R.H.B. Bouma

Microscopic characterization of defect structure in RDX crystals

Three batches of the commercial energetic material RDX, as received from various production locations and differing in sensitivity towards shock initiation, have been characterized with different microscopic techniques in order to visualize the defect content in these crystals. The RDX crystals are embedded in an epoxy matrix and cross‐sectioned. By a treatment of grinding and polishing of the crystals, the internal defect structure of a multitude of energetic crystals can be visualized using optical microscopy, scanning electron microscopy and confocal scanning laser microscopy. Earlier optical micrographs of the same crystals immersed in a refractive index matched liquid could visualize internal defects, only not in the required detail. The combination of different microscopic techniques allows for a better characterization of the internal defects, down to inclusions of approximately 0.5 μm in size. The defect structure can be correlated to the sensitivity towards a high‐amplitude shock wave of the RDX crystals embedded in a polymer bonded explosive. The obtained experimental results comprise details on the size, type and quantity of the defects. These details should provide modellers with relevant and realistic information for modelling defects in energetic materials and their effect on the initiation and propagation of shock waves in PBX formulations.

Development of fiber optic sensors at TNO for explosion and shock wave measurements

Fiber Optic sensors are found to be very suitable for explosion and shock wave measurements because they are immune to Electromagnetic Interference (EMI). In the past few years, TNO has developed a number of sensor systems for explosion and shock wave measurements in which the optical fiber is a vital component. This paper presents a survey of these optical measurement systems using fiber optics. The basic design of these systems, the test configurations and the experimental results are presented.

On the impact testing of cyclotrimethylene trinitramine crystals with different internal qualities

Impact testing of various cyclotrimethylene trinitramine (RDX) samples has been carried out with the Ballistic Impact Chamber (BIC). Numerical simulations have been performed to guide its experimental design and explain the phenomena that are observed in the measured pressure evolution. The experimental results obtained with the BIC are compared to the RDX crystal quality observed by optical and scanning electron microscopies. The reactivity of RDX in the impact testing with the BIC correlates with the crystal quality and this opens up the possibility of achieving a quantitative determination of the crystal quality.

Simulations of deformation processes in energetic materials

The sensitivity of energetic materials has been studied extensively for more than half a century, both experimentally and numerically, due to its importance for reliable functioning of a munition and avoidance or mitigation of accidents (Bowden & Yoffe, 1952). While the shock initiation of an explosive under nominal conditions is relatively well understood from an engineering perspective, our understanding of initiation due to unintended stimuli (weak shock or fragment impact, fire, damaged explosive charge) is far less complete. As an example, one cannot exclude the ignition of an explosive due to mechanical deformation, potentially leading to lowor even high-order explosion/detonation as a consequence of mechanical stimuli with strain rates and pressures well below the shock sensitivity threshold. During the last two decades there has been an increased interest in the scientific community in understanding initiation sensitivity of energetic materials to weak insults. A relationship between energy dissipation and rate of plastic deformation has been developed for crystalline energetic materials (Coffey & Sharma, 1999). Chemical reactions are initiated in crystalline solids when a crystal-specific threshold energy is exceeded. In this sense, initiation is linked to the rate of plastic deformation. However, practical energetic materials are usually heterogeneous composites comprised of one or more kinds of energetic crystals (the filler, for which the mass fraction can exceed 90%) bound together with a binder matrix that often consists of several different polymer, plasticizer, and stabilizer materials. Clearly, the mechanical behavior of these polymer-bonded (plastic-bonded) explosives (PBXs) is far more complicated than for neat crystals of high explosive. It is necessary in realistic constitutive modeling of energetic compositions to incorporate features reflecting the complex, multiphase, multiscale structural, dynamical, and chemical properties; see, for example, Bennett et al., 1998, and Conley & Benson, 1999. The goal in constitutive modeling is to bridge the particulate nature at the mesoscale to the mechanical properties at the macroscale. The macroscale deformations applied to PBX composites in experiments are generally not the same as the local deformation fields in a component crystal within the composite. This has been demonstrated using grain-resolved mesoscale simulations wherein the individual grains and binder phases in a PBX are resolved within a continuum simulation framework.

Application and Characterization of Nanomaterials in Energetic Compositions

As part of a cooperation between several TNO institutes, including TNO Prins Maurits Laboratory, recently a new initiative on nanotechnology was started. The research subjects within this initiative can be roughly divided into two areas: (1) Instrumentation for analysis and manufacture at nano-scale and (2) Nanoscale engineering techniques to create materials and components (including their applications). Currently the research at TNO Prins Maurits Laboratory is focusing on the application of reactive nanomaterials to decontaminate surfaces from e.g. bacteria or toxic chemicals, the use of plasmas to generate nanomaterials like carbon nanotubes, and the application and characterization of nanomaterials in energetic formulations ( e.g. explosives, propellants and pyrotechnic compositions). In this paper results on the latter subject will be presented in more detail. Also results will be included of other research projects involved with energetic/reactive nanomaterials.

Fiber optic techniques for measuring various properties of shock waves

For the past years we have developed several optical techniques to measure properties of shock waves. The fiber optic probe (FOP) is developed to measure the shock-wave velocity and/or the detonation velocity inside an explosive. The space resolution can be as small as 0.5 mm. Single fibers are used as velocity pins, and as devices to measure the flatness of flyers. Arrays of fibers are used to measure the curvature of a shock or detonation front. Also a Fabry-Perot velocity Interferometer System is constructed to measure the velocity of the flyer of an electric gun and the particle velocity in a shock wave. It is possible to combine these two measurements to determine simultaneously the flyer velocity that induces a shock wave in sample and the particle velocity in a window material at the back in a single streak record.

Processing, Application and Characterization of (Ultra)fine and Nanometric Materials in Energetic Compositions

The energetic materials research at TNO Defence, Security and Safety, The Netherlands is focusing at the development and characterization of explosives (insensitive munitions), gun/rocket propellants and pyrotechnic compositions and their ingredients. The application of reactive, (ultra)fine and nanometric materials in these compositions has gained increased interest over the past few years. Current research topics focus on the processing, application and characterization of (1) (ultra)fine energetic crystals and composite nano‐clusters in plastic bonded explosives, (2) metastable intermolecular composites (MICs) and (3) self‐propagating high‐temperature synthesis (SHS). In this paper these topics will be highlighted in more detail.

Experimental Study and Modelling of Combustion Front Velocity in Ti-2B and Ti-C Based Reactant Mixtures

Experiments on combustion synthesis for the Ti-2B and Ti-C systems diluted with an inert metal are presented. The paper shows the influence of geometry, composition, density and particle size of diluent on the combustion front velocity. A Ti-2B reactant mixture diluted with Al and Cu and a Ti-C reactant mixture diluted with Al are studied. The metallic diluent and its concentration are varied. Besides, each experiment is based on a stack of cylinders with decreasing diameter in order to vary the heat losses. In some experiments the eventual quenching of the combustion reaction has been observed. Furthermore these experimental results are compared with theoretical calculations based on analytical expressions derived for such systems.

Combustion synthesis of electrical contact materials

Various electrical contact materials have been obtained by self-sustained high-temperature synthesis (SHS) followed by a hot compression step. The materials studied are TiB2 and TiC-based cermets with a copper or aluminium metal matrix. The density, electrical conductivity, and hardness of the cermets produced in this way were measured. The microstructure of the cermets was analyzed by X-ray diffraction and optical microscopy. The conductivity and hardness of the TiB2 cermets with 40 wt % aluminium are competitive to the properties of common silver refractory metals for contact applications. TiB2 cermets with 40 wt % of copper possess a larger porosity which may be prevented by an increased copper content to reduce the reaction temperature.