Numerical And Experimental Investigation On RT Instability Of High Purity Copper Under Detonation Loading

Rayleigh-Taylor instability(briefly,the following as RT instability)is an important scientific topic in physics and engineering.When the low-density fluid accelerates the high-density fluid,the RT instability makes the initial perturbation at the interface of the two fluids grow and evolve,eventually leading to the spike of the heavy fluid invading into the light fluid and the bubble of the light fluid rising into the heavy fluid.For a long time,there is still a lack of in-depth understanding of the perturbation growth characteristics and its main control factors of metal interface under detonation loading.This is because,on the one hand,compared with gas or fluid,the correlation of metal strength and the complexity of initial perturbation make the initiation and evolution of metal interface RT instability have significantly different characteristics,which makes it difficult to use relatively mature theoretical analysis methods.On the other hand,due to the bottleneck of experimental measurement and diagnosis technology,the laboratory research on metal interface RT instability is still very limited,which also makes it difficult to interpret the data and understand the regularity.Given these difficulties,this dissertation is aimed at the numerical simulation and experimental study of RT instability under detonation loading for high purity copper.The main research contents and conclusions are as follows.1.Numerical model of RT instability of high purity copper under detonation loadingBased on the finite difference code,a two-dimensional simulation model for the study of RT instability of high purity copper under detonation loading is established.In the model,the equation of state of copper is based on Hugoniot line using Grüneisen equation of state.The Steinberg-Cochran-Guinan(SCG)model is selected to describe the loading characteristics of high strain rate,large deformation and high loading pressure for studying the material strength effect of copper.At the same time,in order to reduce the calculation of explosive detonation process,the loading effect of explosive is replaced by applying pressure boundary condition on the loading surface.Compared with the experimental results in the literature,the model parameters such as the calculation area and mesh size are optimized to ensure the convergence and reliability of the numerical model.2.Numerical simulation of RT instability of high purity copper under detonation loadingThe effects of initial perturbation characteristics,sample thickness and material strength on the evolution of RT instability were studied.The simulation results show that the growth of RT instability is closely related to the initial amplitude of interface perturbation,and there is a critical amplitude.When the initial amplitude is less than the critical amplitude,the growth of the interface perturbation increases with the increase of the initial wavelength,and finally tends to a stable value.When the initial amplitude is larger than the critical amplitude,the interface perturbation increases and the state is unstable.For example,when the initial wavelength is 5.08mm and the sample thickness is 2.54mm,the critical amplitude is 0.25mm when the initial amplitude of perturbation is 0.1,0.2,0.25,0.3,0.4 and 0.6mm.Secondly,there is also a critical thickness for the sample thickness.When the sample thickness is less than the critical thickness,the growth of the interfacial perturbation presents a nonlinear unstable state.In contrast,the material strength is an important stabilizing factor,especially in the later stage of perturbation development,the larger the material strength is,the smaller the increase of perturbation amplitude is.Compared with the stabilizing effect of material yield strength,the effect of shear modulus on the perturbation growth of RT instability can be neglected.3.Experimental technology for study of metal material interface RT instabilityAn experimental device for detonation loading was established,in which high-energy explosive was used to load high purity copper sample,and a gap of about 3.5 mm was set between the main explosive and the high purity copper sample to modulate the front edge and amplitude of loading shock wave.At the same time,the structure design of the sample was optimized,and the angle of inclination was about 5°.The edge bending of the sample in the process of flight motion and its influence on the measurement and diagnosis of perturbation growth are effectively avoided.In the experimental diagnosis technology,the X-ray transient imaging technology is established.The 450 keV pulse X-ray machine and time synchronization technology are used to obtain the perturbation growth images of the high purity copper interface RT instability at different times.In addition,the array Photonic Doppler Velocimetry(PDV)cluster velocity measurement technology is developed,and the high temporal and spatial resolution profile of the evolution velocity of the free surface is obtained.The joint diagnosis technique and the experimental observation data provided an important basis for the calculation and simulation research.4.Detonation experiment study on RT instability of high purity copper interfaceThe experiments under detonation loading are carried out.The thickness and diameter of high purity copper samples are 3mm and 66mm,respectively.The interface perturbation of each experiment includes the same wavelength,and two kind of amplitude(0.3mm and 0.5mm).The experiment was divided into three groups.The first group:the initial perturbation wavelength is 5mm,and three detonation experiments are carried out.The X-ray image of the interface perturbation is obtained at 1.98μs,3.50μs and 5.26μs,respectively.At the same time,PDV velocity profile of the disturbed interface is obtained.The second group:the initial perturbation wavelength is 3mm,and three detonation experiments are carried out.The X-ray image of the interface perturbation is obtained at 2.05μs,3.56μs and 5.33μs,respectively.At the same time,PDV velocity profile of the disturbed interface is obtained.The third group:compared the influence of different loading pressure on the growth of interface perturbation,two detonation experiments were carried out.The thickness of the main explosive was 20mm and the thickness of the explosives compared with the first six tests was 10 mm.The experimental results show that the initial wave length of the high purity copper interface is almost unchanged under the quasi isentropic loading of detonation products,but its amplitude increases with time.At about 5.26μs,the amplitude of the perturbation increases to about 700%of the initial value,and the strain rate can reach 10~5/s.When the initial wavelength is the same,the larger the initial perturbation amplitude is,the larger the amplitude and speed of the interface perturbation increase.But in the range of loading pressure in this work,the increase of interface loading pressure has little influence on the increase of interface perturbation.5.Strength effect on RT instability of high purity copperThe strength of metal materials is an important factor affecting the instability of RT.In essence,material strength reflects the ability of material to resist shear deformation under impact load.Therefore,two important parameters,yield strength and shear modulus,which characterize the elastic-plastic mechanical behavior of materials,are comparatively studied in this work.Based on the experimental measurement and numerical simulation,the results show that the yield strength parameter of SCG model commonly used in literature underestimates the increase of interface perturbation amplitude,and it is necessary to increase the yield strength of high purity copper from 0.12GPa to 0.3GPa,an increase of about 150%,in order to make the calculated results consistent with the experimental results.The results fully show that under the coupling action of high strain rate,large deformation and high loading pressure,the strengthening effect of metal materials is obvious,and it has an important influence on the growth of RT instability.In contrast,the effect of shear modulus on the amplitude growth of perturbation could be neglected.