Browsing by Author "Edwards, Michael R."
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Item Open Access Ballistic and physical properties of highly fractured alumina(Maney Publishing, 2010-11-01T00:00:00Z) Horsfall, Ian; Edwards, Michael R.; Hallas, M. J.It is known that ceramic materials in armour are often shattered by a penetrating projectile and the resistance to penetration is therefore dependent upon the properties of this failed material. A small explosive charge was used to shatter a confined 95% alumina tile by passing the stress wave from an explosive detonation through a steel cover plate and into the ceramic. This fractured material was shown to have an elastic modulus of 130 GPa compared to 330 GPa for the monolithic alumina. Impact indentation tests and ballistic depth of penetration tests were performed on the intact ceramic, shattered ceramic and a pressed ceramic powder compact. It was found that the fractured material had a ballistic efficiency approximately 70% of that of the monolithic alumina and was also able to cause significant erosion to the projectItem Open Access Effect of heat treatment on small scale fragmentation of aluminium alloy(Maney Publishing, 2011-01-31T00:00:00Z) Edwards, Michael R.; Deal, CSmall scale explosions, using a detonator, of 7075 aluminium alloy cylinders, 15-100 mm outside diameter, were carried out to investigate the effects of heat treatment on fragmentation. This was the finest for the strongest as received alloy and coarsest for the softest overaged alloy. This effect was similar to that seen in investigations of the fragmentation of steel. Cylinders of 50 and 100 mm in diameter did not fragment but plastically deformed with maximum deformation at the cylinder bottom. Fragmentation of 33 and 42 mm diameter cylinders produced long fragments typical of the break-up of thick walled cylinders. At smaller diameters, break-up gave fragments of several shapes, finer fragments being largely associated with the smallest diameter cylinders and the highest strength alloys. Results followed those seen in large scale studies of cylinder break-up and suggest the possibility of using small scale fragmentation experiments in the investigation of the effects of composition, heat treatment and processing on natural fragmentation.Item Open Access Enhanced radiopacity austenitic stainless steel foil(Cranfield University, 2007-12-12T15:52:58Z) Craig, Charles Horace; Friend, Clifford M.; Edwards, Michael R.Austenitic stainless steel designed for implant applications is used to fabricate balloon expandable coronary stents. The alloy was not designed for this purpose but has been found to work well except for relatively low radiopacity in the energy range used for stent deployment, typically 80kV to 100kV. Stents made of more dense elements such as tantalum exhibit high radiopacity in this energy range. Low radiopacity is due to a combination of tubular stents having a thin wall (strut) thickness (less than 0.13mm) and the alloy being comprised of low-density elements, approximately 2/3 iron and 1/3 chromium and nickel. To retain the desired thickness and increase radiopacity, alloy density may be increased by partial substitution with dense element(s). The new alloy must maintain the biocompatibility, corrosion resistance, non-ferromagnetic structure, strength, ductility, and fatigue- and fracture-resistant characteristics that made the original alloy attractive to stent designers. Coronary stents are subject to intensive review by regulatory authorities prior to being approved for human use, thus stent designers are hesitant to depart from accepted standards in selecting new alloys. Revising an existing alloy is the preferred approach to achieve subtle feature changes. A set of criteria was set that maintained chromium, nickel, and molybdenum within prescribed compositional ranges and diminished iron to its minimum level, allowing platinum to be substituted for approximately 1/3 the total elemental weight (wt%). Above 20wt% platinum, undesirable precipitates were found. An alloy containing 20wt% platinum, in the form of foil and at a thickness of 0.127mm, was found to be free of precipitates not found in the base or original alloy and to provide approximately 20% radiopacity increase at 80kV and 15% radiopacity increase at 100kV, exceeding minimum programme goals at 80kV and equaling those at 100kV.Item Open Access The mechanical and ballistic properties of polycarbonate(2008-05-27T15:05:19Z) Edwards, Michael R.; Waterfall, H.Polycarbonate is commonly used as the material of riot shields. Firings of 8.3 g steel ball bearings at velocities of 23-98 ms-1 produced damage in the form of dents. Subsequent tensile testing showed a small drop in yield strength and ductility for impact velocities greater than 58 ms-1. This drop in yield strength and ductility was more marked for specimens that had been soaked in acetone for 10 minutes after ballistic impact. Soaking in n- heptane produced no similar effect to that seen with acetone. Measurements were taken of the velocities of projectiles likely to be thrown by rioters. These showed that the measured velocities were lower than those velocities that caused macroscopic damage to polycarbonate when hit by a ball bearing. Thus susceptibility to brittle failure is likely to be associated with shields that have been hit by sharp objects and exposed to solvents after impact. Mechanical stressing of polycarbonate at the point of impact by the ball bearing had no effect on the subsequent mechanical properties. This is in contrast to the embrittlement seen in polycarbonate when internal strains are present produced on fast cooling from the melt.Item Open Access Modelling shocks using molecular dynamics(2011-09-07) Park, Nigel; Edwards, Michael R.The study of shocks in solid, crystalline metals has been ongoing since the early works of Rankine and Hugoniot in the latter half of the 19th century. However, the understanding of the behaviour of such materials under these extreme conditions remains an area of active research because of the paucity with which models can predict experimental observations. The modern era has seen a huge increase in the ability to solve many of the problems of this area of study by numerical, rather thatn analytic, means. One of these tools has been the use of computers to provide a numerical solution to the many–body problem posed by consideration of the medium as being composed of interacting atoms. The issue, then, has been transferred from one of dealing with many particles (which remains a problem for some aspects) to one of being able to develop a model which correctly describes the atomic interactions. However, it has been found that approximately correct models provide sufficient fidelity to enable qualitative studies to be undertaken. The study undertaken here has used this advantage to consider the behaviour of metallic materials under weak shock conditions. A comparison with some previous studies is given, which shows that, in order to avoid certain behaviours not observed experimentally, the simulation must contain thermal motion equivalent to at least room temperature. This thermal motion, and its resultant misalignment of the atoms, prevents spurious transfer of uni-directional momentum into rebounding translational supersonic waves. Further examination of the initial generation of dislocations indicates differences in the behaviour of not only the three high symmetry directions, but in the way that shear stress is relieved initially in low symmetry crystals as well. This behaviour gives some indication as to how the elastic precursor, commonly observed in weak shock experiments, decays from the level predicted by the Rankine–Hugoniot conservation relations to the much lower level observed experimentally. However, a very large discrepancy exists between the amplitude of the elastic wave observed in these simulations and that of experiments. It is shown that the existence of defects within the crystal can account for at least some of this discrepancy. However, computational limitations not only prevent the creation of realistic sample sizes, but also prevent the simulation of realistic defect densities and microstructures. This computational limtation, then, means that it is not currently possible to recreate the low Hugoniot elastic limits observed experimentally. The inability of atomistic simulations to recreate experimental data notwithstanding, useful analysis of shock behaviour is demonstrated. This fortuity is used to examine the behaviour of bicrystals under shock loading. It is shown that the difference in shock speed, together with the difference in response of the two crystal orientations leads to an interaction which modifies the behaviour from that observed in single crystal simulations. Further use is made of the ability of modern simulation methods to recreate salient features of dynamic processes to examine the behaviour of metallic substrates under high–speed impact from nanometer sized particles. Here the plasticity of the substrate is shown to be vital to ensuring that the simulation results are faithful to experiment, and hence to space science work. In order to capture this behavioour correctly, issues of substrate size and boundary behaviour are seen to be key.Item Open Access Penetration of a glass-faced transparent elastomeric resin by a lead-antimony-cored bullet(Elsevier , 2009-12-31T00:00:00Z) Hazell, Paul J.; Edwards, Michael R.; Longstaff, H.; Erskine, J.The penetration of the lead antimony-cored 7.62 mm × 51 mm bullet into a glass- faced polyurethane elastomeric polymer resin has been studied. The resulting craters in the resin contained elongated bullet core material that had a significant amount of porosity. A simple linear viscoelastic model was applied to AUTODYN-2D to describe the behaviour of the resin and numerical results of the penetration mechanism and depth-of-penetration appeared to match experimental observations well. Analysis of the high speed photography and a numerical model of this bullet penetrating a viscoelastic polymer showed that during the initial stages of penetration, the projectile is essentially turned inside out. Furthermore, the shape of the cavity was defined by the elastic relaxation of the polymer that led to compression of the core material. A weight analysis of the penetrated materials showed that using a thicker tile of glass resulted in better ballistic performance.Item Open Access Safety cases and safety: safety case elicitation tool for light unmanned air vehicles(2010-10-12) Harries, Rhydian; Edwards, Michael R.Item Open Access Stab resistant body armour(Cranfield University, 2000-03) Horsfall, Ian; Edwards, Michael R.There is now a widely accepted need for stab resistant body armour for the police in the UK. However, very little research has been done on knife resistant systems and the penetration mechanics of sharp projectiles are poorly understood. This thesis explores the general background to knife attack and defence with a particular emphasis on the penetration mechanics of edged weapons. The energy and velocity that can be achieved in stabbing actions has been determined for a number of sample populations. The energy dissipated against the target was shown to be primarily the combined kinetic energy of the knife and the arm of the attacker. The compliance between the hand and the knife was shown to significantly affect the pattern of energy delivery. Flexibility and the resulting compliance of the armour was shown to have a significant effect upon the absorption of this kinetic energy. The ability of a knife to penetrate a variety of targets was studied using an instrumented drop tower. It was found that the penetration process consisted of three stages, indentation, perforation and further penetration as the knife slides through the target. Analysis of the indentation process shows that for slimmer indenters, as represented by knives, frictional forces dominate, and indentation depth becomes dependent upon the coefficient of friction between indenter and sample. Analytical models are demonstrated to provide a reasonable estimate of energy absorption during and after penetration for a wide variety of knives and armour materials. The key armour parameters are shown to be the frictional interaction with the blade and the strength of the target material. The performance of knife blades is shown to increase with increasing sharpness, slimness, and surface finish. No single knife design performs best against all types of armour, and no single armour is best against all knife blades.Item Open Access Thermal and catalytic processing of solid fuels to minimise carbon monoxide emissions(2019-11) Nyombi, Antony; Wessling, Roland; Edwards, Michael R.This research was conducted to evaluate the amount of carbon monoxide (CO) emissions from the incomplete combustion of solid fuels, the factors responsible for its release and methods to minimize its release and thus the human exposure to CO in different environments. Unintentional CO poisoning has resulted in many deaths, injuries and chronic ailments. Previous efforts to minimise exposure to CO include awareness campaigns, domestic CO alarms, improved designs for cooking stoves, space heating appliances, and postcombustion CO oxidation using catalysts. However, these methods have not fully solved the challenge of CO exposure. In this study, different sold fuels were processed and analysed to investigate their behaviour under different oxidative environments and to establish ways to reduce the amount of CO released during combustion. Areas of solid fuel research of particular interest include physicochemical properties, reactivity and surface chemistry, CO emission properties, and catalyst impregnation on solid fuels to minimise CO emissions. This was achieved by using raw and pyrolysed wood biomass and coal. The results reported in this thesis show that the degradation profile of wood sawdust involved a combination of mechanisms, including diffusion, geometrical contraction, nucleation, and reaction-order models. Pyrolysed charcoal was found be microporous with a low specific surface area. The charcoal contained high quantities of • C-C and • C-O free radicals, and low-temperature oxidation (< 200 °C) produced persistent peroxyl radicals. Mild-temperature (300 – 650 °C) oxidation showed that the charcoal was highly reactive, with low pre-exponential factors and activation energies. At temperatures < 550 °C, charcoal did not appear to glow to the naked eye. However, this temperature range was shown to correspond to the emission of large amounts of CO. In confined environments, the measured CO emissions increased with decrease in ventilation and increase in height above the floor. ii Catalyst impregnation on charcoal reduced CO emissions by up to 97 % and increased the free active sites on the solid fuel surfaces, which decreased at temperatures > 500 oC possibly due to thermal deactivation of the catalyst.