyond simple load matching. The 30° difference between inner and outer ply angles creates a gradual stiffness transition that minimises interlaminar stress concentrations. Furthermore, the balanced and symmetric nature of the layup eliminates thermal warping during manufacture, ensuring dimensional stability. The configuration also provides redundancy: if the outer plies experience damage from impact or erosion, the inner plies can maintain pressure containment, albeit at reduced capacity. 6. Conclusions This study has presented the analytical framework for the design and optimisation of multi-layered composite pipes under combined loading conditions, addressing gaps between academic research and industrial practice. The two-level analytical model, rigorously validated against high-fidelity FEM simulations, demonstrates exceptional accuracy with maximum deviations below 3% whilst achieving computational efficiency improvements of two orders of magnitude. The parametric study revealed profound insights into the complex interplay between design parameters and structural performance under combined loads. The key finding that no universally optimal winding angle exists challenges decades of industry practice using ‘ standard ’ angles and underscores the necessity of load-specific design approaches. The stacking sequence analysis proved that strategic ply positioning can enhance structural performance by 20% without altering the manufacturing costs, a finding with immediate economic implications for industry. The practical implications extend substantially beyond immediate oil and gas applications. The analytical framework provides engineers with a rapid, accurate tool for preliminary design and optimisation, potentially reducing development cycles from months to weeks. The identification of robust design plateaus addresses a long-standing challenge in composite manufacturing: balancing optimal performance with production reliability. References [1] Reddy, J.N., 2003. Mechanics of laminated composite plates and shells: theory and analysis. CRC Press. [2] Wang, T., Menshykov, O. and Menshykova, M., 2024. Novel computational model for the failure analysis of composite pipes under bending. Composites Science and Technology 256, 110757. DOI: 10.1016/j.compscitech.2024.110757 [3] Zheng, Z., Lv, Y., Fan, W., Wei, Z., Zhang, S., Wei, J., Menshykova, M., Menshykov, O. and Wang, T., 2025. Mechanical behavior and design optimization of composite and hybrid metal-composite drill pipes under combined loading conditions. Engineering Structures 334, 120288. DOI: 10.1016/j.engstruct.2025.120288 [4] Xia, M., Takayanagi, H. and Kemmochi, K., 2001. Analysis of multi-layered filament-wound composite pipes under internal pressure. Composite Structures 53(4), 483 – 491. DOI: 10.1016/S0263-8223(01)00061-7 [5] Shi, C., Xia, H., Wang, J., Bao, X., Li, H. and Fu, G., 2022. Partially-plastic theoretical model of thermoplastic composite pipes and comparison of composite failure criteria. Composite Structures 280, 114834. DOI: 10.1016/j.compstruct.2021.114834 [6] Xia, M., Kemmochi, K. and Takayanagi, H., 2001. Analysis of filament-wound fiber-reinforced sandwich pipe under combined internal pressure and thermomechanical loading. Composite Structures 51(3), 273 – 283. https://doi.org/10.1016/S0263-8223(00)00137-9 [7] Xia, M., Takayanagi, H. and Kemmochi, K., 2002. Bending behavior of filament-wound fiber-reinforced sandwich pipes. Composite Structures 56(2), 201 – 210. DOI: 10.1016/S0263-8223(01)00181-7 [8] Hastie, J.C., Kashtalyan, M., Guz, I.A., 2019. Failure analysis of thermoplastic composite pipe (TCP) under combined pressure, tension and 519 8