Peter Cawley

The interaction of Lamb waves with defects

The interaction of individual Lamb waves with a variety of defects simulated by notches is investigated using finite-element analysis, and the results are checked experimentally. Excellent agreement is obtained. It is shown that a 2-D Fourier transform method may be used to quantify Lamb wave interactions with defects. The sensitivity of individual Lamb waves to particular notches is dependent on the frequency-thickness product, the mode type and order, and the geometry of the notch. The sensitivity of the Lamb modes a/sub 1/, alpha /sub 0/, and s/sub 0/ to simulated defects in different frequency-thickness regions is predicted as a function of the defect depth to plate thickness ratio and the results indicate that Lamb waves may be used to find notches when the wavelength to notch depth ratio is on the order of 40. Transmission ratios of Lamb waves across defects are highly frequency dependent.<<ETX>>

DISPERSE: A GENERAL PURPOSE PROGRAM FOR CREATING DISPERSION CURVES

The application of guided waves in NDT can be hampered by the lack of readily available dispersion curves for complex structures. To overcome this hindrance, we have developed a general purpose program that can create dispersion curves for a very wide range of systems and then effectively communicate the information contained within those curves. The program uses the global matrix method to handle multi-layered Cartesian and cylindrical systems. The solution routines cover both leaky and non-leaky cases and remain robust for systems which are known to be difficult, such as large frequency-thicknesses and thin layers embedded in much thicker layers. Elastic and visco-elastic isotropic materials are fully supported; anisotropic materials are also covered, but are currently limited to the elastic, non-leaky, Cartesian case.

Optimization of lamb wave inspection techniques

Abstract Lamb waves can propagate over long distances which means that they are attractive for the quick, long range inspection of large structures, and they can also be useful for localized inspection, particularly in thin structures. This paper discusses the selection of the appropriate mode and frequency range for different inspection requirements and reviews the possible methods of excitation, response measurement and signal processing. It is usually desirable to transmit a single, non-dispersive mode, and excitation methods to achieve this are discussed. A variety of signal processing techniques from simple time domain to relatively complex two-dimensional Fourier analysis are available. Time domain processing can often be applied satisfactorily in low frequency-thickness regions where only two modes can propagate, but tends to be unreliable above the cut-off frequency of the a 1 mode. As an example of the design of a Lamb wave testing regime, a set of tests on a butt-welded steel plate with simulated weld defects of different depths is described. It is shown that by operating below the a 1 cut-off frequency with judicious selection of testing technique, the presence of defects can be detected reliably from changes in the shape of the received waveform.

The use of Lamb waves for the long range inspection of large structures

Lamb waves are very attractive for the quick inspection of large structures because they can propagate long distances along plates and shells. The chief drawback of Lamb wave inspection is that at least two modes exist at all frequencies and the modes are generally dispersive, which means that the received signals can be very complicated. As a result, the potential advantages of Lamb wave inspection have seldom been realized in practice. The key to the successful application of Lamb waves is the excitation of a single mode in a non-dipersive region. This paper discusses the selection of an appropriate mode and its excitation and reception. Examples of the use of Lamb waves for the detection of delaminations in composite materials and corrosion in pipes are then given.

The Potential of Guided Waves for Monitoring Large Areas of Metallic Aircraft Fuselage Structure

The potential for long-range propagation of ultrasonic guided waves through metallic aircraft fuselage structure has been investigated using dispersion analysis and numerical modelling, validated by experiment. In order to satisfy the pressing need for integrated structural health monitoring of ageing metallic aircraft, it is likely that an active guided wave system based on current technology must feature efficient propagation over distances of at least 1m with an attenuation of not more than about 40dB/m. Propagation was examined across free skin, tapering skin, skin loaded with sealant and paint, double skin jointed with either sealant or adhesive, and lap and stringer joints, which together adequately characterise metallic monocoque fuselage construction. Whilst the simple and tapering skins allow long range propagation of non-dispersive modes with little reflection at the transition to tapering skin, the attenuation caused by application of a sealant layer generally leaves no viable modes. Guided wave propagation through double skin features the inevitable generation of twin modes with similar phase velocity, which interact with each other during propagation. This interaction crucially determines the efficiency of propagation across narrow joints and effectively precludes propagation across a succession of joints. This work leads to the conclusion that an active aircraft system that relies on guided wave propagation of more than 1m is not feasible, whereas localised guided wave monitoring of structurally significant areas is a more practical approach.

A Study of the Interaction between Ultrasound and a Partially Contacting Solid--Solid Interface

The measurement of the reflection of ultrasonic waves from a partially contacting solid-solid interface can be used to study the contact conditions at that interface. This paper describes measurements and predictions of the reflection of ultrasonic waves from partially contacting aluminium-aluminium interfaces, performed in the low frequency regime where the wavelength of the ultrasound is large compared to the size of the gaps. The proportion of the incident wave which is reflected at the interface (the reflection coefficient) was measured as a function of frequency with a single wideband ultrasonic transducer. When load was applied across the interface three regions of contact can be seen; no contact, partial contact and perfect contact. In the no contact region the measured reflection coefficient was unity at all frequencies. In the partial contact region the measured reflection coefficient increased with frequency. No measurements were taken in the perfect contact region in which the reflection coefficient is known to be zero at all frequencies as this state is the same as a continuous piece of aluminium. The reflection coefficient variation with frequency was modelled using a spring model, good agreement between experiments and predictions being achieved. Reflection coefficient measurements were then used to study the contact between two aluminium surfaces under repeated loading and unloading cycles. Plastic flow on first loading was evident while subsequent loading cycles revealed largely elastic behaviour. Both elastic and plastic statistical contact models, as well as a numerical contact model, were used to predict the variation of interfacial stiffness with pressure. These models agreed qualitatively with the experimentally determined stiffness variations and the predicted stiffness was within an order of magnitude of the measured value in all cases.

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.

The excitation of Lamb waves in pipes using dry-coupled piezoelectric transducers

The development of a dry-coupled piezoelectric transducer system for the detection of corrosion in chemical plant pipework using cylindrical Lamb waves is described. It is shown that the axisymmetricL(0,2) mode at a frequency of about 70 kHz is an attractive mode to use for longdistance propagation. The results show that a ring of piezoelectric length-expander elements can be used to excite theL(0,2) mode and to suppress all the nonaxisymmetric modes. Tests have been carried out both with the piezoelectric elements bonded to the pipe and fabricated into a simple transducer which was clamped against the pipe. The performance of the dry-coupled system was very similar to that of the bonded elements. In pulse echo tests, the noise floor obtained with the dry-coupled system was less than 1% of the amplitude of the propagatingL(0,2) mode. The drycoupled transducers provide a simple, light, readily detachable system for the long-range inspection of pipework.

The Non-destructive Testing of Adhesively Bonded Structure: A Review

Abstract The types of defect encountered in adhesive joints and the non-destructive testing techniques available to detect them are reviewed. Three types of defect: complete voids or dis-bonds, poor cohesive strength of the adhesive layer and poor adhesion between the adhesive layer and adherend are commonly present. It is shown that a variety of techniques is available for dis-bond and void detection, ultrasonics and sonic vibration being the most commonly used. The detection of poor cohesive and adhesive properties, however, is much more difficult than void and dis-bond detection and is the subject of current research. At present there is only one commercially available instrument which claims to predict cohesive strength. There is no reliable non-destructive test to detect poor adhesion.

The low-frequency reflection and scattering of the S0 Lamb mode from a circular through-thickness hole in a plate: Finite Element, analytical and experimental studies

A study of the interaction of the S0 Lamb wave with a circular through-thickness hole in a plate is presented. The study is limited to the nondispersive frequency range of this wave, in which the distributions of stress and displacement are simple. This allows a Finite Element analysis to be undertaken using a two-dimensional membrane discretization. Predictions of the direct reflection of the S0 mode and the lateral scattering of the SH0 mode are made for a range of diameters of the hole. At the same time, an analytical solution based on modal superposition is developed, and this is also used to predict the reflection and scattering coefficients. Both sets of predictions are validated by experimental measurements. It is found that the trends of the reflection coefficients for different hole diameters, frequencies and distances from the hole satisfy a simple normalization. On a detailed scale, the functions exhibit undulations which are shown to result from the interference of the direct reflection with secondary reflections which arrive slightly later.