The plateau stress and energy absorption of low density (≤300 kg/m3) polyurea (PU) foams and expanded polystyrene (EPS) were measured at deformation rates ranging from 0.004 s−1 to 5000 s−1. Low (≤10−1 s−1) strain rate testing was performed using an Instron load frame, intermediate (101–102 s−1) strain rates using a drop-weight impact tower, and high (≥103 s−1) strain rate conditions using a modified split-Hopkinson pressure bar. The plateau stress and energy absorption of low density PU foams exhibit a strong rate dependence across all deformation rates. This result has been previously unreported for low density polymer foams under low and intermediate strain rates. The strain rate sensitivity of PU foams was found to be strongly dependent on cell size for low strain rates and cell wall aperture size for intermediate and high strain rates. EPS type foam, however, remained nearly insensitive to strain rate. At low and intermediate strain rates, the plastic crushing in the EPS and the high plateau stress yield a much higher energy absorption capability than the viscoelastic dissipation in the PU foams. However, PU foams were found to display similar energy absorption properties as EPS based foams under high strain rates. Thus, controlling the strain rate sensitivity of PU foams through aperture diameter can lead to an increase in energy absorption properties at high strain rates, while simultaneously maintaining the peak stress below certain injury thresholds. Additionally, unlike EPS, which undergo plastic crushing after first impact, flexible polyurea foams will recover fully after each impact and thus will have multiple hit capabilities. This will allow these materials to have a wide range of applications, in advance body armors and protective headgears to use in low-cost protection systems for a wide range of military platforms, civilian, and space applications.

In this work we report experimentally observed inelastic recovery within a bulk directionally-cast Ag-Cu eutectic system tailored to possess a high density of incoherent twin interfaces. Incremental high strain-rate compression experiments were performed along the [101] crystallographic direction. Initial loading to a compressive strain of 0.10 was followed by inelastic strain recovery of 0.02 to 0.035. Further incremental loading saw decreasing inelastic strain recovery with each additional loading increment. Once a total compressive stain of 0.22 was reached there was cessation of inelastic strain recovery. Quasi-static compression and thermal experiments were conducted to elucidate the necessary loading conditions for inelastic recovery to occur in the Ag-Cu material and to provide insight of possible active recovery mechanisms.

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