David Alleyne

Defect detection in pipes using guided waves

The detection of corrosion in insulated pipes is of major importance to the oil and chemical industries. Current methods involving point-by-point inspection are expensive because of the need to remove the insulation. An alternative method which is being developed at Imperial College is to propagate guided waves in the walls of the pipes, and to look for reflections from defects. The test configuration is essentially pulse-echo; the insulation is removed at just one location on a pipe and the signals are then transmitted and received using a single transducer unit. The technique is currently undergoing field trials. This paper presents a review of the studies of the propagation of the waves and their sensitivity to defects which have been conducted in order to provide a sound scientific basis for the method. Issues of importance were the selection of the optimum guided wave modes and the establishment of relationships between the defect size and the strength of wave reflection. Analytical and numerical studies were conducted in parallel with an extensive experimental programme.

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 Reflection of Guided Waves From Circumferential Notches in Pipes

The reflection of the L(0, 2), axially symmetric guidea elastic wave from notches in pipes is examined, using laboratory experiments and finite element simulation The result show that the reflection coefficient of this mode is very close to a linear function of the circumferential extent of the notch, and is a stronger function of the through thickne depth of the notch. The motivation for the work was the development of a technique for inspecting chemical plant pipework, but the study addresses the nature of the reflection function and has general applicability.

The Mode Conversion of a Guided Wave by a Part-Circumferential Notch in a Pipe

A study of the reflection of mode-convertea guided waves from notches in pipes has been carried out. Measurements were made on a 76-mm bore diameter (nominal 3-inch ), 5.5-mm wall thickness pipe with circumferentially oriented through-thickness notches of various lengths. In parallel, a finite element model was used to simulate the experiments The axially symmetric L(0, 2) mode was incident on the notches and the L(0, 2), F( 1, 3), and F(2, 3) modes were received in reflection. The results showed excellent agreement between the measurements and the predictions for all three modes. They also showed that the F( 1, 3) mode reflects as strongly as the L(0, 2) mode when the notch length is short. Finally, it has been shown that a very simple analysis based on an assumed crack-opening profile may be used to make accurate predictions of the mode conversion.

A 2-dimensional Fourier transform method for the quantitative measurement of Lamb modes

The key problem associated with the quantitative measurement of the characteristics of propagating Lamb waves is that more than one wave mode can exist at any given frequency. Therefore, a simple Fourier transformation from the time to the frequency domain cannot distinguish between the different modes. A 2-D Fourier transform technique involving both spatial and time transformations from which the required information can be obtained is presented. The results obtained from both numerical and experimental investigations of Lamb waves propagating in steel plates are presented using an isometric projection, which gives a 3-D view of the wave-number dispersion curves. The results show the effectiveness of using the 2-D Fourier transform method to identify and measure the amplitudes of individual Lamb modes.<<ETX>>

A signal regeneration technique for long-range propagation of dispersive Lamb waves

Abstract A technique which enables a dispersive Lamb wave to be propagated over a considerable distance to give a simple waveform at the receiver position is described. The method works by launching a signal which, by superposition of its frequency components, will recombine to form a signal with a simple shape (a pulse or tone burst) at the measurement position. The technique has been tested on thin plates both numerically and experimentally, simple windowed tone bursts being received at the measurement position even though the propagating modes were grossly dispersive. The advantage of this method in NDE applications is that the simple form of the signal received on a good plate potentially makes it easy to detect any small changes due to defects along the propagation path, and also that the signal/noise ratio is maximized at the measurement position.

The Long Range Detection of Corrosion in Pipes Using Lamb Waves

Ultrasonic nondestructive testing techniques are available for the detection of general wall loss associated with pitting and corrosion, but unfortunately they tend to be very slow, single position measurements, making the inspection of the kilometres of pipeline typically found in industrial plants virtually impossible. Flash radiography and eddy current methods are also used, but for complete coverage of the pipe system they also tend to be relatively slow. An alternative to point or single location measurements, which require the physical movement of the transducer, would be to excite stress waves which propagate along the pipe (in the form of cylindrical Lamb waves, see for example, Silk and Bainton [1]) and to monitor the response of the pipe for changes in the received signal at a remote position; alternatively a pulse-echo system in which echoes returning to the source transducer are monitored could be used. In each case changes in the response signal will indicate the presence of an impedance change (the presence of a possible defect) in the pipe. This technique does not require the transducer to be indexed from point to point and it minimises the amount of data processing required to determine the presence of defects or other unwanted features. Access to the inside of the pipe is not required as the propagating modes may also be excited from the pipe outer wall and only a keyhole in the insulation needs to be removed to allow access of a transducer system. The technique is particularly suited to applications where the critical defect size is relatively large so the reduced resolution and sensitivity of the method compared with standard single point ultrasonic inspection is not critical. Lamb waves have been shown to travel tens of meters in steel and this testing method has been used in the steel industry for the post-production inspection of tubing by Bottger et al [2]. The wave~ are sensitive to

The design of a vibration transducer to monitor the integrity of dental implants

Abstract Bone-anchored titanium implants are being used increasingly to provide support for prostheses replacing missing teeth in edentulous and partially dentate patients. A technique is required to monitor bone formation at the implant-tissue interface during healing, and also to check whether there has been bone loss from around the top of the implant. One possible method is to screw a beam into the implanted fixture and to measure the first flexural resonance frequency of the resulting system. This resonance frequency is affected by both the exposed length of fixture and the stiffness of the interface between the implant and the bone. This paper describes the design of a beam-like transducer for clinical trials of the technique. The sensitivity of the transducer resonance frequency to the changes of interest is dependent on the thickness and length of the beam element. However, the choice of these dimensions is constrained by the need to avoid closely spaced resonances. The performance of different transducer shapes and the influence of the thickness and length of the beam element in the transducer has been studied. The results have been used to finalize a transducer design for the clinical trials.