119 research outputs found
An inverse problem of reconstructing the electrical and geometrical parameters characterising airframe structures and connector interfaces
No full textThis article is concerned with the detection of environmental ageing in adhesively bonded structures used in the aircraft industry. Using a transmission line approach a forward model for the reflection coefficients is constructed and is shown to have an analytic solution in the case of constant permeability and permittivity. The inverse problem is analysed to determine necessary conditions for a unique recovery. The main thrust of this article then involves modelling the connector and then experimental rigs are built for the case of the air-filled line to enable the connector parameters to be identified and the inverse solver to be tested. Some results are also displayed for the dielectric-filled line
Non destructive examination of composite structures using dielectric examination
No full textDielectric measurements are widely used in the laboratory to probe the dynamics of molecules, particularly the dynamics of polymer molecules. The dielectric technique exploits the fact that many molecules, although electrically neutral posses an asymmetric distribution of charges which can be approximated to an electric dipole. The (usually thermal) motion of the molecule can be detected by the interaction of this dipole with a time varying electric field. The great advantage of the technique is that no transducers or sensors are required; the direct application of an electric field produces a directly measurable electric response over a frequency range of MHz to GHz. This paper discusses the practical application of dielectric measurements to composite structures and the information that can be obtained on the state of the polymer in polymer composite matrix materials
Dynamic characteristics and processing of fillers in polyurethane elastomers for vibration damping applications
No full textPolyurethane elastomers have the potential of being used to reduce vibrational noise in many engineering applications. The performance of the elastomer is directly related to matching the nature of the mechanical loss characteristics to the frequency and temperature dependence of the source of the vibration. Materials with a broad frequency response and good mechanical properties are desirable for situations were load bearing and isolation becomes an issue. Because automobile, and other related vehicles operate over a broad temperature range, it is desirable for the damping characteristics of the elastomer to ideally be independent of temperature and frequency. In practice, this is not possible and the creation of materials with a broad spectrum response is desirable. In this paper, the effects of various fillers on the breadth and temperature dependence of the vibration damping characteristics of a filled and crosslinked polyurethane elastomer are explored. The fillers studied are wollastonite, barium sulphate and talc. These materials have different shapes, sizes and surface chemistry and undergo different types of interaction with the matrix. The vibration damping characteristics were further varied by the use of a crosslinking agent. Data presented on the rheological characteristics indicate the strength of the filler-polyol interactions. Dielectric relaxation and dynamic mechanical thermal analysis demonstrate the way in which changes in the type of filler, concentration and amount of crosslinker lead to changes in the location and breadth of the energy dissipation process in these elastomers. The vibration damping characteristics of a selected material are presented to demonstrate the potential of these materials
Performance of periodic piezoelectric composite arrays incorporating a passive phase exhibiting anisotropic properties
No full textThis paper explores the minimisation of interelement cross talk in 1-D and 2-D periodic composite array structures through the incorporation of a passive phase exhibiting anisotropic elastic properties. Initially the PZFlex finite element code was used to monitor array aperture response as a function of material properties. It is shown that in array structures comprising passive polymer materials possessing low longitudinal loss and high shear loss, inter-element mechanical cross talk is reduced, without a concomitant reduction in element sensitivity. A number of polymer materials with the desired properties were synthesised and their elastic character confirmed through a program of materials characterisation. Finally, a range of experimental devices exhibiting improved directional response, as a result of a significant reduction in interelement cross talk, are presented and the predicted array characteristics are shown to compare favourably in each case
Non-destructive evaluation (NDE) of composites: dielectric techniques for testing partially or non-conducting composite materials
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Non-destructive evaluation (NDE) of composites: dielectric methods for testing adhesive bonds in composites
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