8 research outputs found
Shock formation and the ideal shape of ramp compression waves
We derive expressions for shock formation based on the local curvature of the
flow characteristics during dynamic compression. Given a specific ramp adiabat,
calculated for instance from the equation of state for a substance, the ideal
nonlinear shape for an applied ramp loading history can be determined. We
discuss the region affected by lateral release, which can be presented in
compact form for the ideal loading history. Example calculations are given for
representative metals and plastic ablators. Continuum dynamics (hydrocode)
simulations were in good agreement with the algebraic forms. Example
applications are presented for several classes of laser-loading experiment,
identifying conditions where shocks are desired but not formed, and where long
duration ramps are desired
Equation of state and strength of diamond in high pressure ramp loading
Diamond is used extensively as a component in high energy density
experiments, but existing equation of state (EOS) models do not capture its
observed response to dynamic loading. In particular, in contrast with first
principles theoretical EOS models, no solid-solid phase changes have been
detected, and no general-purpose EOS models match the measured ambient
isotherm. We have performed density functional theory (DFT) calculations of the
diamond phase to ~10TPa, well beyond its predicted range of thermodynamic
stability, and used these results as the basis of a Mie-Greuneisen EOS. We also
performed DFT calculations of the elastic moduli, and calibrated an algebraic
elasticity model for use in simulations. We then estimated the flow stress of
diamond by comparison with the stress-density relation measured experimentally
in ramp-loading experiments. The resulting constitutive model allows us to
place a constraint on the Taylor-Quinney factor (the fraction of plastic work
converted to heat) from the observation that diamond does not melt on ramp
compression