The Woodruff School

SUMMARY

Polymer-bonded explosives (PBXs) consist of solid energetic crystals, polymer binder, and other ingredients such as metallic fuels or oxidizers. Safety in handling these explosives and the vulnerability to accidental mechanical stimuli are principally governed by their impact sensitivity. However, the ignition mechanisms of PBX and other solid high explosives are complex and still not well understood. A generally accepted view is that mechanical energy deposited in a heterogeneous PBX by impact is converted into heat in localized regions, known as hotspots, and they initiate rapid decomposition of solid explosives into gaseous products. Hotspot formation mechanisms are heavily influenced by factors such as material anisotropy, constituent properties, morphology of constituents� boundaries, and defects. These characteristics of heterogeneous microstructures cause hotspot generations to be stochastic.

The proposed research is concerned with the effects of microstructure attributes and loading intensity on the hotspot development and ignition sensitivity. A computational framework will be developed based on the previously established cohesive finite element method (CFEM). This framework will be used to determine the hotspot criticality and analyze the ignition sensitivity using a statistical approach accounting for randomness in the microstructure. The threshold for hotspot criticality will be defined based on hotspot size and temperature. Multiple microstructures having quantitatively similar attributes based on a real PBX will be generated in a controlled manner through the variation of grain size distribution and the crystal volume fraction. The relationship between microstructural attributes and impact sensitivity will be first obtained in the low intensity loading regime (P < 1 GPa). Then, the analysis will be extended to high intensity loading regime (P ~ 5 GPa). The goal of this research is to computationally understand the relations between microstructure attributes and ignition behavior of PBX over a wide range of loading intensity. The microstructure-loading-response relations obtained in this research may be used to assess the sensitiveness of PBXs.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Seokpum Kim

TIME:	Tuesday, August 12, 2014, 1:00 p.m.

PLACE:	Love Building, 210

TITLE:	Effects of Microstructure and Loading Intensity on Impact Ignition of PBX

COMMITTEE:	Dr. Min Zhou, Chair (M.E.) Dr. Shuman Xia (M.E.) Dr. H. Jerry Qi (M.E.) Dr. Julian Rimoli (A.E.) Dr. Suhithi Peiris (DTRA) Dr. Woo Il Lee (SNU, Korea)

SUMMARY Polymer-bonded explosives (PBXs) consist of solid energetic crystals, polymer binder, and other ingredients such as metallic fuels or oxidizers. Safety in handling these explosives and the vulnerability to accidental mechanical stimuli are principally governed by their impact sensitivity. However, the ignition mechanisms of PBX and other solid high explosives are complex and still not well understood. A generally accepted view is that mechanical energy deposited in a heterogeneous PBX by impact is converted into heat in localized regions, known as hotspots, and they initiate rapid decomposition of solid explosives into gaseous products. Hotspot formation mechanisms are heavily influenced by factors such as material anisotropy, constituent properties, morphology of constituents� boundaries, and defects. These characteristics of heterogeneous microstructures cause hotspot generations to be stochastic. The proposed research is concerned with the effects of microstructure attributes and loading intensity on the hotspot development and ignition sensitivity. A computational framework will be developed based on the previously established cohesive finite element method (CFEM). This framework will be used to determine the hotspot criticality and analyze the ignition sensitivity using a statistical approach accounting for randomness in the microstructure. The threshold for hotspot criticality will be defined based on hotspot size and temperature. Multiple microstructures having quantitatively similar attributes based on a real PBX will be generated in a controlled manner through the variation of grain size distribution and the crystal volume fraction. The relationship between microstructural attributes and impact sensitivity will be first obtained in the low intensity loading regime (P < 1 GPa). Then, the analysis will be extended to high intensity loading regime (P ~ 5 GPa). The goal of this research is to computationally understand the relations between microstructure attributes and ignition behavior of PBX over a wide range of loading intensity. The microstructure-loading-response relations obtained in this research may be used to assess the sensitiveness of PBXs.