Enhanced mechanical energy absorption via localized viscoplasticity of nano - cellular polymer coating under supersonic impact loading

Materials under viscoplastic deformation at ultrahigh strain rates ( > 10(6) s(-1 )) often demonstrate anomalous properties due to thermal and stress localization. As a model system, thermoplastic nanocellular material (NCM) coatings are produced by consolidating microscopic polystyrene shells of nanoscale thin walls using self-limiting electrospray deposition. As both the spatial and temporal characterization scales are crucial, the NCMs are characterized by laser-induced projectile impact test (LIPIT) for understanding their ultrahigh-strain-rate plasticity originating from their porous structures. In LIPIT, supersonic collisions of rigid microspheres create these extreme physical conditions at the microscale. Viscoplastic vertical densification without the Poisson effect is the foremost process in the ultrahigh-rate plastic deformation of the NCM coatings. When the extreme nature of the viscoplastic deformation is promoted by increasing the porosity and reducing the thickness of NCM coatings, significantly more energy dissipation is observed without more material due to the localized feedback between adiabatic plastic deformation and thermal softening. Despite the stochastic and isotropic structural architecture of the NCM, the specific energy absorption of the NCM is high as 170 kJ/kg at the deformation speed of 400 m/s, which is attributed to the nanoscale effects from the thin wall thickness of NCM coatings. The findings suggest the general design rule for enhancing specific energy absorption by creating viscoplastic hot spots under impact loading.