Variable-amplitude fatigue behavior and life assessment of preload-deficient bolts in T-stub connections

Bolts are critical load-transfer components in steel structural connections, and their fatigue performance directly affects the safety and durability of steel structures subjected to cyclic service loading. In practical engineering, bolt preload may be insufficiently introduced or progressively lost because of installation deviations, contact surface embedment, vibration or relaxation. However, fatigue data for bolts under non-ideal conditions remain limited, and their fatigue behavior under realistic variable-amplitude service loading is still insufficiently understood. In this study, 15 variable-amplitude fatigue tests were conducted on unpreloaded M20 bolts in T-stub connections under axial tension. The results show that, at a survival probability of 97.72%, the fatigue strength at 2 × 10 6 cycles under variable-amplitude loading is 86.5 MPa, which is approximately 0.9 times that under constant-amplitude loading. This indicates that variable-amplitude loading reduces the fatigue resistance of the bolts. Nevertheless, this value remains 1.5–1.8 times higher than the corresponding design values specified in current standards, suggesting that existing code-based fatigue assessments are generally conservative for this type of bolted connection. Finite element analysis and fractographic observations confirm that fatigue cracking is governed by severe stress concentration at the first engaged thread root. A local strain-based life assessment procedure was further validated against the test results, with all predictions falling within a 2-fold scatter band. This study establishes an engineering-oriented fatigue life assessment method that is grounded in experimental data and supported by local mechanical responses. The findings provide data support and methodological reference for the fatigue design and life evaluation of bolted connections in steel structures.