Document Type

Dissertation

Date of Degree

Fall 2015

Degree Name

PhD (Doctor of Philosophy)

Degree In

Mechanical Engineering

First Advisor

H.S. Udaykumar

Abstract

Heterogeneous energetic materials such as plastic bonded explosives (PBX), pressed explosives etc have very detailed and non-uniform microstructure. The heterogeneities are usually present in the form of binder, voids, microcracks etc. Shock interaction with these heterogeneities leads to local heated regions known as hot spots. It is widely accepted that these hot spots are predominantly the cause of triggering reaction and eventually ignition in these energetic materials. There are various physical mechanisms operating at mesoscale through which hot spot can be created such as void collapse, inter-granular friction in energetic crystals, shock heating of HMX crystals and binder etc. Hence, microstructural heterogeneities can play a vital role for shock initiation in heterogeneous explosives. In the current work, a general framework is established for performing mesoscale simulations on heterogeneous energetic materials. The numerical framework is based on a massively parallel Cartesian grid based Eulerian solver. Narrow band level set approach is used for sharp tracking of the material interfaces. The interfacial conditions are applied using modified ghost fluid method. The use of level set method for interface tracking provides an inherent advantage of using level set based image segmentation algorithm(active contouring) for the representation of explosives microstructure. The image processing approach allows to perform simulation on real geometries than the idealized shapes. The image processing framework is incorporated in the Eulerian solver. The energetic material considered in the current work is HMX. The chemical decomposition of HMX is modeled using Henson Smilowitz chemical kinetic law. Shock analysis is performed on two different samples of HMX based pressed explosives. Also, both two dimensional and three dimensional shock analysis on mock sugar geometry are performed. The effect of shock strength and relative positioning of voids on ignition threshold of porous HMX is studied. The current work is focused towards the development of a computational framework which can replicate the experimental way of studying the shock initiation behavior of energetic materials i.e. using flyer plate simulations.

Public Abstract

Heterogeneous energetic materials are commonly used in various engineering applications such as mining, munitions, propellants design etc. Examples of such materials are plastic bonded explosives (PBX), pressed explosives etc. Energetic materials are useful because it can release large amount of energy in a very short span of time through initiation of chemical reactions, ignition and detonation. However, rapid release of energy in energetic materials makes them unsafe to handle as even small external disturbances may cause runaway thermal heat release. The behavior of energetic materials is highly unpredictable, which poses challenges related to the controlled design. The controlled design of heterogeneous energetic materials requires understanding of the physical mechanisms which govern the chemical reaction initiation and ignition process. The chemical reaction in heterogeneous explosives initiates because of various physical mechanisms operating at the microstructural scale (mesoscale). The current work is aimed towards understanding of these governing physical mechanisms and sensitivity characterization of heterogeneous energetic materials under shock load using mesoscale numerical simulations.

Keywords

publicabstract

Pages

xv, 173 pages

Bibliography

Includes bibliographical references (pages 165-173).

Copyright

Copyright 2015 Nirmal Kumar Rai

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