Study On Delayed Fracture Of High Strength Bolts For Bridges

In order to effectively control the occurrence probability of delayed fracture of highstrength bolts,research work on delayed fracture of high-strength bolts has been carried out from four aspects: investigation and analysis of bolt fracture diseases,research and analysis of reasons and mechanism of delayed fracture of bolts,analysis of influencing factors of bolt bearing stress state,and research on improvement of delayed fracture resistance of high-strength bolts.Based on the investigation of 11 bridges with delayed fracture of high-strength bolts,the delayed fracture diseases and failure characteristics of high-strength bolts are summarized and analyzed from the aspects of steel materials for fractured bolts,fracture positions of bolts,number of fractured bolts and structural positions of bridges with bolt fracture.Statistical analysis shows that the fracture ratio of high-strength bolts is mostly on the order of 0.01%.The bolt fracture position is mainly at the lower fillet of the bolt head,at the junction of the bolt straight rod and the screw rod,and at the first fastening thread of the bolt nut screwing position.For bridge structural parts,bolt fracture occurs at the node plates of main truss beam members,bridge horizontal joints and transverse joints,and bolt delayed fracture has random distribution.Through chemical analysis,macroscopic and microscopic fracture analysis,metallographic analysis and other methods,the fracture reason of the bolt dropped from a bridge site was analyzed.The research shows that the chemical composition of the fractured bolt meets the specification requirements.The crack of the fractured bolt starts from the surface of the thread root.The source region and the extension region are intergranular fracture surfaces.The tear morphology of chicken claw pattern can be observed at some grain boundaries,showing typical hydrogen-induced delayed fracture characteristics.According to this,when the bolt is loaded and plastically deformed,the dislocations generated by the bolt will carry hydrogen and hydrogen will diffuse and enrich to the stress concentration place under the action of stress.At the same time,the locally enriched hydrogen atoms will reduce the interfacial bonding force,allowing slip bands to penetrate the grain boundaries,thus accelerating the initiation and propagation of cracks and eventually causing hydrogen-induced delayed fracture.In order to master the mechanism of delayed fracture of high-strength bolts,a model of stress-induced hydrogen movement is established based on Eshelby equivalent inclusion theory,and a theoretical model of the relationship between stress concentration at the root of the defect and hydrogen content is proposed,thus providing evidence for hydrogen-induced delayed fracture.Through theoretical analysis and numerical simulation on the distribution of bearing stress of high-strength bolts,the main factors affecting the distribution of bearing stress of bolts are explored.The finite element analysis shows that the stress distribution of the bolt can be optimized and the stress concentration phenomenon can be relieved by appropriately increasing the arc radius R of the thread bottom and the fillet radius r of the bolt head.In order to study the delayed fracture sensitivity of 20 MnTiB and 35 VB steels for high strength bolts,the delayed fracture sensitivity of specimens with different tensile strength was analyzed by using constant load delayed fracture test and slow strain rate tensile test.The test results show that when the tensile strength is between 1040 MPa and 1240 MPa,the hydrogen embrittlement sensitivity decreases first and then increases with the increase of the tensile strength.The breaking points of 20 MnTiB steel and 35 VB steel are 1157 MPa and 1179 MPa respectively.Therefore,it is recommended to control the tensile strength of 20 MnTiB steel and 35 VB steel for high strength bolts to 1040~1240 MPa.Based on the current technical conditions and production process of high-strength bolts,a study on improving the delayed fracture resistance of high-strength bolts was carried out.It was proposed that the upper limit of bolt tensile strength should be reduced from 1240 MPa to 1190 MPa,the content of P and S should be controlled below 0.015% and 0.025%,the thickness of washer should be increased,and external chamfer should be set.According to the above requirements,bolt trial production has been carried out.The yield of bolt trial production is relatively high and the data of various test indexes are more stable than before.Therefore,large-scale production is feasible.