Session Z4: Inelastic Deformations, Fracture and Spall XIV

Relative importance of plasticity and fracture/friction in ignition of polymer-bonded explosives (PBXs)

The ignition of energetic materials (EM) under dynamic loading is mainly controlled by localized temperature spikes known as hotspots. Hotspots occur due to several dissipation mechanisms, including viscoplasticity, viscoelasticity, and internal friction along interfaces and crack surfaces. To analyze the contributions of these mechanisms, we carry out a computational study of the damage evolution, energy dissipation, and ignition behavior polymer-bonded explosives (PBXs) with different levels of plasticity in their energetic grains. The analysis uses a Lagrangian cohesive finite element framework (CFEM) that explicitly accounts for finite-strain elastic-viscoplasticity, viscoelasticity, crack initiation and propagation, contact, friction, heat generation, and heat conduction. To determine the ignition, a criterion based on a hotspot size-temperature criticality threshold obtained from chemical kinetics calculations is used. It is found that samples with higher levels of plasticity (lower yield strengths) are less likely to ignite due to interplays among the dissipation mechanisms. In particular, calculations show that plastic deformation reduces fracture and subsequent heating caused by friction along crack faces, leading to lower temperature in hotspots.   

Coupling crystal plasticity and phase-field damage to simulate $\beta$-HMX-based polymer-bonded explosive under shock load

The development of high explosive materials requires constitutive models that are able to predict the influence of microstructure and loading conditions on shock sensitivity. In this work a model at the continuum-scale for the polymer-bonded explosive constituted of $\beta$-HMX particles embedded in a Sylgard matrix is developed. \\ It includes a Murnaghan equation of state, a crystal plasticity model, based on power-law slip rate and hardening, and a phase field damage model based on crack regularization. The temperature increase due to chemical reactions is introduced by a heat source term, which is validated using results from reactive molecular dynamics simulations. \\ An initial damage field representing pre-existing voids and cracks is used in the simulations to understand the effect of these inhomogeneities on the damage propagation and shock sensitivity. We show the predictions of the crystal plasticity model and the effect of the HMX crystal orientation on the shock initiation and on the dissipated plastic work and damage propagation. The simulation results are validated with ultra-fast dynamic transmission electron microscopy experiments and x-ray experiments carried out at Purdue University.   

Oblique impact and friction of HMX and/or TATB-based PBXs

Transportation, handling, vibrations can lead to moderate compressive but dynamic loadings requiring the characterization of the safety of PBXs submitted to such scenarios. Knowing that ignition can occur at a lower critical height during a fall on an inclined surface than a normal impact, the attention is focused in this paper on the heating due to the friction between PBXs and surfaces. A lot of experiments have been made using free-falling samples in vertical drop configurations on inclined targets or pendulum (skid) drop configurations (Green et al. 1971; Randolph et al. 1976). Data obtained on our HMX and/or TATB-based plastic-bonded explosives using pendulum drop configurations will be detailed. Evaluation of the heating due to friction requires the determination of the tangential projectile/target relative displacement and the contact pressure. The pressure is related to the normal force during the impact and the evolving contact surface, the latter being evaluated using a series of normal impacts. The aim of our paper is to compare the experimental diameter of the contact zones to (i) the classical Hertz's theory of contacting elastic solids and (ii) a spring-mass description of the impact. Data and models are then used to evaluate the increase of the temperature at the projectile/target interface for our explosives. We highlight the experimental bias which has already been attributed to the grits used to mimic the roughness of the surfaces.   

Continuum damage modeling in ductile materials using level sets

Ductile materials under high-velocity impact undergo large deformation and eventually damage. Damage alters the mechanical behavior of the materials and can lead to fracture and fragmentation. This work proposes a general Eulerian framework to model fracture and interfacial debonding in ductile materials. The current effort focuses on a plate impact problem, where a crack forms due to damage accumulation causing a discontinuity in the material. Damage accumulation is described by the continuum damage models. The level set approach is adopted for both tracking the sharp material interfaces and creating the crack. Results are found to be in good agreement with experimental data and two other commercial codes, CTH and EPIC. Also, damage is considered at the interfaces between two bonded materials, such as particles embedded in a matrix in a composite material. The progressive decohesion of the interfaces due to dynamic loading is simulated via a cohesive zone model. The result shows the ability of the code to handle the separation of the interfaces and create voids.