Feed-reflector design for large adaptive reflector antenna (LAR)

A novel feed-reflector system for large Cassegrain antennas of radio astronomy and deep-space communication applications is investigated. This feed-reflector is used to illuminate a hyperboloid subreflector with a 5-10 m diameter located 500 m above the ground. Because the subreflector is located in the near field of the feed-reflector antenna, a theory based on the near field focusing properties of paraboloid reflectors is established. The focusing at near distance is formed by moving the feed horn away from the focal point of the feed-reflector. In this theory, the properties of axial defocused paraboloid reflectors at near distance are investigated in more detail. By using equivalence path law, the subreflector shape is obtained. It is found that the hyperbola can approximate the subreflector well. A detailed ray tracing is performed on the entire system which reveals that the feed system uses some part of the subreflector three times. The gain, side lobe level, cross polarization, and aperture distribution are calculated for different feed horn locations and taper at the edge of the feed-reflector and also for different sizes and eccentricities of the subreflector. Peak efficiency in excess of 74.8% and side lobe level around -20 dB are obtained for an unshaped system. The performance of the system over the operating band (1-22 GHz) is also studied and shown that the lower-frequency limit is dependent on subreflector and feed-reflector sizes     

Diagonal horn integrated with micromachined waveguide forsubmillimetre applications

The feasibility of coupling silicon micromachines rectangular waveguide to a modified diagonal horn antenna using a model at 110 GHz is demonstrated. The antenna patterns obtained are similar to those obtained with a standard diagonal horn. A method of reducing the on-axis cross polarization response of diagonal horns is also presented     

Transient bending of a piezoelectric circular plate

Thin, piezoelectric circular plates are frequently used as active components in transducer and smart materials applications. This paper reports on the exact, explicit solution for the transient motion of a piezoelectric circular plate, built-in or simply supported on the edge and electrically grounded over the entire surface. Expressed by elementary Bessel functions and obtained via exact inverse Laplace transforms, the solution enables the efficient calculation of accurate system parameters.     

An experimental and finite element poroelastic creep response analysis of an intervertebral hydrogel disc model in axial compression

A hydrogel intervertebral disc (IVD) model consisting of an inner nucleus core and an outer anulus ring was manufactured from 30 and 35% by weight Poly(vinyl alcohol) hydrogel (PVA-H) concentrations and subjected to axial compression in between saturated porous endplates at 200 N for 11 h, 30 min. Repeat experiments (n = 4) on different samples (N = 2) show good reproducibility of fluid loss and axial deformation. An axisymmetric nonlinear poroelastic finite element model with variable permeability was developed using commercial finite element software to compare axial deformation and predicted fluid loss with experimental data. The FE predictions indicate differential fluid loss similar to that of biological IVDs, with the nucleus losing more water than the anulus, and there is overall good agreement between experimental and finite element predicted fluid loss. The stress distribution pattern indicates important similarities with the biological IVD that includes stress transference from the nucleus to the anulus upon sustained loading and renders it suitable as a model that can be used in future studies to better understand the role of fluid and stress in biological IVDs.     

Viscoelastic tensile and shear properties of the 62 wt% Sn–36 wt% Pb–2 wt% Ag solder alloy

In this paper the results of the experimental determination of the viscoelastic tensile and shear properties of the eutectic 62 wt% Sn–36 wt% Pb–2 wt% Ag solder alloy in the temperature range between –20 and 100 °C are presented. The dynamic tensile and shear moduli and the viscous damping coefficients were measured using a phase-sensitive resonance technique. The results show that the temperature dependence of the dynamic Young's and shear moduli can be approximated by linear functions and a quadratic polynomial can be used to define a lower bound of the temperature dependence of the viscous damping coefficients of the material. For a temperature rise from 20 to 65 °C the dynamic Young's and shear moduli decrease by approximately 6% and the damping coefficient increases by approximately 80%.     

Pseudoelastic behaviour of cast magnesium AZ91 alloy under cyclic loading–unloading

Large stress–strain hysteresis loops are observed under cyclic loading after a small plastic prestrain. Loops have been observed in sand-cast material in a variety of tempers under tension or compression, and in high-pressure die-cast material with different cross-section thickness tested in tension. The loops are first observed after a nucleation strain of between 0.001 and 0.01% and grow to a maximum width after 1–2% plastic strain, becoming slightly narrower afterwards. When fully developed, the loops add a large (0.3–0.45%) pseudoelastic strain to the elastic strain, effectively decreasing the elastic modulus of the alloy by up to 70%. In sand-cast material of a given temper, the effects tend to be more pronounced in compression than in tension. Further, the effect is slightly larger in die-cast or aged sand-cast as compared to as-cast sand-cast material. The phenomenon is discussed in terms of the partial reversal of twins upon unloading.     

The fracture behaviour of a welded tubular joint: an ESIS round robin on failure assessment methods: Part V: screening method by required toughness and plastic stability considerations

A simplified failure assessment method, which is meant to be appropriate for a first screening of the considered structure with respect to its defect sensitivity, is applied to the T-joint of the ESIS round robin [Fracture behaviour of a welded tubular joint--round robin on failure assessment methods, First Information Package GKSS, 1994; Proceedings of 10th European Conference on Fracture, Berlin, vol. 1, 1994, p. 787]. The method enables one to identify fracture critical parts and to predict roughly the behaviour of a crack-like defect. It consists of an overall required toughness criterion to predict whether or not cleavage fracture can be initiated by a surface crack, and of a simplified stability analysis of the subsequent tearing process. This information can provide the basis to decide whether or not a more detailed failure assessment is required. Combined with analytical calculations of the overall plastic limit loads, the loading and fracture behaviour of the T-joint could be predicted by this simple and cost-effective method with reasonably good accuracy.     

Modelling of Lamb waves in composite laminated plates excited by interdigital transducers

The technique of permanently attaching interdigital transducers (IDT) to either flat or curved structural surfaces to excite single Lamb wave mode has demonstrated great potential for quantitative non-destructive evaluation and smart materials design. In this paper, the acoustic wave field in a composite laminated plate excited by an IDT is investigated. On the basis of discrete layer theory and a multiple integral transform method, an analytical–numerical approach is developed to evaluate the surface velocity response of the plate due to the IDTs excitation. In this approach, the frequency spectrum and wave number spectrum of the output of IDT are obtained directly. The corresponding time domain results are calculated by applying a standard inverse fast Fourier transformation technique. Numerical examples are presented to validate the developed method and show the ability of mode selection and isolation. A new effective way of transfer function estimation and interpretation is presented by considering the input wave number spectrum in addition to the commonly used input frequency spectrum. The new approach enables the simple physical evaluation of the influences of IDT geometrical features such as electrode finger widths and overall dimension and excitation signal properties on the input–output characteristics of IDT. Finally, considering the convenience of Mindlin plate wave theory in numerical computations as well as theoretical analysis, the validity is examined of using this approximate theory to design IDT for the excitation of the first and second anti-symmetric Lamb modes.     

Guided wave damage characterisation in beams utilising probabilistic optimisation

This paper introduces a probabilistic optimisation approach to the characterisation of damage in beams using guided waves. The proposed methodology not only determines the multivariate damage characteristics, but also quantifies the associated uncertainties of the predicted values, thus providing essential information for making decisions on necessary remedial work. The damage location, length and depth and the Young’s modulus of the material are treated as unknown model parameters. Characterisation is achieved by applying a two-stage optimisation process that uses simulated annealing to guarantee that the solution is close to the global optimum, followed by a standard simplex search method that maximises the probability density function of a damage scenario conditional on the measurement data. The proposed methodology is applied to characterise laminar damage and is verified through a comprehensive series of numerical case studies that use spectral finite element wave propagation modelling with the consideration of both measurement noise and material uncertainty. The methodology is accurate and robust, and successfully detects damage even when the fault is close to the end of the beam and its length and depth are small. The particularly valuable feature of the proposed methodology is its ability to quantify the uncertainties associated with the damage characterisation results. The effects of measurement noise level, damage location, length and depth on the uncertainties in damage detection results are studied and discussed in detail.     

Tonality evaluation of wind turbine noise by filter-segmentation

Wind turbine (WT) noise is one of the major hindrances for the widespread use of wind energy. Noise tone can greatly increase the annoyance factor and the negative impact on human health. The national standard for measuring WT noise tonality IEC 61400-11 does not ensure consistency and accuracy in the tonality analysis of strongly non-stationary noise. The aim of this study is to explore a new tonality evaluating methodology that is based on noise separation and stationary processing. An effective Gabor filtering method is proposed to remove background noise embedded in the sound signal of operating wind turbine. One-minute wind-turbine sound signal without background noise is optimally divided into relatively stationary sub-segments, by making full use of the close relationship between the wind-turbine rotor speed and the tonal noise emitted from WT. The proposed method’s theoretical advantages over conventional evaluation in IEC are illustrated. Results of experiments for GOLDWIND 77/1500 WT are presented, which show the capability of the proposed methodology.