Investigation Of Stress-strain Relationships Of Metallic Thin-walled Tubes In Impact Loading

Liquid impact forming(LIF)is an improved forming technology based on Hydraulic forming technology.LIF is a high speed forming technology.With the liquid as a medium,the energy can be released instantaneously and the parts be formed.The technology not only keeps the advantages of hydraulic bulging technology,but also improves the forming efficiency.Therefore,once it is put forward,it has gained the favor of research.However,the flow stress of material in LIF is different from the flow stress of material in THF.Therefore,the investigation of flow stresses of thin-walled tubes in LIF has the important theoretical significance engineering application.In this study,the researches of the flow stress of tubes in LIF are proceed by using a combined method of theoretical analysis and experiment study.The specific topics of this study are as follows:(1)the mechanical model of the tubes is built.Combining with the membrane theory and the force equilibrium equation,the formula for solving the data of the equivalent stress,the equivalent strain and strain rate are derived.(2)A novel approach of the investigation of flow stress of thin-walled tubes in LIF,according to this method,a test system was development.(3)The free-bulged tests and the tube forming tests in LIF are performed based on the experiment tooling,and the impact velocity are 6 mm/s,8 mm/s,10 mm/s and 150 mm/s.Actually,the flow stresses of metallic thin-walled tubes in LIF are determined based on the experimental data and formulas.(4)The developed methods are validated by means of FE simulation of the tube forming in LIF.The results display that:(1)the formula derived based on the mechanical model,the membrane theory and the force equilibrium equation can be well applied to determine the flow stress of tubes.(2)With the impact velocity improving,the forming of tubes is more difficult.With the internal pressure is same,the maximum bulging height of is higher when the impact velocity is faster.In order to make the tube bulge better,greater internal pressure is required.(3)The impact speed is the faster;the strain rate of tubes is the higher.And the data of the strain rate is increasing when the impact speed is the faster.(4)In this study,the experiments of tube hydraulic bulging and the tube forming tests in LIF with the impact velocity are 6 mm/s,8 mm/s,10 mm/s and 150 mm/s are performed,respectively.It is found that the phenomenon of strain rate hardening is more obvious with the increase of impact velocity,and the yield stress is also increasing with the increase of impact velocity.(5)In this study,the developed method is validated by means of FE simulation of the tube forming in LIF.The data of bulging height of tubes are extracted from the results of FE simulations.Compared with the data of bulging height of tubes are extracted from the results of experiments.It’s founded that the deviation rate of the maxim bulging height of tubes which are from the results of FE simulation and experiments is within 7 %.It’s indicted that the stress-strain relationships determined based on the experimental results and related formulas in this study can describe the dynamic mechanical response of the tubes well.