Imperial College London

ProfessorPeterCawley

Faculty of EngineeringDepartment of Mechanical Engineering

Associate Dean for Enterprise for the Faculty of Engineering
 
 
 
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Contact

 

+44 (0)20 7594 7069p.cawley

 
 
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Assistant

 

Ms Nina Hancock +44 (0)20 7594 7068

 
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Location

 

568City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

515 results found

Vogt T, Heinlein S, Milewczyk J, Mariani S, Jones R, Cawley Pet al., 2021, Guided Wave Monitoring of Industrial Pipework – Improved Sensitivity System and Field Experience, Pages: 819-829, ISBN: 9783030645939

Low frequency guided wave inspection using the torsional, T(0,1), mode is routinely used in the petrochemical and other industries for the detection of corrosion patches, the detection threshold being typically around 5% cross section loss, though better sensitivity is obtained on simple pipe configurations not suffering from general corrosion. It has been shown in a blind trial that switching to a permanently installed system operating in SHM mode can improve the sensitivity to about 1% cross section loss and this is very attractive in corrosion monitoring applications. Later work has shown that the detection limit could be reduced to below 1% cross section loss if the compensation for environmental changes, particularly temperature, could be improved. This paper presents a new temperature compensation method involving both overall signal stretching, analogous to the well-known baseline stretch technique, and a further, location-by-location adjustment; this gives significant further improvements in performance. A commercial permanently installed monitoring system giving both local thickness measurements at the transducer location and long-range monitoring for corrosion over 10 s of metres from the transducer position is described. The system enables frequent measurements to be taken, the results being delivered to the operator via a wireless link. The benefits of the frequent readings enabled by the automatic data collection and transmission are discussed. Initial results presented here indicate that this enables defects as small as 0.1% cross section loss to be detected.

Book chapter

Mariani S, Cawley P, 2020, Change detection using the generalized likelihood ratio method to improve the sensitivity of guided wave structural health monitoring systems, Structural Health Monitoring: an international journal, ISSN: 1475-9217

The transition from one-off ultrasound–based non-destructive testing systems to permanently installed monitoring techniques has the potential to significantly improve the defect detection sensitivity, since frequent measurements can be obtained and tracked with time. However, the measurements must be compensated for changing environmental and operational conditions, such as temperature, and careful analysis of measurements by highly skilled operators quickly becomes unfeasible as a large number of sensors acquiring signals frequently is installed on a plant. Recently, the authors have developed a location-specific temperature compensation method that uses a set of baseline measurements to remove temperature effects from the signals, thus producing “residual” signals on an unchanged structure that are essentially normally distributed with zero-mean and with standard deviation related to instrumentation noise. This enables the application of change detection techniques such as the generalized likelihood ratio method that can process sequences of residual signals searching for changes caused by damage. The defect detection performance offered by the generalized likelihood ratio when applied to guided wave signals adjusted either via the newly developed location-specific temperature compensation method or the widely used optimal baseline selection technique is investigated on a set of simulated measurements based on a set of experimental signals acquired by a permanently installed pipe monitoring system designed to monitor tens of meters of pipe from one location using the torsional, T(0,1), guided wave mode. The results presented here indicate that damage on the order of 0.1% cross section loss can reliably be detected with virtually zero false calls if the assumptions of the study are met, notably the absence of sensor drift with time. This represents a factor of 20–50 improvement over that typically achieved in one-off inspection and makes suc

Journal article

Cawley P, 2020, A Development Strategy for SHM Applications, Journal of Nondestructive Evaluation, ISSN: 0195-9298

Permanently installed SHM systems are now a viable alternative to traditional periodic inspection(NDT). However, their industrial use is limited and this paper reviews the steps required indeveloping practical SHM systems. The transducers used in SHM are fixed in location, whereas inNDT they are generally scanned. The aim is to reach similar performance with high temporalfrequency, low spatial frequency SHM data to that achievable with conventional high spatialfrequency, low temporal frequency NDT inspections. It is shown that this can be done via changetracking algorithms such as the Generalized Likelihood Ratio (GLR) but this depends on the inputdata being normally distributed, which can only be achieved if signal changes due to variations inthe operating conditions are satisfactorily compensated; there has been much recent progress onthis topic and this is reviewed. Since SHM systems can generate large volumes of data, it isessential to convert the data to actionable information, and this step must be addressed in SHMsystem design. It is also essential to validate the performance of installed SHM systems, and amethodology analogous to the model assisted POD (MAPOD) scheme used in NDT has beenproposed. This uses measurements obtained from the SHM system installed on a typicalundamaged structure to capture signal changes due to environmental and other effects, and tosuperpose the signal due to damage growth obtained from finite element predictions. There is asubstantial research agenda to support the wider adoption of SHM and this is discussed.

Journal article

Wilcox P, Croxford A, Budyn N, Bevan R, Zhang J, Kashubin A, Cawley Pet al., 2020, Fusion of multi-view ultrasonic data for increased detection performance in non-destructive evaluation, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 476, ISSN: 1364-5021

State-of-the-art ultrasonic non-destructive evaluation (NDE) uses an array to rapidly generate multiple, information-rich views at each test position on a safety-critical component. However, the information for detecting potential defects is dispersed across views, and a typical inspection may involve thousands of test positions. Interpretation requires painstaking analysis by a skilled operator. In this paper, various methods for fusing multi-view data are developed. Compared with any one single view, all methods are shown to yield significant performance gains, which may be related to the general and edge cases for NDE. In the general case, a defect is clearly detectable in at least one individual view, but the view(s) depends on the defect location and orientation. Here, the performance gain from data fusion is mainly the result of the selective use of information from the most appropriate view(s) and fusion provides a means to substantially reduce operator burden. The edge cases are defects that cannot be reliably detected in any one individual view without false alarms. Here, certain fusion methods are shown to enable detection with reduced false alarms. In this context, fusion allows NDE capability to be extended with potential implications for the design and operation of engineering assets.

Journal article

Pialucha T, Pavlakovic B, Alleyne D, Cawley Pet al., 2020, Quantitative measurement of remnant thickness in corrosion under pipe supports, INSIGHT, Vol: 62, Pages: 642-648, ISSN: 1354-2575

Journal article

Cawley P, Mariani S, Liu Y, 2020, Improving sensitivity and coverage of SHM using bulk ultrasonic waves, Structural Health Monitoring: an international journal, ISSN: 1475-9217

Journal article

Cawley P, Chua C, 2020, Crack growth monitoring using fundamental shear horizontal guided waves, Structural Health Monitoring: an international journal, Vol: 19, Pages: 1311-1322, ISSN: 1475-9217

Monitoring cracks in critical sections of steel structures is a topic of growing interest. Existing high frequencyultrasonic techniques have good detection sensitivities but poor inspection coverage, requiring an impractical numberof transducers to monitor large areas. Low frequency guided waves are used for corrosion detection in pipelines,but are insufficiently sensitive for many crack detection applications. The sensitivity can be improved by using higherfrequencies and by placing the receiving transducers closer to the defect. This study evaluates the monitoringperformance of an SH0 mode system at frequencies just below the high-order mode cut-off. Baseline subtractionwith temperature compensation was applied to experimental data generated by a ring of transducers on a 6-inchdiameter pipe. It was found that the residual signals after baseline subtraction were normally distributed so therandom fluctuations could be reduced by coherent averaging; it was thereby possible to reliably detect a 2x1 mmnotch simulating a crack located one pipe diameter along the pipe from the transducer ring. The damage detectionperformance at different locations along the pipe was assessed by analysing receiver operating characteristic (ROC)curves generated by adding simulated defects to multiple experimental measurements without damage. At a fixedstandoff distance, the damage detection performance increases with the square root of the number of averaged signals,and is also improved by averaging the signals received by transducers covering the main lobe of the reflection fromthe defect. When the defect is located more than about one pipe circumference from the transducer ring, the optimalperformance is obtained by averaging across all the transducers in the ring, corresponding to monitoring the T(0,1) pipemode. Therefore, an SH0 mode monitoring system has great potential for crack monitoring applications, particularly forwelds in pipes.

Journal article

Liu Y, Nagy P, Cawley P, 2020, Design optimisation of permanently installed monitoring system for polycrystalline materials, Structural Health Monitoring: an international journal, ISSN: 1475-9217

This paper presents a design procedure for structural health monitoring systems based on bulk wave ultrasonic sensors for structures fabricated from polycrystalline materials. When designing a monitoring system, maximum coverage pertransducer is a general requirement in order for the system to be economic. For coarse grained polycrystalline materials,monitoring is often made challenging by low signal to noise ratios caused by grain scattering. Therefore, when designing a monitoring system for these materials, in addition to the economic requirement, it needs to be ensured that an adequate signal to noise ratio can be obtained throughout the monitoring volume. This typically introduces a trade-off between volume coverage per transducer and sensitivity that must be investigated. In this paper, this trade-off is studied and a methodology using signal to noise maps is presented to design the system, i.e. choose the optimal transducer parameters and placement. Firstly, a combined analytical-and-numerical approach is used to generate a signal to noise map. Then, the influence of various factors on signal to noise ratio is investigated. Finally, two representative examples,with different criteria, are given to illustrate the methodology. In one example, the full surface area of the test piece is covered with transducers and the optimum gives the deepest coverage. The other one aims to achieve the minimum fractional surface area that has to be covered with transducers to monitor a narrow depth range far from the surface,which has a potential application in weld monitoring. Results show that the optimum is likely to be at much lower frequency than typically used in inspection, as tracking signals with time gives sensitivity gains. Experiments were carried out to illustrate that higher volume coverage can be obtained at lower frequencies.

Journal article

Corcoran J, Davies CM, Cawley P, Nagy PBet al., 2020, A quasi-DC potential drop measurement system for materials testing, IEEE Transactions on Instrumentation and Measurement, Vol: 69, Pages: 1313-1326, ISSN: 0018-9456

Potential drop measurements are well established for use in materials testing and are commonly used for crack growth and strain monitoring. Traditionally, the experimenter has a choice between employing direct current (DC) or alternating current (AC), both of which have strengths and limitations. DC measurements are afflicted by competing spurious DC signals and therefore require large measurement currents (10’s or 100’s of amps) to improve the signal to noise ratio, which in turn leads to significant resistive Joule heating. AC measurements have superior noise performance due to utilisation of phase-sensitive detection and a lower spectral noise density, but are subject to the skin-effect and are therefore not well suited to high-accuracy scientific studies of ferromagnetic materials. In this work a quasi-DC monitoring system is presented which uses very low frequency (0.3-30 Hz) current which combines the positive attributes of both DC and AC while mitigating the negatives. Bespoke equipment has been developed that is capable of low-noise measurements in the demanding quasi-DC regime. A creep crack growth test and fatigue test are used to compare noise performance and measurement power against alternative DCPD equipment. The combination of the quasi-DC methodology and the specially designed electronics yields exceptionally low-noise measurements using typically 100-400 mA; at 400mA the quasi-DC system achieves a 13-fold improvement in signal to noise ratio compared to a 25A DC system. The reduction in measurement current from 25A to 400mA represents a ~3900 fold reduction in measurement power, effectively eliminating resistive heating and enabling much simpler experimental arrangements.

Journal article

Mariani S, Heinlein S, Cawley P, 2020, Compensation for temperature-dependent phase and velocity of guided wave signals in baseline subtraction for structural health monitoring, Structural Health Monitoring: an international journal, Vol: 19, Pages: 26-47, ISSN: 1475-9217

Baseline subtraction is commonly used in guided wave structural health monitoring to identify the signal changes produced by defects. However, before subtracting the current signal from the baseline, it is essential to compensate for changes in environmental conditions such as temperature between the two readings. This is often done via the baseline stretch method that seeks to compensate for wave velocity changes with temperature. However, the phase of the signal generated by the transduction system is also commonly temperature sensitive and this effect is neglected in the usual compensation procedure. This article presents a new compensation procedure that deals with both velocity and phase changes. The results with this new method have been compared with those obtained using the standard baseline stretch technique on both a set of experimental signals and a series of synthetic signals with different coherent noise levels, feature reflections, and defect sizes, the range of noise levels and phase changes being chosen based on initial experiments and prior field experience. It has been shown that the new method both reduces the residual signal from a set baseline and enables better defect detection performance than the conventional baseline signal stretch method under all conditions examined, the improvement increasing with the size of the temperature and phase differences encountered. For example, in the experimental data, the new method roughly halved the residual between baseline and current signals when the two signals were acquired at temperatures 15°C apart.

Journal article

Mariani S, Heinlein S, Cawley P, 2020, Location specific temperature compensation of guided wave signals in structural health monitoring, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 67, Pages: 146-157, ISSN: 0885-3010

In guided wave structural health monitoring, defects are typically detected by identifying high residuals obtained via the baseline subtraction method, where an earlier measurement is subtracted from the ‘current’ signal. Unfortunately, varying environmental and operational conditions, such as temperature, also produce signal changes and hence, potentially, high residuals. While the majority of the temperature compensation methods that have been developed target the changed wave speed induced by varying temperature, a number of other effects are not addressed, such as changes in attenuation, the relative amplitudes of different modes excited by the transducer and the transducer frequency response. A temperature compensation procedure is developed whose goal is to correct any spatially dependent signal change that is a systematic function of temperature. At each structural position, a calibration function that models the signal variation with temperature is computed and is used to correct the measurements, so that in the absence of a defect the residual is reduced to close to zero. This new method was applied to a set of guided wave signals collected in a blind trial of a guided wave pipe monitoring system employing the T(0,1) mode, yielding residuals de-coupled from temperature and reduced by at least 50% compared to those obtained using the standard approach at positions away from structural features, and by more than 90% at features such as the pipe end. The method therefore promises a substantial improvement in the detectability of small defects, particularly at existing pipe features.

Journal article

Chua C, Cawley P, Nagy P, 2019, Scattering of fundamental shear guided waves from a surface-breaking crack in plate-like structures, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 66, Pages: 1887-1897, ISSN: 0885-3010

Cracks in critical sections of steel structures pose a major safety concern in many industries. Existing high frequency ultrasonic techniques offer high detection sensitivity to cracks but have poor inspection volume coverage, limiting their practical use for monitoring large areas of structures. Low frequency guided waves have relatively high inspection area coverage and are currently used in pipeline monitoring for corrosion defects, but face challenges in detecting critical cracks which often cause over an order of magnitude lower cross sectional area loss. A study of scattering from small cracks in a thin-walled (<12 mm) section with an incident plane SH0 guided wave at higher frequencies but remaining below the SH1 cut-off is presented here using quasistatic approximations, the aim being to explore the possibility of using this regime for crack growth monitoring applications. A 3D solution was developed using dimensional analysis, which showed that the SH0 reflection ratio is proportional to frequency to the power 1.5, to the effective crack size cubed, and is inversely proportional to the plate thickness and to the square root of the distance from the crack to the receiving sensor. Finite element analysis was used to validate these power coefficients and to calculate the proportionality constant. The results show that a higher inspection frequency offers improved sensitivity but the validity of the results here is limited to the SH1 cut-off frequency. The predicted 3D solution was validated by measurements on a pipe with a progressively grown notch.

Journal article

Heinlein S, Cawley P, Vogt T, 2019, Validation of a procedure for the evaluation of the performance of aninstalled structural health monitoring system, Structural Health Monitoring, Vol: 18, Pages: 1557-1568, ISSN: 1475-9217

Validation of the performance of guided wave structural health monitoring systems is vital if they are to be widely deployed; testing the damage detection ability of a system by introducing different types of damage at varying locations is very costly and cannot be performed on a system in operation. Estimating the damage detection ability of a system solely by numerical simulations is not possible as complex environmental effects cannot be accounted for. In this study, a methodology was tested and verified that uses finite element simulations to superimpose defect signals onto measurements collected from a defect-free structure. These signals are acquired from the structure of interest under varying environmental and operational conditions for an initial monitoring period. Measurements collected in a previous blind trial of an L-shaped pipe section, onto which a number of corrosion-like defects were introduced, were utilised during this investigation. The growth of three of these defects was replicated using finite element analysis and the simulated reflections were superimposed onto signals collected on the defect-free test pipe. The signal changes and limits of reliable detection predicted from the synthetic defect reflections superimposed on the measurements from the undamaged complex structure agreed well with the changes due to real damage measured on the same structure. This methodology is of great value for any structural health monitoring system as it allows for the minimum detectable defect size to be estimated for specific geometries and damage locations in a quick and efficient manner without the need for multiple test structures while accounting for environmental variations.

Journal article

Leung M, Corcoran J, Cawley P, Todd MDet al., 2019, Evaluating the use of Rate-based Monitoring for Improved Fatigue Remnant Life Predictions, International Journal of Fatigue, Vol: 120, Pages: 162-174, ISSN: 0142-1123

The ability to perform accurate remnant life predictions is crucial to ensure the integrity of engineering components that experience fatigue loading during operation. This is conventionally achieved with periodic inspections, where results from non-destructive evaluation and estimation of the operating conditions are obtained to perform remnant life predictions using empirical crack growth laws. However, remnant life predictions made with this approach are very sensitive to their input parameters; uncertainty in each parameter would aggregate and result in great uncertainty in the final prediction. With the increasing viability of permanently-installed systems, it is proposed that the rate of damage growth can be used to more accurately and confidently gauge the integrity of an engineering component and perform remnant life predictions using the Failure Forecast Method. A statistical analysis of an example fatigue crack growth test was performed to compare the uncertainties of the remnant life predictions made using the conventional inspection approach and the proposed rate-based monitoring approach. It is shown that the Failure Forecast Method produces significantly more accurate and confident predictions compared to the inspection approach. The use of the Failure Forecast Method under non-constant amplitude loading conditions was also investigated. An equivalent cycles method is introduced to accommodate step changes in operating conditions. The effect of load interactions was also studied through a fatigue test with isolated overloads and a random variable amplitude loading test. Overall, the study has shown that the frequent data obtained from permanently installed monitoring systems provides new opportunities in remnant life estimates and potentially opens the way to increasing the intervals between outages and safely reducing conservatism in life predictions.

Journal article

Liu Y, Van Pamel A, Nagy P, Cawley Pet al., 2019, Investigation of ultrasonic backscatter using three-dimensional finite element simulations, Journal of the Acoustical Society of America, Vol: 145, Pages: 1584-1595, ISSN: 0001-4966

Theoretical models are commonly used to describe ultrasonic backscattering in polycrystalline materials. However, although a full multiple scattering formalism has been derived, due to the difficulty in evaluation, currently only the single and double scattering effects have been evaluated. Three-dimensional finite element (3D FE) models have recently been demonstrated to be capable of predicting ultrasonic attenuation in polycrystalline materials and thereby show great potential in overcoming this limitation. In this paper, the application of 3D FE models is extended to the backscatter problem. First, longitudinal-to-longitudinal backscattering amplitudes from single grains are predicted, where the setup and configuration of the finite element (FE) model are verified with an isotropic spherical inclusion for which an exact solution is available. Subsequently, backscatter in terms of the root-mean-square noise levels in two different pulse-echo scenarios is investigated; the first is an idealised configuration with plane wave transmission and point reception; the second represents a more realistic finite-size transducer acting with the same apodization in both transmission and reception. Comparisons of FE predictions and approximate theoretical solutions within a range of validity show good agreement; however, the results demonstrate that 3D FE is useful where the simple Independent Scatterer models break down. As computing power increases, 3D FE is an increasingly viable tool to further the understanding of wave propagation in polycrystalline materials.

Journal article

Mariani S, Cawley P, 2019, Location specific temperature compensation of guided wave signals applied to pipe inspections, Pages: 2427-2433

The baseline subtraction method is widely used to detect defect signatures in guided wave structural health monitoring. In essence, an earlier measurement is subtracted from the 'current' signal, and high residuals might indicate damage occurrence. However, varying environmental and operational conditions, such as temperature, also produce signal changes and hence, potentially, high residuals. A number of temperature compensation methods have been developed, which typically targets the varying wave speed due to varying temperature. Nevertheless, other, subtler effects caused by temperature variations are often overlooked, such as changes in attenuation, in the transducer frequency response and in the relative amplitudes of different modes excited by the transducer. A novel temperature compensation method has been recently presented by the authors, which compensates for temperature induced changes of wave speed and signal phase. The compensated signals can then be fed to a second temperature compensation procedure that has been newly developed. This will correct any spatially dependent signal change that is a systematic function of temperature, hence producing residuals less affected by temperature variations. This new method was applied to a set of T(0,1) guided wave signals collected by a pipe monitoring system, yielding residuals reduced by at least 50% compared to those obtained using the standard approach at positions away from structural features, and by more than 90% at features such as the pipe end. The method therefore promises a substantial improvement in the detectability of small defects, particularly at existing pipe features.

Conference paper

Cawley P, 2019, Ultrasonic structural health monitoring - current applications and potential, IEEE International Ultrasonics Symposium (IUS), Publisher: IEEE, Pages: 2107-2109, ISSN: 1948-5719

Conference paper

Combaniere J, Cawley P, McAughey K, Giese Jet al., 2019, Interaction between SH0 guided waves and tilted surface-breaking cracks in plates, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 66, Pages: 119-128, ISSN: 0885-3010

The interaction between SH0 guided waves and simple defects is well understood and documented, and the SH0 and related torsional guided waves are commonly used in inspection. However, tilted and branching cracks, for which vertical notches are a poor approximation, are found in some environments, particularly when pipes are buried in alkaline soils. This paper studies the interaction between SH0 guided waves and tilted, surface-breaking cracks, investigating the effect of the tilt and depth of the defect. The incident wave interacts with the tilted crack to generate a transmitted wave, a reflected wave and a wave trapped below the crack. It is shown that the direction of the tilt of the crack relative to the incident wave direction does not affect the scattering behaviour. Additionally, the axial extent of the crack plays a major role in the reflectivity of the crack, leading to transmission nulls in some configurations. These transmission nulls appear for all crack depths, the frequency range over which the transmission is significantly reduced increasing with crack depth. This behaviour is shown to be analogous to the acoustic energy flow in a duct when a Helmholtz resonator is introduced. The null is not seen above the SH1 cut-off as the propagating signals are no longer mono-modal. The existence of a transmission null and corresponding reflection maximum is promising for the detection of small defects and measurement of the frequency at which the null occurs will assist with defect characterisation. Experimental validations of the key results are presented.

Journal article

Khalili P, Cawley P, 2018, The choice of ultrasonic inspection method for the detection of corrosion at inaccessible locations, NDT and E International, Vol: 99, Pages: 80-92, ISSN: 0963-8695

Inspection for corrosion and pitting defects in the petrochemical industry is vital and forms a significant fraction of the operating expenditure. Low frequency guided wave inspection is frequently employed as it gives large area coverage from a single transducer position. However, detection becomes problematic at inaccessible regions such as pipe supports or beyond T-joints since the low frequency guided waves produce a significant reflection from the feature itself, hence limiting the defect detectability of the method. This suggests testing at higher frequencies which helps to minimise the reflection from the feature and also improves the sensitivity to smaller defects. There are a number of guided wave and related techniques implemented for corrosion inspection including the S0 mode (at ∼ 1 MHz-mm), SH0 and SH1 modes (at ∼ 3 MHz-mm), CHIME, M-skip and Higher Order Mode Cluster (A1 mode at ∼ 18 MHz-mm). This paper presents a systematic analysis of the defect detection performance of each method with sharp and gradual defects, as well as their sensitivity to attenuative coatings, liquid loading, surface roughness and ability to test beyond features such as T-joints. It is shown by finite element analysis backed up by experiments that the A1 mode provides the best overall performance when dealing with surface features such as T-joints and coatings because of its low surface motion. Additionally a combination of two or more methods is suggested for corrosion inspection at inaccessible locations: The A1 mode in reflection for severe, sharp, pitting type defects; long range guided waves in reflection for large-area thinning and the SH1 mode in transmission for shallow, gradual defects.

Journal article

Cawley P, 2018, Structural health monitoring: closing the gap between research andindustrial deployment, Structural Health Monitoring, Vol: 17, Pages: 1225-1244, ISSN: 1475-9217

There has been a large volume of research on structural health monitoring since the 1970s but this research effort has yielded relatively few routine industrial applications. Structural health monitoring can include applications on very different structures with very different requirements; this article splits the subject into four broad categories: rotating machine condition monitoring, global monitoring of large structures (structural identification), large area monitoring where the area covered is part of a larger structure, and local monitoring. The capabilities and potential applications of techniques in each category are discussed. Condition monitoring of rotating machine components is very different to the other categories since it is not strictly concerned with structural health. However, it is often linked with structural health monitoring and is a relatively mature field with many routine applications, so useful lessons can be read across to mainstream structural health monitoring where there are many fewer industrial applications. Reasons for the slow transfer from research to practical application of structural health monitoring include lack of attention to the business case for monitoring, insufficient attention to how the large data flows will be handled and the lack of performance validation on real structures in industrial environments. These issues are discussed and ways forward proposed; it is concluded that given better focused research and development considering the key factors identified here, structural health monitoring has the potential to follow the path of rotating machine condition monitoring and become a widely deployed technology.

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

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

Conference paper

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

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

Journal article

Todd MD, Leung M, Corcoran J, Cawley Pet al., 2018, Fatigue prognosis using the uncertainty-quantified failure forecast method, Pages: 129-137

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.

Conference paper

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

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

Heinlein S, Cawley P, Vogt TK, 2017, Reflection of torsional T(0,1) guided waves from defects in pipe bends, NDT and E International, Vol: 93, Pages: 57-63, ISSN: 0963-8695

This paper investigates the reflection of the torsional T(0,1) mode from defects in pipe bends. The effect of varying circumferential and angular position along the pipe bend, as well as the influence of the bend radius, is investigated via 3D finite element simulations. The results show that the reflection expected from a small defect varies significantly with position, the minimum reflection coefficient being about 10% of that from a comparable defect in a straight pipe, while maxima of around four times the straight pipe value are seen. The areas of low detectability are mainly found on the bend intrados and those of high detectability close to its extrados; similar effects are seen in bends with radii varying from one to twenty pipe diameters. It is shown that the reflection from a defect at a given location is roughly proportional to the square of the von Mises stress produced by the transmitted wave at that position. This holds for defects such as circumferential cracks, the detailed subject of this investigation, and is also expected to be valid for corrosion patches; it will not hold for axial cracks. The results explain the low reflection seen from a simulated corrosion defect at a bend in a previous investigation.

Journal article

Liu Y, Chang L, van Pamel A, Cawley Pet al., 2017, Feasibility and reliability of grain noise suppression in monitoring of highly scattering materials, Journal of Nondestructive Evaluation, Vol: 36, ISSN: 1573-4862

A feasibility study on grain noise suppression using baseline subtraction is presented in this paper. Monitoring is usually done with permanently installed transducers but this is not always possible; here instead monitoring is conducted by carrying out repeat C-scans and the feasibility of grain noise suppression by subtracting A-scans extracted from the C-scans is investigated. The success of this technique depends on the ability to reproduce the same conditions for each scan, including a consistent stand-off, angle, and lateral position of the transducer relative to the testpiece. The significance of errors are illustrated and a 3D cross correlation is used which enables the same lateral position to be located within successive C-scans. The experimental results show that a noise reduction of around 15 dB is obtained after baseline subtraction, which will significantly improve the defect detection sensitivity. In practice however, successive C-scans may be conducted at different temperatures and with different transducers of similar specifications but a varying frequency response. Compensation techniques to reduce the impact of such variations are then presented and their effectiveness is verified experimentally. It is shown that it is feasible to obtain an overall improvement of around 10 dB in the signal to noise ratio via baseline subtraction, where a temperature difference of up to 10 ∘C and a peak frequency shift of as much as ±250 kHz from a baseline value of around 7 MHz can be tolerated. However, this improvement was obtained in laboratory conditions with no changes to the surface of the specimen due to oxidation or corrosion. It is shown that differences in temperature and transducer frequency response are more difficult to compensate for than changes in test geometry and position.

Journal article

Jarvis R, Cawley P, Nagy P, 2017, Performance evaluation of a magnetic field measurement NDE technique using a model assisted probability of detection framework, NDT & E International, Vol: 91, Pages: 61-70, ISSN: 0963-8695

Receiver Operating Characteristics (ROC) are a powerful tool used to evaluate the performance of NDE methods; however, the need to manufacture and scan many test pieces with realistic defects means that they are expensive and time-consuming to produce. Advances in computational power now mean that it is possible to use numerical models to greatly increase the efficiency of producing ROC for practical applications. A Model Assisted Probability of Detection (MAPOD) framework has been developed to predict the performance of magnetic field measurement NDE techniques. The MAPOD method is used to predict the performance of a promising new technique relying on the deflection of a current injected into a pipe at remote locations, and measurement of the resulting magnetic field perturbations due to defects. A significant proportion of pipes cannot be inspected by pigging methods, and external inspection often requires complete coating removal; therefore, an NDE method that functions outside pipe coatings and cladding is attractive. In this method, changes in the radial and axial components of the field are measured and attributed to defects, but a strong azimuthal component means that misalignment can give significant apparent radial and axial signals due to the azimuthal field apparently having a component in these directions. This requires that the second-order gradient of the magnetic field be measured to maximise sensitivity. Fluctuations in the sensitivity and orientation of the gradiometer during the scan are expected to determine the maximum sensitivity of the technique in most practical applications; however, the flexibility of the framework allows performance to be rapidly predicted and quantified for many test scenarios. Results suggest good detection performance for defects greater than 15% of the wall thickness (T = 7.1 mm) in a 6″ pipe with 2 A (200 A/m2) current injected when measuring above typical insulation thickness (25–50 mm).

Journal article

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