Publications
74 results found
Simillides Y, Huthwaite P, Kalkowski MK, et al., 2024, A displacement-based finite element formulation for solving elastic wave problems in coupled fluid-solid media on a GPU, Computers and Structures, Vol: 299, ISSN: 0045-7949
Ultrasonic wave propagation and scattering involving both solids and fluids underpins many key configurations in non-destructive testing and underwater acoustics. The resulting interactions are highly dependent on both material parameters and geometries and are difficult and expensive to investigate experimentally. Modelling capabilities are often used to overcome this, but these are also complex and computationally expensive due to the complexity of the fluid-solid interactions. We introduce a novel explicit time-domain finite element method for simulating ultrasonic waves interacting with fluid-solid interfaces. The method is displacement-based, and relies on classical hourglassing control, in addition to a modified time-stepping scheme to damping out shear motion in an inviscid fluid. One of the key benefits of the displacement-based approach is that nodes in the fluid have the same number of degrees of freedom as those in the solid. Therefore defining a fluid-solid model is as easy as defining an all-fluid or all-solid model, avoiding the need for any special treatments at the interfaces. It is thus compatible with typical elastodynamic finite element formulations and ready for implementation on a graphical processing unit. We verified the method across a range of problems involving millions of degrees of freedom in fields such as non-destructive testing and underwater acoustics.
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.
Bikos D, Samaras G, Cann P, et al., 2023, Destructive and non-destructive mechanical characterisation of chocolate with different levels of porosity under various modes of deformation, Journal of Materials Science, Vol: 58, Pages: 5104-5127, ISSN: 0022-2461
Chocolate exhibits a complex material response under the varying mechanical loads present during oral processing. Mechanical properties such as Young’s modulus and fracture stress are linked to sensorial attributes such as hardness. Apart from this link with hardness perception, these mechanical properties are important input parameters towards developing a computational model to simulate the first bite. This study aims to determine the mechanical properties of chocolate with different levels of micro-aeration, 0–15%, under varying modes of deformation. Therefore, destructive mechanical experiments under tension, compression, and flexure loading are conducted to calculate the Young’s modulus, yield, and fracture stress of chocolate. The values of Young’s modulus are also confirmed by independent ultrasonic mechanical experiments. The results showed that differences up to 35% were observed amongst the Young’s modulus of chocolate for different mechanical experiments. This maximum difference was found to drop with increasing porosity and a negligible difference in the Young’s modulus measurements amongst the different mechanical experiments is observed for the 15% micro-aerated chocolate. This phenomenon is caused by micro-pores obstructing the microscopic inelastic movement occurring from the early stages of the material’s deformation. This work provides a deeper understanding of the mechanical behaviour of chocolate under different loading scenarios, which are relevant to the multiaxial loading during mastication, and the role of micro-aeration on the mechanical response of chocolate. This will further assist the food industry’s understanding of the design of chocolate products with controlled and/or improved sensory perception.
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.
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.
Zuo 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
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.
Abel R, Behforootan S, Boughton O, et al., 2021, Ultrasound and Bone Disease: A Systematic Review, World Journal of Surgery and Surgical Research
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.
Zimmermann 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: 1364-5021
Quantifying 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.
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.
Hutchins DA, Huthwaite P, Davis LAJ, et al., 2020, Mid infrared tomography of polymer pipes, Journal of Nondestructive Evaluation, Vol: 39, Pages: 1-9, ISSN: 0195-9298
Mid-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.
Eckel 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: 0963-8695
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.
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.
Jones GA, Huthwaite P, 2020, Fast binary CT using Fourier null space regularization (FNSR), Inverse Problems, Vol: 36, ISSN: 0266-5611
X-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.
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.
Eckel S, Huthwaite P, Zscherpel U, et al., 2019, 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.
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.
Eckel SF, Huthwaite P, Lowe M, et al., 2018, Establishment and validation of the channelized hotelling model observer for image assessment in industrial radiography, NDT and E International, Vol: 98, Pages: 1-7, ISSN: 0963-8695
A new method for industrial radiography is presented to assess image quality objectively. The assessment is performed by a modelled observer developed to interpret radiographic images in order to rate the detectability of structural defects. For the purpose of qualifying radiographic NDE procedures, computational tools simulate the image, but should additionally automatically assess the associated image quality instead of relying on human interpretation. The Channelized Hotelling Model Observer (CHO) approach, originally developed for medical imaging, is here developed for industrial NDE applications to measure objectively the defect's detectability. A validation study based on a comparison of the model's efficiency of observing circular and elongated flaws shows that the CHO outperforms other detectability models used by industry. Furthermore, the model's reliability was verified by comparing it to psychophysical data.
Zhang C, Huthwaite P, Lowe M, 2018, Eliminating backwall effects in the phased array imaging of near backwall defects, Journal of the Acoustical Society of America, Vol: 144, Pages: 1075-1088, ISSN: 0001-4966
Ultrasonic array imaging is widely used to provide high quality defect detection and characterization. However, the current imaging techniques are poor at detecting and characterizing defects near a surface facing the array, as the signal scattered from the defect and the strong reflection from the planar backwall will overlap in both time and frequency domains, masking the presence of the defect. To address this problem, this paper explores imaging algorithms and relevant methods to eliminate the strong artefacts caused by the backwall reflection. The half-skip total focusing method (HSTFM), the factorization method (FM) and the time domain sampling method (TDSM) are chosen as the imaging algorithms used in this paper. Then, three methods, referred to as full matrix capture (FMC) subtraction, weighting function filtering, and the truncation method, are developed to eliminate or filter the effects caused by the strong backwall reflection. These methods can be applied easily with few tuning parameters or little prior knowledge. The performances of the proposed imaging techniques are validated in both simulation and experiments, and the results show the effectiveness of the developed methods to eliminate the artefacts caused by the backwall reflections when imaging near backwall defects.
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