National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

The Distribution of Load in Screw Threads

Abstract: The distribution of load along the length of a nut is not uniform owing to the strains set up in the bolt and nut under load. These strains are analysed and the load distribution along the thread helix deduced. It is shown that the load may be taken as concentrated at mid-depth of the threads. In the normal bolt and nut case the maximum intensity of loading occurs at the bearing face of the nut, and may be from two to four or more times the mean, depending on the thread form, the proportions of the members, and the degree of lubrication; the maximum intensity is almost independent of the length of nut (unless yielding occurs). Possible methods of improving the load distribution are discussed—differential pitch, tapered threads, and the use of a softer material for the nut. The load distribution in the turnbuckle case is more favourable than in the bolt and nut case, and can theoretically be made uniform by suitably boring out the inner and shaping the outer member; this condition can be approached in prac...

Analysis of hydrogen atom transport in a two phase alloy.

Abstract: The theoretical basis for analysing hydrogen atom transport in a two-phase alloy is evaluated. Experimental measurements of hydrogen atom transport in a duplex stainless steel, thermally treated to give a varying volume fraction of austenite, are described and analysed to ascertain the relative effect on the effective diffusivity of interfacial trapping and of the reduced diffusivity but enhanced solubility of the austenite phase. The effective diffusivity of the as-received duplex stainless steel is a factor of 400 less than that for the fully ferritic steel. It is demonstrated that diffusion through the austenite has no effect on the hydrogen transport, despite the higher solubility of hydrogen in this phase. However, the presence of austenite creates a more tortuous path for the hydrogen transport. The enhanaced solubility of hydrogen atoms in the austenite phase, relative to that in the ferrite phase, can be considered to induce a trapping effect on the hydrogen atom transport; however, the effect is small relative to the trapping associated with the austenite-ferrite interface. The binding energy of the interfacial traps is estimated to be about 52 kJ mol −1 .

Non-equilibrium flow of an ideal dissociating gas

Abstract: The theory of an ‘ideal dissociating’ gas developed by Lighthill (1957) for conditions of thermodynamic equilibrium is extended to non-equilibrium conditions by postulating a simple rate equation for the dissociation process (including the effects of recombination). This equation contains the ‘equilibrium’ parameter of the Lighthill theory plus a further ‘non-equilibrium’ parameter which determines the time scale of the dissociation phenomena.The behaviour of this gas is investigated in flow through a strong normal shock wave and past a bluff body. The assumption is made that the gas receives complete excitation of its rotational and vibrational degrees of freedom in an infinitesimally thin region according to the familiar Rankine-Hugoniot shock wave relations before dissociation begins. The variation of the relevant thermodynamic variables downstream of this region is then computed in a few particular cases. The method used in the latter case is an extension of the ‘Newtonian’ theory of hypersonic inviscid flow. In particular, the case of a sphere is treated in some detail. The variation of the shock shape and the ‘stand-off’ distance with the coefficient Λ, which is the ratio of the sphere diameter to the length scale of the dissociation process, is exhibited for conditions extending from completely undissociated flow to dissociated flow in thermal equilibrium. Results would indicate that significant and observable changes from the undissociated values occur, although values for the non-equilibrium parameter are not, at present, available.