Ratchet Limit Study Of The Pressurized Pipe Under Cyclic Thermo-mechanical Loadings

Pressurized pipes or cylindrical pressurized vessels have been widely used in chemical and petrochemical engineering,military,power plant some other process industries.These components are usually subjected to the combination of sustained and cyclic loadings.Ratcheting is the typical failure mode for pressurized pipes under these cases and affects the safety of components due to excessive accumulation of plastic deformation.The standards of various countries only contain ratchet limit analysis solution of thin-wall cylinder under cycle thermal and internal pressure load combination(Bree).But it is difficult to obtain the analytical solution of ratchet limit for pressurized piping under other complex loading conditions due to the complexity of the theoretical derivation.In order to ensure the safety of the pressure vessel and pressure pipe,analytical solution of ratchet limit for pressurized piping under these complex load conditions are performed based on the developed Non-Cyclic Method in this paper which is common in the engineering field.The corresponding design method is also proposed.The main work and conclusions are as follows:(1)The axial compressive stress caused by gravity and other factors can also significantly reduce the ratchet limit of the structure.Ratchet limit of cylindrical pressure vessels and piping increases obviously with the increment of radius ratio for various load combinations.(2)Von-Mises yield criterion is adopted in the design method proposed in this paper.The combined load is divided into two parts: steady load and cyclic load.The analytical ratcheting limits for pressurized pipes under these complex load conditions include cyclic temperature gradient and constant internal pressure,cyclic temperature gradient and constant internal pressure with compressed ends,cyclic temperature gradient and constant bending moment,cyclic temperature gradient and constant torque,cyclic temperature gradient and constant bending and torsion,cyclic torque and constant internal pressure,cyclic torque and constant bending moment,cyclic bending moment and constant torque,internal pressure straight pipe under cyclic bending moment.(3)Uniaxial tensile test and uniaxial ratchet test of 20 CrMo steel at 500 ? are conducted.The evolution law of the ratchet effect of 20 CrMo steel is analyzed according to the experimental data,and the range of the ratchet limit is 200MPa~300 MPa.(4)Several Bree-like diagrams under the corresponding loading conditions of this paper were obtained,which can be used to estimate ratchet limit of cylindrical pressure vessels and piping conveniently for engineering design.It has good value and lays the theoretical foundation for the ratchet limit design of pressure pipe under complex loading condition in engineering.