Low-cycle fatigue life analysis and prediction of M24 high-strength bolts

To investigate the low-cycle fatigue damage characteristics and life patterns of high-strength bolts used in assembled steel structures, constant-amplitude axial tension low-cycle fatigue tests were conducted on fifteen M24 Grade 10.9 high-strength bolts using a self-designed loading apparatus. The fatigue failure characteristics of typical bolt specimens under different displacement amplitudes were analyzed, and combined with life prediction models, the applicability of the power-law function and the Coffin-Manson formula for predicting the low-cycle fatigue life of high-strength bolts was evaluated. The results show that increasing displacement amplitude intensifies stress concentration in the bolts, resulting in rougher fracture surfaces, a reduced fatigue crack propagation region, and an enlarged instantaneous fracture area. Fatigue failure mainly occurs at the first engaged thread between the bolt and nut. No significant monotonic relationship is observed between low-cycle fatigue life and cyclic displacement amplitude, as similar fatigue lives may occur under different displacement levels. When the displacement amplitude approaches the ultimate load-bearing capacity, the failure mode gradually resembles static tensile failure. Furthermore, the power-law function formula provides more accurate fatigue life predictions than the Coffin-Manson formula, especially when δ/δ_y is less than 2.45.