Study On The Loading Method Of Tube Hydroforming Under Proportional Loading

The proportional loading conditions are as follows: in the bulging process,the stress component at the middle section always increases in the same proportion,and the total theory is suitable for proportional loading,at present,many scholars have discussed how to establish the plastic hardening model of pipes by using the total theory under different loading paths,and how the external load affects the forming process.However,the total theory is suitable for plastic forming under proportional loading,which has certain particularity.There are few researches on the hydraulic bulging based on the hydraulic pressure curve(the relation curve matching with the axial feed)under the condition of proportional loading.Therefore,this paper will study: in the process of TP2 tube forming,the ratio of flow stress component(axial stress component and circumferential stress component)at the highest point of forming zone always increases in a certain proportion,and the hydraulic pressure curve is established and tested under the total theory.In this paper,(1)Because of the establishment of the plastic hardening model under the total theory,based on the stress conditions,the total theory and the principle of plastic deformation work,the plastic hardening relationship of the tube is constructed.(2)Because the proportional loading condition is to make the stress component of the section in the bulging zone increase in the same proportion all the time,this requires the hydraulic pressure curve to change in a certain way,so the hydraulic pressure curve should be obtained to realize the proportional loading condition.(3)Because the calculation of equivalent stress-strain model based on total theory needs deformation data such as strain and displacement field,Therefore,in order to obtain the deformation zone data,the test method is used to realize the tube hydroforming under proportional loading.(4)Based on the deformation data extracted from the test,the equivalent stress-strain model of the pipe is established based on the total theory.(5)Based on the equivalent stress-strain model,the constitutive relation is obtained.The strength coefficient K and hardening index n are the main factors to reflect the material properties,therefore,the parameters of each plastic hardening model are compared;The accuracy of the constitutive relation is evaluated;The effect of proportional loading conditions on tube bulging quality is revealed.The results show that:(1)The order of strength coefficient K from high to low is uniaxial tension,proportion increment and proportion total.The order of hardening index n from high to low is: proportion total,proportion increment and uniaxial tension.(2)The three constitutive relations are used to simulate the bulging process,and the hydraulic pressure curve under proportional loading is used to simulate the bulging process,Comparing the experimental and simulated bulging zone contour and maximum bulging height,it is concluded that the order of accuracy of three kinds of plastic hardening models is as follows: proportion total,proportion increment and unidirectional tension.(3)In order to further reveal that the proportional loading condition can improve the tube bulging quality,the hydraulic pressure curve and linear loading curve under the proportional loading condition were used for bulging test,the bulging profile and maximum bulging height are compared.The results show that the proportional loading conditions can effectively improve the hydraulic bulging quality of the pipe,improve the stress form of the pipe,and thus improve the bulging quality.