Research Articles

Journal article

Egerton JS, Lowe MJS, Huthwaite P, 2019,

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

, NDT & E INTERNATIONAL, Vol: 105, Pages: 35-45, ISSN: 0963-8695

Journal article

Shipway NJ, Huthwaite P, Lowe MJS, Barden TJet al., 2019,

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

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

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

Journal article

Isla J, Cegla F, 2019,

Simultaneous transmission and reception on all elements of an array: binary code excitation

, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 475, Pages: 1-23, ISSN: 1364-5021

Pulse-echo arrays are used in radar, sonar, seismic, medical and non-destructive evaluation. There is a trend to produce arrays with an ever-increasing number of elements. This trend presents two major challenges: (i) often the size of the elements is reduced resulting in a lower signal-to-noise ratio (SNR) and (ii) the time required to record all of the signals that correspond to every transmit–receive path increases. Coded sequences with good autocorrelation properties can increase the SNR while orthogonal sets can be used to simultaneously acquire all of the signals that correspond to every transmit–receive path. However, a central problem of conventional coded sequences is that they cannot achieve good autocorrelation and orthogonality properties simultaneously due to their length being limited by the location of the closest reflectors. In this paper, a solution to this problem is presented by using coded sequences that have receive intervals. The proposed approach can be more than one order of magnitude faster than conventional methods. In addition, binary excitation and quantization can be employed, which reduces the data throughput by roughly an order of magnitude and allows for higher sampling rates. While this concept is generally applicable to any field, a 16-element system was built to experimentally demonstrate this principle for the first time using a conventional medical ultrasound probe.

Journal article

Liu Y, Van Pamel A, Nagy P, Cawley Pet al., 2019,

Investigation of ultrasonic backscatter using three-dimensional finite element simulations

, Journal of the Acoustical Society of America, Vol: 145, Pages: 1584-1595, ISSN: 0001-4966

Theoretical models are commonly used to describe ultrasonic backscattering in polycrystalline materials. However, although a full multiple scattering formalism has been derived, due to the difficulty in evaluation, currently only the single and double scattering effects have been evaluated. Three-dimensional finite element (3D FE) models have recently been demonstrated to be capable of predicting ultrasonic attenuation in polycrystalline materials and thereby show great potential in overcoming this limitation. In this paper, the application of 3D FE models is extended to the backscatter problem. First, longitudinal-to-longitudinal backscattering amplitudes from single grains are predicted, where the setup and configuration of the finite element (FE) model are verified with an isotropic spherical inclusion for which an exact solution is available. Subsequently, backscatter in terms of the root-mean-square noise levels in two different pulse-echo scenarios is investigated; the first is an idealised configuration with plane wave transmission and point reception; the second represents a more realistic finite-size transducer acting with the same apodization in both transmission and reception. Comparisons of FE predictions and approximate theoretical solutions within a range of validity show good agreement; however, the results demonstrate that 3D FE is useful where the simple Independent Scatterer models break down. As computing power increases, 3D FE is an increasingly viable tool to further the understanding of wave propagation in polycrystalline materials.

Journal article

Leung M, Corcoran J, Nagy PB, 2019,

The influence of the dynamic magnetoelastic effect on potential drop measurements

, NDT & E International, Vol: 102, Pages: 153-160, ISSN: 0963-8695

Alternating Current Potential Drop (ACPD) measurements are routinely used for monitoring crack length in laboratory based fatigue tests, and so measurements will be taken on components which are exposed to cyclic dynamic stresses. It has been empirically observed that cyclic stresses cause a strong increase (above 10% is shown in this paper) in measured resistance that is both AC inspection frequency and loading frequency dependent. The excess resistance will result in erroneous measurements; this paper investigates the cause and provides recommendations to limit the influence. Applied stresses influence ACPD measurements through the magnetoelastic effect; elastic strain induces magnetization in ferromagnetic materials, which in turn influences the magnetic permeability and therefore skin depth. Further, it has recently been realised that cyclic magnetization results in a frequency dependent concentration of the magnetic flux at the surface of the component, and consequently a non-uniform spatial distribution of magnetic permeability. In this study it is found that the interaction between the non-uniform spatial distribution of both the current density and magnetic permeability results in significant non-linear modulation of the measurement signal. A combination of finite element simulations and experimental results are used to explore this phenomenon.

Journal article

Herdovics B, Cegla F, 2019,

Compensation of phase response changes in ultrasonic transducers caused by temperature variations

, Structural Health Monitoring, Vol: 18, Pages: 508-523, ISSN: 1475-9217

One of the biggest challenges in structural health monitoring is the compensation of monitored data for environmental and operational conditions. In order to reliably estimate the changes in the structure, it is essential that the effects of environmental and operational conditions on the ultrasonic signal are compensated for before the signals are further analysed. The temperature-induced propagation speed change has the biggest effect on the ultrasonic signal and has been thoroughly investigated. This article investigates the subtler, yet also very important, changes in transducer output resulting from changes in the operating temperature. A compensation method is proposed which compensates for both the transducer phase response change and the wave’s propagation speed change. A key practical feature of the presented compensation method is that it uses only the ultrasonic signal itself for compensation estimation and can be used for any type of ultrasonic wave regardless of the type of transducer. For demonstration purposes, in this article, the results are shown for zero-order torsional guided waves, acquired by a purpose-built electromagnetic acoustic transducer. For signals with a 41.5°C temperature difference, the proposed compensation method was able to reduce the effect of environmental and operational conditions by 20 dB further (7 dB at the tail of the echo) compared to standard methods. This results in a much higher sensitivity to defects in areas where strong reflections are received. Furthermore, for the presented measurement setup, the precision to which the temperature-dependent change in wave propagation speed could be estimated was improved by 15%.

Journal article

Leung M, Corcoran J, Cawley P, Todd MDet al., 2019,

Evaluating the use of Rate-based Monitoring for Improved Fatigue Remnant Life Predictions

, International Journal of Fatigue, Vol: 120, Pages: 162-174, ISSN: 0142-1123

The ability to perform accurate remnant life predictions is crucial to ensure the integrity of engineering components that experience fatigue loading during operation. This is conventionally achieved with periodic inspections, where results from non-destructive evaluation and estimation of the operating conditions are obtained to perform remnant life predictions using empirical crack growth laws. However, remnant life predictions made with this approach are very sensitive to their input parameters; uncertainty in each parameter would aggregate and result in great uncertainty in the final prediction. With the increasing viability of permanently-installed systems, it is proposed that the rate of damage growth can be used to more accurately and confidently gauge the integrity of an engineering component and perform remnant life predictions using the Failure Forecast Method. A statistical analysis of an example fatigue crack growth test was performed to compare the uncertainties of the remnant life predictions made using the conventional inspection approach and the proposed rate-based monitoring approach. It is shown that the Failure Forecast Method produces significantly more accurate and confident predictions compared to the inspection approach. The use of the Failure Forecast Method under non-constant amplitude loading conditions was also investigated. An equivalent cycles method is introduced to accommodate step changes in operating conditions. The effect of load interactions was also studied through a fatigue test with isolated overloads and a random variable amplitude loading test. Overall, the study has shown that the frequent data obtained from permanently installed monitoring systems provides new opportunities in remnant life estimates and potentially opens the way to increasing the intervals between outages and safely reducing conservatism in life predictions.

Journal article

Gu X, Cegla F, 2019,

The effect of internal pipe wall roughness on the accuracy of clamp-on ultrasonic flow meters

, IEEE Transactions on Instrumentation and Measurement, Vol: 68, Pages: 65-72, ISSN: 0018-9456

Clamp-on transit-time ultrasonic flowmeters (UFMs) suffer from poor accuracy compared with spool-piece UFMs due to uncertainties that result from the in-field installation process. One of the important sources of uncertainties is internal pipe wall roughness which affects the flow profile and also causes significant scattering of ultrasound. This paper purely focuses on the parametric study to quantify the uncertainties (related to internal pipe wall roughness) induced by scattering of ultrasound and it shows that these effects are large even without taking into account the associated flow disturbances. The flowmeter signals for a reference clamp-on flowmeter setup were simulated using 2-D finite element analysis including simplifying assumptions (to simulate the effect of flow) that were deemed appropriate. The validity of the simulations was indirectly verified by carrying out experiments with different separation distances between ultrasonic probes. The error predicted by the simulations and the experimentally observed errors were in good agreement. Then, this simulation method was applied on pipe walls with rough internal surfaces. For ultrasonic waves at 1 MHz, it was found that compared with smooth pipes, pipes with only a moderately rough internal surface (with 0.2-mm rms and 5-mm correlation length) can exhibit systematic errors of 2 in the flow velocity measurement. This demonstrates that pipe internal surface roughness is a very important factor that limits the accuracy of clamp on UFMs.

Conference paper

Mariani S, Cawley P, 2019,

Location specific temperature compensation of guided wave signals applied to pipe inspections

, Pages: 2427-2433

The baseline subtraction method is widely used to detect defect signatures in guided wave structural health monitoring. In essence, an earlier measurement is subtracted from the 'current' signal, and high residuals might indicate damage occurrence. However, varying environmental and operational conditions, such as temperature, also produce signal changes and hence, potentially, high residuals. A number of temperature compensation methods have been developed, which typically targets the varying wave speed due to varying temperature. Nevertheless, other, subtler effects caused by temperature variations are often overlooked, such as changes in attenuation, in the transducer frequency response and in the relative amplitudes of different modes excited by the transducer. A novel temperature compensation method has been recently presented by the authors, which compensates for temperature induced changes of wave speed and signal phase. The compensated signals can then be fed to a second temperature compensation procedure that has been newly developed. This will correct any spatially dependent signal change that is a systematic function of temperature, hence producing residuals less affected by temperature variations. This new method was applied to a set of T(0,1) guided wave signals collected by a pipe monitoring system, yielding residuals reduced by at least 50% compared to those obtained using the standard approach at positions away from structural features, and by more than 90% at features such as the pipe end. The method therefore promises a substantial improvement in the detectability of small defects, particularly at existing pipe features.

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Journal article

Shipway N, Barden T, Huthwaite P, Lowe Met 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.

Journal article

Combaniere J, Cawley P, McAughey K, Giese Jet al., 2019,

Interaction between SH0 guided waves and tilted surface-breaking cracks in plates

, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 66, Pages: 119-128, ISSN: 0885-3010

The interaction between SH0 guided waves and simple defects is well understood and documented, and the SH0 and related torsional guided waves are commonly used in inspection. However, tilted and branching cracks, for which vertical notches are a poor approximation, are found in some environments, particularly when pipes are buried in alkaline soils. This paper studies the interaction between SH0 guided waves and tilted, surface-breaking cracks, investigating the effect of the tilt and depth of the defect. The incident wave interacts with the tilted crack to generate a transmitted wave, a reflected wave and a wave trapped below the crack. It is shown that the direction of the tilt of the crack relative to the incident wave direction does not affect the scattering behaviour. Additionally, the axial extent of the crack plays a major role in the reflectivity of the crack, leading to transmission nulls in some configurations. These transmission nulls appear for all crack depths, the frequency range over which the transmission is significantly reduced increasing with crack depth. This behaviour is shown to be analogous to the acoustic energy flow in a duct when a Helmholtz resonator is introduced. The null is not seen above the SH1 cut-off as the propagating signals are no longer mono-modal. The existence of a transmission null and corresponding reflection maximum is promising for the detection of small defects and measurement of the frequency at which the null occurs will assist with defect characterisation. Experimental validations of the key results are presented.

Conference paper

Cawley P, 2019,

Ultrasonic structural health monitoring - current applications and potential

, IEEE International Ultrasonics Symposium (IUS), Publisher: IEEE, Pages: 2107-2109, ISSN: 1948-5719

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Citations: 1

Journal article

Lan B, Carpenter MA, Gan W, Hofmann M, Dunne FPE, Lowe MJSet al., 2018,

Rapid measurement of volumetric texture using resonant ultrasound spectroscopy

, Scripta Materialia, Vol: 157, ISSN: 1359-6462

This paper presents a non-destructive evaluation method of volumetric texture using resonant ultrasound spectroscopy (RUS). It is based on a general theoretical platform that links the directional wave speeds of a polycrystalline aggregate to its texture through a simple convolution relationship, and RUS is employed to obtain the speeds by measuring the elastic constants, where well-established experimental and post-processing procedures are followed. Important lower-truncation-order textures of representative hexagonal and cubic metal samples with orthorhombic sample symmetries are extracted, and are validated against independent immersion ultrasound and neutron tests. The successful deployment of RUS indicates broader applications of the general methodology.

Journal article

Lan B, Britton TB, Jun T-S, Gan W, Hofmann M, Dunne F, Lowe Met al., 2018,

Direct volumetric measurement of crystallographic texture using acoustic waves

, Acta Materialia, Vol: 159, Pages: 384-394, ISSN: 1359-6454

Crystallographic texture in polycrystalline materials is often developed as preferred orientation distribution of grains during thermo-mechanical processes. Texture dominates many macroscopic physical properties and reflects the histories of structural evolution, hence its measurement and control are vital for performance optimisation and deformation history interogation in engineering and geological materials. However, exploitations of texture are hampered by state-of-the-art characterisation techniques, none of which can routinely deliver the desirable non-destructive, volumetric measurements, especially at larger lengthscales. Here we report a direct and general methodology retrieving important lower-truncation-order texture and phase information from acoustic (compressional elastic) wave speed measurements in different directions through the material volume (avoiding the need for forward modelling). We demonstrate its deployment with ultrasound in the laboratory, where the results from seven representative samples are successfully validated against measurements performed using neutron diffraction. The acoustic method we have developed includes both fundamental wave propagation and texture inversion theories which are free from diffraction limits, they are arbitrarily scalable in dimension, and can be rapidly deployed to measure the texture of large objects. This opens up volumetric texture characterisation capabilities in the areas of material science and beyond, for both scientific and industrial applications.

Journal article

Khalili P, Cawley P, 2018,

The choice of ultrasonic inspection method for the detection of corrosion at inaccessible locations

, NDT and E International, Vol: 99, Pages: 80-92, ISSN: 0963-8695

Inspection for corrosion and pitting defects in the petrochemical industry is vital and forms a significant fraction of the operating expenditure. Low frequency guided wave inspection is frequently employed as it gives large area coverage from a single transducer position. However, detection becomes problematic at inaccessible regions such as pipe supports or beyond T-joints since the low frequency guided waves produce a significant reflection from the feature itself, hence limiting the defect detectability of the method. This suggests testing at higher frequencies which helps to minimise the reflection from the feature and also improves the sensitivity to smaller defects. There are a number of guided wave and related techniques implemented for corrosion inspection including the S0 mode (at ∼ 1 MHz-mm), SH0 and SH1 modes (at ∼ 3 MHz-mm), CHIME, M-skip and Higher Order Mode Cluster (A1 mode at ∼ 18 MHz-mm). This paper presents a systematic analysis of the defect detection performance of each method with sharp and gradual defects, as well as their sensitivity to attenuative coatings, liquid loading, surface roughness and ability to test beyond features such as T-joints. It is shown by finite element analysis backed up by experiments that the A1 mode provides the best overall performance when dealing with surface features such as T-joints and coatings because of its low surface motion. Additionally a combination of two or more methods is suggested for corrosion inspection at inaccessible locations: The A1 mode in reflection for severe, sharp, pitting type defects; long range guided waves in reflection for large-area thinning and the SH1 mode in transmission for shallow, gradual defects.

Journal article

Cawley P, 2018,

Structural health monitoring: closing the gap between research andindustrial deployment

, Structural Health Monitoring, Vol: 17, Pages: 1225-1244, ISSN: 1475-9217

There has been a large volume of research on structural health monitoring since the 1970s but this research effort has yielded relatively few routine industrial applications. Structural health monitoring can include applications on very different structures with very different requirements; this article splits the subject into four broad categories: rotating machine condition monitoring, global monitoring of large structures (structural identification), large area monitoring where the area covered is part of a larger structure, and local monitoring. The capabilities and potential applications of techniques in each category are discussed. Condition monitoring of rotating machine components is very different to the other categories since it is not strictly concerned with structural health. However, it is often linked with structural health monitoring and is a relatively mature field with many routine applications, so useful lessons can be read across to mainstream structural health monitoring where there are many fewer industrial applications. Reasons for the slow transfer from research to practical application of structural health monitoring include lack of attention to the business case for monitoring, insufficient attention to how the large data flows will be handled and the lack of performance validation on real structures in industrial environments. These issues are discussed and ways forward proposed; it is concluded that given better focused research and development considering the key factors identified here, structural health monitoring has the potential to follow the path of rotating machine condition monitoring and become a widely deployed technology.

Journal article

Corcoran J, Nagy PB, 2018,

Magnetic stress monitoring using a directional potential drop technique

, Journal of Nondestructive Evaluation, Vol: 37, ISSN: 0195-9298

An alternating current potential drop technique is presented that exploits anisotropic magnetostriction to monitor changes inapplied stress in steel. The background to the technique is provided together with an ad hoc approximation that describes thesensitivity of the sensor to the underlying properties. A uniaxial loading experiment has been conducted on duplex and mildsteel specimens showing that changes in stress are measureable. Saturation and hysteresis afflict the measurement, which, inaddition to sensitivity to temperature and magnetisation, may undermine inversion. With the capabilities and limitations ofthe proposed technique introduced, guidance on possible suitable applications are given, concluding that use for monitoringthe number and relative size of fatigue stress cycles may be a suitable and attractive opportunity.

Journal article

Choi W, Shi F, Lowe MJS, Skelton EA, Craster RV, Daniels WLet al., 2018,

Rough surface reconstruction of real surfaces for numerical simulations of ultrasonic wave scattering

, NDT and E International, Vol: 98, Pages: 27-36, ISSN: 0963-8695

The scattering of waves by rough surfaces plays a significant role in many fields of physical sciences including ultrasonics where failure surfaces are often rough and their accurate identification is critical. The prediction of the strength of scattering can be hampered when the roughness is not adequately characterised and this is a particular issue when the surface roughness is within an order of the incident wavelength. Here we develop a methodology to reconstruct, and accurately represent, rough surfaces using an AutoRegressive (AR) process that then allows for rapid numerical simulations of ultrasonic wave rough surface scattering in three dimensions. Gaussian, exponential and AR surfaces are reconstructed based on real surface data and the statistics of the surfaces are compared with each other. The statistics from the AR surfaces agree well with those from actual rough surfaces, taken from experimental samples, in terms of the heights as well as the gradients, which are the two main factors in accurately predicting the wave scattering intensities. Ultrasonic rough surface scattering is simulated numerically using the Kirchhoff approximation, and comparisons with Gaussian, exponential, AR and real sample surfaces are performed; scattering intensities found using AR surfaces show the best agreement with the real sample surfaces.

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.

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