References and abstracts to PhD theses are also available, and some of the later publications can be downloaded in pdf format.

A review of the state of the art of guided waves for long range inspection is also available.

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  • Journal article
    Heinlein S, Cawley P, Vogt T, Burch Set al., 2018,

    Blind Trial Validation of a Guided Wave Structural Health Monitoring System for Pipework

    , MATERIALS EVALUATION, Vol: 76, Pages: 1118-1126, ISSN: 0025-5327
  • Journal article
    Jarvis R, Farinha A, Kovac M, Cegla Fet al., 2018,

    NDE sensor delivery using unmanned aerial vehicles

    , INSIGHT, Vol: 60, Pages: 463-467, ISSN: 1354-2575
  • Journal article
    Phillips R, Duxbury D, Huthwaite P, Lowe Met al., 2018,

    Simulating the ultrasonic scattering from complex surface-breaking defects with a three-dimensional hybrid model

    , NDT and E International, Vol: 97, Pages: 32-41, ISSN: 0963-8695

    Modelling is increasingly relied on for the design and qualification of ultrasonic inspections applied to safety-critical components. Numerical methods enable the simulation of the ultrasonic interaction with realistic defect morphologies; however, the computational requirements often limit their deployment. The hybrid simulation technique, which combines semi-analytical and numerical methods, realises the potential of high fidelity numerical modelling without the limiting computational factors. The inspection of thick section components for near-backwall surface-breaking defects results in large propagation distances, making them a key application of hybrid modelling. This work presents a methodology for efficiently simulating the ultrasonic inspection of complex surface-breaking defects using a hybrid model. The model is initially verified against full numerical simulation; further validation is presented by comparison to an experimental scan over an artificially machined surface-breaking notch. The potential of the new hybrid method is then demonstrated by carrying out a Monte Carlo analysis on the scattered field from surface-breaking defects with randomly rough surfaces and the results are compared to the Kirchhoff approximation.

  • Conference paper
    Cegla F, Herdovics B, 2018,

    Coded excitation, motion and signal-to-noise ratio

    , IEEE International Symposium on Circuits and Systems (ISCAS), Publisher: IEEE, ISSN: 0271-4302

    Previous work has shown that coded excitation can be used to considerably improve the signal-to-noise ratio (SNR) of signals received by transducers of poor sensitivity such as electromagnetic acoustic transducers (EMATs). EMATs are usually driven with signal powers of the order of kWs so that adequate SNR is achieved. With coded excitation these powers can be reduced to as low as 1-5W. A particular feature of the transmitted codes is that they are temporally long and contain intermittent intervals in which reception takes place. Because of the signal length there is concern that excessive movement of the probe or target can result in deterioration of the performance of such a system. Therefore, in this paper we investigate the effect that physical motion of the test piece can have on the acquired signals. Simulated results will be presented and discussed here.

  • Journal article
    Corcoran J, Davies C, 2018,

    Monitoring power-law creep using the failure forecast method

    , International Journal of Mechanical Sciences, Vol: 140, Pages: 179-188, ISSN: 0020-7403

    Creep is considered to be the life limiting damage mechanism in many load bearing high temperature components. A range of different parameters determine the creep life of a component, many of which are unlikely to be known to sufficient accuracy to enable satisfactory estimation of remnant life. Instead,the integrity of a component shouldbe established through direct measurement of the response of the component to the operating conditions. Creep deformation is shown to be a positive feedback mechanism; anincreasein strain leadsto an increasing strain rate. It has recently been shown that as a consequence of positive feedback the Failure Forecast Method, a generalised framework for predicting time to criticality based on rates of change of damage, may beapplied for remnant life calculations. A range of strain rate based assessments have been proposed in the literature but it is proposed that the Failure Forecast Method unifies many of these techniques and provides additional insight into creep behaviour by virtue of the underlying positive feedback. The methodology has been demonstrated using experiment datasets that are pertinent to creep in high temperature pressure vessels and piping; it is shown that failure times are accurately predicted shortly after the minimum creep strain rate.

  • Journal article
    Van Pamel A, Sha G, Lowe MJS, Rokhlin Set al., 2018,

    Numerical and analytic modelling of elastodynamic scattering within polycrystalline materials

    , Jornal of the Acoustical Society of America, Vol: 143, Pages: 2394-2408, ISSN: 0001-4966

    The elastodynamic behavior of polycrystalline cubic materials is studied through the fundamental propagation properties, the attenuation and wave speed, of a longitudinal wave. Predictions made by different analytical models are compared to both numerical and experimental results. The numerical model is based on a three-dimensional Finite Element (FE) simulation which provides a full-physics solution to the scattering problem. The three main analytical models include the Far-Field Approximation (FFA), the Self-Consistent Approximation (SCA) to the reference medium, and the herein derived Second Order Approximation (SOA). The classic Stanke and Kino model is also included, which by comparison to the SOA, reveals the importance of the distribution of length-scales described in terms of the two-point correlation function in determining scattering behavior. Further comparison with the FE model demonstrates that the FFA provides a simple but satisfactory approximation, whereas the SOA shows all-around excellent agreement. The experimental wave velocity data evaluated against the SOA and SC reveal a better agreement when the Voigt reference is used in second order models. The use of full-physics numerical simulations has enabled the study of wave behavior in these random media which will be important to inform the ongoing development of analytical models and the understanding of observations.

  • Conference paper
    Howard R, Cegla F, 2018,

    The Effect of Pits of Different Sizes on Ultrasonic Shear Wave Signals

    , 44th Annual Conference on Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
  • Conference paper
    Budyn N, Bevan R, Croxford AJ, Zhang J, Wilcox PD, Kashubin A, Cawley Pet al., 2018,

    Sensitivity Images for Multi-View Ultrasonic Array Inspection

    , 44th Annual Conference on Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
  • Journal article
    Shi F, Huthwaite P, 2018,

    Ultrasonic wave-speed diffraction tomography with undersampled data using virtual transducers

    , IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol: 65, Pages: 1226-1238, ISSN: 0885-3010

    Ultrasonic diffraction tomography (DT) offers a way to achieve high-resolution imaging of the wavespeed map, and hence has strong potential applications in medical diagnosis and Nondestructive evaluation (NDE). Ideal images can be obtained with a complete array of sensors surrounding the scatterer, provided that the measurement data are fully sampled in space, obeying the Nyquist criterion. Spatial undersampling causes the image to be distorted and introduce unwanted circular artefacts. In this paper we propose an iteration approach using virtual transducers to achieve high-resolution tomographic imaging with undersampled measurements. At each iteration stage, the extent constraint estimated from the shape of the object of interest is applied on the image space to obtain a regularized image, based on which the ultrasonic measurement data at virtual transducers are calculated using a forward model. The full dataset composed of original and virtual measurements is then used for tomography in the next stage. A final image with sufficiently high resolution is obtained after only a few iterations. The new imaging method yields improvements in the robustness and accuracy of ultrasonic tomography with undersampled data.We present numerical results using complicated wavespeed maps from realistic corrosion profiles. In addition, an experiment using guided ultrasonic waves is performed to further evaluate the imaging method.

  • Journal article
    Cawley P, Khalili P, 2018,

    Relative ability of wedge coupled piezoelectric and meander coil EMAT probes to generate single mode Lamb waves

    , IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 65, Pages: 648-656, ISSN: 0885-3010

    Ultrasonic guided waves are used extensively when checking for defects in petrochemical and other industries and are mostly generated using piezoelectric transducers on an angled wedge or EMATs in different configurations. Low frequency inspection allows for long distance propagation but it is best suited for detecting relatively large defects, while at higher frequencies, the presence of multiple wave modes limit defect detectability, so achieving practical single Lamb mode excitation via careful transduction is very beneficial. This paper investigates the relative ability of angled piezoelectric and meander coil EMAT probes to produce single mode transduction in the medium (~1 to 5 MHz-mm) and high (> 5 MHz-mm) frequency-thickness regions of the dispersion curves. The nature of each transducer is studied analytically by simulating the corresponding surface forces, followed by the use of a Fourier transform in time and space (2-D FFT) to highlight the excitation region in wavenumber-frequency space. With angled wedge excitation there is a linear relationship between the excitation frequency and the wavenumber which means the excitation tends to track typical dispersion curves, allowing for easier pure mode generation. In contrast, the EMAT controls frequency and wavenumber separately which makes it more difficult to generate a pure mode when dispersion curves are close together; however, by narrowing the frequency bandwidth via a large number of cycles in the excitation signal, pure mode generation via an EMAT was shown to be possible even in areas of closely spaced modes. As example cases, analytical results, backed up by experiments, showed that signals dominated by the A0 mode at 1.5 MHz-mm and also the A1 mode at 18 MHz-mm can be generated with both angled piezoelectric and EMAT probes.

  • Journal article
    Shi F, Lowe M, Skelton EA, Craster RVet al., 2018,

    A time-domain finite element boundary integral approach for elastic wave scattering

    , Computational Mechanics, Vol: 61, Pages: 471-483, ISSN: 0178-7675

    The response of complex scatterers, such as rough or branched cracks, to incident elastic waves is required in many areas of industrial importance such as those in non-destructive evaluation and related fields; we develop an approach to generate accurate and rapid simulations. To achieve this we develop, in the time domain, an implementation to efficiently couple the finite element (FE) method within a small local region, and the boundary integral (BI) globally. The FE explicit scheme is run in a local box to compute the surface displacement of the scatterer, by giving forcing signals to excitation nodes, which can lie on the scatterer itself. The required input forces on the excitation nodes are obtained with a reformulated FE equation, according to the incident displacement field. The surface displacements computed by the local FE are then projected, through time-domain BI formulae, to calculate the scattering signals with different modes. This new method yields huge improvements in the efficiency of FE simulations for scattering from complex scatterers. We present results using different shapes and boundary conditions, all simulated using this approach in both 2D and 3D, and then compare with full FE models and theoretical solutions to demonstrate the efficiency and accuracy of this numerical approach.

  • Journal article
    Zhang C, Huthwaite P, Lowe M, 2018,

    The application of the Factorization Method to the subsurface imaging of surfacebreaking cracks

    , IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 65, Pages: 497-512, ISSN: 0885-3010

    A common location for cracks to appear is at the surface of a component; at the near surface, many nondestructive evaluation techniques are available to inspect for these, but at the far surface this is much more challenging. Ultrasonic imaging is proposed to enable far surface defect detection, location, and characterization. One specific challenge here is the presence of a strong reflection from the backwall, which can often mask the relatively small response from a defect. In this paper, the factorization method (FM) is explored for the application of subsurface imaging of the surface-breaking cracks. In this application, the component has two parallel surfaces, the crack is initiated from the far side and the phased array is attached on the near side. Ideally, the pure scattered field from a defect is needed for the correct estimation of the scatterer through the FM algorithm. However, the presence of the backwall will introduce a strong specular reflection into the measured data which should be removed before applying the FM algorithm. A novel subtraction method was developed to remove the backwall reflection. The performance of the FM algorithm and this subtraction method were tested with the simulated and experimental data. The experimental results showed a good consistency with the simulated results. It is shown that the FM algorithm can generate high-quality images to provide a good detection of the crack and an accurate sizing of the crack length. The subtraction method was able to provide a good backwall reflection removal in the case of small cracks (1-3 wavelengths).

  • Journal article
    Zou F, Cegla F, 2018,

    On quantitative corrosion rate monitoring with ultrasound

    , Journal of Electroanalytical Chemistry, Vol: 812, Pages: 115-121, ISSN: 1572-6657

    Wall-thickness loss rate (WTLR) is an important parameter that defines a corrosion process. The speed at which a WTLR can be determined is directly related to how quickly one can intervene in a process that is heading in the wrong direction. Ultrasonic testing has been widely used as a convenient and efficient technique for online corrosion monitoring. One of the key performance parameters of ultrasonic corrosion monitoring is detection speed. While WTLRs can be determined by fitting linear lines to wall-thickness loss (WTL) measurements, the presence of noise in the measurements makes it difficult to judge the confidence levels of the slopes that are calculated this way. In this paper, a statistics based approach for assessing the detection speeds that are achievable by ultrasonic corrosion monitoring systems is presented. Through the statistical analysis of experimental data, a state-of-the-art laboratory setup is shown to be able to detect both WTLRs and changes in WTLR that are of interest to industry (i.e. 0.1–0.2 mm/year) within 1–2 h.

  • Journal article
    Jarvis R, Cawley P, Nagy PB, 2018,

    Permanently installed corrosion monitoring using magnetic measurement of current deflection

    , STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL, Vol: 17, Pages: 227-239, ISSN: 1475-9217
  • Conference paper
    Todd MD, Leung M, Corcoran J, Cawley Pet al., 2018,

    Fatigue prognosis using the uncertainty-quantified failure forecast method

    , Pages: 129-137

    © APWSHM 2018. All rights reserved. Several material failure modes such as fatigue have been noted to occur, after initiation phases, as a consequence of a positive-feedback mechanism. Positive feedback systems continually "accelerate" their underlying physics until failure, and as such, are unstable processes that result in the tendency of the rate of change in the observable data (or more generally, damage-sensitive features) to approach infinity. Models have been proposed based on this asymptotic property of positive feedback mechanisms for predicting the time to criticality, generally now known as the "failure forecast method". The typical implementation of this approach is to compute the inverse time rate-of-change in the features and linearly regress that data vs. time; inevitable uncertainties in the data, measurement process, or environmental contamination noise will corrupt the regression, leading to distributions in the parameters used to make the failure forecast. This study will look at a parametric implementation of the failure forecast method using a probability density function computed from the regression process and evaluating its predictive performance on fatigue data, considering regression window, sampling time, noise level, and predictor. A comparison is also drawn between the Failure Forecast Method and a conventional periodic inspection realization.

  • Journal article
    Wang Y, Zou F, Cegla F, 2017,

    Acoustic waveguides: an attractive alternative for accurate and robust contact thermometry

    , Sensors and Actuators A: Physical, Vol: 270, Pages: 84-88, ISSN: 0924-4247

    We report a robust and very precise method of measuring temperature using ultrasonic waves. Solid stainless steel waveguides are used to provide well-defined and stable ultrasonic wave propagation paths. Ultrasonic wave velocity is strongly temperature dependent. The arrival times of the ultrasonic wavepackets along a waveguide are used to infer the average temperature of the waveguide. Our ultrasonic temperature measurements exhibit a high precision (i.e. ±0.015 ⁰C) that is more than two times better than the quoted accuracy of 1/10 DIN resistance temperature detectors (RTDs). The responsiveness of the waveguides was also investigated. While ultrasonic measurements can be made at very high frequencies, the responsiveness is limited by the heat transfer into the active sensing area. The waveguides make it easy to customise the dimension of the active sensing area and a shorter response time than those of RTDs has been demonstrated. The technique presented in this paper is a robust and cost effective alternative to other contact temperature measurements.

  • Journal article
    Corcoran J, Raja S, Nagy PB, 2017,

    Improved thermoelectric power measurements using a four-point technique

    , NDT and E International, Vol: 94, Pages: 92-100, ISSN: 0963-8695

    The Seebeck coefficient of a material is dependent on its composition and microstructure and is consequently sensitive to service related material degradation; of particular interest is the sensitivity to thermal and irradiation embrittlement which may be exploited for the material characterisation of in service components. Conventionally, thermoelectric measurements are taken using a two-point contact technique which introduces a temperature differential in the test component through a heated ‘hot tip’ electrode; it is argued that measurements using this methodology are sensitive to the thermal contact resistance between the component and the electrodes. An alternative three- or four-point technique is proposed where heat is introduced to the component remotely which leads to much less sensitivity to contact condition. An experiment is presented that compares the two techniques and demonstrates the improved performance of the four-point technique. Aside from the improved accuracy, the modified technique also facilitates a ‘passive’ implementation that could be used from continuous monitoring of components in service.

  • Journal article
    Li Z, Dixon S, Cawley P, Jarvis R, Nagy PB, Cabeza Set al., 2017,

    Experimental studies of the magneto-mechanical memory (MMM) technique using permanently installed magnetic sensor arrays

    , NDT & E INTERNATIONAL, Vol: 92, Pages: 136-148, ISSN: 0963-8695
  • Journal article
    Li Z, Jarvis R, Nagy PB, Dixon S, Cawley Pet al., 2017,

    Experimental and simulation methods to study the Magnetic Tomography Method (MTM) for pipe defect detection

    , NDT & E INTERNATIONAL, Vol: 92, Pages: 59-66, ISSN: 0963-8695
  • Journal article
    Zou F, Cegla F, 2017,

    High Accuracy Ultrasonic Corrosion Rate Monitoring

    , Corrosion, Vol: 74, Pages: 2663-2679, ISSN: 0010-9312

    Ultrasonic testing with permanently installed transducers is widely used for online corrosion monitoring in the field. In this paper, a carefully optimized ultrasonic corrosion monitoring technique for carrying out measurements in the laboratory is presented. It is shown that for thickness measurements of a 10 mm steel component, a repeatability of ~40 nm can be maintained over the period of a day. The technique has been applied to monitoring the wall losses of a steel sample during forced and unforced corrosion experiments. All ultrasonic wall loss measurements reported have been validated by optical surface profile scans and, where possible, by analytical predictions based on Faraday’s law. Further analysis of the results shows that wall loss rates in the order of 0.1 – 0.2 mm/year can be detected within 1 – 2 hours. This state-of-the-art laboratory technique is highly accurate and responsive, and possesses the potential for becoming a powerful alternative corrosion assessment tool that is convenient to use.

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