Design Research On Shape Memory Alloy Pipe Joints

Pipeline transport has advantages of saving land resources,reducing environmental pollution,being safe and reliable,so it has become one of the most widely used material transport modes in our country.To reduce the occurrence of pipeline leakage accidents,scholars at home and abroad have made a great deal of researches on causes of pipeline leakage accidents and found that pipe joints are the weakest point of pipeline.Comparing with traditional methods of connection such as welding and flange connection which have some drawbacks,shape-memory-alloy(i.e.SMA)pipe joints which have many speial advantages such as shape memory effect,corrosion resistance,high stiffness,etc.can reduce incidence of pipeline leaks.At present,overseas and domestic scholars have made some achievements in research on SMA pipe joints,but there are still many related issues needed further study in order to explore function and characteristics of SMA pipe joints deeply.The main research contents of this paper are as follows.(1)Advantages and disadvantages of various pipe connections,characteristics of SMA,application fields and development of SMA pipe joints were introduced in detail.(2)Several kinds of constitutive models of SMA,including Tanaka model,Liang-Rogers model,Brinson model,Ivshin-Pence model,Graesser model and Landau-Devonshire model,which are widely used by overseas and domestic scholars,were summarized.After analyzing advantages and disadvantages of each constitutive model comprehensively,Brinson model with more advantages and more convenient to apply in numerical simulation was selected.(3)Axial pull-out test of SMA pipe joints was designed to test the applicability of Fe-based SMA pipe joints.Influence rules of heat-training and fitting gap on property of Fe-based shape memory alloy pipe joints were explored.Results show that Fe-based SMA pipe jonits after heat-training and with 0.2mm fitting clearance which between the Fe-base SMA pipe joints and connected steel pipe,have the best property.(4)Finite element analysis software(ABAQUS)was used to establish finite element model of Fe-based SMA pipe joints and connected steel pipes.Brinson model of SMA was programmed into executable program by programming software(FORTRAN).This executable program can be accessed to ABAQUS by UMAT,a subroutine of ABAQUS.First of all,a numerical model same as specimen 3 in axial pull-out test was established,and it simulated the process of low-temperature expansion,temperature recovery and axial pull-out test of Fe-based SMA pipe jonits,which verified the correctness of this numerical model.Numerical simulation results show that the Mises stress of Fe-based SMA pipe joints increases gradually from the middle to both of its ends in the axial direction,and the edges has been in the yielded state.Therefore,while designing Fe-based SMA pipe joints,the thickness at the edges can be appropriately weakened to reduce the stress there.The Mises stress of the steel pipe to be connected increase gradually in the axial direction from the loading end to the free end(the free end is in the covering state of the Fe-based SMA pipe joints,and its hoop stress and radial stress are both large),and the maximum Mises stress has been very close to the yield stress of 304 steel.The connected steel pipes will be easily damaged if they're in this state for a long time.Therefore,in practical application,the length of Fe-base SMA pipe joints can be properly extended and the thickness can be properly reduced,so as to prolong the service life of the connected steel pipes.Reducing the thickness of Fe-base SMA pipe joints is to reduce the hoop stress and radial stress of connected steel pipes.Increasing the length of Fe-base SMA pipe joints is to assure its connection performance by increasing the contacting area.(5)By changing the parameters of Fe-based SMA pipe joints in this numerical model,influence rules of the fitting clearance,the thickness and the length of Fe-base SMA on its connection performance were investigated.The results show that the pull-out force decreases with the increase of the fitting clearance between Fe-base SMA joints and the connected pipes.Increasing the thickness of Fe-base SMA pipe joints within a certain range is very helpful to improve its connection performance,but beyond this range,the influence will be reduced.Moreover,the excessive thickness will often lead to the yield phenomenon of the connected steel pipes,resulting in leakage and so on.The fastening force of Fe-based SMA pipe joints increases with its length,but the growth rate will decrease gradually.The purpose of this study is to provide a theoretical basis for the design standards of SMA pipe joints,and to promote the use of SMA pipe joints in the field of oil and gas pipelines instead of traditional connection methods like welding and flange connection.