Imperial College London

DrPeterHuthwaite

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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p.huthwaite Website

 
 
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568City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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49 results found

Egerton JS, Lowe MJS, Huthwaite P, 2019, Automated and antidispersive coherent and incoherent noise reduction of waveforms that contain a reference pulse, NDT and E International, Vol: 105, Pages: 35-45, ISSN: 0963-8695

© 2019 Fully automated and antidispersive noise reduction of a waveform containing a reference pulse is often necessary for more efficient, accurate, and reliable interpretation of signals – such as highly attenuated waves in ultrasonic NDE. We propose two methods, pulse specification (PS) and coherent enhanced pulse specification (CEPS), for improving the signal-to-noise ratio (SNR) of coherent pulse amplitudes against both incoherent and coherent noise, as well as apply complementary antidispersive pulse generation (APG) to localise the pulse upon reception. We have demonstrated PS and CEPS effectiveness, with and without APG, using two examples of ultrasonic inspection, by comparison with established methods and demonstrated improvements in SNR up to a factor of 2 (6.02 dB) and never less than 1 (0 dB) for favoured pulse specifications. This has benefits in applications such as NDE, medical ultrasonics, seismology, and acoustics. We deconvolve waveforms using a reference pulse time windowed from said waveforms. These waveforms are convolved with pulses that have centre frequencies and bandwidths that are optimally and automatically specifiable for that unprocessed waveform. We combine independent measurements of the waveforms using PS and CEPS to maximise the coherent pulse amplitude of known scatterers and minimise the noise content. CEPS also enhances the coherent scatterer amplitude to significantly greater than achieved using an established filtered time shift method – where the time shift is subjective. Unlike such time shift methods, PS and CEPS do not reduce the amplitude of the coherent pulse signals when reducing noise. Automated and antidispersive coherent and incoherent noise reduction by pulse specification has been demonstrated to be effective in ultrasonic NDE of both damping and scattering media. Suggestions for further applications of this method have already included application to ultrasonic NDE of composite media, application t

JOURNAL ARTICLE

Shipway NJ, Huthwaite P, Lowe MJS, Barden TJet al., 2019, Performance based modifications of random forest to perform automated defect detection for fluorescent penetrant inspection, Journal of Nondestructive Evaluation, Vol: 38, ISSN: 0195-9298

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

JOURNAL ARTICLE

Shipway N, Barden T, Huthwaite P, Lowe Met al., Automated defect detection for Fluorescent Penetrant Inspection using Random Forest, NDT and E International, ISSN: 0963-8695

JOURNAL ARTICLE

Eckel SF, Huthwaite P, Lowe M, Schumm A, Guérin Pet 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.

JOURNAL ARTICLE

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.

JOURNAL ARTICLE

Phillips R, Duxbury D, Huthwaite P, Lowe Met al., 2018, Simulating the ultrasonic scattering from complex surface-breaking defects with a three-dimensional hybrid model, NDT and E International, Vol: 97, Pages: 32-41, ISSN: 0963-8695

Modelling is increasingly relied on for the design and qualification of ultrasonic inspections applied to safety-critical components. Numerical methods enable the simulation of the ultrasonic interaction with realistic defect morphologies; however, the computational requirements often limit their deployment. The hybrid simulation technique, which combines semi-analytical and numerical methods, realises the potential of high fidelity numerical modelling without the limiting computational factors. The inspection of thick section components for near-backwall surface-breaking defects results in large propagation distances, making them a key application of hybrid modelling. This work presents a methodology for efficiently simulating the ultrasonic inspection of complex surface-breaking defects using a hybrid model. The model is initially verified against full numerical simulation; further validation is presented by comparison to an experimental scan over an artificially machined surface-breaking notch. The potential of the new hybrid method is then demonstrated by carrying out a Monte Carlo analysis on the scattered field from surface-breaking defects with randomly rough surfaces and the results are compared to the Kirchhoff approximation.

JOURNAL ARTICLE

Shi F, Huthwaite P, 2018, Ultrasonic wave-speed diffraction tomography with undersampled data using virtual transducers, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, ISSN: 0885-3010

Ultrasonic diffraction tomography (DT) offers a way to achieve high-resolution imaging of the wavespeed map, and hence has strong potential applications in medical diagnosis and Nondestructive evaluation (NDE). Ideal images can be obtained with a complete array of sensors surrounding the scatterer, provided that the measurement data are fully sampled in space, obeying the Nyquist criterion. Spatial undersampling causes the image to be distorted and introduce unwanted circular artefacts. In this paper we propose an iteration approach using virtual transducers to achieve high-resolution tomographic imaging with undersampled measurements. At each iteration stage, the extent constraint estimated from the shape of the object of interest is applied on the image space to obtain a regularized image, based on which the ultrasonic measurement data at virtual transducers are calculated using a forward model. The full dataset composed of original and virtual measurements is then used for tomography in the next stage. A final image with sufficiently high resolution is obtained after only a few iterations. The new imaging method yields improvements in the robustness and accuracy of ultrasonic tomography with undersampled data.We present numerical results using complicated wavespeed maps from realistic corrosion profiles. In addition, an experiment using guided ultrasonic waves is performed to further evaluate the imaging method.

JOURNAL ARTICLE

Zhang C, Huthwaite P, Lowe M, 2018, The application of the Factorization Method to the subsurface imaging of surfacebreaking cracks, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 65, Pages: 497-512, ISSN: 0885-3010

A common location for cracks to appear is at the surface of a component; at the near surface, many nondestructive evaluation techniques are available to inspect for these, but at the far surface this is much more challenging. Ultrasonic imaging is proposed to enable far surface defect detection, location, and characterization. One specific challenge here is the presence of a strong reflection from the backwall, which can often mask the relatively small response from a defect. In this paper, the factorization method (FM) is explored for the application of subsurface imaging of the surface-breaking cracks. In this application, the component has two parallel surfaces, the crack is initiated from the far side and the phased array is attached on the near side. Ideally, the pure scattered field from a defect is needed for the correct estimation of the scatterer through the FM algorithm. However, the presence of the backwall will introduce a strong specular reflection into the measured data which should be removed before applying the FM algorithm. A novel subtraction method was developed to remove the backwall reflection. The performance of the FM algorithm and this subtraction method were tested with the simulated and experimental data. The experimental results showed a good consistency with the simulated results. It is shown that the FM algorithm can generate high-quality images to provide a good detection of the crack and an accurate sizing of the crack length. The subtraction method was able to provide a good backwall reflection removal in the case of small cracks (1-3 wavelengths).

JOURNAL ARTICLE

Jones GA, Huthwaite P, 2017, Limited view X-ray tomography for dimensional measurements, NDT and E International, Vol: 93, Pages: 98-109, ISSN: 0963-8695

The growing use of complex and irregularly shaped components for safety-critical applications has increasingly led to the adoption of X-ray CT as an NDE inspection tool. Standard X-ray CT methods require thousands of projections, each regularly distributed evenly through 360∘ to produce an accurate image. The time consuming acquisition of thousands of projections can lead to significant bottlenecks. Recent developments in medical imaging driven by both increasing computational power and the desire to reduce patient X-ray exposure have led to the development of a number of limited view CT methodologies. Thus far these limited view algorithms have been applied to basic synthetic data derived from simple medical phantoms. Here, we use experimental data to rigorously test the capability of limited view algorithms to accurately reconstruct and precisely measure the dimensional features of an additive manufactured sample and a turbine blade. Our findings highlight the importance of prior information in producing accurate reconstructions capable of significantly reducing X-ray projections by at least an order of magnitude. In the turbine blade example a dramatic reduction in projections from 5000 to 24 was observed while still demonstrating the same level of accuracy as standard CT methods. The findings of the study also suggest the importance of sample complexity and the presence of sparsity in the X-ray projections in order to maximise the capabilities of these limited algorithms. With the ever increasing computational power limited view CT algorithms offer a method for reducing data acquisition time and alleviating manufacturing throughput bottlenecks without compromising image accuracy and quality.

JOURNAL ARTICLE

Egerton JS, Lowe MJS, Huthwaite P, Halai HVet al., 2017, A multiband approach for accurate numerical simulation of frequency dependent ultrasonic wave propagation in the time domain, JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 142, Pages: 1270-1280, ISSN: 0001-4966

Finite element (FE) simulations are popular for studying propagation and scattering of ultrasonic waves in nondestructive evaluation. For a large number of degrees of freedom, time domain FE simulations are much more efficient than the equivalent frequency domain solution. However, unlike frequency domain simulations, time domain simulations are often poor at representing the speed and the attenuation of waves if the material is strongly damping or highly dispersive. Here, the authors demonstrate efficient and accurate representation of propagated and scattered waves, achieved by combining a set of time domain solutions that are obtained for a set of frequency ranges known as bands, such that, in combination, the authors' multiband solution accurately represents the whole wave spectrum. Consequently, high accuracy is achieved, at minor computational cost, using a modest number of bands. The multiband technique is implemented for ultrasonic wave propagation in highly attenuating polyethylene material, using three frequency bands, and can yield a reduction in empirical acoustic properties fractional error compared with respective time domain simulations, in propagation duration, of a factor of 1.4, and in full-width-half-maximum, of a factor of 10. Last, the accuracy of this approach is further exemplified in a wave scattering simulation.

JOURNAL ARTICLE

Egerton JS, Lowe MJS, Huthwaite P, Halai HVet al., 2017, Ultrasonic attenuation and phase velocity of high-density polyethylene pipe material, JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 141, Pages: 1535-1545, ISSN: 0001-4966

Knowledge of acoustic properties is crucial for ultrasonic or sonic imaging and signal detection in nondestructive evaluation (NDE), medical imaging, and seismology. Accurately and reliably obtaining these is particularly challenging for the NDE of high-density polyethylene (HDPE), such as is used in many water or gas pipes, because the properties vary greatly with frequency, temperature, direction and spatial location. Therefore the work reported here was undertaken in order to establish a basis for such a multiparameter description. The approach is general but the study specifically addresses HDPE and includes measured data values. Applicable to any such multiparameter acoustic properties dataset is a devised regression method that uses a neural network algorithm. This algorithm includes constraints to respect the Kramers-Kronig causality relationship between speed and attenuation of waves in a viscoelastic medium. These constrained acoustic properties are fully described in a multidimensional parameter space to vary with frequency, depth, temperature, and direction. The resulting uncertainties in acoustic properties dependence on the above variables are better than 4% and 2%, respectively, for attenuation and phase velocity and therefore can prevent major defect imaging errors.

JOURNAL ARTICLE

Huthwaite P, 2017, Ultrasonic finite element simulations on GPUs with Pogo

CONFERENCE PAPER

Haith MI, Huthwaite P, Lowe MJS, 2016, Defect characterisation from limited view pipeline radiography, NDT & E INTERNATIONAL, Vol: 86, Pages: 186-198, ISSN: 0963-8695

This work presents a method of characterising pipeline defects using a small number of radiographs taken at different angles around the pipe. The method relies on knowledge of the setup geometry and use of multiple images, and does not require calibration objects to be included in the setup. It is aimed at use in situations where access is difficult such as in subsea pipeline inspections. Given a set of radiographs, a background subtraction method is used to extract defects in the images. Using a ray tracing algorithm and knowledge of the experimental setup, the range of possible locations of the defect in 3D space is then calculated. Constraints are applied on potential defect shapes and positions to further refine the defect range. The method is tested on simulated and experimental flat bottomed hole defects and simulated corrosion patch defects with lateral and axial sizes ranging from 12.5 to 33.8 mm and thickness between 3 mm and 16 mm. Results demonstrate a good, consistent ability to calculate lateral and axial defect dimensions to within ±3 mm of the true size. Defect thickness calculations are more difficult and as such errors are more significant. In most cases defect thickness is calculated to within 4 mm of the actual value, often closer. Errors in thickness are due to overestimation, meaning the calculation could be used to place a maximum limit on potential defect size rather than as an actual estimate of the thickness. This would still be useful, for example in deciding whether a defect requires further investigation.

JOURNAL ARTICLE

Huthwaite P, 2016, Eliminating incident subtraction in diffraction tomography, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1471-2946

Diffraction tomography is a powerful algorithm for producing high-resolution quantitative reconstructions across a wide range of applications. A major drawback of the method is that it operates on the scattered field, which cannot generally be directly measured, but must instead be calculated by subtracting the incident field, i.e. the equivalent field with no scatterer present. Unfortunately, often the incident field is not measurable and hence must be estimated, causing errors. This paper highlights an important, but not widely recognized, result: for particular widely used formulations of the algorithm, the subtraction of the incident field is unnecessary, and the algorithm can actually be applied directly to measured signals. The theory behind this is derived, showing that the incident field will vanish under far-field conditions, and the result is demonstrated in practice. Tests with subsampled arrays show that aliasing artefacts can appear, but can be removed with a filter at the expense of resolution. The incident field also has no effect for a variety of array configurations tested. Finally, the performance in the presence of both correlated and uncorrelated errors is confirmed, in all cases demonstrating that the incident field has a negligible effect on the final reconstruction.

JOURNAL ARTICLE

Haith MI, Ewert U, Hohendorf S, Bellon C, Deresch A, Huthwaite P, Lowe MJS, Zscherpel Uet al., 2016, Radiographic modelling for NDE of subsea pipelines, NDT & E INTERNATIONAL, Vol: 86, Pages: 113-122, ISSN: 0963-8695

This work presents an investigation of the accuracy of a radiographic simulation model applied to subsea pipeline inspections. Experimental measurements of a sample in a water tank are used to develop a set of calibrated simulation parameters for the modelling software aRTist. Image quality parameters such as signal-to-noise ratio, contrast and basic spatial resolution are compared with the aim of matching simulated values to experimental results. With this method signal-to-noise ratio was successfully matched while differences were still found in contrast-to-noise ratio comparisons. This means that measurements depending on absolute intensity are not accurate enough, however wall thickness measurements in tangential images, which are not based on absolute intensity, were found to produce similar results in simulated and experimental cases. The differences in contrast and intensity are thought to be due to detector backscatter and additional scatter from out-of-setup objects within the exposure bay, due to a lack of source collimation. These would affect the experimental results but were not included in the simulated setup. This was investigated by including different proportions of peripheral water and other objects in the modelled setup and examining the effect on image quality parameters. Results show that this additional scatter has a significant impact on the radiograph, particularly on image contrast, and is therefore the likely cause of differences between experimental and simulated images. This implies that it will be very difficult to completely match simulated to experimental results, as including all possible scattering objects in the model would be very complex. An improvement could be made by using real subsea data to estimate this additional scattering, which could then be used to calibrate the model. However there would still be significant uncertainty in the ability of the model to accurately produce realistic intensity and contrast.

JOURNAL ARTICLE

Huthwaite P, 2016, Guided wave tomography with an improved scattering model, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1471-2946

Producing accurate thickness maps of corrosion damage is of great importance for assessing life in the petrochemical industry. Guided wave tomography provides a solution for this, by sending guided waves through the region of interest, then using tomographic imaging techniques to reconstruct the thickness map, importantly eliminating the need to take measurements at all points across the surface. However, to achieve accurate maps, the imaging algorithm must account for the way in which the guided waves interact with corrosion defects, and the complex scattering which occurs. Traditional approaches have exploited the dispersive nature of guided waves: a velocity map is produced from a dataset, then converted to thickness using the dispersion relationship. However, these relationships are derived for plates of constant thickness, which is not the case in the majority of defects, causing significant inaccuracies to exist in the images. This paper develops a more sophisticated inversion solution which accounts for the full-guided wave scattering, enabling more accurate images with resolution better than a wavelength, compared with two wavelengths previously. This is demonstrated with simulated and experimental data. The speed and stability of the algorithm in the presence of random noise and systematic errors is also demonstrated.

JOURNAL ARTICLE

Seher M, Huthwaite P, Lowe MJS, 2016, Experimental Studies of the Inspection of Areas With Restricted Access Using A0 Lamb Wave Tomography, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol: 63, Pages: 1455-1467, ISSN: 0885-3010

Corrosion damage in inaccessible regions presents a significant challenge to the petrochemical industry, and determining the remaining wall thickness is important to establish the remaining service life. Guided wave tomography is one solution to this and involves transmitting Lamb waves through the area of interest and, subsequently, using the received signals to reconstruct a thickness map of the remaining wall thickness. This avoids the need to access all points on the surface, making the technique well suited to inspection for areas with restricted access. The influence of these areas onto the ability to detect and size surface conditions, such as corrosion damage, using guided wave tomography is assessed. For that, a guided wave tomography system is employed, which is based on low-frequency A0 Lamb waves that are excited and detected with two arrays of electromagnetic acoustic transducers. Two different defect depths are considered with different contrasts relative to the nominal wall thickness, both of which are smoothly varying and well-defined. The influence of areas with restricted surface access, support locations, pipe clamps, and STOPAQ(R) coatings is experimentally tested, and their influence assessed through comparison to a baseline reconstruction without the respective restriction in place, demonstrating only a small influence on the detected value of the remaining wall thickness.

JOURNAL ARTICLE

Van Pamel A, Huthwaite P, Brett CR, Lowe MJSet al., 2016, Numerical simulations of ultrasonic array imaging of highly scattering materials, NDT & E International, Vol: 81, Pages: 9-19, ISSN: 0963-8695

JOURNAL ARTICLE

Huthwaite P, 2016, Improving accuracy through density correction in guided wave tomography, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 472, ISSN: 1364-5021

The accurate quantification of wall loss caused by corrosion is critical to the reliable life estimation of pipes and pressure vessels. Traditional thickness gauging by scanning a probe is slow and requires access to all points on the surface; this is impractical in many cases as corrosion often occurs where access is restricted, such as beneath supports where water collects. Guided wave tomography presents a solution to this; by transmitting guided waves through the region of interest and exploiting their dispersive nature, it is possible to build up a map of thickness. While the best results have been seen when using the fundamental modes A0 and S0 at low frequency, the complex scattering of the waves causes errors within the reconstruction. It is demonstrated that these lead to an underestimate in wall loss for A0 but an overestimate for S0. Further analysis showed that this error was related to density variation, which was proportional to thickness. It was demonstrated how this could be corrected for in the reconstructions, in many cases resulting in the near-elimination of the error across a range of defects, and greatly improving the accuracy of life estimates from guided wave tomography.

JOURNAL ARTICLE

Van Pamel A, Brett CR, Huthwaite P, Lowe MJSet al., 2015, Finite element modelling of elastic wave scattering within a polycrystalline material in two and three dimensions, Journal of the Acoustical Society of America, Vol: 138, Pages: 2326-2336, ISSN: 0001-4966

Finite element modelling is a promising tool for further progressing the development of ultrasonic non-destructive evaluation of polycrystalline materials. Yet its widespread adoption has been held back due to a high computational cost, which has restricted current works to relatively small models and to two dimensions. However, the emergence of sufficiently powerful computing, such as highly efficient solutions on graphics processors, is enabling a step improvement in possibilities. This article aims to realise those capabilities to simulate ultrasonic scattering of longitudinal waves in an equiaxed polycrystalline material in both two (2D) and three dimensions (3D). The modelling relies on an established Voronoi approach to randomly generate a representative grain morphology. It is shown that both 2D and 3D numerical data show good agreement across a range of scattering regimes in comparison to well-established theoretical predictions for attenuation and phase velocity. In addition, 2D parametric studies illustrate the mesh sampling requirements for two different types of mesh to ensure modelling accuracy and present useful guidelines for future works. Modelling limitations are also shown. It is found that 2D models reduce the scattering mechanism in the Rayleigh regime.

JOURNAL ARTICLE

Egerton JS, Lowe MJS, Halai HV, Huthwaite Pet al., 2015, Improved FE Simulation of Ultrasound in Plastics, 42nd Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: American Institute of Physics (AIP), ISSN: 1551-7616

Some UK and US nuclear power stations have begun introducing high-density polyethylene (HDPE) pipes to certain cooling water circuits. HDPE offers improved performance over existing pipe materials, such as cast iron, by not corroding in-ternally or externally, yet occasional defects form in HDPE pipe fusion joints at the production stage. This necessitates suitable volumetric NDE to safely and reliably assess joint integrity. Ultrasonic NDE is the most viable current technique, but improved inspection capability is needed, given that the challenges of NDE of plastics differ significantly from those of metals. This also necessitates an accurate and reliable wave propagation simulation technique, such as finite-element (FE) modelling. Accurate FE modelling of ultrasound in high-density polyethylene (HDPE) must account for frequency-dependent behaviour but, the most ap-parent way to do so – frequency domain FE modelling – is prohibitively computationally expensive and potentially impossible to solve for all but the smallest models. Here we present a multiband time domain FE simulation technique to address this. The proposed multiband technique is a computationally efficient and accurate approach to time domain FE modelling of ultrasonic wave propagation. It could, for example, be used to validate the NDE of a large range of candidate fusion joint defects in HDPE. The proposed model uses a small number of time domain FE simulations at individual frequency bands that together cover the bandwidth of interest. The frequency dependence of acoustic properties of ultrasound is accurately represented for HDPE and could readily be applied to other media.

CONFERENCE PAPER

Huthwaite P, Lowe M, Cawley P, 2015, Guided Wave Tomography Performance Analysis, 42nd Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: American Institute of Physics (AIP), ISSN: 1551-7616

Quantifying wall loss caused by corrosion is a significant challenge for the petrochemical industry. Corrosion commonly occurs at pipe supports, where surface access for inspection is limited. Guided wave tomography is pursued as a solution to this: guided waves are transmitted through the region of interest from an array, and tomographic reconstruction techniques are applied to the measured signals in order to produce a map of thickness. There are many parameters in the system which can affect the performance; this paper investigates how the accuracy varies as defect width and depth, operating frequency and guided wave mode are all changed. For the S0 mode, the best performance was seen around 170kHz on the 10mm plate, with poor performance seen at almost all other frequencies. A0 showed better performance across a broad range of frequencies, with resolution improving with frequency as the wavelength reduced. However, it was shown that the resolution limit did drop relative to the wavelength, limiting the performance at high frequencies slightly.

CONFERENCE PAPER

Ewert U, Tschaikner M, Hohendorf S, Bellon C, Haith MI, Huthwaite P, Lowe MJSet al., 2015, Corrosion Monitoring with Tangential Radiography and Limited View Computed Tomography, 42nd Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: American Institute of Physics (AIP), ISSN: 1551-7616

Accurate and reliable detection of subsea pipeline corrosion is required in order to verify the integrity of the pipeline. A laboratory trial was conducted with a representative pipe sample. The accurate measurement of the wall thickness and corrosion was performed with high energy X-rays and a digital detector array. A 7.5 MV betatron was used to penetrate a stepped pipe and a welded test pipe of 3 m length and 327 mm outer diameter, with different artificial corrosion areas in the 24 mm thick steel wall. The radiographs were taken with a 40 x 40 cm² digital detector array, which was not large enough to cover the complete pipe diameter after magnification. A C-arm based geometry was tested to evaluate the potential for automated inspection in field. The primary goal was the accurate measurement of wall thickness conforming to the standard. The same geometry was used to explore the ability of a C-arm based scanner in asymmetric mode for computed tomography (CT) measurement, taking projections covering only two thirds of the pipe diameter. The technique was optimized with the modelling software aRTist. A full volume of the pipe was reconstructed and the CT data set was used for reverse engineering, providing a CAD file for further aRTist simulations to explore the technique for subsea inspections.

CONFERENCE PAPER

Van Pamel A, Huthwaite P, Brett CR, Lowe MJSet al., 2015, Finite Element Modelling of Wave Propagation in Highly Scattering Materials, 42nd Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: American Institute of Physics (AIP), ISSN: 1551-7616

Ultrasonic inspection of highly scattering materials presents challenges for industry. This article describes a Finite Element modelling methodology to simulate wave propagation within polycrystalline materials. Concerns are answered regarding its required mesh sampling and ability to capture the complex scattering physics. It is shown that grain scattering phenomena are closely reproduced across a range of scattering regimes. The procedure is subsequently applied to investigate the optimal configuration of an array inspecting such a material. It is found that in certain situations, separating emitter and receiver can be advantageous as this reduces the received backscatter.

CONFERENCE PAPER

Haith MI, Ewert U, Hohendorf S, Bellon C, Deresch A, Huthwaite P, Lowe MJS, Zscherpel Uet al., 2015, Modelling Based Radiography for NDE of Subsea Pipelines, 42nd Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: American Institute of Physics (AIP), ISSN: 1551-7616

This work presents the use of limited experimental measurements to develop a set of calibrated simulation parameters that can then be used for reliable simulation of subsea pipeline inspections. The modelling software aRTist is used as the simulation tool, and the calibration is through comparison with experimental images of a well characterised sample in a water tank. Image quality parameters such as signal-to-noise ratio, contrast and basic spatial resolution are compared with the aim of matching simulated values to experimental results. Currently the model is partially calibrated, with signal-to-noise ratio successfully matched while differences are still found in contrast-to-noise ratio comparisons. This means that measurements depending on absolute intensity are not accurate enough in the simulation at this stage. However, the simulation is found to be accurate for wall thickness measurements in tangential images, which are not based on absolute intensity, with simulated and experimental cases producing similar results

CONFERENCE PAPER

Seher M, Huthwaite P, Lowe MJS, Nagy PBet al., 2015, Model-Based Design of Low Frequency Lamb Wave EMATs for Mode Selectivity, Journal of Nondestructive Evaluation, Vol: 34, ISSN: 1573-4862

A low-frequency, omni-directional A0 Lamb wave ElectroMagnetic Acoustic Transducer (EMAT) is developed for applications in guided wave tomography, operating at 50 kHz on a 10 mm thick steel plate. The key objective is to excite an acceptably pure A0 wave mode in relation to the S0 mode, which can also be present at this operating point and is desired to be suppressed by approximately 30 dB. For that, a parametric Finite Element (FE) model of the design concept is implemented in a commercially available FE software, where the bias magnetic field is calculated initially, then combined with the eddy current caused by the induction coil to produce a force. A numerical optimization process employing a genetic algorithm is set up and the EMAT design is optimized to yield an improved A0 mode selectivity. The parameters subjected to optimization are the magnet diameter and the magnet lift-off, which control the direction of the exciting force in the skin depth layer and therefore the mode selectivity. Although there are three possible electromagnetic acoustic interaction mechanisms, the optimisation considers only the Lorentz force, as its performance surface contains a clear optimum and from the optimised design a physical prototype is built. The FE model is validated against measurements on an aluminium plate for the Lorentz force excitation mechanism and on a steel plate for both the Lorentz and magnetisation force. For the steel plate, it is found that only considering the Lorentz force leads to a significant overestimation of the mode selectivity, as the S0 amplitude is underestimated by the Lorentz force, but the A0 amplitude remains mainly uninfluenced. Further, it has been found that additionally including the magnetisation force into the optimisation leads to a better mode selectivity, however, the optimisation drives the optimum to a minimum magnet diameter and therefore reduces the EMAT sensitivity. In a numerical study robustness is shown for fair

JOURNAL ARTICLE

Huthwaite P, Seher M, 2015, Robust Helical Path Separation for Thickness Mapping of Pipes by Guided Wave Tomography, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, Vol: 62, Pages: 927-938, ISSN: 1525-8955

Pipe wall loss caused by corrosion can be quantifiedacross an area by transmitting guided Lamb waves throughthe region and measuring the resulting signals. Typically thedispersive relationship for these waves, which means thatwave velocity is a known function of thickness, is exploited,enabling the wall thickness to be determined from a velocityreconstruction. The accuracy and quality of this reconstructionis commonly limited by the angle of view available from thetransducer arrays. These arrays are often attached as a pairof ring arrays on either side of the inspected region, and dueto the cylindrical nature of the pipe, waves are able to travelin an infinite number of helical paths between any two transducers.The first arrivals can be separated relatively easily bytime gating, but by using just these components the angle ofview is severely restricted. To improve the viewing angle, it isnecessary to separate the wavepackets. This paper provides anoutline of a separation approach: initially the waves are backpropagatedto their source to align the different signals, then afiltering technique is applied to select the desired components.The technique is applied to experimental data and demonstratedto robustly separate the signals

JOURNAL ARTICLE

Huthwaite P, Seher M, 2015, Helical Path Separation For Guided Wave Tomography, 41st Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: AMER INST PHYSICS, Pages: 761-770, ISSN: 0094-243X

CONFERENCE PAPER

Seher M, Huthwaite P, Lowe M, Cawley Pet al., 2015, Experimental Study of A0 Lamb Wave Tomography, 41st Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: AMER INST PHYSICS, Pages: 245-253, ISSN: 0094-243X

CONFERENCE PAPER

Huthwaite P, Shi F, Van Pamel A, Lowe MJSet al., 2015, High-Speed GPU-Based Finite Element Simulations for NDT, 41st Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: AMER INST PHYSICS, Pages: 1815-1819, ISSN: 0094-243X

CONFERENCE PAPER

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