Showing papers by "National Physical Laboratory published in 1987"

Mechanics of matrix cracking in brittle-matrix fibre-reinforced composites

Abstract: Energy-balance calculations for a continuum model of cracking in a uniaxially fibre-reinforced composite having a brittle matrix are presen­ted. It is assumed that the fibres are strong enough to remain intact when the matrix cracks across the entire cross section of the composite. By equating the energy availability for the cracking of continuum and discrete fibre models it is shown how the crack boundary condition relating fibre stress to crack opening must be selected. It is confirmed that the Griffith fracture criterion is valid for matrix cracking in composites. By considering the energy balance of long cracks it is shown that the limiting value of the stress intensity factor is independent of crack length and that it predicts a matrix-cracking strain that is consistent with the known result. An improved numerical method is described for solving a crack problem arising from the study of the cracking of brittle-matrix composites. Numerical results of high accuracy are obtained, which show how the cracking stress is related to the size of a pre-existing defect. Of special significance is the prediction of the correct threshold stress (i.e. matrix­-cracking stress) below which matrix cracking is impossible no matter how large the pre-existing defect.

OSI conformance testing

Abstract: Now that many international standards for Open Systems Interconnection (OSI) services and protocols have reached at least the Draft International Standard stage, there is much international activity in standardizing OSI conformance test suites. This new activity is based on the emerging OSI conformance testing methodology and framework standards being progressed by ISO and CCITT. This paper presents the major aspects of this methodology and framework.

Pressure waveforms generated by a dornier extra-corporeal shock-wave lithotripter

Abstract: Pressure waveforms in the acoustic field generated by a Dornier (HM3) shock-wave lithotripter have been measured using a bilaminar shielded PVDF membrane hydrophone in water. Using these waveforms, values of the peak-positive (p+) and peak-negative pressure (p-) at various positions in the field have been estimated. At the focus, p+ is 38.6 MPa (standard deviation = 9.0 MPa) and p- is 10.1 MPa (standard deviation = 1.0 MPa) at 20 kV discharge potential and an electrode separation in the range 1.3 to 2.4 mm. The peak-positive pressure is found to fall to 50% (-6 dB level) at about 60 mm either side of the focus on the major axis of the reflector and on a 10 mm radius circle around the focus in the focal plane. A shot-to-shot variation of +/- 25% in p+ is attributed to the inherent variability of the electrical discharge which may result in changes in the exact position and strength of the acoustic field. The results reported are considered to be more accurate than those of previous measurements due to the relatively flat frequency response of this type of hydrophone.

Dispersive Fourier Transform Spectroscopy

Abstract: A review is presented of recent developments in the methods of dispersive Fourier transform spectroscopy that have demonstrated the unique value of this broad band method for determining the optical constants of gases, liquids and solids.

Ada language and methodology

Abstract: From the combination of knowledge and actions, someone can improve their skill and ability. It will lead them to live and work much better. This is why, the students, workers, or even employers should have reading habit for books. Any book will give certain knowledge to take all benefits. This is what this ada language and methodology tells you. It will add more knowledge of you to life and work better. Try it and prove it.

Pulse averaging methods for a laser remote monitoring system using atmospheric backscatter

Abstract: Three methods for averaging differential absorption lidar (DIAL) measurements are discussed. They are compared using experimental data acquired with a dual-laser direct-detection DIAL system operating in the near UV using atmospheric backscatter. The data set was acquired with the two lasers tuned to the same wavelength to eliminate any spectral variations and fluctuations of differential absorbers from the measurement. The results are compared by evaluating both the standard deviations and the means of the integrated columns. The results suggest that shot noise on the backscattered signals dominates the measurement statistics and considerably reduces the necessity for short interpulse delay times.

Fractography of a SiC particulate reinforced aluminium metal matrix composite

Abstract: Etude par microscopie electronique a balayage des materiaux a matrice metallique d'alliage Al-6061 renforces par SiC rompus par traction. Analyse et comprehension du mecanisme de rupture

Analysis of the Sommer technique for measurement of the mobility for charges in two dimensions

Abstract: The technique for measuring the low-frequency ac mobility of free surface charges first employed by Sommer is analyzed for arbitrary values of driving frequency, charge mobility, and effective mass. Analytical expressions for the cell admittance are given for both rectangular and circular geometries in the absence of edge corrections.

Modelling creep-corrosion interactions in nickel-base superalloys

Abstract: Revue et classification des plus recents travaux experimentaux sur les interactions entre la corrosion et le fluage dans le cas des superalliages base nickel. Modelisation de ces phenomenes permettant d'obtenir une approche valable du comportement des composants en service

Mapping The Performance Of Surface-Measuring Instruments

Abstract: The performance of a dimensional surface-measuring instrument is usually limited by a small number of critical components or specifications. For example, parameters such as those specifying ranges and resolutions, and probe geometry are needed to define the capabilities of the instrument. The response of the instrument to sinusoidal perturbations of varying amplitude and wavelength, can be mapped in amplitude-wavelength space to show the effect of each parameter and the overall limits of performance.

The relationship between the SI Ohm, the Ohm at NPL, and the quantized Hall resistance

Abstract: New measurements relating the quantized Hall resistance R H (= h/ie2), International System (SI) Ohm (Ω SI ), and the National Physical Laboratory maintained ohm (Ω NPL ) have now been completed at NPL in the U.K. with improvements and simplifications in the cryogenic current comparator measurements and 1000-Ω dc resistance measurements. From the measurements over the past four years the relationship between Ω NPL and Ω SI can be described by the equation Ω NPL − Ω SI = −1.049(0.020) − 0.0478(0.0074)[t − 1986.0] μΩ in which t is measured in years. For the previous two years the equivalent relationship between R H and Ω NPL is R H = 25 812.8(1 + 1.452(0.038) × 10 −6 + 0.0694(0.0772) · [t − 1986.0] × 10−6) Ω NPL in which the uncertainties (in parentheses) are one-standard-deviation (1σ) random uncertainties of the least squares fit to the data. Combining the most recent measurements of R H and Ω SI , using a more direct method of measurement R H = 25 812.8106(17) Ω SI in which the relative combined uncertainty is 0.067 × 10−6.

Embedded user models

Abstract: A global definition of user models is presented and the characteristics which define a particular form of model (an ‘embedded user model’) are highlighted and discussed. The distinctions between various forms of conceptual, application and student models are described and examined. These definitions are located within a general categorisation of user modelling techniques, based on implementation methodology, and on ‘ownership’ of the model itself. An extensive literature review of recent research in modelling techniques is referenced and a number of techniques described. The architecture of embedded user models is contrasted to that of User Interface Management Systems and an outline design specification for a system encompassing such a model is proposed.

Individual observer data for the 1955 Stiles-Burch 2 degrees pilot investigation.

Abstract: Although the results of the 2 degrees Stiles-Burch investigation were published in 1955 in the form of mean data, individual results have not been available. It is only recently that they have been in wide demand. Since in 1955 this demand was not anticipated, individual results were left in an unprocessed form; the processing has now been completed. Tables are given for each of 10 observers and their mean for color-matching functions, unit (WDW) coordinates, and relative luminous efficiency functions.

Physicochemical properties and catalytic behavior of manganese oxides

Abstract: Two series of manganese oxides have been prepared by the thermal decomposition of manganese carbonate and oxalate under vacuum (300–900°C temperature). The kinetics of aqueous H 2 O 2 decomposition on these oxide surfaces have been studied. The reaction was found to be first order with respect to [H 2 O 2 ]. Catalytic activities were found to be decreasing with the increasing temperature of decomposition. X-ray diffraction studies of the powdered samples show that the samples contain mainly two types of structures. NaCl(MnO) and calcinite (Mn 3 O 4 ). Infrared spectra show a characteristic band at 796 cm −1 for the sample obtained by the decomposition of both Mn oxalate and Mn carbonate at 750 and 900°C. Samples prepared at other temperatures show similar characteristic bands, i.e., at 1124, 604, and 484 cm −1 , etc. Surface areas vary from 112 to 4 m 2 /g. Pores are mainly of cylindrical shape. There is an agglomeration of the particles as the temperature of decomposition is increased. Conductivity measurements indicate that most of the samples show p - type semiconductivity.

A taxonomy of evaluation techniques for hci

Abstract: The results of an extensive literature search are presented and a rationale given for the existence of a number of diverse and distinct techniques used to evaluate the human-computer interface. A prescriptive taxonomy of the evaluation techniques uncovered in the literature search is proposed, and the emergence of an ‘evaluation environment’ as a necessary part of the taxonomy is described. Although much work needs to be done, the environment represents a first pass at a tool for matching techniques to requirements.

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

Abstract: The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa2Cu3O7/YBa2Cu3O7 point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa2Cu3O7.