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Journal articleZuo P, Huthwaite P, 2022,
Quantitative mapping of thickness variations along a ray path using geometrical full waveform inversion and guided wave mode conversion
, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 478, ISSN: 1364-5021 -
Journal articleSarris G, Haslinger SG, Huthwaite P, et al., 2021,
Attenuation of Rayleigh waves due to surface roughness
, Journal of the Acoustical Society of America, Vol: 149, Pages: 4298-4308, ISSN: 0001-4966Rayleigh waves are well known to attenuate due to scattering when they propagate over a rough surface. Theoretical investigations have derived analytical expressions linking the attenuation coefficient to statistical surface roughness parameters, namely, the surface's root mean squared height and correlation length and the Rayleigh wave's wavenumber. In the literature, three scattering regimes have been identified—the geometric (short wavelength), stochastic (short to medium wavelength), and Rayleigh (long wavelength) regimes. This study uses a high-fidelity two-dimensional finite element (FE) modelling scheme to validate existing predictions and provide a unified approach to studying the problem of Rayleigh wave scattering from rough surfaces as the same model can be used to obtain attenuation values regardless of the scattering regime. In the Rayleigh and stochastic regimes, very good agreement is found between the theory and FE results both in terms of the absolute attenuation values and for asymptotic power relationships. In the geometric regime, power relationships are obtained through a combination of dimensional analysis and FE simulations. The results here also provide useful insight into verifying the three-dimensional theory because the method used for its derivation is analogous.
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Journal articleAbel R, Behforootan S, Boughton O, et al.,
Ultrasound and Bone Disease: A Systematic Review
, World Journal of Surgery and Surgical Research -
Journal articleHuang M, Sha G, Huthwaite P, et al., 2021,
Longitudinal wave attenuation in polycrystals with elongated grains: 3D numerical and analytical modeling
, Journal of the Acoustical Society of America, Vol: 149, Pages: 2377-2394, ISSN: 0001-4966This work develops a second-order approximation (SOA) model and a three-dimensional (3D) finite element (FE) model to calculate scattering-induced attenuation for elastic wave propagation in polycrystals with elongated grains of arbitrary crystal symmetry. The SOA model accounts for some degree of multiple scattering, whereas the 3D FE model includes all scattering possibilities. The SOA model incorporates the accurate geometric two-point correlation function obtained from the FE material systems to enable comparative studies between the two models. Also, the analytical Rayleigh and stochastic asymptotes are presented to provide explicit insights into propagation behaviors. Quantitative agreement is found between the FE and analytical models for all evaluated cases. In particular, the FE simulations support the SOA model prediction that grain shape does not exert influence on attenuation in the Rayleigh regime and its effect emerges as frequency increases to the stochastic regime showing anisotropy in attenuation. This attenuation anisotropy intensifies with the increase in frequency, but it exhibits a complicated behavior as frequency transits into the geometric regime. Wavefield fluctuations captured from the FE simulations are provided to help observe these complex scattering behaviors. The proportionality of attenuation to elastic scattering factors is also quantitatively evaluated.
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Journal articleShipway NJ, Huthwaite P, Lowe MJS, et al., 2021,
Using ResNets to perform automated defect detection for Fluorescent Penetrant Inspection
, Independent Nondestructive Testing and Evaluation (NDT and E) International, Vol: 119, Pages: 102400-102400, ISSN: 0963-8695Fluorescent Penetrant Inspection (FPI) is a popular Non-Destructive Testing (NDT) method which is used extensively in the aerospace industry. However, the nature of FPI means results are susceptible to the effects of human factors and this can lead to variable results, making automation desirable. Previous work has investigated the use of established machine learning method Random Forest to perform automated defect detection for FPI. Whilst good results were obtained, there was still a significant number of false positives being identified as defective. This paper presents work done to investigate the potential of using deep learning methods to perform automated defect detection.A dataset was obtained from a set of 99 titanium alloy test pieces with cracks induced using thermal fatigue loading. These test pieces were repeatedly processed and using data augmentation a large dataset was obtained. This data was used to train a ResNet34 and ResNet50 architecture as well as a Random Forest. Two significant results were obtained. Firstly, the ResNet50 is able to create a network capable of detecting 95% of defects with a false call rate of 0.07. This result far exceeded that obtained using the Random Forest method despite both methods only having access to a small dataset. This demonstrated the strong capability of deep learning architectures. The second result was that increasing the amount of data obtained from non defective regions significantly increases performance. This result is encouraging as this data, obtained from non-cracked parts, can be quickly and cheaply obtained by reprocessing test pieces.
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Journal articleZimmermann A, Huthwaite P, Pavlakovic B, 2021,
High-resolution thickness maps of corrosion using SH1 guided wave tomography
, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 477, ISSN: 1364-5021Quantifying corrosion damage is vital for the petrochemical industry, and guided wave tomography can provide thickness maps of such regions by transmitting guided waves through these areas and capturing the scattering information using arrays. The dispersive nature of the guided waves enables a reconstruction of wave velocity to be converted into thickness. However, existing approaches have been shown to be limited in in-plane resolution, significantly short of that required to accurately image a defect target of three times the wall thickness (i.e. 3 T) in each in-plane direction. This is largely due to the long wavelengths in the fundamental modes commonly used, being around 4 T for both A0 and S0 at the typical operation points. In this work, the suitability of the first-order shear-horizontal guided wave mode, SH1, has been investigated to improve the resolution limit. The wavelength at the desired operating point is significantly shorter, enabling an improvement in resolution of around 2.4 times. This is first verified by realistic finite-element simulations and then validated by experimental results, confirming the improved resolution limit can now allow defects of maximum extent 3T-by-3T to be reliably detected and sized, i.e. a long-pursued goal of guided wave tomography has been achieved.
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Journal articleHaslinger SG, Lowe MJS, Craster R, et al., 2021,
Prediction of reflection amplitudes for ultrasonic inspection of rough planar defects
, Insight, Vol: 63, Pages: 28-36, ISSN: 2156-485XThe characteristics of planar defects (no loss of material volume) that arise during industrial plant operation are difficult to predict in detail, yet these can affect the performance of non-destructive testing (NDT) used to manage plant structural integrity. Inspection modelling is increasingly used to design and assess ultrasonic inspections of such plant items. While modelling of smooth planar defects is relatively mature and validated, issues have remained in the treatment of rough planar defect species. The qualification of ultrasonic inspections for such defects is presently very conservative, owing to the uncertainty of the amplitudes of rough surface reflections. Pragmatic solutions include the addition of large sensitivity thresholds and more frequent inspection intervals, which require more plant downtime. In this article, an alternative approach has been developed by the authors to predict the expected surface reflection from a rough defect using a theoretical statistical model. Given only the frequency, angle of incidence and two statistical parameter values used to characterise the defects, the expected reflection amplitude is obtained rapidly for any scattering angle and size of defect, for both compression and shear waves. The method is applicable for inspections of isotropic media that feature surface reflections such as pulse-echo or pitch-catch, rather than for tip signal-dependent techniques such as time-of-flight diffraction. The potential impact for inspection qualification is significant, with the new model predicting increases of up to 20 dB in signal amplitude in comparison with models presently used in industry. All mode conversions are included and rigorous validations using numerical and experimental methods have been performed. The model has been instrumental in obtaining new statistically significant results related to the effect of tilt; the expected pulse-echo backscattered amplitude for very rough planar defects is independent of til
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Conference paperWest G, Harris E, Lowe M, et al., 2021,
Multi-band finite element simulation of ultrasound attenuation by soft tissue
, IEEE International Ultrasonics Symposium (IEEE IUS), Publisher: IEEE, ISSN: 1948-5719 -
Journal articleHuang M, Sha G, Huthwaite P, et al., 2020,
Elastic wave velocity dispersion in polycrystals with elongated grains: Theoretical and numerical analysis
, Journal of the Acoustical Society of America, Vol: 148, Pages: 3645-3662, ISSN: 0001-4966The phase velocity dispersion of longitudinal waves in polycrystals with elongated grains of arbitrary crystallographic symmetry is studied in all frequency ranges by the theoretical second-order approximation (SOA) and numerical three-dimensional finite element (FE) models. The SOA and FE models are found to be in excellent agreement for three studied polycrystals: cubic Al, Inconel, and a triclinic material system. A simple Born approximation for the velocity, not containing the Cauchy integrals, and the explicit analytical quasi-static velocity limit (Rayleigh asymptote) are derived. As confirmed by the FE simulations, the velocity limit provides an accurate velocity estimate in the low-frequency regime where the phase velocity is nearly constant on frequency; however, it exhibits dependence on the propagation angle. As frequency increases, the phase velocity increases towards the stochastic regime and then, with further frequency increase, behaves differently depending on the propagation direction. It remains nearly constant for the wave propagation in the direction of the smaller ellipsoidal grain radius and decreases in the grain elongation direction. In the Rayleigh and stochastic frequency regimes, the directional velocity change shows proportionalities to the two elastic scattering factors even for the polycrystal with the triclinic grain symmetry.
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Journal articleHuang M, Sha G, Huthwaite P, et al., 2020,
Maximizing the accuracy of finite element simulation of elastic wave propagation in polycrystals
, Journal of the Acoustical Society of America, Vol: 148, Pages: 1890-1910, ISSN: 0001-4966Three-dimensional finite element (FE) modelling, with representation of materials at grain scale in realistic sample volumes, is capable of accurately describing elastic wave propagation and scattering within polycrystals. A broader and better future use of this FE method requires several important topics to be fully understood, and this work presents studies addressing this aim. The first topic concerns the determination of effective media parameters, namely, scattering induced attenuation and phase velocity, from measured coherent waves. This work evaluates two determination approaches, through-transmission and fitting, and it is found that these approaches are practically equivalent and can thus be used interchangeably. For the second topic of estimating modelling errors and uncertainties, this work performs thorough analytical and numerical studies to estimate those caused by both FE approximations and statistical considerations. It is demonstrated that the errors and uncertainties can be well suppressed by using a proper combination of modelling parameters. For the last topic of incorporating FE model information into theoretical models, this work presents elaborated investigations and shows that to improve agreement between the FE and theoretical models, the symmetry boundary conditions used in FE models need to be considered in the two-point correlation function, which is required by theoretical models.
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Journal articleHutchins DA, Huthwaite P, Davis LAJ, et al., 2020,
Mid infrared tomography of polymer pipes
, Journal of Nondestructive Evaluation, Vol: 39, Pages: 1-9, ISSN: 0195-9298Mid-infrared signals in the 2–5 μm wavelength range have been transmitted through samples of polymer pipes, as commonly used in the water supply industry. It is shown that simple through-transmission images can be obtained using a broad spectrum source and a suitable camera. This leads to the possibility of tomography, where images are obtained as the measurement system is rotated with respect to the axis of the pipe. The unusual 3D geometry created by a source of finite size and the imaging plane of a camera, plus the fact that refraction at the pipe wall would cause significant ray bending, meant that the reconstruction of tomographic images had to be considered with some care. A result is shown for a thinning defect on the inner wall of a polymer water pipe, demonstrating that such changes can be reconstructed successfully.
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Journal articleCawley 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: 1475-9217Monitoring 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.
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Journal articleCorcoran J, Leinov E, Jeketo A, et al., 2020,
A guided wave inspection technique for wedge features
, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol: 67, Pages: 997-1008, ISSN: 0885-3010Numerous 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.
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Journal articleSha G, Huang M, Lowe MJS, et 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: 0001-4966Better 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.
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Journal articleCorcoran 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: 0018-9456Potential 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.
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Journal articleParra-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: 0963-8695The 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.
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Journal articleHaslinger SG, Lowe MJS, Huthwaite P, et al., 2020,
Elastic shear wave scattering by randomly rough surfaces
, Journal of the Mechanics and Physics of Solids, Vol: 137, Pages: 1-20, ISSN: 0022-5096Characterizing 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.
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Journal articleEckel S, Zscherpel U, Huthwaite P, et 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: 0963-8695Extracting 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.
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Journal articleJones GA, Huthwaite P, 2020,
Fast binary CT using Fourier null space regularization (FNSR)
, Inverse Problems, Vol: 36, ISSN: 0266-5611X-ray CT is increasingly being adopted in manufacturing as a non destructive inspection tool. Traditionally, industrial workflows follow a two step procedure of reconstruction followed by segmentation. Such workflows suffer from two main problems: (1) the reconstruction typically requires thousands of projections leading to increased data acquisition times. (2) The application of the segmentation process a posteriori is dependent on the quality of the original reconstruction and often does not preserve data fidelity. We present a fast iterative x-ray CT method which simultaneously reconstructs and segments an image from a limited number of projections called Fourier null space regularization (FNSR). The novelty of the approach is in the explicit updating of the image null space with values derived from a regularized image from the previous iteration, thus compensating for any missing projections and effectively regularizing the reconstruction. The speed of the method is achieved by directly applying the Fourier Slice Theorem where the non-uniform fast Fourier transform (NUFFT) is used to compute the frequency spectrum of the projections at their positions in the image k-space. At each iteration a segmented image is computed which is used to populate the null values of the image k-space effectively steering the reconstruction towards a binary solution. The effectiveness of the method to generate accurate reconstructions is demonstrated and benchmarked against other iterative reconstruction techniques using a series of numerical examples. Finally, FNSR is validated using industrial x-ray CT data where accurate reconstructions were achieved with 18 or more projections, a significant reduction from the 5000 needed by filtered back projection.
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Journal articleMariani 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: 0885-3010In 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.
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