Dynamic response of metal-polymer bilayers subjected to blast loading




Albrecht, Aaron Berkeley

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The use of compliant coatings, in particular polyurea, for improved blast protection of structures has been reported recently in the literature. The goal of this research is to develop a comprehensive understanding of the reasons for improved performance of coated structures through experimentation and correlation with simulation. The different factors influencing the response of an elastomer coated ductile metal subjected to a blast load have been examined and quantified. First, dynamic strain localization in the metal is a precursor to ductile failure; this was characterized for the metal of interest with and without the polymer coating. Experiments with the expanding ring/tube and experiments have demonstrated that for Al 6061-O and Al 3003-H14, the localization strain is unaffected by both deformation rate and the polymer coating; however, two important effects of the coating have been explored. First the additional mass of the coating provides an inertial resistance. Second, the flow resistance of the polymer provides continued dissipation of energy even after the metal has yielded potentially preventing failure in the metal, or at least containing fragments. These effects were examined for two different types of polymers – polyurea, an elastomer and polycarbonate, a thermoplastic shear yielding polymer. It is shown that these two effects can be used to tailor the coating to optimize blast protection of the bilayer system. In order to take advantage of this optimization, the constitutive behavior of the elastomer coating must be determined at strain rates and loading conditions that are experienced in the blast loading; these strain rates are in the range of 1000 to 10,000 per second. This has been accomplished through a hybrid method that combines measurements with numerical simulations to extract the constitutive response of the material. The strain rate dependent behavior of polyurea for rates in the range of 800-8000 per second has been determined by measuring the spatio-temporal evolution of the particle velocity and strain in a thin strip subjected to high speed impact loading that generates uniaxial stress conditions and comparing this with numerical simulations of the one-dimensional problem using the method of characteristics. A similar scheme to track the particle velocity and strain during the axisymmetric deformation of a membrane subjected to high speed loading has also been developed; this requires two projections of the deformation to be obtained in order to facilitate the measurement of axial and kink waves in the membrane. The finite volume method is adapted for simulations of these dynamic uniaxial and axisymmetric problems with a view towards simulating shock waves that are expected to form in some loading conditions. The hybrid method is used once again to characterize the constitutive response. The axisymmetric experiments have demonstrated the inability of the uniaxial models for both polyisoprene rubber and polyurea to completely capture their behavior during a more complex loading, and left a need for further work on characterizing the dynamic constitutive response of these polymers.



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