39 research outputs found
Modifications and Applications of the HERMES model: June - October 2010
The HERMES (High Explosive Response to MEchanical Stimulus) model has been developed to describe the response of energetic materials to low-velocity mechanical stimulus, referred to as HEVR (High Explosive Violent Response) or BVR (Burn to Violent Reaction). For tests performed with an HMX-based UK explosive, at sample sizes less than 200 g, the response was sometimes an explosion, but was not observed to be a detonation. The distinction between explosion and detonation can be important in assessing the effects of the HE response on nearby structures. A detonation proceeds as a supersonic shock wave supported by the release of energy that accompanies the transition from solid to high-pressure gas. For military high explosives, the shock wave velocity generally exceeds 7 km/s, and the pressure behind the shock wave generally exceeds 30 GPa. A kilogram of explosive would be converted to gas in 10 to 15 microseconds. An HEVR explosion proceeds much more slowly. Much of the explosive remains unreacted after the event. Peak pressures have been measured and calculated at less than 1 GPa, and the time for the portion of the solid that does react to form gas is about a millisecond. The explosion will, however, launch the confinement to a velocity that depends on the confinement mass, the mass of explosive converted, and the time required to form gas products. In many tests, the air blast signal and confinement velocity are comparable to those measured when an amount of explosive equal to that which is converted in an HEVR is deliberately detonated in the comparable confinement. The number of confinement fragments from an HEVR is much less than from the comparable detonation. The HERMES model comprises several submodels including a constitutive model for strength, a model for damage that includes the creation of porosity and surface area through fragmentation, an ignition model, an ignition front propagation model, and a model for burning after ignition. We have used HERMES in computer simulations of US and UK variants of the Steven Test. We have recently improved some of the submodels, and report those developments here, as well as the results of some additional applications
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Multi-scale Computer Simulations to Study the Reaction Zone of Solid Explosives
We have performed computer simulations at several different characteristic length scales to study the coupled mechanical, thermal, and chemical behavior of explosives under shock and other pressure loadings. Our objective is to describe the underlying physics and chemistry of the hot-spot theory for solid explosives, with enough detail to make quantitative predictions of the expected result from a given pressure loading
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Computer Simulations to Study the Effects of Explosive and Confinement Properties on the Deflagration-to-Detonation Transition (DDT)
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Cumulative-strain-damage model of ductile fracture: simulation and prediction of engineering fracture tests
A cumulative-strain-damage criterion is used to predict the initiation and propagation of fracture in ductile materials. The model is consistent with a model of ductile rupture that involves void growth and coalescence. Two- and three-dimensional finite difference computer codes, which use incremental-plasticity theory to describe large strains with rotation, are used to trace the history of damage in a material due to external forces. Fracture begins when the damage exceeds a critical value over a critical distance and proceeds as the critical-damage state is reached elsewhere. This unified approach to failure prediction can be applied to an arbitrary geometry if the material behavior has been adequately characterized. The damage function must be calibrated for a particular material using various material property tests. The fracture toughness of 6061-T651 aluminum is predicted
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Computational Studies of the Skid Test: Evaluation of the Non-Shock Ignition of LX-10 Using HERMES
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Reduced yield detonation characteristics in large failure diameter materials
We have made detailed measurements of the approach to steady, self-supported propagating shock waves at greatly reduced yield in composite propellants. Propa- gation velocities are less than one half the theoretical value expected for full reac- tion at the sonic plane. Previous experimental studies 1 have given evidence of similar behavior. Also, previous theoretical work 2 in an analytic form has shown the possibility of reduced yield detonations. We have developed a reaction model coupled with a hydrody- namic code that together provide a description of the coupling of the complex reac- tion behavior with shock propagation and expansion in energetic materials. The model results show clearly that if the dependence of reaction rate on pressure is of sufficiently low order and the mode of consumption is by "grain burning" the calcu- lated detonation behavior closely parallels the observed non-ideal results. We describe the experiments, the reaction model, and compare experimental and calculational results. We also extend the model to predict results in the unexplored regime of very large size charges
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Experiments and computer simulations of the dynamic cavity formed by a particulated shaped-charge jet in sand
Experiments have been carried out to measure the dynamic cavity growth of dry sand during penetration by particulated jets from Viper 65-mm-diameter, Cu-lined conical shaped charges at 1,000-mm standoff. The sand target was instrumented with foil switches, piezoelectric pins, and pressure transducers. Flash radiography at 450-keV was used to characterize the jets before impact and to image the target hole during jet penetration. The authors have developed a dry sand equation of state based on existing Hugoniot data as input to a porous material model incorporated in the 2-D arbitrary Lagrangian-Eulerian hydrocode CALE. They have carried out sand penetration simulations in which the particulated jet is modeled as hot copper rods. By varying parameters in the sand and copper descriptions they identify those features that affect the dynamic cavity formation
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Hazards Response of Energetic Materials - Developing a Predictive Capability for Initiation and Reaction under Multiple Stimuli
We present our approach to develop a predictive capability for hazards--thermal and nonshock impact--response of energetic material systems based on: (A) identification of relevant processes; (B) characterization of the relevant properties; (C) application of property data to predictive models; and (D) application of the models into predictive simulation. This paper focuses on the last two elements above, while a companion paper by Maienschein et al focuses on the first two elements. We outline models to describe the both the microscopic evolution of hot spots for detonation response and thermal kinetic models used to model slow heat environments. We show examples of application to both types of environments