Study On The Mechanical Properties And Monitoring Of Low Temperature Resistant Modified Laminated Polyurethane Isolation Bearing

With the development of large bridge structure technology,the demand for bearing capacity of bearings is increasing.Laminated polyurethane isolation bearings have attracted the attention of experts and scholars,owing to their high bearing capacity and simple manufacturing process.However,the horizontal stiffness of the bearing increases sharply when it encounters low temperature,which seriously affects its mechanical properties and limits its engineering application in cold regions.Therefore,based on the composition of polyurethane in the bearing,this paper explored the influence of three polyether glycol modifiers on the low-temperature resistance of polyurethane through microscopic characterization and mechanical property tests.The mechanical properties of modified bearings and their seismic isolation effect on bridges at low temperatures were studied.In addition,the lateral surface strain range of laminated isolation bearings is large during shear deformation,and the lateral surface bulges during compression deformation.Traditional strain sensors cannot effectively monitor their deformation.In this paper,a flexible strain sensor was prepared with polyurethane as the polymer substrate and multiwall carbon nanotubes(MCNTs)as the nano-conductive particle.The sensing behavior at low temperature was improved by introducing modifiers.The sensing stability at large strain was improved by introducing silane coupling agent.The force-electric sensing behavior of the sensor was systematically investigated.The deformation monitoring effect of the laminated isolation bearing was verified.The main studies are as follows:(1)The micro mechanism and mechanical properties of modifying low temperature resistance of polyurethane were studied.Low-temperature modification of laminated isolation bearings prepared from polyurethane with raw materials PTHF,TDI,and MOCA was carried out.The effect of three polyether glycol modifiers with different chemical structural symmetry and number of carbon atoms in the repeating units of the main chain on the low-temperature resistance of polyurethane was investigated.A control group was set up to conduct microscopic characterization tests on four polyurethane samples.The principle of impacts of modifiers on the degree of microphase separation and glass transition temperature of polyurethane was explored.The mechanical properties of modified polyurethane specimens were tested at 23℃,0℃,-20℃,and-40℃,respectively.It was found that 3-methyltetrahydrofuran/tetrahydrofuran co-polyether diol(3MTHF)had the best effect on the low-temperature resistance of polyurethane.The polyurethane specimens with various 3MTHF mass fractions were tested and analyzed.The optimum content of 3MTHF was determined to be 25 wt.% in combination with the performance requirements of the bearings.(2)The low temperature resistance and basic mechanical properties of modified laminated polyurethane isolation bearings were studied.The results showed that the horizontal equivalent stiffness(K)of the bearing before modification increased by 34.0%,64.7%,and 95.1% at 0℃,-20℃,and-40℃,respectively,compared to 23℃;The K of the modified bearings increased by 14.6%,29.2%,and 45.1%,respectively.It was revealed that 3MTHF can enhance the frost resistance of laminated polyurethane isolation bearings by limiting the low-temperature hardening of polyurethane.Besides,sufficient energy dissipation capacity of the bearings was maintained.The performance indicators of the bearing met the requirements of the specification,with the ultimate shear strain reaching over 300% and the 60-year creep only being 5.05%.(3)The seismic isolation effectiveness of modified laminated polyurethane isolation bearings on bridges at low temperatures was investigated.The Bouc-Wen model was chosen to simulate mechanical behaviors of the bearing,and 20 near-field seismic waves were selected.The seismic response analysis of a three-span continuous beam bridge under different temperatures was conducted through Opensees,verifying that 3MTHF can improve the seismic isolation effect of the bearing at low temperatures.The seismic vulnerability curve was drawn through incremental dynamic analysis.The results proved that 3MTHF can improve the low temperature resistance of the bearing,maintain sufficient energy consumption capacity at low temperatures,enhance the protection effect of bridge piers under earthquake action,and significantly reduce the risk of bridge collapse at low temperatures.(4)Aiming at the fact that traditional strain sensors cannot be applied to deformation monitoring of laminated isolation bearings with large strain ranges and complex deformation,this study prepared a flexible strain sensor with large strain sensing ranges and high sensitivity,using polyurethane as the matrix and MCNTs as conductive fillers.It was proved that MCNTs were uniformly dispersed in polyurethane through microscopic characterization tests.Moreover,it was found that the sensor follows the classical percolation theory with a percolation value of 1.0 wt.% through conductivity testing.The low-temperature sensing performance of the sensor was improved by adding 3MTHF.Besides,the sensing stability at 150% strain was enhanced by introducing silane coupling agents.It was revealed that the sensor has strong linear and stable sensing behavior through tensile and bending sensing tests.The gauge factor reached 8.26 as the content of MCNTs exceeds 3.0 wt.%.The good deformation monitoring effect of the sensor on laminated isolation bearings was achieved.