Thesis ( Ph.D.) -- McMaster University, 2000. / Includes bibliographical references. Also available via World Wide Web.

Thesis (Ph. D.)--Lehigh University, 2006. / Includes vita. Includes bibliographical references (leaves 675-691).

The bulge forming process is a method for shaping tubular components using an internal hydrostatic pressure combined with a compressive axial load. Initial investigations involved carrying out an extensive literature survey to determine the components which could be formed and the effects of using lubricants and different tube materials. Die-blocks were designed to produce tee pieces, cross joints and off-set joints, and electronic on-line instrumentation was incorporated so that the forming pressures and loads could be accurately monitored. A series of tests were carried out in the forming of: (1) tee pieces, cross joints and off-set joints from copper tubes of two different wall thicknesses, (2) tee pieces using different types of plungers, (3) tee pieces using die-blocks coated with various lubricants, (4) tee pieces from aluminium, copper and steel tubes, (5) tee pieces using die-blocks with various branch radii. From the resulting components, formed with various combinations of internal pressure and compressive axial load, the limits for a successful forming operation were established. Further analysis of these components was then undertaken to evaluate the effects of the internal pressure and axial load on the bulge height and the wall thickness in the deformation zone. From these results, which have been illustrated graphically, the greatest effect on the resulting bulge can be seen to be the compressive axial load. Theoretical analyses are presented, which predict the wall thickness distribution around the bulge zone and also the axial loads required in the forming process. Comparison of these predictions with the experimental results shows fairly good agreement.

Novel segmented-electrode discharge tubes which operate as sealed-off He-Cd hollow cathode lasers and which do not require a flowing gas system to distribute cadmium vapour are reported. The electrode geometry is arranged to produce a net axial electric field so that cadmium distribution is enhanced by cataphoresis,' a process which is not normally associated with He-Cd hollow cathode lasers. The conditions under which the segmented-electrode tubes exhibit the properties of hollow cathode discharges are determined from their current and voltage characteristics. One system studied consists of alternate anode and cathode segments with glass insulating sections arranged axially, and behaves as a series of hollow cathode discharges under appropriate conditions. Each anode/hollow cathode pair is separated from its neighbouring segments by insulating glass sections of length D, which provide a recombination site and prevent adjacent discharges from overlapping each other. A calculation shows that adjacent discharges will remain isolated provided that D > 2.85cm, in helium at 10 Torr and 700 K. Another segmented-electrode system studied consists of a series of metal segments equal in length. This system does not have discrete anode segments and exhibits the positive Volt-Ampere (VI) characteristics normally expected of, hollow cathode discharges only at low values of discharge current. A transition point in the characteristics exists, and, for discharge currents above this, the VI slope is negative. This behaviour is explained in terms of the plasma length, which is defined as the axial extent of the negative glow within a cylindrical segment, and which increases with increasing discharge current. At low values of discharge current (several mA), the plasma length is less than the length of the metal segments, and the discharge tube operates as a series of hollow cathodes. Under these conditions, each segment has a negative glow discharge partially penetrating the bore from one end, while the other end performs as an anode for the next segment, and so on. The last segment is at power supply earth potential and has no anodic function. The transition points in the VI characteristics occur when the discharge current is increased to a value such that the plasma length equals the length of the metal segments. For discharge currents above this value, only the last segment at power supply earth potential continues to operate as a hollow cathode. The results of these studies provide guidelines for the design of segmented-electrode He-Cd hollow cathode laser discharge tubes in terms of the axial order and optimum spacing of anode and cathode segments. It is shown that large axial temperature variations can exist in thin-walled (0.9mm) hollow cathode structures, implying that the current density distribution at the cathode surface is not uniform. A differential equation is derived to describe heat flow within a cylindrical conductor heated at one end and cooled by natural convection and radiation along its length. Approximate solutions are obtained for both thick-walled and thin-walled conductors. The results of the thick-walled approximation yield thermal design criteria for hollow cathodes, where moderate cathode temperatures (350°C) and minimal axial temperature variations are required. The thin-walled approximation yields results which provide design criteria for the protection from high temperature failure of components such as cylindrical glass/metal seals and 'O' rings.

This thesis describes experimental and theoretical aspects of low pressure gas discharges confined by metallic walls. Simple considerations predict that the current between the anode and the cathode of a discharge tube which contains isolated cylindrical metal segments is conducted through the metal. Under certain conditions, however, a glow discharge may form along the axis of the tubes conduction occurs along the path which results in the lowest sustaining VO 1tage. The energy losses at insulating and conducting walls in the region of the positive column are investigated theoretically for rare gas discharges at high current densities. The energy losses are found to be greater when the walls are conducting, resulting in a higher axial electric field. A possible criterion for the maximum length of a metal segment is proposed. The maximum length may be defined as the length at which the radial electric field in the positive ion sheath at one end of a metal segment is equal to the breakdown field. A relationship between the maximum length of a metal segment and the tube radius and pressure has been derived. The form of this relationship agrees qualitatively with experimental results. Measurements of the anode-cathode breakdown voltage of a gas in a tube containing metal segments of small bore are presented. The anode-cathode breakdown voltage is found to be approximately equal to the sum of the breakdown voltages of the inter-segment and electrode-segment gaps. When the system is operating on the left of the Paschen minimum, the anode-cathode breakdown voltage may be increased by the addition of metal segments in such away as to reduce the effective gap. Individually the product's of pressure and inter-segment gap (pd) are less than the product (pd) min at the Paschen minimum. Hence, the anode-cathode breakdown voltage will increase as the number of gaps (and segments) increases and pd decreases. Discharges in dual-bore (alternate narrow and wide bore) metal tubes have been investigated. The anode-cathode breakdown voltages and sustaining voltages of low current density discharges, confined by dual-bore tubes of various lengths, are shown to depend upon the system geometry. A laser system using dual-bore metal discharge tubes, has been designed and tested for producing high current densities in rare gas-metal vapour mixtures.

This thesis describes an investigation the aim of which was the development of low pressure gas switches with the advantages of zero standby power consumption and instant readiness. Hydrogen thyratrons use a hollow anode to give the switch a convenient reverse conduction capability. The hollow anode structure has been shown to pass a 4 kA pulse current at 500 Hz for 10¹⁰shots. The use of the hollow anode structure as a cold cathode for a low pressure switch is proposed and triggering of the structure by ions is demonstrated. Under conditions of low gas pressure and high discharge voltage, electrons make few collisions in the cathode dark space of a glow discharge and form extensive beams which travel many centimetres in the gas. Current/voltage characteristics of this 'electron beam' type of discharge are presented for deuterium at pressures between 0.2 and 1.0 torr. The electron beam discharge was found to be space-charge limited with I V3/2 at pressures below about 0.25 torr and I V3/2 at pressures above about 0.25 torr. It is proposed that the current in the electron beam discharge is limited by the flow of positive ions in the cathode dark space. Control of the emission area of a discharge in a hollow metal cylinder is demonstrated and is used as a triggering method for a new type of low pressure gas switch. Tests in a pulse modulator at repetition rates up to 1 kHz show that the switch operates satisfactorily. The triggering mechanism is shown to depend on the properties of the cold cathode glow discharge which, in certain circumstances, leads to the unusual phenomenon of post trigger-pulse firing of the switch. The phenomenon is shown to result from the interaction of the trigger discharge cathode dark space and the geometry of the switch. The glow discharge electron beam is successfully applied as a triggering method in several new low pressure gas switches. In one arrangement, the electron beam is used to pre-ionise the switch and subsidiary grids are used to trigger main conduction. In another arrangement, the electron beam is directed into the high voltage region to trigger conduction directly. The designs of these switches are discussed and their operation is demonstrated.

An investigation into aggregate breakage has been carried out using suspensions of uniform polystyrene spheres destabilised by addition of salt or polymer. A controlled flow apparatus was used to expose the suspended aggregates to turbulent pipe flow of variable intensity and duration. Analysis of aggregate size distribution was carried out using the HIAC light obscuration technique.

Energy absorption mechanisms have been investigated intensively for the past decades by various authors and institutions, and numerous articles and other literature sources are available in print, as well as on the Internet. Energy absorbers and crashworthiness structures are two main research components in the energy absorption field under investigation today. In this research geometric changes are introduced on Al 6063-T6 circular tubes in the form of horizontal and spiral grooves to asses their influence on energy absorption characteristics. The horizontal and spiral grooves were cut into the tube to a cut depth of half the wall thickness of the tubes. The pitch was varied for both the horizontal and spiral grooves, while the cut width was kept constant. A specially designed static impact sleeve was used to compress the test specimens axially in an Instron 250 kN universal hydraulic testing system. Load vs. displacement graphs were generated from the captured experimental data for the uncut, horizontal and spiral grooved tubes. Energy vs. displacement graphs were created from the experimental data. The final deformed tubes were visually examined to determine the effect the geometric change had on the circular tube form, as well as the deformation pattern of the crushed tube. A Finite Element Method model is presented for each of the experimentally investigated tube impact models. A two dimensional (2D) model for the uncut as well as horizontally grooved tube is generated and analysed using a quasi static loading approach. Non-linear material properties are assigned to the model, and the Riks algorithm is used to model the non-linear post buckling behaviour of the various tubes. The results from the FEM analysis are used to generate load vs. displacement and energy vs. displacement graphs that are compared with the experimental data. Three dimensional (3D) FEM models of the normal, spiral and horizontal cut tubes were also generated in a CAD environment. A dynamic explicit non-linear analysis was done for each of the models to determine the reaction force and energy output values of each of the models. All analyses extend into the plastic material domain. Reaction force vs. displacement and energy vs. displacement graphs are generated from these analyses. A comparison is made between the numerically and experimentally determined gradients of the energy vs. displacement graphs of each of the tubes investigated. This forms the basis for an energy absorber design with application in the transport industry. Unique geometric imperfections were investigated experimentally and numerically for aluminium tubes. A lower buckling load than that for the normal tubes was achieved with the introduction of these geometric imperfections. New deformation patterns on tubes with imperfections not previously observed were described and analysed extensively. The load vs displacement graphs showed a constant increase in the load for the spiral grooved tubes. From the comparison between the numerically and experimentally investigated geometric imperfections a design guide line was esthablished and used in the conceptual design of an energy absorber for the automotive industry. / Prof. L. Pretorius Prof. R.F. Laubscher

No description available.

No description available.