| With the rapid development of infrastructure construction in China,steel structure bridges have become an important part of the transportation system.High-strength bolts are widely used in the connection of steel structure bridges because of its good stiffness and strength,easy disassembly and maintenance,mature technology and other advantages.However,in the service process,the bridge bolts have been in a complex stress environment and outdoor atmospheric environment for a long time result in the breaking and falling off,which seriously affects the safety of the bridge.This article focuses on the fracture failure behavior of 20 Mn Ti B steel high-strength bolts of a steel structure bridge,clarifies the failure mechanism and main factors of bolt fracture,and provides the basis for the selection of steel structure bridge bolts and the safety maintenance of bridges.The main research contents and results are as follows:The optical emission spectrometer,EDS energy spectrometer and X-ray diffractometer were used to detect the fracture composition of the failed bolts.The results show that the chemical composition of the matrix of the failed bolts meets the standard composition requirements.The fracture surface is mainly composed of Fe,C,and O elements,and the presence of S,Si,Cl,and K elements is detected.The results of phase analysis are mainly iron oxides.However,the corresponding position of the new bolts is mainly composed of Fe,C,Mn,Si elements.According to the comparison of element content and distribution,the Si content on the fracture of the failed bolts increases,and component segregation and inclusion elements appear on the fracture.The fracture morphology and microstructure of the failed bolts were systematically studied.The research shows the bolt fracture has corrosion fatigue fracture and stress corrosion fracture from the macro and micro morphology.Through the analysis of metallographic structure and structure,there are micro-defects near the crack source of the failed bolts,and the metallographic structure of the fracture has changed.Compared with the tempered sorbite of the new bolts,there are massive ferrite in the failed bolts.The analysis and research results of the mechanical properties of the failed bolts show that the hardness value of the failed bolts meets the standard requirements,but its impact energy and impact toughness are significantly reduced compared with the new bolts.The stress corrosion test was carried out by simulating the climatic environment of the bridge location,coupling the corrosive medium and stress conditions.Then,the physical and chemical properties of the bolts after corrosion testing were compared with those of the failed bolts.The results show that the color of fracture appearance,phase analysis results and EDS composition test results of the corroded bolt and the failed bolts are similar,and there are corrosion pits on the micro-morphology.After corrosion,the mechanical properties of the bolt have some changes,but the degree of change is very small.Therefore,atmospheric environmental corrosion is not the main cause of bolt failure,but it will accelerate bolt failure and fracture.Combined with the above research and analysis results,there are micro-defects near the crack source of the fracture surface of the 20 Mn Ti B failed bridge high-strength bolts.There are corrosion marks on the fracture surface,and S,Cl,K and other elements are detected.It shows that micro-defects are the main reason for the failure of 20 Mn Ti B high-strength bolts,and atmospheric corrosion will accelerate the failure of bolts. |
