Paul D Wilcox

The effect of dispersion on long-range inspection using ultrasonic guided waves

The dispersion of ultrasonic guided waves causes wave-packets to spread out in space and time as they propagate through a structure. This limits the resolution that can be obtained in a long-range guided wave inspection system. A technique is presented for quickly predicting the rate of spreading of a dispersive wave-packet as it propagates. It is shown that the duration of a wave-packet increases linearly with propagation distance. It is also shown that the duration of a wave-packet after a given propagation distance can be minimised by optimising the input signal. A dimensionless parameter called minimum resolvable distance (MRD) is defined that enables a direct comparison to be made between the resolution attainable at different operating points. Some conclusions are made concerning the resolution of various operating points for the case of Lamb waves in an aluminium plate.

Flexible interdigital PVDF transducers for the generation of Lamb waves in structures

Abstract The development of flexible, cheap PVDF transducers for the generation and detection of Lamb waves in plates is described. Lamb waves offer a possible means of inspecting a large area of structure using a small number of sensors, so these transducers may be used in the development of ‘smart’ structures. In order to obtain a simple signal which can readily be interpreted, it is essential to generate a single Lamb mode over a controlled frequency bandwidth. This has been achieved by using an interdigital design. The process of designing transducers for the generation of a particular mode is discussed and experimental results showing a variety of modes on aluminium, steel, perspex and composite plates are presented. It is shown that the transducers can be operated over the frequency range 0.5–4 MHz. They exert primarily out-of-plane forces on the plate which means that they are more suited to the generation and detection of the lower antisymmetric Lamb modes than the symmetric modes. The antisymmetric modes propagate with minimal attenuation in metal plates so the PVDF interdigital transducers can be used satisfactorily in long range testing applications. However, in perspex and composites, the attenuation in the antisymmetric modes is much higher than in the symmetric modes so ways of increasing the strength of excitation of the symmetric modes must be developed.

Dispersion and excitability of guided acoustic waves in isotropic beams with arbitrary cross section

A finite element (FE) technique for computing the properties of guided waves that can exist in an isotropic beam of arbitrary cross section is presented. The FE model uses a two-dimensional mesh to represent a cross section through the beam and cyclic axial symmetry conditions to prescribe the displacement field perpendicular to the mesh. FE results are presented for plate and angle sections. Excitability functions are calculated and implications for transducer placement are considered.

Development of a computer model for an ultrasonic polymer film transducer system

Abstract A piezoelectric polymer film such as polyvinylidene fluoride (PVDF) offers the possibility of making cheap ultrasonic transducers which can be permanently bonded to structures for NDE applications. However, before this can be achieved it is necessary to have a full understanding of the behaviour of PVDF in the context of a transducer application. This paper describes the development of an interactive computer program to implement a one-dimensional model of a thin film transducer system mounted on a solid substrate. Starting from a typical transducer system mounted on a solid substrate. Starting from a typical tranducer equivalent circuit, this paper explains how that circuit can be expanded into a complete model representing a PVDF transducer with one or more backing layers bonded to an arbitrary solid substrate. The computer program uses this model to provide immediate predictions of electrical input impedence, acoustic response and pulse-echo response. A series of experiments has been performed using PVDF transducers bonded to a variety of materials in order to validate the model. This has enabled the degree of accuracy required for various parameters within the model, such as mechanical and dielectric loss values, to be assessed. Once validated, the model has been used as a tool to predict the effect of physical parameters, such as bond and electrode thickness, on the performance of a transducer.

Flexible Interdigital PVDF Lamb Wave Transducers for the Development of Smart Structures

Conventional ultrasonic or eddy current inspection of structures requires a probe to be scanned over the whole area to be tested. This is extremely time consuming, and hence costly, when large areas such as aircraft wings or pressure vessels are to be covered. A further disadvantage of ultrasonic inspection is that a coupling fluid between the transducer and the structure is generally required. The most reliable coupling method is to use immersion, the testpiece being fully immersed in a water bath. However, with large structures this is frequently not practical and they are often tested using jet probe assemblies, the ultrasound being propagated down jets of water directed at the structure. However, this is generally only practical at the manufacture stage and field inspection is often carried out manually using contact transducers, coupling being achieved by applying gel to the surface of the structure. Many structures also have critical areas which are difficult to inspect because access is impeded. For example, spars and other stiffeners in aircraft pose problems because once the aircraft is built they are covered by the fuselage or wing skin.

The Use of Huygens’ Principle to Model the Acoustic Field from Interdigital Lamb Wave Transducers

From a non-destructive evaluation point of view, Lamb waves are a highly attractive means of inspecting a large area of a structure from a single location. Interdigital transducers (IDTs) which make use of the piezoelectric properties of the polymer PVDF have been used previously in signal processing applications [1] to generate acoustic waves in piezoelectric substrates. Over the last two years, work has been carried out at Imperial College [2,3] to design PVDF IDTs that are capable of transmitting and receiving ultrasonic Lamb waves in non-piezoelectric plates for non-destructive testing purposes. Such transducers can be permanently bonded to a structure and hence have potential uses in smart structure applications. A schematic diagram of a simple PVDF IDT is shown in Figure 1. The wavelength and frequency of a single Lamb wave mode in the structure are selected by the spacing of the fingers in the IDT and the frequency at which the IDT is excited. Further details of their construction can be found in [2].

An Example of the Use of Interdigital PVDF Transducers to Generate and Receive a High Order Lamb Wave Mode in a Pipe

From a non-destructive evaluation point of view, Lamb waves are a highly attractive means of inspecting a large area of a structure from a single point. Interdigital PVDF transducers have been used previously in signal processing applications [1] to generate acoustic waves in piezoelectric substrates. This paper in conjunction with that of Monkhouse et al [2] aims to provide an overview of the work accomplished so far at Imperial College in the use of interdigital PVDF transducers to transmit and receive Lamb waves in certain structures for non-destructive evaluation purposes. Interdigital PVDF transducers may be permanently bonded to either flat of curved surfaces and this attribute together with their low cost means that they are potentially suitable for“smart structure” applications.

On the sensitivity of corrosion and fatigue damage detection using guided ultrasonic waves

Constant, long-time monitoring of large plate-like structures, e.g., oil storage tanks, can be performed using permanently attached remote sensors. A guided ultrasonic wave array consisting of piezoelectric transducer elements for the excitation and reception of the first antisymmetric Lamb wave mode A0, has been designed and built. Laboratory measurements for a steel plate containing various defects have been performed. The results are compared to theoretical predictions and the sensitivity of the array device for defect detection is ascertained.

A Guided Ultrasonic Waves Array for Structural Integrity Monitoring

Constant, long‐term monitoring of large plate‐like structures, e.g., oil storage tanks, can be performed using permanently attached remote sensors. A guided ultrasonic waves array, consisting of piezoelectric transducer elements for the excitation and reception of the first antisymmetric Lamb wave mode A0, has been designed and built. Laboratory measurements for a steel plate containing various defects have been performed. The results are compared to theoretical predictions and the sensitivity of the array device for defect detection is ascertained.

A structural model for high pressure helical wire-wound thermoplastic hose

This paper develops a model for a specific type of hose construction designed to withstand very high operating pressures. The model is based on a model previously developed by Entwistle and White (Int. J. Mech. Sci. 19 (1977) 193) with two significant modifications. Firstly the compressible inner core is included in the model using Lames thick walled cylinder theory. Secondly the model allows for the squeezing effect on wires when a hose gets shorter under pressurisation. The model calculates whether the wires in a particular layer will be squeezed together and when this occurs, the behaviour is modelled using Hertzian contact theory. The governing equations are solved using a minimising Newton Raphson technique. Model predictions are compared with experimental results obtained for pressure deformation response in terms of hose axial strain and wire strain and show good agreement. Considerable hysteretical behaviour is seen in the hose axial strain and it is suggested that this may be due to the twisting contact movements between different layers and as such may be a good indicator of the amount of fretting taking place. It is also suggested that, when a hose is designed to get shorter on pressurisation, length change may be a good indicator of manufacturing quality in terms of wire packing efficiency.