Research On Mechanism Of Axial-tensile Constant-amplitude Fatigue Fracture Of M24 High-strength Bolts With Under-pretension State

With the proposal of the goals of peak carbon emissions and carbon neutrality,the prefabricated steel structure buildings,as one of the significant structural types of green buildings,have attracted considerable attention and been promoted significantly.It was widely used in industrial buildings,civil buildings,wind turbines,power transformer substations,and other structures.The end-plate high-strength bolted connection is the primary joint type of the prefabricated steel structure,hence its reliability is directly related to the monolithic mechanical performance of the structure.The high-strength bolt is likely to become the weak part of the structure under alternating cyclic loadings such as wind load and mechanical vibration load,which may cause fatigue failure.So far,there has been limited research on the constantamplitude fatigue fracture mechanism and crack propagation process of high-strength bolts.Additionally,there still exist deficiencies in fatigue estimation methods for end-plate highstrength bolts in relevant standards.Therefore,it is necessary to further investigate its fatigue performance and reveal the fatigue fracture mechanism.The research was jointly supported by National Natural Science Foundation of China(Grant No.12172244),Fund Program for the Scientific and Technological Activities of Returned Overseas Scholars of Shanxi Province(Grant No.DC1900000602),and Basic Research Program of Shanxi Province(Grant No.202103021223105).This paper took M24high-strength bolts with the performance of grade 8.8 as the research object,and the fatigue performance and fracture mechanism were investigated through the constant-amplitude fatigue test under stress ratio R = 0.5 on the bolts with the owe-pre-tightening state.Furthermore,the fatigue crack propagation mechanism was simulated and analyzed through numerical simulation combined with test results.The constant-amplitude fatigue characteristics of highstrength bolts were discussed based on the previous research.The primary research contents and conclusions are as follows.(1)By summarizing and comparing the fatigue calculation methods of bolt details in relevant steel structure design standards and 238 sets of fatigue test results in the research literature,it could be concluded that bolt diameter and material significantly influence its fatigue strength.Meanwhile,the influence of geometric characteristics,stress ratio,pretightening force,working environment,surface treatment,and other factors on the fatigue performance of high-strength bolts and their mechanism of action were reviewed.(2)Constant-amplitude fatigue test under a stress ratio of 0.5 was performed on 30 specimens of grade 8.8 owe-pre-tightening force high-strength bolts,and the fatigue S-N curves and fatigue strength were obtained.The S-N curves of different pre-tightening torque were fitted,and it was concluded through comparison analysis that the fatigue performance of the bolt enhances with the increase of pre-tightening force.The fatigue life estimation formula of highstrength bolts considering the pre-tightening torque was further fitted based on the traditional Basquin equation.By comparing with current research results of fatigue performance influencing factors,it was found that the fatigue strength of high-strength bolt is negatively related to stress ratio and stress range,while is little influence by pre-tightening force.(3)The bolt specimens exhibit three typical fatigue fracture modes,with 80% of specimens fracturing at the bottom of the first loaded thread screwed into the nut,which is the main fatigue failure mode.While some specimens fractured at the bottom of the bolt head and the run-out region of the thread The fatigue fracture mechanism was revealed based on the scanning electron microscopic analysis of macro and micro fatigue fracture morphology.The fatigue features regions of macroscopic fracture morphology have distinct boundaries,and the area of fatigue propagation region is positively correlated with fatigue life while negatively correlated with stress range and stress ratio.The microscopic fracture morphology is influenced by the loading scheme,from which the fatigue process could be inferred.Based on the element energy spectroscopy analysis,the main ingredients of inclusions were obtained,which may reduce the fatigue performance.(4)Based on finite element numerical simulation,the stress of the high-strength bolt demonstrates gradient distribution along the axial direction.It was found that crack length and relative position significantly influence the surface stress distribution of the bolt,based on which the crack prediction formula considering changing rate of strain was proposed.Parametric analysis was conducted combined with the extended finite element method(XFEM)to investigate the influence of pre-crack length,pre-crack angle,and pre-crack aspect ratio on stress intensity factor and crack propagation angle.Furthermore,the sensibility of the affecting factors was discussed through orthogonal analysis.The 3-D fatigue crack propagation path and crack surface morphology of the FE model were numerically simulated based on XFEM,which agrees with the experimental results to some extent.