Study On Bearing Capacity Of Friction-Type High Strength Bolted Connection Under Corrosive Environment

With the rapid development of steel structures,there are more and more steel structures in long-term service.The shortcoming of poor corrosion resistance of steel structures is very prominent,especially the steel structure bridge exposed to the atmosphere.With the gradual failure of anti-corrosion means,the invasion of corrosive media,resulting in section loss and change of geometric parameters,and will lead to deterioration of steel material to become brittle,eventually leading to a decline in bearing capacity,surface damage,ductility,etc.Corrosion influences mainly from steel,component,connection and structure.This paper discusses the influence of corrosion from steel properties and bearing capacity of frictiontype high-strength bolt connection.The accelerated corrosion test method adopted in this paper is electrolytic corrosion,which is mostly used to simulate the corrosion of reinforced concrete structures,but is less studied for steel structures.Therefore,the corrosion effect of Q235 B steel plate after electrolytic corrosion is first studied,and then it is used for corrosion friction type highstrength bolt connection.Study on mechanical properties of Q235 B steel plate after electrolytic corrosion,mainly study the influence of electrolytic corrosion method on the corrosion form and mechanical properties of steel plate surface.The results show that(1)uniform corrosion and pitting occur simultaneously in the electrolytic corrosion condition of Q235 B steel plate,and the pitting corrosion phenomenon becomes more obvious with the increase of corrosion degree.The corrosion depth at the junction of the corroded section and the uncorroded section is greater than that of the corroded section.Electrolytic corrosion can better simulate the corrosion form in the natural atmosphere.(2)With the increase of corrosion degree,the mechanical properties of Q235 B steel plate change to different degrees.The yield strength and tensile strength show an obvious linear degradation trend,while the strength-yield ratio shows an obvious upward trend.By analyzing the test data,the relationship between yield strength,tensile strength,strength/yield ratio and corrosion rate was established.Friction type high strength bolt pretension in the bolt connection is a very important parameter,closely associated with resistance to sliding coefficient and bearing capacity,so in order to measure the relaxation and corrosion of high strength bolt pretension,in high strength bolt axial force calibration experiment through the opening in the center of the bolt axis running through the round hole will be in the form of optical fiber Bragg grating sensor encapsulated within the bolt to measure bolt pretension,The sensor installation position was determined by finite element simulation.The relationship between the axial force of the high strength bolt and the strain at the measuring point of the sensor is determined through the calibration test of the axial force of the high strength bolt.The results show that when the axial force of the bolt is below 170 k N,the linear relationship between the axial force of the bolt and the strain measured by the sensor completely conforms to the linear relationship.The high strength bolt is in the elastic stage,and it is completely feasible to obtain the axial force of the bolt from the strain measured by the sensor.When the bolt axial force reaches the set value of 170 k N,the strain at each measuring point of the sensor is different,which is caused by the different installation positions of the sensor.This indicates that there is an error in the installation method presented in this paper,so the bolt needs to be calibrated for the bolt axial force.Friction type high strength bolt connection in corrosion environment bearing capacity study,first of all,according to the specifications produced friction type high strength bolt joint specimens,and then by monitoring optical fiber Bragg grating strain sensor will be completed specimen assembly,then measured the bolt pretension of relaxation,and then different degrees of corrosion of high strength bolt connection,shear tests are carried out at last.By analyzing the test results and data,the following conclusions are drawn :(1)The bolt relaxes more than 50% of the total relaxation after 1 hour of final screw,and decreases and becomes very slow after 5 hours,when the relaxation reaches 80% of the total relaxation;Relaxation was nearly complete after 48 hours.(2)There is almost no corrosion on the nut,washer,screw thread part of the bolt and the contact surface between the bolt and the cover plate and the mother plate,and the solution almost does not penetrate from the contact surface around the bolt;The contact surface is mainly red rust,and there is no corrosion around the bolt hole of the contact surface compared with other areas of the contact surface.(3)the corrosion depth around the cover is greater than the corrosion depth around the bolt hole.(4)With the increase of corrosion depth,the bolt axial force decreases.(5)The load-displacement curve of the specimen has four distinct stages:(1)friction force transfer stage,(2)slip stage,(3)yield strengthening stage and(4)failure stage.(6)The sliding load of the first group of specimens is less than the calculated bearing capacity due to the relaxation effect;However,under the combined action of corrosion and relaxation,the sliding load is higher than the calculated bearing capacity,and the corrosion effect increases the bearing capacity of the specimen.(7)With the increase of corrosion time,the area of red rust caused by corrosion on the contact surface becomes larger and larger,and the red rust becomes thicker,which is the reason why the slip coefficient increases with the increase of corrosion depth.On the basis of the experimental study,this paper carries out the finite element simulation of the corroded friction-type high-strength bolt connection,and considers the loss of cover section,the variation of friction coefficient and the variation of bolt axial force caused by the corrosion.By comparing the finite element results with the test results,the applicability and effectiveness of using the finite element numerical simulation method to analyze the corroded friction high-strength bolt connection are verified.