3 – Testing, Experiments and Properties of HSREP

Normal-incidence and pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer, polyurea P-1000, at high strain rates – 105–106 s−1 – and high pressures – up to 18 GPa. Configurations with samples sandwiched between two hard plates have been used to conduct constant-pressure, pressure-change, and low-pressure experiments to investigate the pressure dependence of the shearing resistance. A symmetric pressure-shear plate impact configuration has been used to measure directly the thickness-averaged nominal strain rates of the sample – as well as the tractions on both of its interfaces with linear elastic plates. Release wave experiments have been used to capture the behavior at compressive stresses below 0.5 GPa, and still lower until tensile failure occurs. From these experiments, the quasi-isentrope of polyurea is obtained as well as its high-strain-rate shearing resistance at pressures up to 18 GPa. The experimental results show that the shearing resistance of polyurea increases strongly – essentially proportionally – with increasing pressure. Based on these experimental results, a quasi-linear viscoelasticity model is introduced to capture the observed nonlinear pressure-volume behavior, the strong dependence of shearing resistance on pressure, and the strong relaxation of deviatoric stresses. This model has been implemented in Abaqus to simulate the response of polyurea P-1000 under the impact conditions of the various experiments. Results of these simulations are compared with the experimental results to gain understanding of the viability of the proposed model. Finally, an assessment is made of current understanding of the mechanical response of polyurea P-1000 and what further high-strain-rate and high-pressure research is required. Certain elastomers, when present on the front (strike-face) side of steel, significantly enhance the resistance to ballistic penetration. The mechanism is primarily twofold: (1) impact induces a viscoelastic phase transition in the polymer, with consequent large energy absorption; and (2) the transient hardening of the rubber (by approximately. three orders of magnitude) spreads the impact force laterally, reducing the local pressure. The result is armor having improvements in ballistic performance of approximately 50% without additional weight (or substantial reductions in areal density without loss of penetration resistance). Variations on this approach, using multiple rubber or steel layers, or replacing the elastomer coating with a laminate composed of thin elastomer and metal layers, are described. The question of the range of applicability of the time–temperature superposition principle or of time–temperature equivalence has often been questioned. Specifically, the assurance that the “extrapolation” of data acquired at strain rates and time scales appropriate for quasistatic deformations may be appropriate under conditions involving very high rate or extremely short time scales has not been guaranteed in the past. Evidence is presented here showing that the use of data derived from quasistatic deformation histories is applicable to time scales that are shorter by factors of 106–108 which cover most of the deformation rates germane to explosive environments.