A Multi-Criteria Structural–Durability Assessment Methodology for Steel, Aluminium, and Hybrid Semi-Trailer Chassis under Service-Oriented Loading Envelopes

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This study presents a structural–durability assessment methodology for S355JR steel, AA6082-T6 aluminium-intensive and steel–aluminium hybrid semi-trailer chassis architectures under a common loading framework. Finite-element structural response is combined with literature-derived constituent stress–life (S–N) relationships, constituent-level Goodman mean-stress treatment, cumulative-damage concepts, welded-detail fatigue admissibility and interface stress-concentration sensitivity. Constituent S–N relationships are used only as material-level comparative references, whereas welded details and mechanically fastened steel–aluminium transitions are treated as independent local durability constraints. Accordingly, the hybrid architecture is not assigned an artificial averaged S–N curve; its fatigue response is represented through load-path-dependent constituent behaviour subject to welded-detail and interface limitations. Relative to the S355JR reference chassis, the aluminium-intensive and hybrid architectures reduce mass by 39.57% and 12.25%, respectively, while retaining 85.4% and 97.6% of the reference global stiffness. Over the common 10 4 –10 6 -cycle interval, the AA6082-T6 constituent fatigue-strength retention relative to S355JR ranges from 41.9% to 58.0%, with a mean retention factor of 0.497. The corresponding mean constituent-level retention domain for the hybrid architecture is therefore 0.497–1.000. The resulting stiffness–mass–fatigue Design Efficiency Index is 1.000 for steel, 0.703 for aluminium and 0.553–1.112 for the hybrid architecture. Break-even analysis shows that the hybrid configuration exceeds the aluminium-intensive architecture when its governing constituent fatigue-retention factor exceeds approximately 0.633 and exceeds the steel reference in the proposed screening index above approximately 0.900. These thresholds are necessary but not sufficient durability conditions because welded-detail and interface performance may remain governing. The proposed framework therefore separates constituent fatigue potential from local structural-detail durability and provides a physically consistent basis for load-path-oriented material allocation in lightweight heavy-vehicle chassis.