| Metal materials have completely different dynamic response characteristics under different impact loading conditions(such as plastic deformation,impact phase change,melting,damage and lamellar fracture,micro-matter injection,etc.),and understanding these dynamic responses is particularly important for discovering new materials,new phenomena,new physical state.The dynamic response of metallic materials to loading at different strain rates has been investigated both nationally and internationally,however,only a single strain rate or component has been designed to achieve a specific strain rate loading.There are no experimental and simulation techniques to systematically compare the path,microstructure,etc.changes of shock loading and ramp loading.At the same time,the dynamic response mechanism of the material under ramp loading and the microstructure evolution mechanism are not clear enough.Therefore,in this paper,molecular dynamics simulation,hydrodynamic simulation,and ramp loading experiment of gradient flyer are combined to study the microstructure evolution mechanism,shock response and spalling behavior of single crystal Al under ramp loading.Molecular dynamics simulations were used to study the shock response and spalling behavior of single crystal Al at different initial temperatures(such as spalling strength,pore nucleation,growth and merging processes,dislocation density evolution,thermodynamic pathways,etc.).First,the Rakine-Hugoniot conservation theory and the Hugoniot temperature equation are used to explain the reduction in impact pressure and the Hugoniot temperature increment due to higher initial temperatures;the spalling strength of single crystal Al decreases with increasing initial temperature.For the same temperature,as the impact strength increases,the spalling strength first increases and then decreases due to a competitive mechanism between temperature softening and strain rate hardening;the number of holes and the degree of damage increases with increasing initial temperature.At low impact velocities(v=1.0 km/s),the changes in the number of holes and the degree of damage are highly dependent on the initial temperature,whereas at high impact velocities the opposite result is obtained;the dislocation density of single crystal Al increases with increasing initial temperature and decreases when the temperature increases to a certain level;the thermodynamic path of single crystal Al during impact compression can be divided into compression,stretching or release and damage phases.Melting can occur in all three phases,depending on the initial temperature and impact strength,with melting of the target material occurring earlier as the impact strength and initial temperature increase.Molecular dynamics simulations are used to study the dynamic response behavior of single crystal Al at different strain rates(such as free end velocity profiles,damage and hole number evolution,spalling strength,etc.).From the free end velocity curves of single crystal Al,it can be seen that under impact loading and ramp loading,the same peak velocity can be reached eventually by maintaining a certain time duration,and the higher the compressive strain rate will lead to an earlier elastic-plastic transition time of single crystal Al.When t_r=25 ps,the spalling strength of single crystal Al is only 7.0GPa,when t_r>25 ps,the spalling strength starts to increase;with the decrease of the ramp loading time t_r,the damage degree and the number of holes of single crystal Al will gradually increase.However,when t_r=0 ps(impact loading),the damage degree of single crystal Al gradually increases but the number of holes decreases;at the same ramp loading time t_r,both peak pressure and temperature decrease as the distance d to the free surface decreases.The peak pressure in the central material element is up to 40GPa,while the peak pressure near the free surface is only 15 GPa;the compressive strain rate and dislocation density in the target material gradually increases as the ramp loading time t_rdecreases,but the dislocation density decreases when t_r=0 ps.The MLEP software was used to conduct hydrodynamic simulations to explore loading with different strain rates using Al-Cu gradient flyers and to investigate the dynamic response behavior of single crystal Al under the above loading conditions.Firstly,the model and physical parameters of the Al-Cu gradient flyer impacting the Al target were determined according to the designed curves;then,the effect of the layer thickness of the Al-Cu gradient flyer on the strain rate in the ramp loading was analyzed by MLEP,and it was concluded that the strain rate would increase as the layer thickness of the flyer increased;then,the effect of the impact velocity on the strain rate in the ramp loading was analyzed by MLEP,and it was concluded that the strain rate would increase as the impact.Finally,by combining the effects of layer thickness and impact velocity on strain rate in ramp loading,it can be concluded that both thinning the layer thickness and increasing the impact velocity further increase the strain rate variation range.The free-surface waveform characteristics of Al-Cu gradient flyer and Cu loading processes,as well as the structural characteristics of single-crystal Al after loading,were investigated to analyze the link between ramp loading and impact loading.The results show that when the impact velocities are the same,the spalling curve time of single crystal Al under ramp loading is large and the peak free surface velocity is low compared with that of impact loading;the hole bands corresponding to single crystal Al under ramp loading and impact loading are both distributed in a straight line,but the hole bandwidth of impact loading is smaller than that of ramp loading,and the holes formed during impact loading have irregular shapes,but the holes during ramp loading The shape of the holes during ramp loading is more regular;under ramp loading and impact loading,the grain boundaries of single crystal Al recovery samples are small-angle grain boundaries,and the percentage of small-angle grain boundaries is higher than 75%,but compared with ramp loading,the percentage of small-angle grain boundaries of impact loading recovery samples is greater than 3°,and the energy convergence around the holes is greater,which leads to the refinement of the grains around the holes,smaller average grain size,greater grain deflection,and greater grain orientation angle.The grain orientation angle is greater.The analysis suggests that the strain rate and temperature rise effect of oblique wave loading is lower than that of impact loading,which are the main reasons for the change of spalling strength of single crystal Al. |
