Cranfield Defence and Security
Permanent URI for this community
Browse
Browsing Cranfield Defence and Security by Supervisor "Akhavan, Jacqueline"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Open Access The development of a laser detonator system(2015-06-26) Bowden, M.; Akhavan, Jacqueline; Drake, R. C.Laser detonators offer several advantages over traditional electrical detonators, such as exploding bridgewire and slapper detonators, in terms of both safety and performance. Laser detonators remove the electrical conduction path to the energetic material, providing immunity from threats such as electrostatic discharge and lightning. A larger separation between the initiating energy source and the explosive devices is possible, up to several tens of metres, compared to a few metres for electrical slapper detonators. A laser detonator system has been developed, with laser-driven flyer plates used to shock initiate the explosive. All aspects of the system, including coupling into an optical fibre, used to transmit the laser energy to the detonator, the optical fibre, the flyer plate launch and acceleration and subsquent shock into the explosive, and the explosive initiation have been investigated, with an understanding of the underlying principles and processes developed. Shock initiation of two secondary explosives, hexanitrostilbene and pentaerythritol tetranitrate, has been studied at extremely high shock pressures, comparable to the detonation pressure, and the critical energy fluence required for initiation established. The laser detonator system is robust and optimised, with design tools developed to enable efficient design of future systemItem Open Access The development of Novel, Low Sensitivity, Gas-generating Formulations for Hotwire Ignited Devices(2013-04) McAteer, Daniel; Akhavan, Jacqueline; Contini, AlessandroA novel, low sensitivity, gas-generating composition was developed to replace an existing primary explosive, potassium picrate, for use in a generic squib device. The novel composition was based on energetic materials researched from the open literature. The chosen materials were synthesised or purchased and combined in order to produce formulations which would reproducibly ignite when contacted with a hotwire. The formulations were assessed for their chemical compatibility and homogeneity by small-scale hazard testing, differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spectroscopy. The compositions were test fired within squibs coupled to a pressure bomb and the pressure generation and rise time was measured using a piezoelectric gauge.Item Open Access The effect of ionising radiation on the explosives: TATB, HMX and PETN(Cranfield University, 2019) Lewis, D. A.; Akhavan, Jacqueline; Cook, M.; Wilson, I; Stennett, C.The effect ionizing radiation, specifically gamma, has on three of the most well known explosive materials; HMX, PETN and TATB, has been studied experimentally and computationally. Samples of these explosives that had been irradiated in vacuum to a total dose (equivalent to water) of 200 kGy by an average incident energy of 1.25 MeV, showed changes to their explosive and physical properties to varying degrees, with sensitivity to impact being most notably changed for PETN and HMX. Changes to the thermal properties of HMX and TATB, alongside the detection of long lived radicals for TATB and PETN, suggests alterations to each material’s chemistry. Changes were only detectable in solid state analytical methods, indicating that alterations to these materials are isolated within this state. Through comparison with other data, these changes appear to be highly dependant on the gaseous environment in which they are irradiated, with vacuum having the most significant effect. The widely reported greening of TATB under the influence of gamma (and ultraviolet and x-Ray) irradiation was found to be reversible upon re-crystallisation, with the process also appearing to remove the long lived radical that had existed in the material for over eight years. Radical concentration appears to correlate with total absorbed dose along with the level of green within the material. Computational investigation attributes the discolouration of TATB to the cationic radical derivative of TATB, which is stabilised by de-localised pi-bonding resonance of the constitutive aromatic ring, it is also suggested that this is the source of the radical signal observed in ESR analysis.Further computational investigation suggests that purported decomposition products of TATB such as the mono-furazan, mono-nitroso and phenoxyl radical are not the source of the discolouration and are also not thermodynamically favourable, unless the monofurazan or phenoxyl derivatives are in a cationic form. Thermodynamic evaluation of potential decomposition pathways for PETN and HMX yield a selection of energetically favourable products, however the significant majority are, like TATB, in the cationic radical form. Simulated ESR spectra for purported HMX decomposition products did not agree with those observed in literature, attributed to their short lived nature. However a measured long lived radical in TATB is attributed to the cationic radical form of TATB, the purported source of the discolouration. A long lived radical in PETN was also measured and assigned to the cation derivative of PETN by comparison with computational predictions.Item Open Access The effects of Gamma radiation on a PBX containing TATB and the fluoropolymer FK-800(2014-06-10) Connors, S. C.; Akhavan, Jacqueline; Padfield, JamesThe polymer bonded explosive TCV is analogous to PBX compositions used in some nuclear weapons where the PBX will be exposed to high energy ionising gamma radiation. It is therefore important to study how gamma radiation affects the mechanical and chemical properties of the PBX. In this study 60Co was used to irradiate samples of the TCV, its FK-800 binder and TATB explosive filler, at 37.5 °C, to total doses up to 200 kGy in air and under vacuum. Post irradiation analysis consisted of mechanical, thermal and chemical analysis of the irradiated materials. Results from the radiolysis of the FK-800 showed predominant main chain scission taking place, these results in the release of volatile fluorine containing products and an increase in the polymer’s crystallinity. The changes to the FK-800’s structural properties result in an increase in Young’s modulus and yield stress whilst reducing both ultimate tensile strength and elongation at maximum stress. Dynamic mechanical analysis shows the material softening initially upon irradiation, then stiffening as crystallinity increased. TATB was found to turn green and have increased sensitiveness to impact and electric spark discharge with gamma radiation. Analysis by HPLC and LC-MS identified a decomposition product with a mass of 240 g mol-1. No significant changes to the mechanical properties of TCV were identified; however, its sensitiveness to impact and electric spark discharge were found to increase similar to TATB’s. In conclusion, gamma irradiation of TCV has identified significant changes to the mechanical and chemical properties of the FK-800 binder, have little to no effect on mechanical properties of PBX up to 100 kGy. The predominant radiolytic effects on TCV were to its hazard characteristics caused by changes in the TATB. A mono-furazan derivative of TATB has been suggested as the decomposition product identified, and as a possible cause for the increase in sensitiveness of TATB upon gamma irradiation.Item Open Access The Ignition Behaviour of Energetic Materials Under Confined Cookoff(2018-09) Stennett, Christopher; Akhavan, JacquelineIn a series of experiments and theoretical work, the process of ignition in confined energetic materials has been explored and understanding gained. Early work focused on the direct observation of the cook-off process but was hampered by the available technology. In more recent work, dynamic measurements of the deformation of the confinement have been explored, and refinements to the direct observation method have been made that make use of recent advances in camera technology. We have observed, for the first time, complex melting and development of gas spaces within which the early stages of ignition take place, and propose a new mechanism by which violent cook-off responses might develop in certain explosivesItem Open Access Material strength evolution of FCC metals under high strain rates(2020-12) Gilroy-Hirst, Caitlin; Hazael, Rachael; Wood, David; Akhavan, JacquelineCopper and stainless steel 21-6-9, two face-centred cubic (FCC) materials, were shock loaded in partially recovered uniaxial strain flyer-plate impact experiments at varying impact pressures (6 GPa - 16 GPa). These samples were collected post impact via three differing recovery configurations: (1) standard or ‘free boundary’; (2) partial shock recovery, i.e. using both ‘momentum trapping’ and ‘soft recovery’ techniques but with a flyer plate thickness of 10 mm to see the effects on partial tensile release in the rear spall plates of an established recovery target design; finally (3) a traditional full recovery comparison with a 2 mm flyer impact. These samples were then analysed using a suite of analytical techniques including X ray diffraction, optical microscopy, hardness testing and compression after impact testing, to observe the material deformation characteristics in the simplified partially recovered shock and release condition compared to the more complex ‘standard’ release wave situation. A control sample of a ‘ fully recovered’ sample was also carried out for comparative purposes. The rear spall plates were modified on these partially recovered samples to see the effect that this had on the tensile release waves and how this changed the material deformation properties overall. Results for the stainless steel 21-6-9 demonstrated the ability of the partial shock recovery technique (hereafter called ‘partial-recovery’) to mitigate the reverberations compared to that of the standard samples. Upon analysis of the targets, it was observed that the dislocation density was generally lower for the partially recovered samples but varied with impact pressure for both the standard and recovered configuration. For the stainless steel recovery experiments an increasing dislocation density from 4 x 1015 m-2 at 6 GPa to 6.5 x 1015 m-2 at 12 GPa was observed which then decreased to 5.5 x 1015 m-2 at 16 GPa. The twinning density to grain ratio obtained from optical microscopy showed a linear increase (y= 1.5345x - 8.3508) with an R2 value of 0.8295 from 7 GPa at 4 twins per grain to 17 twins per grain at 16 GPa. Twinning density is indicative of a successful recovery, where a successful recovery is a reduction in reverberations in the sample and in a full recovery is a 1D shock. The dataset both verifies the use and success of the recovery technique as well as demonstrating the materials characteristics under partial-recovery, while observing reverberation effects.