8 - Modeling and Simulations, Applications in Ballistic and Blast

Abstract

Three polyureas of decreasing soft segment molecular weights of 1000, 650, and a 250/1000 blend, with increasing modulus and hysteresis, were molded onto circular steel plates and then impacted with a high-speed (275 m/s) pointed projectile. The polyurea layer of the post mortem bilayers was characterized on a molecular level by small angle synchrotron X-ray scattering (SAXS) at the Advanced Photon Source at Argonne National Laboratory. Analysis revealed that hard domains of the polyureas of the lower molecular weight soft segment reformed over a greater area of the coating that resulted in less out-of-plane bilayer deformation. This supports the hypothesis that polymer strain hardening is a mechanism that retards necking failure of the metal plate. The mechanical behavior of elastomeric copolymer polyureas at extreme loading conditions is further discussed in this chapter. Here the Taylor impact behavior of the exemplar polyureas (PU1000 and PU650), where extreme deformation and deformation rates are incurred, is elucidated in experiments and numerical simulations using the constitutive modeling framework of the two polyurea copolymers detailed in Chapter 4. The hybrid glassy and rubber nature of the elastomeric copolymers is addressed by examining the extreme deformation of “model” glassy and rubbery polymers, for which the constitutive laws have been partially selected from the original polyurea models. Then the extreme behavior of copolymeric polyureas is rationalized in the Taylor impact experiments and simulations, revealing how the elastomeric copolymer polyurea can take advantages from both glassy and rubbery polymeric features in terms of resilience, shape recovery and energy dissipation under such extreme deformation conditions. Polyurea is of particular interest to armor systems designers due to its unique physical behavior, low density, and favorable manufacturing and application techniques. Inherent rate and pressure dependencies exhibited by polyurea allow this otherwise soft material to exert large resistive forces and dissipate large amounts of energy under ballistic impact conditions. The design and evaluation of armor systems using complex materials such as polyurea may be enhanced with improved analytical material models implemented within appropriate computational tools. This work details a modified temperature and pressure dependent viscoelastic constitutive model for polyurea under ballistic impact conditions. The constitutive model is implemented and demonstrated within the shock physics hydrocode CTH. Demonstration of the models performance is first given through comparison with high-rate confined compression and pressure–shear plate impact test data. Second, model performance and design optimization considerations are demonstrated through comparison with various experimentally studied polyurea coated armor configurations.