Susan Bartyczak

DYNAMIC PROPERTIES OF POLYUREA 1000

The shock response of the viscoelastic polymer material polyurea 1000 has been investigated. Sabots carrying Al or Cu metal disks were launched into target assemblies containing the polyurea material. The target consisted of a thin metal disk on the impact side, a 6.5‐mm‐thick polyurea disk, and a thick metal backup disk. 50‐Ω manganin gauges were epoxied between the metal/polymer and polymer/metal interfaces to measure the interface stresses and shock transit time. Measured longitudinal stresses ranged from 6 to 43 kbar. The measured shock velocity‐particle velocity relationship was linear over this stress range. Maximum volume compression was about 30% for the initial shock wave. Several reshock waves were also measured for each experiment.

Investigation of microstructural changes in impacted polyurea coatings using small angle X-ray scattering (SAXS)

Three polyureas with decreasing soft segment molecular weights of 1000, 650, and a 250/1000 blend were molded onto circular steel plates and then impacted with a high speed (275 m/s) conical-shaped steel cylinder. 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 the Argonne National Laboratory. Analysis revealed that the hard domains of the polyureas with lower molecular weight soft segments reformed and oriented over a greater area of the coating, thus increasing the polymer strain hardening and resulting in visibly less out of plane bilayer deformation. This agrees with the hypothesis that polymer strain hardening is a mechanism that retards necking failure of the metal plate.

Mechanical Characterization and Numerical Material Modeling of Polyurea

The mechanical behavior of four unique blends of polyurea materials has been investigated through a combined experimental and computational study. Mechanical characterization of each material was evaluated under both tensile and compressive loading at strain rates ranging from 0.01 to 100 strains per second (1/s). Planar blast wave experiments utilizing a 40 mm light gas gun were also conducted which imparted strain rates up to 104 strains per second (1/s). The material testing results showed that stress-strain response is a function of loading, strain level, and strain rate. These results were utilized to define a non-linear rubber material model in Ls-Dyna which was validated against the test data through a series of “block” type simulations for each material. Each material model was shown to replicate both the tensile and compressive behavior as well as the strain rate dependence. The material models were subsequently extended to the simulations of the blast wave experiments. The blast wave simulations were shown to accurately capture wave propagation resulting from a shock type pressure loading as well as the stress magnitudes of the transmitted waves after passing through the respective polyurea materials. The current study has resulted in the mechanical characterization of four polyurea materials under tensile/compressive loading at increasing strain rates, a suitably validated numerical material model, and suitable correlations between experimental and simulation results.

8 – Modeling and Simulations, Applications in Ballistic and Blast

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.

7 – Properties of Hard and Soft Viscoelastic Polymers Under Blast Wave Loading

Abstract In the present work the properties of hard and soft polymers are measured under blast wave loading at 0.5, 1.3, and 2.1 bar. A new target assembly was designed for an existing gas gun to perform these experiments. The hard polymers are Kevlar KM2 and polyurea 1000; the soft polymers are Zorbium 83i and 110i, and Sorbothane 30, 50, and 70. Kevlar KM2, Zorbium 83i, and Zorbium 110i are used in the Lightweight Marine Corps and Army Advanced Combat Helmets. The remaining four materials were investigated for comparison with these materials and for possible inclusion in future helmet designs. Bilayer attenuation experiments indicate that inclusion of polyurea 1000 and Sorbothane 30 in the helmet may improve attenuation. Longitudinal blast wave velocity versus particle velocity was determined for each polymer. The largest increase in wave velocity occurred for Sorbothane 30 and Zorbium 83i and 110i. Stress–strain relations were also determined. The maximum μstrain values for Kevlar KM2, polyurea 1000, and the Sorbothane polymers were 100–200, whereas the values for the more compressible Zorbium foams were 20,000–60,000. As the typical blast wave risetime was 1 μs, strain rates were 100–200 s-1 for the harder polymers, and 20,000–60,000 s-1 for the Zorbium foams. The stress–strain relations were used to determine the blast energy absorbed at the three incident blast pressures for each polymer. At 2.1 bar blast pressure, polyurea 1000 and Kevlar KM2 absorbed the least at 2 and 11%, respectively, and Zorbium 83i and 110i absorbed the most at 25 and 46%, respectively. As blast wave input and output stresses were measured for the polymer targets, it was possible to determine an attenuation coefficient for the decrease in blast amplitude as a function of polymer thickness. Kevlar KM2 showed the least attenuation (56% for a 6.35 mm thickness) and Zorbium 83i showed the most attenuation (89% for a 6.35 mm thickness). These new measured properties can be used to improve blast wave simulations of the helmet-head system.