Table of contents

In this note we present some results of a theoretical investigation into the propagation of stress waves in a carbon fibre-epoxy resin (CE) composite. The results indicate that a CE composite can be used as a collimator or filter of stress waves and further more suggest a possible failure mechanism of a CE composite, particularly when it is exposed to strong and periodic mechanical disturbances.

A study of gas phase flow, with and without an additional surface flux, in a three-dimensional cubic network of interconnected tubes is described. The anisotropy and connectivity of the model can be modified by elimination of tubes from each unit cubic cell. For a network in which all tubes have the same radius, the anisotropy can be related to gas phase structure factors. Computer calculations enable the properties of networks with Gaussian radius distributions to be investigated and it is established that estimates of structure factors for infinitely large networks can be made using networks of limited size. The effect of distribution breadth and of the amount of interconnection at a junction on the structure factors is discussed. It is found that radius distribution is less important in network systems than in the serial model described in an earlier paper. This difference is attributed to the dominance of constrictions in the former model in contrast to the multiple paths available at each junction in the latter.

Considerable interest has recently been shown in new rheometer designs which make use of a steady flow to determine the complex viscosity of an elasticoviscous liquid. Here, we develop the necessary mathematical background to a number of new rheometers of this sort. The suggested measurements involve the components of the force or the components of the couple acting on one of the instrument members. Since commercial rheometers are available which make such measurements, it is anticipated that these instruments will be adapted by some experimentalists to include the new geometries suggested in this paper.

Consideration is first given to the generation of Poiseuille flow, for an elastico-viscous liquid, caused by a constant pressure gradient, and then to the corresponding decay of the fully developed flow when the pressure gradient is suddently removed. It is found that the velocity profiles are strongly dependent on the elastic parameters and can oscillate about their terminal states. The results are interpreted to give a possible qualitative explanation of anomalies found by Oliver and Macsporran in jet thrust measurements.

The requirement for specific vibrational modes is mentioned in relation to certain metal working operations. A detailed study of the propagation of radial waves in an annular disk when resonance occurs is presented and the accurate solution found to coincide closely with a simplified model in which the mean ring circumference is made equal to an integral number of whole wavelengths. Experiments showed that the resonant frequency could be accurately predicted for such a system.

The theory of the previously described thermoelectric transformer is developed in general terms and applied to a transformer that is based on the Nernst and Ettingshausen effects. It is shown that the thermomagnetic (Nernst-Ettingshausen) transformer possesses certain advantages over the Peltier-Seebeck transformer. Thus, it is in principle possible to obtain voltage multiplication exceeding unity from a thermomagnetic transformer with a single-element primary and a single-element secondary. However, it is considered preferable to obtain high voltage multiplication by employing a multi-element secondary, since a very short response time can then be achieved. A simple reciprocal thermomagnetic transformer made from a bismuth alloy has been tested.

Measurements of the laser radiation power density P_th required to cause ionization growth and breakdown in the pressure ranges 1800 less-than-or-eq, slant p less-than-or-eq, slant 16 000 torr and 200 less-than-or-eq, slant p less-than-or-eq, slant 20 000 torr have been made in helium and argon, respectively. Three Q-switched lasers (ruby, Nd, and dye) were used to investigate the dependence of P_th upon pressure, radiation frequency ω and flash duration 2τ. Values of P_th lie between 10¹¹ W cm⁻² at low pressures and 10¹⁰ W cm⁻² at high pressures.

The experimental results are compared with expressions based upon an extrapolation of classical microwave breakdown theory to optical frequencies. Reasonably good agreement between measured and calculated threshold intensities is obtained.

The interaction between the laser radiation and the gas proceeds via inelastic electron-atom collisions and photoionization. Three pressure regimes may be specified: a low pressure regime in which the threshold intensity and ionization growth rate are governed principally by diffusion losses, an intermediate régime in which the laser flash duration is the dominant factor and a high pressure régime where electron-ion recombination controls the rate of ionization growth and the onset of breakdown. The ionization rate is shown to be proportional to the laser beam intensity and, in the intermediate régime, the threshold intensity P_th is proportional to ω²/τ at constant pressure. A value for the electron-atom momentum transfer collision frequency of about 3-4×10⁹p and of about 5 eV for the electron mean energy is derived for argon at the onset of breakdown.

Using the shock wave technique, radial distributions of gas temperature have been measured in cylindrical, wall stabilized, arc columns under both steady state and transient conditions. The arc atmosphere was air and the current range 0·5-100 A. The transient measurements included the case of free recovery following sudden current interruption.

The temperature profiles have been used in conjunction with published electrical conductivity data to determine electrical conductance values and these show good agreement with direct electrical measurements and theory. This result confirms that the arc plasma is in approximate thermal equilibrium so that the column behaviour may be described solely in terms of thermal processes.

The importance of natural radial flow under transient conditions has been estimated by calculating flow velocity profiles from the measured temperature distributions and published gas density data. The results are in good agreement with theory, and show the increasing importance of the radial flow at higher currents.

Atomic absorption spectroscopy has been used to detect the atoms sputtered from nickel and iron cathodes in glow discharges of argon. The observed spatial distribution of sputtered atoms differs from that predicted by diffusion theory suggesting that some sputtered material is ejected from the cathode as molecules or groups of atoms. These are undetectable by atomic absorption until dissociated in the cathode dark space of the discharge. A relationship for calculating the deposition rate of sputtered atoms from the spectroscopic measurements has been obtained, but uncertainties in some quantities prevent an accurate calculation in this case. However, an upper limit to the deposition rate of atoms is lower than the gravimetric deposition rate measured under the conditions studied. Thus, atomic absorption spectroscopy, which detects only individual atoms, cannot be used to measure total sputtering deposition rates while there is evidence that some sputtered material may be ejected as molecules.

It is shown experimentally that the mean statistical time-lag for electric breakdown in n-hexane is inversely proportional to the area of a planar electrode system and independent of the electrode spacing. This is consistent with the conjecture that for given materials the rate of initiation of breakdowns per unit area is a unique function of stress.

A computer program based on a procedure discussed by Powell has been developed for the resolution of overlapping peaks in data output from x-ray diffraction, ion-exchange chromatography with spectroscopic detection, and infrared spectroscopy. A polynomial background is also fitted to the data so that recourse to arbitrary graphical methods for separating peaks and background is no longer necessary.

The directional behaviour of Young's modulus, shear modulus, and Poisson's ratio are expressed for a number of crystallographic planes for cubic materials. Their behaviour as a function of direction on a particular plane is shown to be a simple sum of sines and cosines of twice and sometimes of four times the angle. A general expression gives the coefficients for the sines and cosines and a table gives the coefficients for 19 planes on the standard triangle of the stereographic projection. Superposed on the stereographic projection are shown polar plots of the shear modulus Gprime₁₂ (1prime direction normal to the plane) for silicon. Examples are also given showing G₁₂prime for copper for some planes of zones [1bar above 00], [1bar above 10] and [2bar above 10].

The mechanical behaviour of some plastics with and without fibre reinforcement has been investigated experimentally for compressive loading cycles of approximately 100 μs duration and for steady-state compressive loading. For the dynamic measurements the split Hopkinson pressure bar technique has been used in which cylindrical specimens are sandwiched between two rods and deformed under the action of a compressive stress wave induced by impacting the free end of one of the rods with a projectile launched by a light gas gun. The experimental results are compared with the mechanical behaviour predicted by a particular form of nonlinear mechanical equation of state. For all materials two regions of deformation are distinguished, the transition from one state, that is the higher state of the internal structure to the lower state being identified with the phenomenon of thixotropy as observed in the flow of certain types of non-Newtonian liquid.

A method of extracting information about trapping levels in crystals by numerical curve fitting, known for first- and second-order kinetics curves, has now been extended to general-order kinetics glow peaks. This generalized method, suitable for use by digital computer, enables the evaluation of the activation energy as well as the kinetics order. The method has been used on thermoluminescence peaks of SrF₂: Tb crystals. The results are compared with activation energies and kinetics orders found by other methods.

Cooper and Elliott have recently shown that the electric breakdown of KBr single crystals is initiated in the cathode region. An explanation was offered which depended on the accumulation of positive space charge arising from electron impact ionization. Experiments are described in the paper which show that an alternative explanation of cathode field enhancement by polarization due to the migration of positive ions is improbable.

The stress dependence of contact potential has been measured in copper and aluminium at very low stress levels using a sensitive ac method. The results of two experiments are presented and in both cases the effect is found to be much smaller than the free electron estimates calculated from the theory of Dessler et al.

Calculations have been performed to find the optimum thickness for a transparent gallium arsenide photocathode mounted on a sapphire substrate. The performances of other semiconductors with band-gaps as low as 1·0 eV have also been compared with GaAs. The behaviour of such surfaces under starlight and moonlight conditions is also calculated.

A general analysis is given for the determination of the effective dielectric constant of a macroscopically homogeneous and isotropic two-phase composite. Some approximate formulae are presented and their applications discussed in terms of the general features of the sample geometry.

The design of canal ion sources is discussed taking into account the lens effect of the canal aperture, and the results summarized in a universal design curve. Experimental data available falls between this curve and one in which lens effects are neglected. The relationship between theory and experiment is considered.

Exact calculations are presented for the fringing fields inside and outside a hollow partial-cylinder conductor, as used on certain types of plasma pinch machines. The concentration of current density j_z(θ) at the slot edges is also calculated analytically.

Total secondary electron coefficients in helium + 0·1% hydrogen and helium + 1·0% hydrogen have been calculated from the measured dependence of breakdown voltage on gas pressure. OFHC copper and molybdenum concentric cylinder electrodes were used with the inner conductor at positive potential. The variation of the secondary coefficients is presented as a function of E/p0 at the cathode in the range 5-20 V cm⁻¹ torr⁻¹.

This note shows that the observed lack of density variations for formative time lags τ of sparks can be explained by the known pressure and electric field dependences of the first Townsend ionization coefficient and the electron drift velocity. It is demonstrated that τ varies linearly with gap length and inversely as the fractional over-voltage across the spark gap (in agreement with the measurements of Blair and Farish). Finally, it is demonstrated that the accuracy of the error in the measured `restriking' voltage for a recovering spark channel is constant for the complete recovery period.

The use of a refractory material like tungsten for the target electrode causes faster switching of a laser triggered spark gap. This is believed to be due to the faster local heating to a higher temperature which causes a greater injection of plasma into the gap. Reliable switching with a formative time of less than 10 ns has been achieved.

The neutral density in a mercury positive column, modulated at low frequencies, has been monitored using the 2537 Å absorption technique and the results compared with predictions and microwave measurements reported in an earlier paper. The agreement between the various results is presented as confirmatory evidence that low-frequency current modulation in positive columns is associated with perturbations in neutral density of the appropriate sense and magnitude to cause the clockwise hysteresis previously observed between electron density and discharge current.

Thin films of nickel deposited on silver have been reported to change on annealing in a way suggesting an alloy formation. The inverse experiment, in which thin silver films are deposited on nickel at pressures of 2×10⁻⁵ torr and annealed at 280 °C, has given a similar result. Experiments performed in an ultra-high vacuum plant have not shown any changes in the films.

Induced ac losses and magnetization curves measured at 4·2 K, are reported for a reversible supersaturated solid solution of Pb-11% Sn, and for the same material after the precipitation of pinning centres. The results show lower hysteretic losses in the precipitated material for peak applied fields up to H_fp, as defined by slow-swept magnetization measurements, but more significantly the losses up to this field are lower than in pure lead.