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  • Journal article
    Corcoran J, Leinov E, Jeketo A, Lowe MJSet al., 2020,

    A guided wave inspection technique for wedge features

    , IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol: 67, Pages: 997-1008, ISSN: 0885-3010

    Numerous engineering components feature prismatic wedge-like structures that require Non-destructive Evaluation (NDE) in order to ensure functionality or safety. This paper focuses on the inspection of the wedge-like seal fins of a jet engine drum, though the capabilities presented will be generic. It is proposed that anti-symmetric flexural edge modes, feature guided waves localised to the wedge tips, may be used for defect detection. Although analytical solutions exist that characterise the ultrasonic behaviour of ideal wedges, in practise real wedges will be irregular (containing for example truncated tips, are built onto an associated structure or have non straight edges) and therefore generic methodologies are required to characterise wave behaviour in non-ideal wedges. This paper uses a semi-analytical finite element (SAFE) methodology to characterise guided waves in wedge-like features with irregular cross-sections to assess their suitability for NDE inspection and compare them to edge modes in ideal wedges. The science and methodologies required in this paper are necessary to select an appropriate operating frequency for the particular application at hand. Additionally, this paper addresses the practical challenge of excitation and detection of flexural edge modes by presenting a piezoelectric based dry-coupled transducer system suitable for pulse-echo operation. The paper therefore presents the scientific basis required for industrial exploitation, together with the practical tools that facilitate use. The study concludes with the experimental demonstration of the edge wave based inspection of a seal fin, achieving a signal-to-noise ratio of 28 dB from a 0.75 mm radial tip defect.

  • Journal article
    Sha G, Huang M, Lowe MJS, Rokhlin SIet al., 2020,

    Attenuation and velocity of elastic waves in polycrystals with generally anisotropic grains: Analytic and numerical modeling.

    , Journal of the Acoustical Society of America, Vol: 147, Pages: 2442-2465, ISSN: 0001-4966

    Better understanding of elastic wave propagation in polycrystals has interest for applications in seismology and nondestructive material characterization. In this study, a second-order wave propagation (SOA) model that considers forward multiple scattering events is developed for macroscopically isotropic polycrystals with equiaxed grains of arbitrary anisotropy (triclinic). It predicts scattering-induced wave attenuation and dispersion of phase velocity. The SOA model implements the generalized two-point correlation (TPC) function, which relates to the actual numeric TPC of simulated microstructure. The analytical Rayleigh and stochastic asymptotes for both attenuation and phase velocity are derived for triclinic symmetry grains, which elucidate the effects of the elastic scattering factors and the generalized TPC in different frequency regimes. Also, the computationally efficient far field approximation attenuation model is obtained for this case; it shows good agreement with the SOA model in all frequency ranges. To assess the analytical models, a three-dimensional (3D) finite element (FE) model for triclinic polycrystals is developed and implemented on simulated 3D triclinic polycrystalline aggregates. Quantitative agreement is observed between the analytical and the FE simulations for both the attenuation and phase velocity. Also, the quasi-static velocities obtained from the SOA and FE models are in excellent agreement with the static self-consistent velocity.

  • 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
    Parra-Raad J, Khalili P, Cegla F, 2020,

    Shear waves with orthogonal polarisations for thickness measurement and crack detection using EMATs

    , NDT & E International, Vol: 111, Pages: 1-7, ISSN: 0963-8695

    The use of polarised shear waves to detect the presence of crack-like defects seems to have received little or no attention in the past. The authors believe that the main reason for this appears to be the lack of a device with the capability to excite shear waves of different polarisations. In this paper, the authors, first, present the design of an EMAT that permits the excitation of two orthogonally polarised shear waves in metallic materials by means of two coils that are orthogonal with respect to each other. This is then followed by a 3D finite element analysis of the wavefield generated by the EMAT and its interactions with crack-like defects of different sizes, positions and orientations. Then a methodology of how this EMAT can be used to simultaneously measure material thickness and detect crack-like defects in pulse-echo mode is introduced. Good agreement between the finite element simulation and experimental results was observed which makes the presented technique a potential new method for simultaneous thickness measurements and crack detection.

  • Journal article
    Eckel S, Zscherpel U, Huthwaite P, Paul N, Schumm Aet al., 2020,

    Radiographic film system classification and noise characterisation by a camera-based digitisation procedure

    , Independent Nondestructive Testing and Evaluation (NDT and E) International, Vol: 111, Pages: 1-9, ISSN: 0963-8695

    Extracting statistical characteristics from radiographic films is vital for film system classification and contrast sensitivity evaluation and serves as a basis for film noise simulation. A new method for digitising radiographic films in order to extract these characteristics is presented. The method consists of a camera-based setup and image processing procedure to digitise films. Correct optical density values and granularity can be extracted from the digitised images, which are equal to results obtained by standardised measurement procedures. Specific statistical characteristics of film noise are theoretically derived and subsequently verified by the obtained data, including characteristics such as Gaussianity and spatial spectral characteristics of the optical density fluctuations. It is shown that the presented method correctly measures the granularity of film noise and can therefore replace time-consuming microdensitometer measurements traditionally required for film system classifications. Additionally, the inherent unsharpness of film systems was investigated and compared with literature data. This comparison serves as another validation approach of the presented method.

  • Journal article
    Haslinger SG, Lowe MJS, Huthwaite P, Craster RV, Shi Fet al., 2020,

    Elastic shear wave scattering by randomly rough surfaces

    , Journal of the Mechanics and Physics of Solids, Vol: 137, Pages: 1-20, ISSN: 0022-5096

    Characterizing cracks within elastic media forms an important aspect of ultrasonic non-destructive evaluation (NDE) where techniques such as time-of-flight diffraction and pulse-echo are often used with the presumption of scattering from smooth, straight cracks. However, cracks are rarely straight, or smooth, and recent attention has focussed upon rough surface scattering primarily by longitudinal wave excitations.We provide a comprehensive study of scattering by incident shear waves, thus far neglected in models of rough surface scattering despite their practical importance in the detection of surface-breaking defects, using modelling, simulation and supporting experiments. The scattering of incident shear waves introduces challenges, largely absent in the longitudinal case, related to surface wave mode-conversion, the reduced range of validity of the Kirchhoff approximation (KA) as compared with longitudinal incidence, and an increased importance of correlation length.The expected reflection from a rough defect is predicted using a statistical model from which, given the angle of incidence and two statistical parameters, the expected reflection amplitude is obtained instantaneously for any scattering angle and length of defect. If the ratio of correlation length to defect length exceeds a critical value, which we determine, there is an explicit dependence of the scattering results on correlation length, and we modify the modelling to find this dependence. The modelling is cross-correlated against Monte Carlo simulations of many different surface profiles, sharing the same statistical parameter values, using numerical simulation via ray models (KA) and finite element (FE) methods accelerated with a GPU implementation. Additionally we provide experimental validations that demonstrate the accuracy of our predictions.

  • Journal article
    Jones GA, Huthwaite P, 2020,

    Fast binary CT using Fourier null space regularization (FNSR)

    , INVERSE PROBLEMS, Vol: 36, ISSN: 0266-5611
  • Journal article
    Herdovics B, Cegla F, 2020,

    Long-term stability of guided wave electromagnetic acoustic transducer systems

    , Structural Health Monitoring, Vol: 19, Pages: 3-11, ISSN: 1475-9217

    This article evaluates the long-term stability of a Lorentz force guided wave electromagnetic acoustic transducer. The specific application of the investigated electromagnetic acoustic transducer is pipeline health monitoring using low-frequency (27 kHz) long-range torsional guided waves. There is a concern that repeated swings in the temperature of the structure can cause irreversible changes in the transduction mechanism and therefore pose a risk to the long-term stability of transducers. In this article we report on guided wave signals acquired on a custom-built transducer while it was exposed to more than 90 heating cycles. The highest temperature that was reached during cycling was 80°C and the measurements were acquired over a 14-month period. At the end of the 1-year period, the transducer phase had changed by 23.32° and its amplitude by 3.7%. However, this change was not gradual and most of the change occurred early on, before the highest temperature was first reached in the temperature cycling process. The observed change after this was 6.08° phase shift and 0.9% amplitude change. The possible sources of output changes were investigated, and it was found that the mechanical properties of the contact layer between the electromagnetic acoustic transducer and the pipe surface was very important. A soft silicone interlayer performed best and was able to reduce temperature-induced phase changes in the monitored signals from a maximum of 80 degrees phase change to about 20 degrees phase change, a fourfold reduction.

  • 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
    Pesaresi L, Fantetti A, Cegla F, Salles L, Schwingshackl CWet al., 2020,

    On the use of ultrasound waves to monitor the local dynamics of friction joints

    , Experimental Mechanics, Vol: 60, Pages: 129-141, ISSN: 0014-4851

    Friction joints are one of the fundamental means used for the assembly of structural components in engineering applications. The structural dynamics of these components becomes nonlinear, due to the nonlinear nature of the forces arising at the contact interface characterised by stick-slip phenomena and separation. Advanced numerical models have been proposed in the last decades which have shown some promising capabilities in capturing these local nonlinearities. However, despite the research efforts in producing more advanced models over the years, a lack of validation experiments made it difficult to have fully validated models. For this reason, experimental techniques which can provide insights into the local dynamics of joints can be of great interest for the refinement of such models and for the optimisation of the joint design and local wear predictions. In this paper, a preliminary study is presented where ultrasound waves are used to characterise the local dynamics of friction contacts by observing changes of the ultrasound reflection/transmission at the friction interface. The experimental technique is applied to a dynamic friction rig, where two steel specimens are rubbed against each other under a harmonic tangential excitation. Initial results show that, with a controlled experimental test procedure, this technique can identify microslip effects at the contact interface.

  • Journal article
    Shi F, Huthwaite P, 2019,

    Waveform-based geometrical inversion of obstacles

    , Physical Review Applied, Vol: 12, Pages: 1-15, ISSN: 2331-7019

    Full-waveform inversion (FWI) can produce previously unobtainable levels of accuracy and is revolutionizing the field of wave imaging. The basic principle is that a numerically produced data set is matched to the measured waveforms, enabling a high-resolution image to be produced since the model being inverted fully captures the physical behavior without approximation. This is achieved by gradually updating the numerical model using optimization algorithms. Currently, most FWI methods aim to recover material properties of a medium containing penetrable scatterers; however, there are many applications that, instead, require the boundary shapes of impenetrable objects to be reconstructed. Conventional velocity-style FWI will be trapped in local minima, with such problems being due to the extremely sharp contrast at the boundary. We propose a FWI procedure to directly recover the geometrical parameters of impenetrable obstacles via shape optimizations. The geometry is reconstructed by iteratively deforming the boundary of the target, following the negative direction of the geometrical boundary gradient. The boundary gradient is calculated from the shape derivatives of mass and stiffness matrices of a finite-element (FE) representation, when distorting the elements attached at the boundary. In addition, multiple-scattering events, which are more likely to occur between impenetrable obstacles, can be utilized automatically to provide substantial information for the inversion. Numerical and experimental results are shown to demonstrate the accuracy of the procedure for an example taken from the field of nondestructive evaluation, giving sizing within fractions of a wavelength for the tested cases; this step change in accuracy could be critical in sizing defects, enabling significantly more reliable decisions to be made about whether it is safe to continue using a component. Mathematical derivations and physical reasons for the success of our approach are illustrated.

  • 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
    Eckel S, Huthwaite P, Zscherpel U, Schumm A, Paul Net al., 2020,

    Realistic film noise generation based on experimental noise spectra

    , IEEE Transactions on Image Processing, Vol: 29, Pages: 2987-2998, ISSN: 1057-7149

    Generating 2D noise with local, space-varying spectral characteristics is vital where random noise fields with spatially heterogeneous statistical proper-ties are observed and need to be simulated. A realistic, non-stationary noise generator relying on experimental data is presented. That generator is desired in areas such as photography and radiography. For example, before performing actual X-ray imaging in practice, output imag-es are simulated to assess and improve setups. For that purpose, realistic film noise modelling is crucial because noise downgrades the detectability of visual signals. The presented film noise synthesiser improves the realism and value of radiographic simulations significantly, allowing more realistic assessments of radiographic test setups. The method respects space-varying spectral characteristics and probability distributions, locally simulating noise with re-alistic granularity and contrast. The benefits of this ap-proach are to respect the correlation between noise and image as well as internal correlation, the fast generation of any number of unique noise samples, the exploitation of real experimental data, and its statistical non-stationarity. The combination of these benefits is not available in exist-ing work. Validation of the new technique was undertaken in the field of industrial radiography. While applied to that field here, the technique is general and can also be utilised in any other field where the generation of 2D noise with local, space-varying statistical properties is necessary.

  • Journal article
    Haslinger SG, Lowe MJS, Huthwaite P, Craster R, Shi Fet al., 2019,

    Appraising Kirchhoff approximation theory for the scattering of elastic shear waves by randomly rough defects

    , Journal of Sound and Vibration, Vol: 460, Pages: 1-16, ISSN: 0022-460X

    Rapid and accurate methods, based on the Kirchhoff approximation (KA), are developed to evaluate the scattering of shear waves by rough defects and quantify the accuracy of this approximation. Defect roughness has a strong effect on the reflection of ultrasound, and every rough defect has a different surface, so standard methods of assessing the sensitivity of inspection based on smooth defects are necessarily limited. Accurately resolving rough cracks in non-destructive evaluation (NDE) inspections often requires shear waves since they have higher sensitivity to surface roughness than longitudinal waves. KA models are attractive, since they are rapid to deploy, however they are an approximation and it is important to determine the range of validity for the scattering of ultrasonic shear waves; this range is found here. Good agreement between KA and high fidelity finite element simulations is obtained for a range of incident/scattering angles, and the limits of validity for KA are found to be much stricter than for longitudinal wave incidence; as the correlation length of rough surfaces is reduced to the order of the incident shear wavelength, a combination of multiple scattering and surface wave mode conversion leads to KA predictions diverging from those of the true diffuse scattered fields.

  • 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
    Corcoran J, Nagy P, 2019,

    Passive thermoelectric power monitoring for material characterisation

    , Structural Health Monitoring, Vol: 18, Pages: 1915-1927, ISSN: 1475-9217

    Monitoring deterioration of material properties is important for assessing the structural integrity of engineering components, as it may indicate susceptibility to damage. This article focusses on the example of thermoelectric power measurements, which are known to be indicative of thermal and irradiation embrittlement and may therefore act as a proxy metric for material integrity. A passive thermoelectric power–monitoring technique is proposed which is suitable for permanent installation on engineering components. In passive measurements, the active perturbation (in this case, the heating required to create a temperature gradient) is replaced by incidental perturbation from the environment. The reduction in the ‘signal’ amplitude associated with relying on incidental perturbations may be compensated by increasing the number of individual measurements, facilitated by the greatly reduced power demand of the passive modality. Experimental studies using a stainless steel tube as a test component demonstrate thermoelectric power accuracy of <0.03 μV/°C is achievable with temperature gradients of the order of 2°C; in many cases of practical importance, this is sufficient to track the anticipated changes in thermoelectric power associated with thermal degradation.

  • Journal article
    Cawley P, Chua C, 2019,

    Crack growth monitoring using fundamental shear horizontal guided waves

    , Structural Health Monitoring: an international journal, 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
    Elliott JB, Lowe MJS, Huthwaite P, Phillips R, Duxbury DJet al., 2019,

    Sizing Subwavelength Defects With Ultrasonic Imagery: An Assessment of Super-Resolution Imaging on Simulated Rough Defects

    , IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL, Vol: 66, Pages: 1634-1648, ISSN: 0885-3010
  • Journal article
    Egerton JS, Lowe MJS, Huthwaite P, 2019,

    Automated and antidispersive coherent and incoherent noise reduction of waveforms that contain a reference pulse

    , NDT & E INTERNATIONAL, Vol: 105, Pages: 35-45, ISSN: 0963-8695
  • Journal article
    Shipway NJ, Huthwaite P, Lowe MJS, Barden TJet al., 2019,

    Performance based modifications of random forest to perform automated defect detection for fluorescent penetrant inspection

    , Journal of Nondestructive Evaluation, Vol: 38, ISSN: 0195-9298

    The established Machine Learning algorithm Random Forest (RF) has previously been shown to be effective at performing automated defect detection for test pieces which have been processed using fluorescent penetrant inspection (FPI). The work presented here investigates three methods (two previously proposed in other fields, one novel method) of modifying the FPI RF based on the individual performance of decision trees within the RF. Evaluating based on the 2 Score, which is the harmonic mean of precision and recall which places a larger weighting on recall, it is possible to reduce the RF in size by up to 50%, improving speed and memory requirements, whilst still gain equivalent results to a full RF. Introducing a performance based weighting or retraining decision trees which fall below a certain performance level however, offers no improvement on results for the increased computation time required to implement.

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