Publications
520 results found
Cawley P, 2024, Guided waves in long range nondestructive testing and structural health monitoring: Principles, history of applications and prospects, NDT and E International, Vol: 142, ISSN: 0963-8695
The early work on long range guided wave nondestructive testing (NDT) was largely done in industry, and multiple applications were developed before a surge in academic interest starting in the early 1990s. This paper reviews the history of the field, covering the topics that must be addressed in the design of a test procedure, as well as applications in both NDT and Structural Health Monitoring (SHM). The use of guided waves in long range NDT is relatively mature, and largely limited to simple structures in applications that are difficult or impossible to address by conventional point inspection methods. Point methods are impractical in most large area SHM applications where it is necessary to detect isolated defects since it is not possible to cover the whole area of interest with transducers. Guided waves therefore have great potential in SHM as they enable a large area of structure to be monitored from a limited number of transduction locations, so reducing the added weight, cost and complexity of the attachment to the structure. It is also possible to improve the sensitivity of the test relative to that obtained in one-off inspection by monitoring the change in signals with time using baseline subtraction. Current SHM deployments are largely in the same types of structure as those where guided wave NDT is used, but there is substantial potential for increasing the range of applications. The paper concludes with a review of the research and development needs for this potential to be realised.
Mariani S, vogt T, Heinlein S, et al., 2023, The performance of a guided wave pipe monitoring system over extended periods of field operation, Materials Evaluation, ISSN: 0025-5327
The performance of a permanently installed guided wave pipe monitoring system over extended periods has been investigated. Other than at a clamped support, the signals obtained in the three installations reported here have been stable to better than 1% (-40dB) over periods of 12-24 months, even in hostile oil and gas facility environments. The use of baseline subtraction means that the sensitivity to defects is substantially better than that obtained in one-off inspection; in the two site examples reported here, improvements of 7 and 12 dB relative to one-off interpretation of the A-scan signal were obtained, and changes can be flagged automatically. The improvement in sensitivity is even larger at features such as welds and bends that give significant reflections even in the absence of damage. The permanently installed system combines long range guided wave monitoring for localised corrosion patches with eight precise thickness measurements around the pipe circumference, making it possible to monitor the general wall loss rate, as well as detecting any more severe, localised corrosion. Permanently installed guided wave monitoring systems are therefore now a commercially attractive tool for the assessment of current and future piping integrity, and provide commercially valuable information for improved operations.
Cawley P, Rodgers E, Mariani S, 2023, The use of circumferential guided waves to monitor axial cracks in pipes, Structural Health Monitoring: an international journal, Vol: 22, Pages: 2609-2625, ISSN: 1475-9217
Guided wave testing is in routine use to detect corrosion, particularly in the oil and gas sector, but the detection of axial cracking is difficult using the axially propagating waves commonly used for corrosion detection. This paper presents a novel guided wave monitoring technique for the detection of axial cracking in piping. Circumferential SH0 waves, travelling around the circumference were used in a monitoring configuration to detect axial defects in a 6” schedule 80 steel pipe. Finite Element Analysis showed that both the defect reflection and the decline in the transmitted SH0 wave can be used for defect detection. Baseline subtraction was utilised to produce residual signals that can be better analysed for small defect reflections. The RMS of the residual signal after baseline subtraction was found to be the most satisfactory means of monitoring defect progression. An amplification effect was identified where the residual signal is compounded by continued interaction with the defect on each revolution of the incident wave. The finite element predictions were validated by an experiment in which an EDM notch was grown in four stages. Temperature compensation using Location Specific Temperature Compensation was applied to the experimental data, allowing residuals to be compared for a ~10oC temperature swing over a monitoring period of over 2 months. It was determined that a 10mm long (~23% of wavelength), 5mm deep (~45% of the wall thickness) defect at an axial offset from the line of the transducers of 1.5 wavelengths (65mm) would be readily detectable with a very high Probability of Detection and virtually no chance of a false call. Therefore, this novel guided wave monitoring system for axial crack detection is likely to be attractive forapplications in a range of industries for its sensitivity to axial cracking combined with large coverage from a single transducer location.
Cawley P, Heinlein S, Mariani S, et al., 2022, Improved thickness measurement on rough surfaces by using guided wave cut-off frequency, NDT&E International, Vol: 132, Pages: 1-7
Substantial variability is seen when thickness measurements using conventional ultrasonic time of flight measurements are carried out on rough surfaces; this makes it difficult to estimate corrosion rates when the corrosion mechanism leads to surface roughness. It has been shown that thickness measurements using guided wave cut-off frequencies (through thickness resonance frequencies) can be done at much lower frequencies than conventional time of flight thickness gauging for the same minimum thickness resolution. The lower frequency measurements are less susceptible to variations caused by surface roughness and so will give more consistent and reliable results in cases where corrosion leads to increased surface roughness. Measurements were carried out using guided wave cut-off frequencies on four plates with different surface roughness. On a plate with root mean square (rms) surface roughness of 0.3 mm, the thickness estimates followed the trend predicted from the plate geometry and probe footprint, whereas higher frequency measurements reported in the literature deviated from this trend at an rms roughness of 0.1 mm. The guided wave cut-offcut-off frequency measurements can be done using the same transduction system as that used for large area guided wave monitoring so it is possible to combine large and small area monitoring in a single unit. Frequent measurements enable the wall thickness obtained with guided wave cut-off measurements to be tracked with time, and the low susceptibility of the measurements to surface roughness means that accurate corrosion rates will be obtained.
Vogt T, Pavlakovic B, Cawley P, 2022, A MULTIPLE-ECHO CALIBRATION TECHNIQUE FOR GUIDED WAVE TESTING, MATERIALS EVALUATION, Vol: 80, Pages: 32-+, ISSN: 0025-5327
Mariani S, Cawley P, 2021, 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, Vol: 20, Pages: 3201-3226, 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
Cawley P, 2021, A development strategy for structural health monitoring applications, Journal of Nondestructive Evaluation, Diagnostics and Prognostics of Engineering Systems, Vol: 4, ISSN: 2572-3898
Permanently installed structural health monitoring (SHM) systems are now a viable alternative to traditional periodic inspection (nondestructive testing (NDT)). However, their industrial use is limited, and this article reviews the steps required in developing practical SHM systems. The transducers used in SHM are fixed in location, whereas in NDT, they are generally scanned. The aim is to reach similar performance with high temporal frequency, low spatial frequency SHM data to that achievable with conventional high spatial frequency, and low temporal frequency NDT inspections. It is shown that this can be done via change tracking algorithms such as the generalized likelihood ratio (GLR), but this depends on the input data being normally distributed, which can only be achieved if signal changes due to variations in the operating conditions are satisfactorily compensated; there has been much recent progress on this topic, and this is reviewed. Since SHM systems can generate large volumes of data, it is essential to convert the data to actionable information, and this step must be addressed in the SHM system design. It is also essential to validate the performance of installed SHM systems, and a methodology analogous to the model-assisted probability of detection (POD) (MAPOD) scheme used in NDT has been proposed. This uses measurements obtained from the SHM system installed on a typical undamaged structure to capture signal changes due to environmental and other effects and to superpose the signal due to damage growth obtained from finite element predictions. There is a substantial research agenda to support the wider adoption of SHM, and this is discussed in this study.
Cawley P, Mariani S, Liu Y, 2021, Improving sensitivity and coverage of SHM using bulk ultrasonic waves, Structural Health Monitoring: an international journal, Vol: 20, Pages: 2641-2652, ISSN: 1475-9217
Practical ultrasonic structural health monitoring systems must be able to deal with temperature changes and some signal amplitude/phase drift over time; these issues have been investigated extensively with low-frequency-guided wave systems but much less work has been done on bulk wave systems operating in the megahertz frequency range. Temperature and signal drift compensation have been investigated on a thick copper block specimen instrumented with a lead zirconate titanate disc excited at a centre frequency of 2 MHz, both in the laboratory at ambient temperature and in an environmental chamber over multiple 20°C–70°C temperature cycles. It has been shown that the location-specific temperature compensation scheme originally developed for guided wave inspection significantly out-performs the conventional combined optimum baseline selection and baseline signal stretch method. The test setup was deliberately not optimised, and the signal amplitude and phase were shown to drift with time as the system was temperature cycled in the environmental chamber. It was shown that the ratio of successive back wall reflections at a given temperature was much more stable with time than the amplitude of a single reflection and that this ratio can be used to track changes in the reflection coefficient from the back wall with time. It was also shown that the location-specific temperature compensation method can be used to compensate for changes in the back wall reflection ratio with temperature. Clear changes in back wall reflection ratio were produced by the shadow effect of simulated damage in the form of 1-mm diameter flat-bottomed holes, and the signal-to-noise ratio was such that much smaller defects would be detectable.
Liu Y, Nagy P, Cawley P, 2021, Design optimisation of permanently installed monitoring system for polycrystalline materials, Structural Health Monitoring: an international journal, Vol: 20, Pages: 1294-1311, 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.
Vogt T, Heinlein S, Milewczyk J, et 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.
Wilcox P, Croxford A, Budyn N, et 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.
Pialucha T, Pavlakovic B, Alleyne D, et al., 2020, Quantitative measurement of remnant thickness in corrosion under pipe supports, INSIGHT, Vol: 62, Pages: 642-648, ISSN: 1354-2575
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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.
Corcoran J, Davies CM, Cawley P, et 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.
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.
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.
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.
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.
Leung M, Corcoran J, Cawley P, et 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.
Liu Y, Van Pamel A, Nagy P, et 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.
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.
Combaniere J, Cawley P, McAughey K, et 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.
Cawley P, 2019, Ultrasonic structural health monitoring - current applications and potential, IEEE International Ultrasonics Symposium (IUS), Publisher: IEEE, Pages: 2107-2109, ISSN: 1948-5719
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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.
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.
Heinlein S, Cawley P, Vogt T, et 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
Budyn N, Bevan R, Croxford AJ, et 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
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.
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
Todd MD, Leung M, Corcoran J, et 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.
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