Damage Mechanism Of Transparent Polymer Materials Under Impact Loading

In modern society,high-speed impact and explosion accidents increase with the devel-opment of technology,and the protection of human body becomes more and more important.Polycarbonate has a remarkable impact resistance,specific strength and transparency,and is an ideal transparent protective material.It is widely used in transparent protective parts such as bulletproof glasses and exposed to the impact of bullets and fragments.In order to improve the design of protection instruments,it is necessary to study the mechanical properties and damage mechanisms of polycarbonate under shock loading.In addition,it is also important to study the damage to human body caused by high-speed impact and explosion accidents.Ballistic gelatin,which has good transparency and dynamic response similar to muscle tissue,is widely used as a substitute for human muscle tissue in dynamic experiments.Therefore,the dynamic response of ballistic gelatin under impact loading is of great interest in studying the damage to human body.In order to study the mechanical property and damage mechanism of polycarbonate un-der impact loading,plate impact experiments are carried out on one-stage light-gas gun and two-stage light-gas gun.Photonic Doppler velocimetry is used to measure the free surface ve-locity history of the sample.Through the Hugoniot experiments,the instants of impact and shock arrival at the rear surface of sample are obtained,and the Hugoniot equation of state of polycarbonate is calculated as u_s=2.39+0.80u_p+1.24u_p²(km/s)within a peak stress range of0.248–1.290 GPa.Our data fill the gap in the Hugoniot equation of state of polycarbonate at low pressure.Through spallation experiments,the free surface particle velocity history of sample is obtained,and tensile strain rate and spallation strength of polycarbonate are then calculated.Polycarbonate undergoes strain softening and release melting under impact loading,and the softening and melting cause changes in spallation strength and tensile strain rate,respectively.X-ray computed tomography(CT)is applied to characterize the postmortem polycarbon-ate samples of spallation experiments,and three-dimensional configurations of voids inside the postmortem samples are obtained via analyzing the CT data.The morphologies of voids are quantified,inclduing sphericity and statistics of voids and damage degree.The morphologies of initial voids and their subsequent development in low-speed experiments(impact velocity?244 m/s)and high-speed experiments(impact velocity>244 m/s)are different.Since only inter-chain breakage occurs at the early stage of damage,the voids in the low-speed experi-ments are slender.Nevertheless,the voids in high-speed experiments become ellipsoidal,due to the breakage of intramolecular forces at the early stage of spallation.The difference in dam-age mechanisms between low-speed experiments and high-speed experiments leads to that in growth and coalescence,and sphericity distribution and damage distribution of voids.In order to study the damage response of ballistic gelatin,penetration experiments are car-ried out on one-stage light-gas gun.High-speed photography is used to captures its dynamic process.The diameter of cavity,penetration depth and the displacement history of projectile are obtained.Nondimensional analysis is used to analyze the penetration depth of projectile,and a general empirical equation is established for penetration depth as a function of projectile diam-eter and projectile incident velocity for low-and high-speed penetration.The nondimensional analysis yields?d=0.550?_D¹.271?v₀^{0.826-12.028/?v20}.The model of drag force is used to analyze the displacement history.The acceleration(drag force)–velocity curve during penetration is‘S'-shaped,and the average drag force per unit cross-sectional area is the same at the same in-stantaneous velocity for different projectile diameters and incident velocities in the middle and late stages of penetration.The finite element method is applied to simulate the penetration experiment on ballistic gelatin.A two-dimensional axisymmetric model is used along with the ALE method,and bal-listic gelatin is described as an elastic-plastic material with a polynomial equation of state.The numerical results are in good agreement with the experimental data and helpful for the analysis of experiments.