Models For Projectile Impact Into Hybrid Multi-Layer Armor Systems With Axisymmetric Or Biaxial Layers With Gaps

摘要

Analytical and numerical modeling of fibrous material resistance to penetration under impact by high-velocity projectiles has been of great interest not only for personnel protection reasons but also because trial-and-error testing is costly and time-consuming. In this thesis, two PC-based models are developed for projectile impact into a multi-layer system of membrane layers with nonzero spacings between them. The projectile is a standard right circular cylinder (RCC) often used in laboratory experiments to compare material systems, and the models blend theoretical analysis and numerical simulation to characterize the interaction between the projectile and the various layers. We first consider a system of axisymmetric layers under impact by an RCC projectile. In particular, we consider such performance measures as the critical strains in layers resulting in their failure, the strains in unfailed layers, critical layer gaps, the number of layers penetrated, and the residual velocities in cases where all layers have been penetrated. The model allows variation of mechanical properties from layer to layer as well as variations in spacings between layers, in order to study their combined effects on the ballistic performance of the system. Case studies are performed on the ballistic impact response of fibrous material systems of particular interest in body armor. These are ultra-high molecular weight polyethylene (UHMWPE) fibers such as DSM's Dyneema SK76, as well as aramid fibers such as duPont's Kevlar-29. We also develop a semi-analytical model for a multi-layered biaxial, elastic membrane system impacted by an RCC projectile. The model builds on a single-layer membrane model, which has been under development by collaborators in the overall body armor work at Cornell University. Key assumptions and parameter values in the single layer model were guided by simulation results using a code based on the finite difference method (FDM) incorporating an algorithm frequently used in molecular dynamics simulations. The code was originally developed by researchers at DSM (makers of Dyneema) and has been modified by the author and several collaborators at Cornell University to suppress local strain concentrations and dynamic artifacts resulting from the descretization of the structure, and to better handle the current geometry. Numerical simulations of impact into a flexible panel are performed where the main emphasis is on a comprehensive understanding of the strain and displacement fields, as well as on the velocity fields versus time. The panel is treated as a single biaxial membrane with negligible shear stiffness compared with the tensile stiffness, and is assigned the properties of Dyneema SK76 or Kevlar 29 biaxial fabrics and flexible composites having about 15 to 20 percent matrix content. Numerical results are obtained through incremental integration of differential equations using small time steps. Compared to simulations using the modified DSM code, there are several important improvements: (1) The calculation time has been accelerated by at least a factor of 1000; (2) Results under different parameter combinations can be obtained for larger geometric sizes and much longer times; and (3) Modeling multi-layer systems is now possible, and we present results for several cases.