Nucleation of a sodium droplet on C60

We investigate theoretically the progressive coating of C60 by several sodium atoms. Density functional calculations using a nonlocal functional are performed for NaC60 and Na2C60 in various configurations. These data are used to construct an empirical atomistic model in order to treat larger sizes in a statistical and dynamical context. Fluctuating charges are incorporated to account for charge transfer between sodium and carbon atoms. By performing systematic global optimization in the size range 1<=n<=30, we find that Na_nC60 is homogeneously coated at small sizes, and that a growing droplet is formed above n=>8. The separate effects of single ionization and thermalization are also considered, as well as the changes due to a strong external electric field. The present results are discussed in the light of various experimental data.Comment: 17 pages, 10 figure

The mechanics of large-strain inhomogeneous deformation of polymeric materials under dynamic loading conditions

Dynamic tension and Taylor impact experiments were performed to study the high-rate inhomogeneous deformations in polymers under a range of conditions. The mechanics of deformation were examined utilizing finite element simulations together with a physically based constitutive model of large-strain high-rate deformation behaviour of polymers. The comparison of the numerical predictions with the high-speed photographic data and final deformed shapes demonstrates the effectiveness of the constitutive model in predicting the progression of complex inhomogeneous deformation events during impact loading scenarios

High-rate thermomechanical behavior of poly(vinyl chloride) and plasticized poly(vinyl chloride)

A combined experimental and analytical investigation was carried out in order to develop predictive capabilities for the rate-dependent behavior of poly(vinyl chloride) (PVC) and a dioctyl phthalate (DOP)-plasticized PVC, with focus on predicting the thermo-mechanically coupled behavior under high rates of deformation. The two materials were studied experimentally using both dynamic mechanical analysis (DMA) and compression testing over a wide range of strain rates (10$^{ - 4}$ s$^{ - 1}$ to 2000 s$^{ - 1})$. DMA testing revealed both an $\alpha$-transition and a low-temperature $\beta$-transition (-56$^{\circ}$C) in the neat PVC; the incorporation of 20wt% DOP in PVC reduced the $\alpha$-transition temperature by 54$^{\circ}$C, and also suppressed the $\beta$-transition peak. In compression testing, rate-sensitivity transitions were observed in both the neat PVC and the PVC-20wt% DOP compound. The transition in PVC is attributed to the shift of the $\beta$-transition, whereas the transition in the 20wt% DOP blend is due to the rubbery-to-glassy transition as the deformation rate goes from low to high. A constitutive model for the finite strain deformation of amorphous polymers, introduced elsewhere [1,2] and tailored here for the two material systems of interest, is shown to capture the large deformation stress-strain behavior at all rates tested

The response of polymethyl methacrylate (PMMA) subjected to large strains, high strain rates, high pressures, a range in temperatures, and variations in the intermediate principal stress

This article presents the response of polymethyl methacrylate (PMMA) subjected to large strains, high strain rates, high pressures, a range in temperatures, and variations in the intermediate principal stress. Laboratory data from the literature, and new test data provided here, are used in the evaluation. The new data include uniaxial stress compression tests (at various strain rates and temperatures) and uniaxial stress tension tests (at low strain rates and ambient temperatures). The compression tests include experiments at ̇ε = 13,000 s−1, significantly extending the range of known strain rate data. The observed behavior of PMMA includes the following: it is brittle in compression at high rates, and brittle in tension at all rates; strength is dependent on the pressure, strain, strain rate, temperature, and the intermediate principal stress; the shear modulus increases as the pressure increases; and it is highly compressible. Also presented are novel, high velocity impact tests (using high-speed imaging) that provide insight into the initiation and evolution of damage. Lastly, computational constitutive models for pressure, strength, and failure are presented that provide responses that are in good agreement with the laboratory data. The models are used to compute several ballistic impact events for which experimental data are available