High Strain-rate Impact Response And Damage Mechanisms Of Aluminum Matrix Alloy And Its Composites

In aerospace and defense industries,structural materials are inevitably subjected to impact loading,which is likely to produce catastrophic consequences in an extremely short period of time.Impact dynamics,as a discipline dedicated to investigate the impact response of materials at high strain rates,has been a major focus of scientific researchers.The investigation of impact dynamics,however,is very complicated,because impact dynamics is an interdisciplinary discipline covering mechanics,materials science and physics,and involves a multiscale range from macroscopic to microscopic scale.Therefore,understanding of the dynamic response and potential mechanism on the material under impact loading is inadequate.Aluminum matrix alloys and their composites,as typical lightweight structural materials,are widely used in several fields with extreme environments.The systematic investigations of aluminum matrix alloys and their composites under impact loading can not only provide a better understanding of the dynamic deformation and damage mechanisms,but also provide scientific guidance for the application of aluminum alloys and their composites in extreme environments.In this thesis,the 2024 aluminum alloy and its composites with different microstructures are selected for plate impact and penetration experiments.The main contents are as follows:Firstly,the spall response of boron-doped aluminum with the simple microstructure is investigated mainly to explore and improve the gas gun loading technology and X-ray computed tomography.Effects of the second-phase boron particles（content 0.07 wt.%and 0.15 wt.%）on the spall strength of high-purity aluminum are discussed.According to the macroscopic free surface velocity histories,the addition of boron particles has no influence on the Hugoniot elastic limit（HEL）,but a significant effect on the spall strength.With increasing boron particle content,the spall strength becomes increasingly lower.In addition,the spall strength of highpurity aluminum increases with increasing impact velocity,but the spall strength of borondoped aluminum increases first and then decreases slightly.Scanning electron microscopy（SEM）and synchrotron X-ray computed tomography（XCT）results and analyses indicate that the spall strength is mainly determined by the weak particle-matrix interface in the materials,and the number of particle-matrix interfaces determines the nucleation and growth pattern of the microvoids.In addition to second-phase particles,carbon nanotube（CNT）is also extensively used as a reinforcing phase in metal matrix composites.To understand the effects of CNTs and interfaces on the spall strength of aluminum alloys,a specially designed 2024-aluminum matrix composite（CNT/2024Al）is investigated.The results indicate that the addition of CNTs increases the spall strength of the 2024Al alloy,contrary to the interfacial damage nucleation mechanism in conventional materials（e.g.,boron-doped aluminum）,which may be attributed to the large amount of energy consumed by the pull-out and fracture of CNTs.Furthermore,the anisotropy of the microstructure shows a negligible effect on the HEL,but has a significant effect on the spall strength.The maximum spall strength is observed when loaded along the extrusion direction（i.e.,the direction of lamellar elongation）.In the experiment,the crack propagation direction and damage characteristics can be attributed to the collinear or non-collinear extension of microcracks.Besides the plate impact,ballistic penetration is also a frequently encountered condition for structural materials in service.However,the stress state of materials is very complex under ballistic impact loading.Therefore,the 2024Al alloy with relatively simple microstructure is chosen for experimental and simulation investigations.Firstly,the multiple ballistic impact experiments are conducted for the 2024Al alloy using spherical projectiles with an impact velocity of 400 m s-1,to investigate the effect of impact number on the dynamic deformation and damage of the 2024Al alloy.The impact process is captured by high-speed photography.Three-dimensional laser scanning technology is applied to obtain high-precision crater parameters（including crater depth,diameter,and volume）.With increasing number of impacts,there is a slight increase in the crater diameter,but a significant increase in the crater depth and volume.The crater parameters are exponentially related to the number of impacts.Microstructural characterizations of the postmortem samples reveal that micron-sized deformation twinning occurs in the 2024Al alloy when the shear strain exceeds a certain threshold,due to the self-pinning and entanglement of dislocations under high strain rate and large shear deformation conditions.Additionally,the {111} pole figures of the deformation twins and the deformation bands show a pair of points overlapping,indicating that the formation of twins is associated with deformation bands.As a natural extension of the multiple impact experiments,the systematic single-shot ballistic impact experiments are carried out on 2024Al alloy with the impact velocity of 105-1930 m s-1.As the impact velocity increases,the normalized crater depth conforms to the Poncelet model,and the normalized crater volume follows a power-law relation.The microstructural characterizations reveal that at the impact velocity of 916 ms-1,micron-sized deformation twins are observed in the high-stacking-fault-energy aluminum alloy,and we believe that the formation of twins is closely related to shear strain and temperature.With increasing impact velocity,the deformation and damage mechanisms are deformation banding,adiabatic shear banding,microcracking,deformation twinning,and melting in order.Furthermore,multiple ballistic impacts are compared with single-shot impact,and it is found that the crater parameters are similar for the single-and multiple-shot impact cases when the total impact energy is the same.However,there are significant differences in the microstructures of postmortem samples.Eventually,both multiple-and single-shot impacts are simulated by finite element models using the Cowper-Symonds constitutive model,to predict crater morphology,instantaneous velocity fields,stress fields,equivalent plastic strain fields and shear strain fields.The simulation results are found to match the experimental data well and can reproduce the experimental process to aid in the interpretation of the experimental phenomena.In addition,the JohnsonCook constitutive model are compared with the Cowper-Symonds constitutive model,and it is found that at lower velocities（<500 m s-1）,the difference between the results obtained by the two models is relatively small,while at higher impact velocities（>500 m s-1）,the two models show increasing differences with increasing impact velocities.It may be because the JohnsonCook constitutive model is derived from experimental results of split Hopkinson bars with relatively low strain rates（102 s-1-104 s-1）,and is not applicable to penetration experiments with relatively high strain rates(10⁴ s^-1-10⁶ s^-1).