Fatigue Life Prediction of Composite–Steel Single-Lap Hybrid Joint with Experimental Calibration Method

Abstract The study explored a fatigue life prediction framework incorporating an experimentally derived calibration approach for composite–steel hybrid single-lap joints. Since the overlap length of hybrid joints governs the load transfer path, stress distribution, and adhesive-bolt load sharing, it is known as one of the most influential parameters determining their mechanical and fatigue performance. Accordingly, quasistatic and tension fatigue tests were performed for joints with different overlap lengths and their corresponding load levels. Inherent scatter in fatigue life data was treated using a two-parameter Weibull distribution, and probabilistic force-life (P-F-N) curves with survival probabilities from 99% to 1% were constructed via maximum likelihood method and Monte Carlo simulation. In general, finite element analysis (FEA) predicts immediate local failure at the overlap edge within a single cycle due to high stress concentration, whereas experiments exhibit failure after gradual damage growth over many cycles. To resolve this discrepancy, an experimental calibration method called the critical fatigue failure length (CFFL) was employed to quantify the experimentally observed damage region where the local FEA stresses matched the experimentally derived fatigue strength. The calculated CFFL for hybrid joints was shown to increase nonlinearly with overlap length and its corresponding load level. Based on this relationship, a second-order polynomial correlation was established for each load level to generalize its prediction. Validation through supplementary fatigue test at a different overlap length demonstrated that the CFFL-calibrated FEA results were in good agreement with the experimental fatigue lives, falling within the 95% to 5% survival probability range of the P-F-N curves. The proposed probabilistic approach effectively bridges the gap between numerical and experimental fatigue behavior, offering a practical and reliable basis for the durability assessment and design optimization of lightweight hybrid joints used in advanced structural applications.