Experiment Study On Mechanical Behavior Of Flexible Joint With Ductile Cast Iron Pipe Under Cyclic Loading

Ductile cast iron pipe,which widely used in most areas of China,has many characteristics like high strength,high toughness,thin but compact pipe wall,and corrosion resistance.It is an ideal material for water and gas supply pipe.The urban undergroundpipe system which composed by ductile cast iron pipes and other common pipes,play an important role in modern society.Accordingly,such a significant system was called city lifeline.With the prosperity and development of modern cities,the status of undergroundpipe system will be more and more prominent in the area with large population and dense buildings for its contributions to ensuring the normal operation of the city and providing an important guarantee for the post disaster relief and governance.pipe joint is the weak links of pipeline seismic.The research on pipe joint is critical as once it is broken,it would make a big difference to the function of the whole pipe,like the leakage of water and gas which means a lot of economic losses.Nowadays,domestic and foreign scholars have done a lot of theoretical and experimental research on pipe joint.However,compared with the abundant theoretical research,the experimental researches,most of which are based on static load drawing and bending,are still not enough.In this paper,three kinds of axial full scale tests,which are drawing test with different rates,cyclic loading test with multicycle,and gradually cyclic loading test,are carried out on the flexible joint of ductile cast iron pipe with different diameters.And all of those tests were conducted under two conditions that filling with water pressure or not.Based on drawing test with different rates,focusing on cyclic loading test,taking socket,spigot,and gasket as the research object,to gain correlation parameters.Through the experiment,some data like the relationship between force and displacement,the characteristics and area of hysteresis curve,the ultimate displacement value and the anti-pulling capacity are obtained.Consulting the area of hysteresis curve under different displacement and data fitting,we established an axial mechanical model of pipeline joint.By analyzing the mechanical properties and failure mechanism of flexible joint under different loading conditions,we can provide some important correlation parameter of flexible joints' seismic behavior for the theoretical study on seismic response analysis of pipe.In this paper,two types of bending full scale tests,longitudinal bending test and longitudinal cyclic loading test,are carried out on the flexible joints of ductile iron pipes with different diameters.Also,those two tests were conducted under two conditions that filling with water pressure or not.Then,we observed the change of joint gasket under bending with different loading conditions and then recorded the data of the bending moment and the angle of the joint from the test.In this account,we made a relation curve between the moment and the rotation angle from the longitudinal bending test and the hysteresis curve of joint moment and rotation angle from the longitudinal cyclic loading test.And then,the area of different hysteretic curves were obtained.By analyzing the bending mechanism of pipeline with different diameters under different loading modes,we can get the joint stress formula under different test conditions.Thus,all of those efforts could provide joint resistance parameters and technical support for the seismic design of underground pipeline.