Experimental Study And Numerical Simulation Of Time-dependent Spring-back Of Typical Metal Tubes Under Rotary Bending

Metal tubes have been widely used in aviation, aerospace, automotive, instrumentation and other fields because of excellent mechanical properties, function diversity and lightweight. Springback is the inevitably phenomenon after unloading processing, while a large number of experimental studies have found that continuous deformation will occur over time, which is called time-dependent springback. Time-dependent springback will result in lower dimensional accuracy, difficulty in assembling, or even security risks. This phenomenon is still in cognitive stage at present. The research of experiments, theoretical study and finite element analysis has been taken by different laboratories in the world. With the time-dependent springback of aerospace used 1Cr18Ni9 Ti stainless steel tube and 5A03 rust-proof aluminum tube, this paper carries out experimental and FEM research based on elastic-plastic and viscoelastic theories.Uniaxial tensile test results of these two tubes indicate that time-dependent springback amounts grow with the increase of pre-strain and strain rate and there are local fluctuations on time-dependent springback curves. The curve can be divided into a fast and then a slow stage, and 1Cr18Ni9 Ti tubes enter the second stage faster than 5A03 ones.It was found that bent tubes’ angles increase rapidly in the early 105 s, and then turn to be stable. With the same ratio of bending radius R and tube diameter d0, time-dependent springback angles increase as the tube diameter rise, and time to reach a steady state are approximately equal. Time-dependent springback angle and time to reach a steady state increase with the growth of R/d0 when the tube diameter is fixed. Stress relief annealing at lower temperature promotes the release of residual stress in bent tubes, which causes the springback occurrence in advance.The two-dissipative visco-plasticity mechanisms model of 1Cr18Ni9 Ti and Wiechert visco-elasticity model of 5A03 are established based on series tensile and stress relaxation tests. A strain hardening law creep model is proposed based on creep tests of these two materials. Tensile time-dependent springback simulation results of these two tubes with corresponding constitutive model show good agreement with the experimental value and trend. Simulated time-dependent springback angle of bent 1Cr18Ni9 Ti tube reaches 73% of test result, and the trend of springback curves are very close. It’s revealed that the recovery of elastic strain after unloading give the rise to residual stress in different components, and then drive the continuous deformation of visco and elastic components result of the time-dependent springback.