Review of experimental techniques for high rate deformation and shock studies

Hot spot ignition mechanisms for explosives

This paper describes the response of explosives to stress and impact and in particular the mechanisms of ‘ hot-spot production. Samples in the form of single crystals, powder layers, pressed pellets, gels, polymer bonded explosives (PBXS) and propellants have been studied. Techniques used include a drop-weight facility with transparent anvils which allows photography at microsecond framing intervals, an instrumented drop-weight machine, a miniaturized Hopkinson bar system for high strain rate property measurement, laser speckle for studying the deformation and fracture of PBXS, an automated system for analysing speckle patterns and heat sensitive film for recording the positions and temperatures of hot spots. Polishing and staining methods have been developed to observe the microstructure of PBXS and failure during quasi-static loading. Ignition, when it occurred, took place at local hot-spot sites. Evidence is discussed for a variety of ignition mechanisms including adiabatic shear of the explosive, adiabatic heating of trapped gases during cavity collapse, viscous flow, friction, fracture and shear of added particles and triboluminescent discharge.

Hot-spot ignition mechanisms for explosives and propellants

The dislocation pile‐up avalanche model is used to explain the crystal size dependence for hot spot‐controlled initiation of chemical decomposition in cyclotrimethylenetrinitramine crystals subjected to drop‐weight impact testing. Deformation‐induced temperature rises, hot spot sizes, and lifetimes are related to previously reported values for direct thermal decomposition. A reasonable chemical reaction yield is estimated from available kinetic data.

Crystal size dependence for impact initiation of cyclotrimethylenetrinitramine explosive

The shock response of single crystals of the explosive pentaerythritol tetranitrate was studied from 3.4 to 19 GPa. Four orientations, 〈110〉, 〈001〉, 〈101〉, and 〈100〉 were studied. The shock Hugoniot and shock initiation sensitivities were measured in wedge tests. Intermediate velocity transitions were noted. A sensitivity anomaly near 4 GPa was discovered for 〈110〉 orientations. Chemiluminescent phenomena associated with this anomaly were studied using framing photography and uv/visible spectroscopy. Both plane‐wave explosive and gas‐gun impact experiments were done. Analysis of the steric hindrance occurring during edge dislocation motion in the shear flow associated with shock‐induced uniaxial strain was done. The results corroborate the hypothesis that the orientation dependence of shock initiation sensitivity is due to the anisotropy of plastic flow, that is, the relative ease of slip in different orientations.

Shock response of pentaerythritol tetranitrate single crystals

In earlier papers we have described the development of photographic and pressure-measuring techniques to study the detailed behaviour of thin layers of explosive when impacted. Ignition, when it occurs, invariably does so at local sites where 'hot spots' have been generated. The present paper gives the evidence for ignition by a variety of mechanisms including adiabatic shear of the explosive, adiabatic heating of trapped gas spaces, viscous flow, friction, fracture or shear of added particles, and triboluminescent discharge. Some of these mechanisms have been suggested earlier though others have resulted from our recent work. The value of the new photographic evidence is that it allows us to assess the viability of each mechanism for different explosives and impact conditions.

Ignition Mechanisms of Explosives during Mechanical Deformation

The behaviour of thin layers of solid materials under drop-weight impact is studied with the aid of high-speed photographic and pressure-measuring techniques. Photographic sequences taken with a high-speed framing camera show that explosive materials suffer large-scale deformation before initiation of explosion. The sample may undergo plastic flow in bulk, show evidence of partial fusion, and even (with PETN) melt completely. There is also evidence of Munroe jetting and instability of flow of material at the anvil/layer interfaces. The flow speed of the sample during these processes is considerable and may reach 300 m/s. When ignition of the layer occurs it does so at a small number of local hot spots, following which rapid combustion develops at speeds of 200-700 m/s. Strain-gauge measurements show that the pressures attained during drop-weight impact are typically 0.5-1 GPa (5–10 kbar) and the duration of impact 300–500 μs. In the course of impact of a thin layer of granular material a sharp pressure drop may occur, frequently from several hundred MPa down to zero. With an explosive layer, ignition occurs immediately following the instant of the pressure drop. The sudden fall in pressure is due to mechanical failure of the sample, and correlation of the two experiments shows that this is the cause of the very high flow speeds attained during impact. On the basis of these results a possible mechanism of ignition is suggested.

S. N. Heavens

Papers

Ignition Mechanisms of Explosives during Mechanical Deformation

The ignition of a thin layer of explosive by impact

Flow, melting and ignition of solid explosives under impact