G.R. Tomlinson

Numerical and experimental uniaxial loading on in-plane auxetic honeycombs

Auxetic honeycombs show in-plane negative Poissons ratio properties; they expand in all directions when pulled in only one, and contract when compressed. This characteristic is due to the reentrant shape of the honeycomb unit cell. The cell convoluteness gives a geometric stiffening effect that affects the linear elastic properties of the whole cellular solid. In this paper finite element simulations are carried out to calculate the in-plane Poissons ratio and Youngs moduli of re-entrant cell honeycombs for different geometric layout combinations (side cell aspect ratio, relative thickness and internal cell angle) subjected to uniaxial loading. The results show a high sensitivity of the mechanical properties for particular ranges of the geometric cell parameters. An image data detection technique is used to extract displacements and strains from an aramid paper re-entrant honeycomb sample in a tensile test. The comparison between numerical and experimental results shows good agreement.

Fail-safe sensor distributions for impact detection in composite materials

This paper studies the problem of optimal sensor placement for impact detection and location in composite materials. The study involves a simple impact experiment on a composite box panel. The time-varying strain data are measured using piezoceramic sensors. An effective impact detection procedure is established using a neural network approach. The procedure determines the location and amplitude of impacts. A genetic algorithm is used to determine the optimum sensor positions for a diagnostic system. The main object of the paper is to study fail-safe distributions, i.e. those whose sub-distributions also have a low probability of detection error. The results are validated against an exhaustive search. The study shows that genetic algorithms combined with neural networks can be effectively used to find near-optimal sensor distributions for damage detection. The methods presented are generic and can be used in similar sensor position problems.

The parametric characteristic of frequency response functions for nonlinear systems

The characteristic of the frequency response functions of nonlinear systems can be revealed and analyzed by analyzing of the parametric characteristics of these functions. To achieve these objectives, a new operator is defined, and several fundamental and important results about the parametric characteristics of the frequency response functions of nonlinear systems are developed. These theoretical results provide a significant and novel insight into the frequency domain characteristics of nonlinear systems and circumvent a large amount of complicated integral and symbolic calculations which have previously been required to perform nonlinear system frequency domain analysis. Several new results for the analysis and synthesis of nonlinear systems are also developed. Examples are included to illustrate potential applications of the new results.

A new procedure for detecting nonlinearity from transient data using the gabor transform

A new technique for the identification of nonlinearity in multi-degree of freedom systems is presented. The technique is based on the joint application of the Gabor and the Hilbert transforms to the transient response of a system. The Gabor transform is used first to identify a time-variant matrix representing the spatial behaviour of the system. This matrix is then used to decouple the transient response into a set of uncoupled quasi-harmonic components. Finally the Hilbert transform is applied to identify the dissipative and restoring forces associated with each component which is equivalent to a single degree of freedom system. Numerical examples are supplied to help clarify the main advantages and the possible limitations of the method in the presence of strong nonlinearities and closely spaced frequencies.

Nonlinearity in experimental modal analysis

Modal analysis is established as one of the fundamental strategies in approaching structural dynamic problems. Despite its universal appeal, it is fundamentally a linear theory and cannot be applied to significantly nonlinear systems without incurring substantial difficulties in implementation and interpretation of results. This paper documents a number of attempts to reconcile nonlinearity with modal analysis. Three main approaches are discussed, each of which is characterized by a different philosophy.

Modelling of a Hybrid Constrained Layer/Piezoceramic Approach to Active Damping

It has been shown that significant reductions in structural vibration levels can be achieved using a hybrid system involving constrained layer damping and active control with piezoceramics. In this paper, mathematical models based on the Rayleigh Ritz approach, are developed to describe the longitudinal and flexural vibration behaviour of a cantilevered beam when excited using piezoceramic patches bonded to a constrained layer damping treatment. Predictions of static and steady state dynamic behaviour, obtained using the models are validated by comparison with results from finite element analysis and laboratory experiments. The models are then used in open loop and closed loop velocity feedback control simulations to demonstrate the improvements in stability and performance achieved using this method over that achieved using conventional active control.

On the Non-Linear Characteristics of Automotive Shock Absorbers

The objectives of this paper are essentially twofold. In the first case an experimental study of a number of shock absorbers is presented; the restoring force surface method of non-linear system identification is applied in order to determine the non-linear characteristics of the absorbers in an easily visualizable manner. In the second part, a new physical model for the absorber is presented which incorporates effects due to compressibility of the fluid in the absorber; this provides a more realistic representation of the stiffness characteristics than previous simple models. The new model is compared with the experimental data.

Characterization of Automotive Shock Absorbers Using Random Excitation

In a previous paper [see reference (4)], it was shown that the restoring force surface (RFS) procedure provides a direct and clear method for characterizing the dynamic properties of automotive shock absorbers or dampers. The procedure was based on repetitive harmonic testing of the absorbers at fixed frequency but with varying amplitude. The current paper describes how the surfaces can be obtained from tests using random excitation. The merits and demerits are discussed relative to the harmonic test procedure. It is shown that the random excitation approach offers a useful alternative but produces force surfaces which are corrupted by small stochastic components; an explanation of the distortion is given in terms of the mathematical model proposed in the previous paper. The implications for identification of shock absorbers are discussed

The Transmissibility of Vibration Isolators With a Nonlinear Antisymmetric Damping Characteristic

In the present study, the concept of the output frequency response function, recently proposed by the authors, is applied to theoretically investigate the force transmissibility of single degree of freedom (SDOF) passive vibration isolators with a nonlinear antisymmetric damping characteristic. The results reveal that a nonlinear antisymmetric damping characteristic has almost no effect on the transmissibility of SDOF vibration isolators over the ranges of frequencies, which are much lower or higher than the isolators resonance frequency. On the other hand, the introduction of a nonlinear antisymmetric damping can significantly reduce the transmissibility of the vibration isolator over the resonance frequency region. The results indicate that nonlinear vibration isolators with an antisymmetric damping characteristic have great potential to overcome the dilemma encountered in the design of passive linear vibration isolators, that is, increasing the level of damping to reduce the transmissibility at the resonance could increase the transmissibility over the range of higher frequencies. These important theoretical conclusions are then verified by simulation studies.

IMPROVED WAVE FORCE CLASSIFICATION USING SYSTEM IDENTIFICATION

An extension to the Morison equation including Duffing oscillator-type force terms is postulated through knowledge of the flow mechanisms. This is used to curve-fit measured force time-histories from velocity time-histories, generated experimentally from various sources: regular oscillatory flow in a U-tube, cylinder oscillation in still water and in a current, random waves in the large De Voorst wave flume and a directional sea state at the Christchurch Bay Tower. The curve fits from the Morison equation are sometimes poor, while the curve fits from the extended equation are always excellent, although the corresponding ‘predictions’ give little or no improvement on the Morison equation. The curve fits obtained by simply adding a term proportional to F|F|, where F is force, are also significant improvements over the Morison fits enabling an improved classification of force in terms of drag, inertia and history (for each flow situation). In unidirectional flows the association of a significant history term with vortex shedding is confirmed by the occurrence of a prominent transverse or lift force. In directional seas, lift (due to vortex shedding) cannot be isolated and it is suggested that the data analysis described here will indicate the significance of vortex shedding through the relative magnitude of the history term.