Methods were developed to determine empirical rate laws for the shock-induced decomposition of condensed explosives. Pressure-field histories are measured with embedded gauges in plane-wave shock-initiation experiments. A Lagrangian analysis is used to integrate the fluid-dynamic conservation relations, giving the histories of density and energy fields in the reactive flow. A reactant-product equation of state is assumed and a global reaction progress variable and the associated reaction rate are calculated. Correlations of the rate to other state variables provide empirical rate laws, which prove successful in the numerical modeling of numerous initiation and detonation phenomena. Heterogeneous explosive rate laws combining three factors - shock-strength, depletion, and heating - are consistent with many shock-initiation observations and the favored nucleation and growth concept of shock-induced decomposition. The strong correlation to a simple Arrhenius heating factor is remarkable, because the temperature is an average, equilibrium quantity calculated from the equation of state, yet the formation of local high-temperature regions, or hotspots, is the dominant reaction mechanism in heterogeneous explosives. Possible physical implication of the Arrhenius correlation, and other choices for the three rate factors are discussed.

It is known that the Chapman-Jouguet theory of detonation is based on the assumption of an instantaneous and complete transformation of explosives into detonation products in the wave front. Therefore, one should not expect from the theory any interpretations of the detonation limits, such as shock initiation of det onation and kinetic instability and propagation (failure diameter). The Zeldovich-Von Neuman-Doring (ZND) theory of detonation appeared, in fact, as a response to the need for a theory capable of interpreting such limits, and the ZND detonation theory gave qualitative interpretations to the detonation limits. These interpretations were based essentially on the theoretical notion that the mechanism of explosives transformation at detonation is a combustion of a layer of finite thickness of shock-compressed explosive behind the wave shock front with the velocity of the front. However, some experimental findings turned out to be inconsistent with the the ory. A very small change of homogeneous (liquid) explosives detonation velocity with explosive charge diameter near the rather sizable failure diameter is one of the findings. The elucidation of the nature of this finding has led to the discovery of a new phenomenon. This phenomenon has come to be known as the breakdown (BD) of the explosive self-ignition behind the front of shock waves under the effect of rarefaction waves.

The facilities used by the Los Alamos Explosives Technology group to study the shock-induced decomposition of high explosives are described.

The fundamental picture that shock initiation in heterogeneous explosives is caused by the linking of hot spots formed at inhomogeneities was put forward by several researchers in the 1950's and 1960's, and more recently. Our work uses the computer hardware and software developed in the Advanced Simulation and Computing (ASC) program of the U.S. Department of Energy to explicitly include heterogeneities at the scale of the explosive grains and to calculate the consequences of realistic although approximate models of explosive behavior. Our simulations are performed with ALE-3D, a three-dimensional, elastic-plastic-hydrodynamic Arbitrary Lagrange-Euler finite-difference program, which includes chemical kinetics and heat transfer, and which is under development at this laboratory. We developed the parameter values for a reactive-flow model to describe the non-ideal detonation behavior of an HMX-based explosive from the results of grain-scale simulations. In doing so, we reduced the number of free parameters that are inferred from comparison with experiment to a single one - the characteristic defect dimension. We also performed simulations of the run to detonation in small volumes of explosive. These simulations illustrate the development of the reaction zone and the acceleration of the shock front as the flame fronts start from hot spots, grow, and interact behind the shock front. In this way, our grain-scale simulations can also connect to continuum experiments directly.