Study On Impact And Penetration In Ballistic Gelatin

The study of impact and penetration phenomena involves a variety of physical and mechanical problems: penetration resistance, penetration cavity, blunt impact shock wave, material behavior and so forth. Its application has been extended to aerospace industry, marine industry, automotive engineering, protection engineering, national defense industry, and biomedical engineering. The study of impact and penetration into ballistic gelatin is a fundamental basis for small arm damage mechanism and efficiency in mechanical trauma and biomedical engineering.Ballistic gelatin is a widely used biological tissue surrogate for establishing physical, numerical and analytical model of impact and penetration, and 10wt% ballistic gelatin at 4℃ is accepted as a standard simulation target both in China and abroad. Ballistic gelatin can stand large elastic deformation without plastic deformation before fracture. It is also viscoelastic. In the present study, rheological experiments of compression and shearing are carried out to obtain the linear and nonlinear viscoelastic parameters and the fracture characters of the ballistic gelatin. Constitutive models are established concerning to different problems in the blunt impact and penetration cavitation.The present work firstly deals with abundant experimental data of steel and tungsten spheres penetrated into ballistic gelatin, considers the penetrating process as penetrator opening cavity process in the target and establishes resistance force model and cavity movement model based on the energy conservation law. The resistance force and penetration cavity are related by a critical parameter: the work needed to open unit volume of cylindrical cavity in gelatin. The experimental results show that this parameter is rate sensitive and can be linearly related with a properly defined strain rate. Since the conception of "opening cavity" is very useful and important in the problem of penetration, the thesis has done a good deal of work to establish the cavity expansion model for soft and elastic materials like ballistic gelatin. The pressures needed to open a spherical or a cylindrical cavity in the soft target are derived and the target is described by Mooney-Rivlin constitutive model. This cavity expansion model can properly describe the penetration cavity in ballistic gelatin.When a bullet cannot penetrate into the target behind a body armor, the blunt impact shock wave can make damage. This thesis studies the linear viscoelastic behavior of ballistic gelatin by using a viscoelastic spherical wave propagation model to describe the impact shock wave in ballistic gelatin. With properly defined boundary conditions, the shock wave front inside the target can be calculated using characteristic-line method. The established viscoelastic model considerably improves the prediction of shock wave attenuation in the ballistic gelatin.When a bullet penetrates into ballistic gelatin with high velocity, the yaw angle grows due to instability. Based on the published studies, a simplified two degrees of freedom model, which the mass center of bullet moves in a straight line and the bullet tumbles around its mass center, is established to describe the velocity retardation and yaw angle movements. The experimental data of two types of bullets penetrating in gelatin are fitted to obtain the resistance coefficients and moment coefficients. It’s found that the yaw angle growth is greatly dependent on the initial yaw angle but independent on the initial penetration velocity.