| Because of its light weight,good product performance and other advantages,metal pipes are widely used in aerospace,automobile manufacturing,mechanical parts and other fields.Precision separation of pipe is the first step in pipe manufacturing.The low-stress precision separation technology based on crack initiation and propagation has attracted much attention in the field of industrial technology because of its advantages of saving raw materials and low energy consumption.At present,the separation technology of pipe is mostly limited to sawing,shearing and other separation processes,and rarely involves the research on the influence law of fatigue crack initiation,expansion,fracture and expansion rate of metal pipe,which hinders the application of low stress in pipe.In this thesis,the fatigue fracture mechanism of metal pipe is studied through the combination of theoretical analysis,simulation and experimental verification through the prefabricated V-notch of metal pipe.The main research work and conclusions include:(1)The principle of rotary precision separation of eccentric is analyzed,and the theoretical model of the loading load of the pipe is obtained,as well as the stress change and influencing factors of the metal pipe during loading.The basic mechanical properties of the material were determined through the room temperature tensile experiment,and the Zerilli-Armstrong(Z-A)constitutive model was established.The influence law of the loading lever,gripping lever and base Angle radius of the pipe structure parameters on the V-notch stress was analyzed by ABAQUS finite element method.According to the analysis results,more optimized structural parameters were obtained:The loading lever ranges from 70mm to 100mm.The clamping lever ranges from 12mm to The base Angle radius ranges from 0.1 mm to 0.4mm.(2)Based on fracture mechanics and fatigue theory,a three-dimensional finite element model for eccentric loading of pipes was established,and the correctness of the model was verified.The numerical simulation results show that when the notch depth is q=0.2,the maximum stress concentration coefficient of the V-shaped gap increases by 29.9%compared with that of the notch depth is q=0,which significantly speeds up the crack initiation time of the pipe.Under alternating load and linear load,the crack propagation rate of the outer surface is faster than that of the inner crack propagation rate,and the crack opening degree is larger under the same analysis step.The crack initiation process is studied.Due to the action of tensile stress and shear stress,the crack occurs on one side of the notch loading load,and the crack initiation Angle increases with the increase of the loading lever,but the effect on the crack initiation position is small.(3)The fatigue test platform of metal pipe was built,the pipe sample was prepared,the separation rule of four materials,20 steel,45 steel,304 stainless steel and copper,was studied under different loading force arms and different loading frequencies,and the section quality of metal pipe was studied,and the profile value of pipe section was tested.When the loading force arm L=100mm,the section was relatively clean.When the loading lever L=70mm,the uneven surface is obvious.Based on the experimental platform,the crack initiation process of metal pipes was studied,and the crack growth rate was studied under different loading frequencies f=3.3Hz and f=6.6Hz.The loading frequency had a great influence on the crack growth rate,and the experimental results were in good agreement with the finite element simulation results. |
