Publications
321 results found
Georgiades E, Lowe MJS, Craster RV, 2024, Computing leaky Lamb waves for waveguides between elastic half-spaces using spectral collocation., J Acoust Soc Am, Vol: 155, Pages: 629-639
In non-destructive evaluation guided wave inspections, the elastic structure to be inspected is often embedded within other elastic media and the ensuing leaky waves are complex and non-trivial to compute; we consider the canonical example of an elastic waveguide surrounded by other elastic materials that demonstrates the fundamental issues with calculating the leaky waves in such systems. Due to the complex wavenumber solutions required to represent them, leaky waves pose significant challenges to existing numerical methods, with methods that spatially discretise the field to retrieve them suffering from the exponential growth of their amplitude far into the surrounding media. We present a spectral collocation method yielding an accurate and efficient identification of these modes, leaking into elastic half-spaces. We discretise the elastic domains and, depending on the exterior bulk wavespeeds, select appropriate mappings of the discretised domain to complex paths, in which the numerical solution decays and the physics of the problem are preserved. By iterating through all possible radiation cases, the full set of dispersion and attenuation curves are successfully retrieved and validated, where possible, against the commercially available software disperse. As an independent validation, dispersion curves are obtained from finite element simulations of time-dependent waves using Fourier analysis.
Kalkowski MK, Lowe MJS, Samaitis V, et al., 2023, Weld map tomography for determining local grain orientations from ultrasound, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 479, ISSN: 1364-5021
Yeoh WY, Lan B, Lowe MJS, 2023, Investigation of the influence of macrozones in titanium alloys on the propagation and scattering of ultrasound, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 479, ISSN: 1364-5021
The presence of macrozones (or micro-textured regions) in Ti-6Al-4V (Ti-64) was shown to be a potential cause to the onset of cold dwell fatigue which reduces fatigue life significantly. Past research has demonstrated the potential of using ultrasonic testing for macrozone characterization, with the variation of ultrasound attenuation, backscatter and velocity in the presence of macrozones. However, due to the complexity of the microstructure, some physical phenomena that were observed are still not well understood. In this study, we propose the use of finite-element polycrystalline models to provide us with a means to systematically study the wave-macrozone interaction. Through this investigation performed using two-dimensional models, we are able to identify important correlations between macrozone characteristics (size, shape and texture) and ultrasound responses (attenuation, backscatter and velocity). The observed behaviours are then validated experimentally, and we also highlight how this understanding can potentially aid with the characterization of macrozones in Ti-64 samples.
Sarris G, Haslinger SG, Huthwaite P, et al., 2023, Attenuation of Rayleigh waves due to three-dimensional surface roughness: a comprehensive numerical evaluation, Journal of the Acoustical Society of America, Vol: 154, Pages: 808-818, ISSN: 0001-4966
The phenomenon of Rayleigh wave attenuation due to surface roughness has been well studied theoretically in the literature. Three scattering regimes describing it have been identified-the Rayleigh (long wavelength), stochastic (medium wavelength), and geometric (short wavelength)-with the attenuation coefficient exhibiting a different behavior in each. Here, in an extension to our previous work, we gain further insight with regard to the existing theory, in three dimensions, using finite element (FE) modeling, under a unified approach, where the same FE modeling techniques are used regardless of the scattering regime. We demonstrate good agreement between our FE results and the theory in all scattering regimes. Additionally, following this demonstration, we extend the results to cases that lie outside the limits of validity of the theory.
Kalkowski MK, Bézi Z, Lowe MJS, et al., 2023, Supporting imaging of austenitic welds with finite element welding simulation— which parameters matter?, Applied Sciences, Vol: 13, Pages: 7448-7448, ISSN: 2076-3417
The basic principle of ultrasound is to relate the time of flight of a received echo to the location of a reflector, assuming a known and constant velocity of sound. This assumption breaks down in austenitic welds, in which a microstructure with large oriented austenitic grains induces local velocity differences resulting in deviations of the ultrasonic beam. The inspection problem is further complicated by scattering at grain boundaries, which introduces structural noise and attenuation. Embedding material information into imaging algorithms usually improves image quality and aids interpretation. Imaging algorithms can take the weld structure into account if it is known. The usual way to obtain such information is by metallurgical analysis of slices of a representative mock-up fabricated using the same materials and welding procedures as in the actual component. A non-destructive alternative to predict the weld structure is based on the record of the welding procedure, using either phenomenological models or the finite element method. The latter requires detailed modelling of the welding process to capture the weld pool and the microstructure formation. Several parameters are at play, and uncertainties intrinsically affect the process owing to the limited information available. This paper reports a case study aiming to determine the most critical parameters and levels of complexity of the weld formation models from the perspective of ultrasonic imaging. By combining state-of-the-art welding simulation with time-domain finite element prediction of ultrasound in complex welds, we assess the impact of the modelling choices on the offset and spatial spreading of defect signatures. The novelty of this work is in linking welding simulation with ultrasonic imaging and quantifying the effect of the common assumptions in solidification modelling from the non-destructive examination perspective. Both aspects have not been explored in the literature to date since solidificati
Sarris G, Haslinger SG, Huthwaite P, et al., 2023, Ultrasonic methods for the detection of near surface fatigue damage, Independent Nondestructive Testing and Evaluation (NDT and E) International, Vol: 135, Pages: 1-13, ISSN: 0963-8695
Fatigue zones in a material can be identified using ultrasonic waves, as it has been shown that their propagation speed will reduce when travelling through such a zone. However, as fatigue damage is usually concentrated in a thin near-surface layer, through-thickness measurements result in very small changes of the average propagation speed across the full thickness, which are potentially difficult to reliably correlate to specific fatigue states. In this study, we have completed fatigue state assessments using Rayleigh waves, which travel on the surface of a material, to maximise those changes. We found that the use of Rayleigh waves amplifies the changes in speed, after propagation in the damaged region, by a factor of up to ten. The monotonic nature of the reduction in wave speed was verified against the theory using dislocation density measurements. Finally, a stiffness-reducing finite-element modelling technique, able to capture the effects of fatigue on the time of flight of longitudinal bulk and Rayleigh waves, was also derived and verified against the experimental measurements.
Sarris G, Haslinger SG, Huthwaite P, et al., 2023, Fatigue state characterization of steel pipes using ultrasonic shear waves, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 70, Pages: 72-80, ISSN: 0885-3010
The phenomenon of the reduction in the propagation speed of an ultrasonic wave when it travels through a fatigue zone has been well-studied in the literature. In addition, it has been established that shear waves are more severely affected by the presence of such a zone, compared with longitudinal waves. Our study uses these phenomena to develop a method able to characterize the fatigue state of steel pipes. Initially, the existing theory regarding the increased sensitivity of shear waves to the presence of fatigue is validated through measuring and comparing the change in propagation speed of both longitudinal and bulk shear waves on flat geometries, at different fatigue states. The comparison is achieved with the aid of ultrasonic speed C-scans of both longitudinal and shear waves, with the latter now being obtainable through our implementation of advances in electromagnetic acoustic transducers (EMATs) technology. EMATs have not been traditionally used for producing C-scans, and their ability do to so with adequate repeatability is demonstrated here; we show that shear wave scanning with EMATs now provides a possibility for inspection of fatigue damage on the inner surface of pressure-containing components in the nuclear power industry. We find that the change in ultrasonic wave speed is amplified when shear waves are used, with the magnitude of this amplification agreeing well with the theory. Following the verification of the theory, the use of EMATs allowed us to tailor the shear wave scanning method to pipe geometries, where C-scans with conventional piezoelectric transducers would not have been possible, with the results successfully revealing the presence of fatigue zones.
Huang M, Rokhlin SI, Lowe MJS, 2022, Appraising scattering theories for polycrystals of any symmetry using finite elements., Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 380, Pages: 20210382-20210382, ISSN: 1364-503X
This paper uses three-dimensional grain-scale finite-element (FE) simulations to appraise the classical scattering theory of plane longitudinal wave propagation in untextured polycrystals with statistically equiaxed grains belonging to the seven crystal symmetries. As revealed from the results of 10 390 materials, the classical theory has a linear relationship with the elastic scattering factor at the quasi-static velocity limit, whereas the reference FE and self-consistent (SC) results generally exhibit a quadratic relationship. As supported by the results of 90 materials, such order difference also extends to the attenuation and phase velocity, leading to larger differences between the classical theory and the FE results for more strongly scattering materials. Alternatively, two approximate models are proposed to achieve more accurate calculations by including an additional quadratic term. One model uses quadratic coefficients from quasi-static SC velocity fits and is thus symmetry-specific, while the other uses theoretically determined coefficients and is valid for any individual material. These simple models generally deliver more accurate attenuation and phase velocity (particularly the second model) than the classical theory, especially for strongly scattering materials. However, the models are invalid for the attenuation of materials with negative quadratic coefficients. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 1)'.
Georgiades E, Lowe MJS, Craster RV, 2022, Leaky wave characterisation using spectral methods, JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 152, Pages: 1487-1497, ISSN: 0001-4966
Szlaszynski F, Lowe MJS, Huthwaite P, 2022, Short range pipe guided wave testing using SH0 plane wave imaging for improved quantification accuracy, Sensors, Vol: 22, Pages: 1-23, ISSN: 1424-8220
Detection and criticality assessment of defects appearing in inaccessible locations in pipelines pose a great challenge for many industries. Inspection methods which allow for remote defect detection and accurate characterisation are needed. Guided wave testing (GWT) is capable of screening large lengths of pipes from a single device position, however it provides very limited individual feature characterisation. This paper adapts Plane Wave Imaging (PWI) to pipe GWT to improve defect characterization for inspection in nearby locations such as a few metres from the transducers. PWI performance is evaluated using finite element (FE) and experimental studies, and it is compared to other popular synthetic focusing imaging techniques. The study is concerned with part-circumferential part-depth planar cracks. It is shown that PWI achieves superior resolution compared to the common source method (CSM) and comparable resolution to the total focusing method (TFM). The techniques involving plane wave acquisition (PWI and CSM) are found to substantially outperform methods based on full matrix capture (FMC) in terms of signal-to-noise ratio (SNR). Therefore, it is concluded that PWI which achieves good resolution and high SNR is a more attractive choice for pipe GWT, compared to other considered techniques. Subsequently, a novel PWI transduction setup is proposed, and it is shown to suppresses the transmission of unwanted S0 mode, which further improves SNR of PWI.
Huang M, Huthwaite P, Rokhlin S, et al., 2022, Finite-element and semi-analytical study of elastic wave propagation in strongly scattering polycrystals, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 478, Pages: 1-22, ISSN: 1364-5021
This work studies scattering-induced elastic wave attenuation and phase velocity variation in three-dimensional untextured cubic polycrystals with statistically equiaxed grains using the theoretical second-order approximation (SOA) and Born approximation models and the grain-scale finite-element (FE) model, pushing the boundary towards strongly scattering materials. The results for materials with Zener anisotropy indices A > 1 show a good agreement between the theoretical and FE models in the transition and stochastic regions. In the Rayleigh regime, the agreement is reasonable for common structural materials with 1 < A < 3.2 but it deteriorates as A increases. The wavefields and signals from FE modelling show the emergence of very strong scattering at low frequencies for strongly scattering materials that cannot be fully accounted for by the theoretical models. To account for such strong scattering at A > 1, a semi-analytical model is proposed by iterating the far-field Born approximation and optimizing the iterative coefficient. The proposed model agrees remarkably well with the FE model across all studied materials with greatly differing microstructures; the model validity also extends to the quasi-static velocity limit. For polycrystals with A < 1, it is found that the agreement between the SOA and FE results is excellent for all studied materials and the correction of the model is not needed.
Haslinger SG, Lowe MJS, Wang Z, et al., 2021, Time of flight diffraction for rough planar defects, NDT & E INTERNATIONAL, Vol: 124, ISSN: 0963-8695
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Liu Y, Kalkowski MK, Huang M, et al., 2021, Can ultrasound attenuation measurement be used to characterise grain statistics in castings?, Ultrasonics, Vol: 115, Pages: 106441-106441, ISSN: 0041-624X
Industrial inspection protocols are qualified using mock-ups manufactured according to the same procedure as the plant part. For coarse-grained castings, known for their low inspectability, relying on mock-ups becomes particularly challenging owing to the variability of grain properties among components. Consequently, there is a keen interest in the capability to verify whether the grain size of the component under test matches the qualification specification in-situ. This paper investigates the potential of an attenuation measurement for assessing the ultrasonic inspectability of coarse-grained components using qualified procedures in a practical setting. The experimental part of the study focuses on an industrial Inconel 600 mock-up with spatially varying attenuation, measured across the entire sample in an immersion tank. Three zones with distinctly different attenuations were examined using metallography, which allowed for calculating classical grain size histograms and two-point correlation functions. For one of the zones, we synthesised the microstructure with the same statistical properties numerically and simulated the propagation of ultrasound using a grain-scale finite element model. The results showed good agreement with the experiment, and lead to several suggestions for the reasons for the discrepancy, the varying grain size statistics being the most likely. A parametric study, which followed, depicted the effect of the mean and standard deviation-to-mean ratio of the log-normal grain size distribution on the attenuation of ultrasound and its frequency dependence. Most notably, we demonstrated the known non-uniqueness of the relationship between the log-normal grain size distribution parameters and the attenuation. We suggested that the correlation length calculated from a single exponential fit to the two-point correlation function is a more robust metric describing grain statistics for this context, which can be obtained from attenuation. The correlat
Kalkowski MK, Lowe MJS, Barth M, et al., 2021, How does grazing incidence ultrasonic microscopy work? A study based on grain-scale numerical simulations, Ultrasonics, Vol: 114, ISSN: 0041-624X
Grazing incidence ultrasonic microscopy (GIUM) is an experimental method for visualising the microstructures of polycrystals with local preferential orientations. It has previously been demonstrated on an austenitic stainless steel weld, exposing grains much smaller than the propagating wavelength, but the physical mechanism of the method has only been proposed as a hypothesis. In this paper, we use grain-scale finite element simulations based on the EBSD measurements to verify the principles behind GIUM images further and to assess how deep does the method penetrate the component under examination. The simulations indicate that while lateral contraction of grains contains microstructure signatures, the free surface effect is the crucial factor contributing to the generation of the images. Further, we show that only features up to the depth in the order of the average grain size in that direction can be visualised.
Sarris 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: 0001-4966
Rayleigh 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.
Huang M, Rokhlin S, Lowe MJS, 2021, Finite element evaluation of a simple model for elastic waves in strongly scattering elongated polycrystals, JASA Express Letters, Vol: 1, Pages: 1-8, ISSN: 2691-1191
A simple semi-analytical model for longitudinal scattering-induced attenuation and phase velocity is proposed for strongly scattering cubic polycrystals with statistically elongated grains. It is formulated by iterating the Born approximation of the far-field approximation model and by empirically increasing the coefficient in the quadratic term for the elastic scattering factor. The comparison with the three-dimensional grain-scale finite element calculations shows excellent performance of the semi-analytical model for both attenuation and phase velocity in all studied frequency ranges and especially in the Rayleigh regime in which, for strongly scattering materials, the existing analytical models significantly disagree with the numerical results.
Huang 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: 0001-4966
This 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.
Shipway 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: 0963-8695
Fluorescent 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.
Haslinger 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: 2156-485X
The 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
West 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
Huang 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: 0001-4966
The 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.
Huang 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: 0001-4966
Three-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.
Corcoran 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: 0885-3010
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.
Sha 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: 0001-4966
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.
Haslinger 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: 0022-5096
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.
Haslinger SG, Lowe MJS, Huthwaite P, et 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.
Elliott JB, Lowe MJS, Huthwaite P, et 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
There is a constant drive within the nuclear power industry to improve upon the characterization capabilities of current ultrasonic inspection techniques in order to improve safety and reduce costs. Particular emphasis has been placed on the ability to characterize very small defects which could result in extended component lifespan and help reduce the frequency of in-service inspections. Super-resolution (SR) algorithms, also known as sampling methods, have been shown to demonstrate the capability to resolve scatterers separated by less than the diffraction limit when deployed in representative inspections and therefore could be used to tackle this issue. In this paper, the factorization method (FM) and the Time Reversal Multiple-Signal-Classification (TR-MUSIC) algorithms are applied to the simulated ultrasonic array inspection of small rough embedded planar defects to establish their characterization capabilities. Their performance was compared to the conventional total focusing method (TFM). A full 2-D finite-element (FE) Monte Carlo modeling study was conducted for defects with a range of sizes, orientations, and magnitude of surface roughness. The results presented show that for subwavelength defects, both the FM and TR-MUSIC algorithms were able to size and estimate defect orientation accurately for smooth cases and, for rough defects, up to a roughness of 100 μm. This level of roughness is representative of the thermal fatigue defects encountered in the nuclear power sector. This contrasted with the relatively poor performance of TFM in these cases which consistently oversized these defects and could not be used to estimate the defect orientation, making through-wall sizing with this method impossible.
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
Shipway NJ, Huthwaite P, Lowe MJS, et 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.
Shipway N, Barden T, Huthwaite P, et al., 2019, Automated defect detection for Fluorescent Penetrant Inspection using Random Forest, NDT and E International, Vol: 101, Pages: 113-123, ISSN: 0963-8695
Fluorescent Penetrant Inspection (FPI) is the most widely used NDT method in the aerospace industry. Inspection of FPI is currently done visually and difficulties arise distinguishing between penetrant associated with defects and that due to insufficient wash-off or geometrical indications. This, in addition to the nature of the inspection process, means inspection is largely influenced by human factors. The ability to perform automated inspection would provide increased consistency, reliability and productivity.The Random Forest algorithm was used to detect defects in a number of flat titanium plates which had been processed with FPI and photographed to produce digital images. This method has demonstrated the ability to correctly distinguish between defects and other non-relevant indications with accuracy comparable to a human inspector with a very small number of training examples. These results show the potential for the Random Forest algorithm to be used to detect defects in aerospace components, allowing the entire FPI line to become autonomous.
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