Dynamic Fragmentation Behavior And Mechanism Of Metallic Materials Under Complex Shock Loading

The dynamic damage and fragmentation of metal materials under complex shock loading are key scientific issues in shock physics.It is of great significance for understanding the structural evolution of materials,analyzing the internal mechanical mechanism and evaluating the material reliability and service life with the important research value in both basic engineering and national weapon applications.By taking molecular dynamics simulations,this thesis focuses on the dynamic evolution processes of metal materials under various complex shock loading conditions and different initial structures,and analyzes the micromechanical behavior and mechanism in different physical phenomena.The contents and insights of this thesis are as following:1.The dynamic fragmentation behaviors of metal lead with grooves on the surface under unsupported shock wave loading are studied.The complex fragmentation process of metal lead matrix near the free surface is analyzed.The results show that the inherent decaying characteristics of unsupported wave and rarefaction waves will lead to the multiple fracture phenomenon beneath the free surface,and finally form slugs.The tensile fragmentation processes of slugs under the velocity gradient are obtained,and the spatial distribution of clusters size is counted.It is found that the medium-size clusters conform to the power-law distribution,and the large-size clusters conform to the exponential distribution.The relationship between their characteristic volume can be predicted based on the strain rate relationship between jets and slugs according to Grady energy principle.2.The influence of polycrystalline effect on the micro-jet processes of copper is investigated.The large-scale molecular dynamics simulations of polycrystalline copper with surface grooves are carried out,and obtain the physical images of jet formation and fragmentation processes of polycrystalline samples under different shock loading conditions.The properties of jet velocities,morphology of micro-jet and micro-jet factor are given based on the analysis of microstructure and melting criterion.Through comparing with results of single-crystal metal,the mechanical mechanism of polycrystalline effect on micro-jet properties and mass are explained.It is found that the loading wave in the polycrystalline copper has a front width when the shock is not strong enough,and the grooves experience a slow compression process.At this time,the micro-jet factor is dominated by the loading waveform,and the micro-jet factor of single crystal sample is in a higher level than that of polycrystalline.With the increase of the intensity of the shock,the front width of the loading wave in the polycrystalline decreases gradually,and the unloading melting phenomenon will occur,which leads to the micro-jet quality increases significantly higher than that of the single crystal sample.When the shock velocity is high enough,the polycrystalline effect in polycrystalline can be ignored.3.The mechanical behavior of spallation damage and recompression processes of copper under double shock loading is analyzed.We firstly construct the spall damage and recompression models of single crystal copper and polycrystalline copper and obtain the evolution of free surface velocities with time.Based on the analysis of propagation of waves and the evolution of voids in the damage region,the characteristics of free surface velocity peaks and its internal mechanism are given.The velocity shoulder of secondary velocity peak is explained by analyzing the two-dimensional distribution image of velocity in the shock direction.The physical mechanism of the recrystallization process is revealed through comparing the microstructure characteristics of the initial and final spallation damage region,combined with the secondary shock response state of the spallation damage region.After recrystallization processes,there are highdensity dislocations in the spallation damage region of single crystal samples,while the different states of the spallation damage region of polycrystalline samples during the secondary shock response will lead to the difference during recrystallization process.The phenomenon of grain sizes reduction can be observed when spall damaged region of the samples undergoes the partially melted states under secondary shock loading.In summary,this thesis uses molecular dynamics simulation method to systematically study the damage and fragmentation processes and internal mechanical mechanism of metal materials under various shock loading conditions,which not only provides new cognition for physical phenomena in experiments and engineering applications,but also provides new ideas for subsequent experimental researches.