Mesoscale Mechanical Responses and Ignition Reaction of High Explosives at the Crystal Scale

Author:Wang Zuo Jie

Supervisor:huang feng lei


Degree Year:2017





HMX(Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)crystals and RDX(Hexahydro-1,3,5-trinitro-1,3,5-triazine)crystals are main constituents of polymer-bonded explosives(PBX)and their mechanical properties as well as mesoscale deformation mechanisms under quasi-static and dynamic conditions greatly affect the thermal-mechanical-chemical and ignition responses of PBX.In the present work,the mechanical properties and anisotropic responses of HMX and RDX single crystal have been analyzed through nanoindentation and plate impact tests and parameters of mechanical constitutive models of HMX and RDX have been calibrated.Based on a dislocation-mediated anisotropic elasto-viscoplastic model,the mesoscale mechanical,damage and failure mechanisms and ignition responses of single crystals and PBX have been analyzed.The elastic modulus and hardness ofβ-HMX andα-RDX single crystal were measured by the nanoindentation test and their indentation depth dependency has been analyzed.The Atomic Force Microscopes(AFM)were used to observe the microscopic damage characteristics,such as the“pop in”and“pop out”in the load displacement curves.The critical load of the radial or lateral cracks was determined.The numerical simulation of nanoindentation was carried out by using ABAQUS finite element code and the parameters of the damaged plasticity model were determined by comparison with the experimental load displacement curves.The dynamic responses of several orientations of HMX and RDX single crystals were studied by the plate impact tests.At the low impact velocity(320 m/s),the crystal/PMMA interface particle velocity profiles show distinct elastic-plastic wave structure.According to the impedance matching method,the dynamic mechanical properties were obtained.Hugoniot elastic limit(HEL)and plastic wave speedUpl of HMX and RDX show anisotropy and pressure/strain rate dependency.The anisotropy of elastoplastic response was explained by analyzing the deformation systems of HMX and RDX crystals.Parameters of an anisotropic elasto-viscoplastic model were calibrated by comparing the experimental interface velocity profiles with the calculated ones.The elastic anisotropy and dislocation motion are responsible for the anisotropy and heterogeneity of pressure,cumulative shear strain and temperature.When the resolved shear stress of some orientations is larger,dislocations are easier to initiate and extend.More plastic work is converted to heat,so these orientations are more sensitive.A damage elasto-viscoplastic model and a damage viscoelastic model have been developed for HMX crystals and the polymer binder respectively.The damage elasto-viscoplastic model considers the anisotropy nature of HMX crystals and dislocation-mediated plasticity as well as the cleavage damage.The damage visoelastic model captures rate dependent damage due to binder rupture and binder delamination during the loading process.The dynamic response,damage and failure mechanisms were analyzed under the dynamic compression and tension.During compression,the dislocations accumulate rapidly inside the crystals,which cause the cleavage fracture while the binder rupture and binder delamination have less effect.In the case of dynamic tension,the damage caused by the interface debonding is more significant than the grain cleavage fracture and the binder fracture,so PBX is more susceptible to intergranular debonding failure.Based on the proposed elasto-viscoplastic model for HMX crystals and a viscoelastic model for the polymer binder,a three-dimensional mesoscale numerical model was established to study the thermomechanical response of plastic bonded explosive(PBX)and granular explosive(GX)and to quantify the interaction between an anisotropic crystal phase and an isotropic polymer binder phase under impact.At the same impact velocity,the average and local stress and temperature are lower than those of GX.The presence of the softer binder can reduce the stress localization.Calculated hot spots number and pressure relations homogenized from mesoscale results shows that GX are more sensitive than PBX.A mesoscopic reactive model has been developed for the HMX crystals,which incorporates nonlinear elasticity,crystal plasticity,and temperature-dependent chemical reaction.Based on this model and the viscoelastic model for the binder,the thermal–mechanical–chemical responses of PBX under impact were quantified.At a critical impact velocity(V≥300 m/s),a chemical reaction is triggered because the temperature contributed by the volumetric and plastic works is sufficiently high.Physical quantities,including stress,temperature,and extent of reaction,are homogenized from those across the microstructure at the mesoscale to compare with macroscale measurements,which will advance the continuum-level models.