T.P. Waters

The Effect of Dual-Rate Suspension Damping on Vehicle Response to Transient Road Inputs

The acceleration response of road vehicles to shock inputs from road irregularities such as bumps and hollows is an important consideration in the design of vehicle suspensions and damping characteristics, in particular. In this paper, the influence of the damper on the shock response of a simple vehicle model is considered. An analysis is presented of a single degree-of-freedom model subjected to a transient displacement input. Simple approximate expressions are given for the peak acceleration during an impulse of both short and long durations compared to the natural period, from which the role of the damper is clearly apparent. For impulses of short duration the peak acceleration occurs during the impulse and is shown to be approximately proportional to the damping ratio. Corollary to this, the peak acceleration can be reduced by switching the damper to a lower value during the impulse. The potential benefits of doing so are illustrated through numerical simulation, and a simple formula is given for the maximum possible reduction in peak acceleration. The results are also contrasted with those of a conventional dual-rate automotive damper model. The switchable damper is found to offer sufficient benefit to warrant further investigation.

Demonstrator to show the effects of negative stiffness on the natural frequency of a simple oscillator

This article describes a demonstrator to show the effects of negative stiffness on the free vibration of a simple oscillator. The test rig consists of a horizontal beam that is hinged at one end and is supported by two coil springs to form a single-degree-of-freedom system. Additional correction springs, which provide negative stiffness, can be attached to lower the natural frequency of the system. The effect of the change in natural frequency can be easily seen visually, and it is shown that for one of the configurations of correction springs, the natural frequency can be reduced by a factor of about 4.

On the Effects of Mistuning a Force-Excited System Containing a Quasi-Zero-Stiffness Vibration Isolator

Nonlinear isolators with high-static-low-dynamic-stiffness have received considerable attention in the recent literature due to their performance benefits compared to linear vibration isolators. A quasi-zero-stiffness (QZS) isolator is a particular case of this type of isolator, which has a zero dynamic stiffness at the static equilibrium position. These types of isolators can be used to achieve very low frequency vibration isolation, but a drawback is that they have purely hardening stiffness behavior. If something occurs to destroy the symmetry of the system, for example, by an additional static load being applied to the isolator during operation, or by the incorrect mass being suspended on the isolator, then the isolator behavior will change dramatically. The question is whether this will be detrimental to the performance of the isolator and this is addressed in this paper. The analysis in this paper shows that although the asymmetry will degrade the performance of the isolator compared to the perfectly tuned case, it will still perform better than the corresponding linear isolator provided that the amplitude of excitation is not too large.

Signal processing for experimental modal analysis

This paper is an overview of signal processing for modal analysis. The approach taken is to summarize some of the types of data that arise in modal analysis, to classify signal processing problems and to attempt to show the relevance of the latter to the former. It is intended to emphasize the diversity of approaches that may be applied and point to advanced methods that offer potential.

Theory of inner product vector and its application to multi-location damage detection

Structural damage detection methods using time domain vibration responses have shown appeal in recent years. In previous papers by the authors, the inner product vector (IPV) was proposed as a damage detection algorithm which uses cross correlation functions between vibration responses under white noise excitation or band pass white noise excitation. The proposed algorithm was verified by some simulative and experimental examples featuring a single damage location. However, damage at multiple locations was not considered. Therefore, this paper extends the application of IPV-based structural damage detection to the problem of multiple damage locations. Firstly, the theory of the IPV and its implementation in a damage detection context is reviewed. Then, two strategies for detecting multiple damages utilizing IPV are proposed. Finally, a damage detection experiment of a honeycomb sandwich composite beam is adopted to illustrate the feasibility and effectiveness of the IPV-based damage detection method.

Damage Assessment of Beams Using Flexural Wave Reflection Coefficients

The wave reflection coefficients of damage such as cracks, notches and slots in otherwise uniform beams depend on frequency and on the size of the damage. Experimental results are presented for the wave power reflection coefficients of transverse slots of various depths sawn into a number of beam specimens. These results are compared with a conventional spring model to estimate the depth of the slot. The method appears to work well for larger slot depths (greater than about 30% of the thickness of the beam) and at higher frequencies, allowing their existence to be inferred and their size to be estimated. This is due to the fact that the reflection coefficients are larger in these regimes. For smaller slots or at low frequencies, noise and experimental errors, such as miscalibration errors and ill-conditioning, become more significant.

The Aeolus project: Science outreach through art

With a general decline in people’s choosing to pursue science and engineering degrees there has never been a greater need to raise the awareness of lesser known fields such as acoustics. Given this context, a large-scale public engagement project, the ‘Aeolus project’, was created to raise awareness of acoustics science through a major collaboration between an acclaimed artist and acoustics researchers. It centred on touring the large singing sculpture Aeolus during 2011/12, though the project also included an extensive outreach programme of talks, exhibitions, community workshops and resources for schools. Described here are the motivations behind the project and the artwork itself, the ways in which scientists and an artist collaborated, and the public engagement activities designed as part of the project. Evaluation results suggest that the project achieved its goal of inspiring interest in the discipline of acoustics through the exploration of an otherworldly work of art.

Application of the wave finite element method to reinforced concrete structures with damage

Vibration based methods are commonly deployed to detect structural damage using sensors placed remotely from potential damage sites. Whilst many such techniques are modal based there are advantages to adopting a wave approach, in which case it is essential to characterise wave propagation in the structure. The Wave Finite Element method (WFE) is an efficient approach to predicting the response of a composite waveguide using a conventional FE model of a just a short segment. The method has previously been applied to different structures such as laminated plates, thinwalled structures and fluid-filled pipes. In this paper, the WFE method is applied to a steel reinforced concrete beam. Dispersion curves and wave mode shapes are first presented from free wave solutions, and these are found to be insensitive to loss of thickness in a single reinforcing bar. A reinforced beam with localised damage is then considered by coupling an FE model of a short damaged segment into the WFE model of the undamaged beam. The fundamental bending, torsion and axial waves are unaffected by the damage but some higher order waves of the cross section are significantly reflected close to their cut-on frequencies. The potential of this approach for detecting corrosion and delamination in reinforced concrete beams will be investigated in future work.

A Combined Finite and Spectral Element Approach to Wave Scattering in a Cracked Beam: Modelling and Validation

This paper concerns flexural and axial wave motion in a cracked beam. A combined finite element (FE) and spectral element (SE) model of a cracked beam is presented. A portion of the beam, which contains the crack, is modelled using FE analysis and combined with semi-infinite SEs. From the combined model the reflection and transmission coefficients of the crack are estimated. To determine the accuracy of this approach, a beam with a mass discontinuity is considered in the first instance. The reflection coefficients are estimated numerically and compared with experimental results. Secondly, a slot-type transverse crack is cut along the width of the beam. The experimental results are compared with both an FE model and a conventional lumpedparameter spring model. The purpose of this work is to investigate further the use of audiofrequency wave propagation as a basis for crack assessment and provide a valid model to use in the development of an assessment procedure.

Delamination of surface accretions with structural waves: Piezo-actuation and power requirements:

Unwanted accretions on structures, such as aircraft and wind turbine icing or deposits in pipes, are a common problem, which can pose a serious safety threat if not treated effectively and punctually. In this article we investigate the capability of piezo-excited structural waves to delaminate accreted material. The core of the concept is to utilise the stress distribution associated with waves propagating through the structure to detach unwanted build-up. We apply a wave-based technique for modelling piezoelectric excitation based on semi-analytical finite elements to calculate the shear stress at the interface between the host structure and the accretion generated by piezo-actuated waves. Our analyses include the effects of the actuator’s dynamics and allow for comparing different types of actuators, identifying the most effective frequency of excitation and formulating realistic power requirements. For the dual purposes of proof of concept and validation of the model, we present a demonstration experiment in which patches of accreted material are removed from a beam-like waveguide with emulated anechoic terminations using ultrasonic excitation.