Investigating Fatigue Mechanisms and Crack Growth in 20mntib Steel High-Strength Bolts: An Experimental and Simulation Study

Abstract

In this paper, axial-tensile, constant-amplitude fatigue experiments are performed on M24 high-strength bolts with grade 8.8 fabricated from 20MnTiB steel with a stress ratio (R) of 0.3, and their crack development is simulated. The stress range-fatigue life (S–N) curve is derived by using the experimental results. The fatigue mechanism is then investigated through strain and fractographic analyses. Moreover, the extended finite element method (XFEM) is applied for assessing the fatigue crack propagation behavior of the high-strength bolts. The findings reveal that the 20MnTiB steel bolt exhibits a threshold fatigue strength of 140.77 MPa at two million loading cycles, which is 1.68 times greater than the corresponding value for 35K steel bolts at the same stress ratio. The bolt's stable deformation stage constitutes 87% of its total fatigue life. The XFEM is capable of accurately predicting the fatigue crack propagation trajectory and the lifespan of the high-strength bolts. Our analysis indicates that the crack initially propagates predominantly along the bolt's circumferential direction, accounting for 85% of the overall crack propagation life, before transitioning to unstable growth and experiencing an exponential increase in length.