Hopkinson Tension Bar Technique For Material Testing In Large Deformation

Nowdays,Hopkinson bar technique is quite mature,it can measure the dynamic performance of materials under the strain rate in the order of 10²-10⁴s^-1.However it is difficult to achieve large deformation of mateials under the low strain rate(10²s^-1).In order to solve the problem that material can't achieve large deformation under the intermadiate strain rate,this paper introduce the new tension technique for materials in large deormation which is improved by traditional Hopkinson tension bar technique.In consideration of one-dimensional stress wave theory,the striker is a little longer than loading bar in this experiment,it can get loads with long time.Secondary loading technique is used in the experiment to reflect the tensile stress wave again and produce secondary loading on the specimens.It can produce loading time about 4L/C and realize large deformation of materials.The rationality of the ecperimental device can be verified by simulation of ABAQUS.The duration of the strain rate obtained in the simulation can reach 1.2ms and the constant load can be ensured when the total length of the system is about 3.3m.It proved that this exprimental technique is useful.We simulate and compare several different models to optimize experimental device including the block and the flange.Optimized block can make the second reflection loading more ideal.Changed the size of flange can reduce the oscillation of strain rate.According to one-dimensional stress wave theory and simulation results,we design a model and then set up an experimenal device.In order to ensure that the loading bar and the striker are coaxial and can load stably at low speed,a design scheme is proposed.This experimental device adopt vertical structure.We have designed a strain gage sticking device,which attaches the strain gage to the inside of the loading rod to measure the signal.The length of long striker can be reduced by using copper material.In low-speed experiment,it can solve the problem that specimen can't be observed.1.2 milliseconds high-time loading can be obtained by the experiment,which inevitably leads to the superposition of stress waves in loading bar.In this paper,a stress wave inversion technique based on transfer function is introduced.The superimposed complex stress waves are separated into single stress waves.The analysis confirms that noise has little effect on stress wave separation technology.The complete test signals at different strain rates were obtained by testing the plate and rod shaped specimens of 45#steel and 6061 aluminum alloy.The engineering stress-strain curve and engineering strain rate curve are obtained by analyzing and processing the experimental data.In this experiment,the material achieve large deformation under low strain rates.We didn't use whole block to reflcet stress wave,so the strain rate decreased,but this experimental technique has basically achieved the expected goal,which deserves further study.