Study On Fatigue Performance Of Grade 8.8 Fastening Bolts For Transmission Towers

Being used in manufacture of 8.8-grade bolts for transmission networks,40Cr steel possesses excellent mechanical properties,which enables to meet the requirements of the fatigue strength higher than 800 MPa and the yield strength higher than 640 MPa.Recently,the collapse accidents of the transmission towers happened frequently due to the failure of fastened bolts with the increase coverage of high-voltage power grid in China.Thus,fatigue performance and service reliability of fastening bolts have been paid great attention,including prestress,fatigue crack growth and also stress relaxation behavior which affect the service reliability of the fastening bolts.In this work,relying on the national grid research and development project,the fatigue properties of 40Cr steel specimens under different prestresses were studied.And the effects of stress concentration conditions,notch positions,on the fatigue crack growth properties of 40Cr steel were studied by using three-point bending specimens,and the notch sensitivity of the material was also investigated simultaneously.The stress relaxation behaviors of different prestressed specimens were also studied.The results have been obtained as follows:It was found that when the prestress increased from 10%,30%to 50%of the material ultimate tensile strength,the fatigue limit of the material was reduced from 370 MPa to 263 MPa through stress-controlled axial loading fatigue tests.The effective stress corresponding to the fatigue limit(107 cycles)of the bolt related to the pretension force and the stress ratio was determined with the value of 562.75 MPa.When the effective stress applied is lower than this value,fatigue failure of the 8.8 bolt will not occur.Effective preload corresponding to the fatigue limit of the 8.8 bolt fatigue limit and relationships of stress ratio versus maximal prestress and pretighting torque of M6 and M27 bolts are required.The effective stress conversion relationship is used to convert the stress amplitude-fatigue life curve of the smooth specimen and the prefabricated notch specimen under the alternating load into the effective stress and fatigue life curve.With the distance increase from the notch to the center of samples,the threshold for fatigue crack initiation,and the deflection angle of the fatigue crack increased.The crack propagation path became more tortuous,and the fatigue crack propagation rate reduced accordingly.Fatigue cracks propagated along the slat bundles,and the intergranular fracture occurred when the notch was near the center of samples.While cracks propagated through the slat bundles if the notches far from the center,and transcrystalline fracture occurred accordingly.The degree of notch sensitivity of 40Cr steel is 0.31.The results of stress relaxation experiments show that in the first stage of bolt stress relaxation,the relaxation rate is high and the duration is short,and in the second linear stage,the stress relaxation rate is slow.With prestress rising from 30%to 64%of the ultimate tensile strength,there was almost no variation of the proportion of remaining prestress after a long period of service.The preload of the bolts should be increased as much as possible with ensuring of fatigue life of the bolts.