Session D1: EM-3: Mechanical Effects and Initiation

Presentation of an approach for the analysis of the mechanical response of propellant under a large spectrum of loadings: numerical and mechanical issues

Many authors claim that to understand the response of a propellant, specifically under quasi static and dynamic loading, the mesostructural morphology and the mechanical behaviour of each of its components have to be known. However the scale of the mechanical description of the behaviour of a propellant is relative to its heterogeneities and the wavelength of loading. The shorter it is, the more important the topological description of the material is. In our problems, involving the safety of energetic materials, the propellant can be subjected to a large spectrum of loadings. This presentation is divided into five parts. The first part describes the processes used to extract the information about the morphology of the meso-structure of the material and presents some results. The results, the difficulties and the perspectives for this part will be recalled. The second part determines the physical processes involved at this scale from experimental results. Taking into account the knowledge of the morphology, two ways have been chosen to describe the response of the material. One concerns the quasi static loading, the object of the third part, in which we show how we use the mesoscopic scale as a base of development to build constitutive models. The fourth part presents for low but dynamic loading the comparison between numerical analysis and experiments.   

Evidence for Friction Between Crack Surfaces During Deformation of Composite Plastic Bonded Explosives

The compressive strength has been found to increase linearly with hydrostatic pressure in a low pressure range in which work softening due to crack damage is observed. Analysis indicates that this linear increase can be attributed to friction between the surfaces of closed cracks and a friction coefficient is obtained from the linear slope and the measured angle of the failure plane. Analysis also indicates that the plane of maximum shear stress, the failure plane, is greater than 45 degrees when friction is present* as observed and a friction coefficient is also calculated directly from this angle. In addition, a relationship between the ratio of compressive to tensile strengths and the friction coefficient has been given by Zuo and Dienes*. The observed ratio is greater than the predicted value without friction and a friction coefficient is obtained which is in agreement with the two values obtained as discussed above. This agreement of three independent measures of the friction coefficient is taken a strong evidence for the presence of friction. This friction can be the source of hot spots and ignition during deformation*. *Zuo, Q. H., and Dienes, J. K., LA-13962-MS (2002).   

Effects of shear strain on initiation of chemical reactions in HMX

We performed a theoretical study of detonation initiation reactions in crystalline $\beta $-HMX using ab-initio methods. A HONO formation and elimination and direct N-NO$_{2}$ bond dissociation are considered to be main mechanisms of detonation initiation in HMX. We calculated the activation barriers of these two reactions using nudged elastic band method. We studied the same reactions in HMX exposed to the shear strain in (001) and (101) directions. The shear strain has been modeled by constructing an interface between different layers of HMX. We observed a significant change in energy barriers with respect to bulk calculations. This indicates that the shear strain plays an important role in detonation initiation in HMX. The results obtained are consistent with previously observed trends in DADNE and TATB and may be used to reveal common features of high explosive initiation behaviors.   

Defect Characterization in Crystalline Explosives

While microstructural defects such as dislocations, voids, and impurities may dramatically affect ignition sensitivity of energetic materials, the use of non-destructive techniques to accurately characterize the nature of internal defects and how they correlate to initiation is lacking. The objective of this work is to investigate the application of various defect characterization methods to crystalline explosives. X-ray Topography imaging, performed at on oriented, crystal slabs of RDX, shows that for a $<210>$ crystal slab, dislocation structure was only observed in the 020 transmission image compared to the 002, 102, 111, and 021 images. XRT images of the $<111>$ sliced sample taken through the 102 and 202 crystal planes show features including dislocations, grain boundaries, and the seed origin. Small and Ultra Small Angle Neutron Scattering experiments were performed on standard grade and reduced sensitivity grade RDX powder samples using the contrast variation method. Significant differences in scattering profiles were observed from these two versions of RDX, likely due to the existence of sub-micron voids or impurity pockets in the standard grade RDX sample. Large Scale Gap Test (LSGT) data from IHDIV NSWC for formulations containing these powder samples were then used to correlate neutron scattering to shock sensitivity.