Imperial College London

DrFredericCegla

Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Mechanical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 8096f.cegla

 
 
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Location

 

567City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

88 results found

Fantetti A, Mariani S, Pesaresi L, Nowell D, Cegla F, Schwingshackl Cet al., 2021, Ultrasonic monitoring of friction contacts during shear vibration cycles, Mechanical Systems and Signal Processing, Vol: 161, ISSN: 0888-3270

Complex high-value jointed structures such as aero-engines are carefully designed and optimized to prevent failure and maximise their life. In the design process, physically-based numerical models are employed to predict the nonlinear dynamic response of the structure. However, the reliability of these models is limited due to the lack of accurate validation data from metallic contact interfaces subjected to high-frequency vibration cycles. In this study, ultrasonic shear waves are used to characterise metallic contact interfaces during vibration cycles, hence providing new validation data for an understanding of the state of the friction contact. Supported by numerical simulations of wave propagation within the material, a novel experimental method is developed to simultaneously acquire ultrasonic measurements and friction hysteresis loops within the same test on a high-frequency friction rig. Large variability in the ultrasound reflection/transmission is observed within each hysteresis loop and is associated with stick/slip transitions. The measurement results reveal that the ultrasound technique can be used to detect stick and slip states in contact interfaces subjected to high-frequency shear vibration. This is the first observation of this type and paves the way towards real-time monitoring of vibrating contact interfaces in jointed structures, leading to a new physical understanding of the contact states and new validation data needed for improved nonlinear dynamic analyses.

Journal article

Huang J, Cegla F, Wickenden A, Coomber Met al., 2021, Simultaneous Measurements of Temperature and Viscosity for Viscous Fluids Using an Ultrasonic Waveguide, SENSORS, Vol: 21

Journal article

Parra-Raad J, Lan B, Cegla F, 2021, Orthogonally polarised shear waves for evaluating anisotropy and cracks in metals, NDT & E INTERNATIONAL, Vol: 121, ISSN: 0963-8695

Journal article

Khalili P, Cegla F, 2021, Excitation of Single-Mode Shear-Horizontal Guided Waves and Evaluation of Their Sensitivity to Very Shallow Crack-Like Defects, IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL, Vol: 68, Pages: 818-828, ISSN: 0885-3010

Journal article

Hall T, Cegla F, van Arkel RJ, 2021, Simple smart implants: simultaneous monitoring of loosening and temperature in orthopaedics with an embedded ultrasound transducer, IEEE Transactions on Biomedical Circuits and Systems, Vol: 15, Pages: 102-110, ISSN: 1932-4545

Implant failure can have devastating consequences on patient outcomes following joint replacement. Time to diagnosis affects subsequent treatment success, but current diagnostics do not give early warning and lack accuracy. This research proposes an embedded ultrasound system to monitor implant fixation and temperature – a potential indicator of infection. Requiring only two implanted components: a piezoelectric transducer and a coil, pulse-echo responses are elicited via a three-coil inductive link. This passive system avoids the need for batteries, energy harvesters, and microprocessors, resulting in minimal changes to existing implant architecture. Proof-of-concept was demonstrated in vitro for a titanium plate cemented into synthetic bone, using a small embedded coil with 10 mm diameter. Gross loosening – simulated by completely debonding the implant-cement interface – was detectable with 95% confidence at up to 12 mm implantation depth. Temperature was calibrated with root mean square error of 0.19 °C at 5 mm, with measurements accurate to ±1 °C with 95% confidence up to 6 mm implantation depth. These data demonstrate that with only a transducer and coil implanted, it is possible to measure fixation and temperature simultaneously. This simple smart implant approach minimises the need to modify well-established implant designs, and hence could enable mass-market adoption.

Journal article

Garriga Casanovas A, Khalili P, Cegla F, 2020, Development of a new, wireless acquisition system for EMATs compatible with the robotics operating system, IEEE Sensors Journal, Vol: 20, Pages: 12783-12790, ISSN: 1530-437X

The deployment of transducers to perform in situ inspections of industrial components can be complicated, and in many cases is still performed manually by a team of operators, which involves significant costs and can be dangerous. Robots capable of deploying probes in difficult to access locations are becoming available. Electromagnetic acoustic transducers (EMAT) are well suited to be used with robots since they are noncontact transducers that do not require a coupling medium, and can easily perform scans. However, existing acquisition systems for EMATs are generally not suitable to be directly mounted on robots. In this paper, a new wireless acquisition system for EMATs is presented. The system is standalone, it transmits the inspection data over WiFi, and is compatible with the robotics operating system (ROS). In addition, it is designed to be modular, small and lightweight so that it can be easily mounted on robots. The system design in terms of hardware and software is described in thispaper. The resulting performance of the system is also reported.

Journal article

Zhang Y, Cegla F, Corcoran J, 2020, Ultrasonic monitoring of pipe wall interior surface temperature, STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL, ISSN: 1475-9217

Journal article

Gu X, Cegla F, 2020, Modeling Surface Roughness-Related Uncertainties of Leaky Lamb Wave Clamp-on Ultrasonic Flowmeters, IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, Vol: 69, Pages: 6843-6852, ISSN: 0018-9456

Journal article

Parra-Raad J, Khalili P, Cegla F, 2020, Shear waves with orthogonal polarisations for thickness measurement and crack detection using EMATs, NDT & E International, Vol: 111, Pages: 1-7, ISSN: 0963-8695

The use of polarised shear waves to detect the presence of crack-like defects seems to have received little or no attention in the past. The authors believe that the main reason for this appears to be the lack of a device with the capability to excite shear waves of different polarisations. In this paper, the authors, first, present the design of an EMAT that permits the excitation of two orthogonally polarised shear waves in metallic materials by means of two coils that are orthogonal with respect to each other. This is then followed by a 3D finite element analysis of the wavefield generated by the EMAT and its interactions with crack-like defects of different sizes, positions and orientations. Then a methodology of how this EMAT can be used to simultaneously measure material thickness and detect crack-like defects in pulse-echo mode is introduced. Good agreement between the finite element simulation and experimental results was observed which makes the presented technique a potential new method for simultaneous thickness measurements and crack detection.

Journal article

Herdovics B, Cegla F, 2020, Long-term stability of guided wave electromagnetic acoustic transducer systems, Structural Health Monitoring, Vol: 19, Pages: 3-11, ISSN: 1475-9217

This article evaluates the long-term stability of a Lorentz force guided wave electromagnetic acoustic transducer. The specific application of the investigated electromagnetic acoustic transducer is pipeline health monitoring using low-frequency (27 kHz) long-range torsional guided waves. There is a concern that repeated swings in the temperature of the structure can cause irreversible changes in the transduction mechanism and therefore pose a risk to the long-term stability of transducers. In this article we report on guided wave signals acquired on a custom-built transducer while it was exposed to more than 90 heating cycles. The highest temperature that was reached during cycling was 80°C and the measurements were acquired over a 14-month period. At the end of the 1-year period, the transducer phase had changed by 23.32° and its amplitude by 3.7%. However, this change was not gradual and most of the change occurred early on, before the highest temperature was first reached in the temperature cycling process. The observed change after this was 6.08° phase shift and 0.9% amplitude change. The possible sources of output changes were investigated, and it was found that the mechanical properties of the contact layer between the electromagnetic acoustic transducer and the pipe surface was very important. A soft silicone interlayer performed best and was able to reduce temperature-induced phase changes in the monitored signals from a maximum of 80 degrees phase change to about 20 degrees phase change, a fourfold reduction.

Journal article

Pesaresi L, Fantetti A, Cegla F, Salles L, Schwingshackl CWet al., 2020, On the use of ultrasound waves to monitor the local dynamics of friction joints, Experimental Mechanics, Vol: 60, Pages: 129-141, ISSN: 0014-4851

Friction joints are one of the fundamental means used for the assembly of structural components in engineering applications. The structural dynamics of these components becomes nonlinear, due to the nonlinear nature of the forces arising at the contact interface characterised by stick-slip phenomena and separation. Advanced numerical models have been proposed in the last decades which have shown some promising capabilities in capturing these local nonlinearities. However, despite the research efforts in producing more advanced models over the years, a lack of validation experiments made it difficult to have fully validated models. For this reason, experimental techniques which can provide insights into the local dynamics of joints can be of great interest for the refinement of such models and for the optimisation of the joint design and local wear predictions. In this paper, a preliminary study is presented where ultrasound waves are used to characterise the local dynamics of friction contacts by observing changes of the ultrasound reflection/transmission at the friction interface. The experimental technique is applied to a dynamic friction rig, where two steel specimens are rubbed against each other under a harmonic tangential excitation. Initial results show that, with a controlled experimental test procedure, this technique can identify microslip effects at the contact interface.

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

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

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.

Journal article

Gu X, Cegla F, 2019, The uncertainties induced by internal pipe wall roughness on the measurements of clamp-on ultrasonic flow meters, IEEE International Ultrasonics Symposium (IUS), Publisher: IEEE, Pages: 1586-1589, ISSN: 1948-5719

Conference paper

Jarvis R, Farinha A, Kovac M, Cegla Fet al., 2018, NDE sensor delivery using unmanned aerial vehicles, Insight (Northampton): non-destructive testing and condition monitoring, Vol: 60, Pages: 463-467, ISSN: 1354-2575

The robotic deployment of NDE sensors has great cost-saving potential in cases where the measurement cost is high due to access restrictions or the need to temporarily decommission the test structure. Unmanned aerial vehicles (UAVs) are able to quickly reach inaccessible components to perform visual inspection and deploy NDE sensors. In this work, a mechanical sensor release mechanism is presented that has enabled electromagnetic acoustic transducers (EMATs) to be deployed onto a ferromagnetic pipe and a plate, after which the component wall thickness measurements can be transmitted wirelessly to a remote location. The reliability of the method and the most promising areas for future development are discussed.

Journal article

Cegla F, Herdovics B, 2018, Coded excitation, motion and signal-to-noise ratio, IEEE International Symposium on Circuits and Systems (ISCAS), Publisher: IEEE, ISSN: 0271-4302

Previous work has shown that coded excitation can be used to considerably improve the signal-to-noise ratio (SNR) of signals received by transducers of poor sensitivity such as electromagnetic acoustic transducers (EMATs). EMATs are usually driven with signal powers of the order of kWs so that adequate SNR is achieved. With coded excitation these powers can be reduced to as low as 1-5W. A particular feature of the transmitted codes is that they are temporally long and contain intermittent intervals in which reception takes place. Because of the signal length there is concern that excessive movement of the probe or target can result in deterioration of the performance of such a system. Therefore, in this paper we investigate the effect that physical motion of the test piece can have on the acquired signals. Simulated results will be presented and discussed here.

Conference paper

Howard R, Cegla F, 2018, The Effect of Pits of Different Sizes on Ultrasonic Shear Wave Signals, 44th Annual Conference on Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Publisher: AMER INST PHYSICS, ISSN: 0094-243X

Conference paper

Zou F, Cegla F, 2018, On quantitative corrosion rate monitoring with ultrasound, Journal of Electroanalytical Chemistry, Vol: 812, Pages: 115-121, ISSN: 1572-6657

Wall-thickness loss rate (WTLR) is an important parameter that defines a corrosion process. The speed at which a WTLR can be determined is directly related to how quickly one can intervene in a process that is heading in the wrong direction. Ultrasonic testing has been widely used as a convenient and efficient technique for online corrosion monitoring. One of the key performance parameters of ultrasonic corrosion monitoring is detection speed. While WTLRs can be determined by fitting linear lines to wall-thickness loss (WTL) measurements, the presence of noise in the measurements makes it difficult to judge the confidence levels of the slopes that are calculated this way. In this paper, a statistics based approach for assessing the detection speeds that are achievable by ultrasonic corrosion monitoring systems is presented. Through the statistical analysis of experimental data, a state-of-the-art laboratory setup is shown to be able to detect both WTLRs and changes in WTLR that are of interest to industry (i.e. 0.1–0.2 mm/year) within 1–2 h.

Journal article

Wang Y, Zou F, Cegla F, 2017, Acoustic waveguides: an attractive alternative for accurate and robust contact thermometry, Sensors and Actuators A: Physical, Vol: 270, Pages: 84-88, ISSN: 0924-4247

We report a robust and very precise method of measuring temperature using ultrasonic waves. Solid stainless steel waveguides are used to provide well-defined and stable ultrasonic wave propagation paths. Ultrasonic wave velocity is strongly temperature dependent. The arrival times of the ultrasonic wavepackets along a waveguide are used to infer the average temperature of the waveguide. Our ultrasonic temperature measurements exhibit a high precision (i.e. ±0.015 ⁰C) that is more than two times better than the quoted accuracy of 1/10 DIN resistance temperature detectors (RTDs). The responsiveness of the waveguides was also investigated. While ultrasonic measurements can be made at very high frequencies, the responsiveness is limited by the heat transfer into the active sensing area. The waveguides make it easy to customise the dimension of the active sensing area and a shorter response time than those of RTDs has been demonstrated. The technique presented in this paper is a robust and cost effective alternative to other contact temperature measurements.

Journal article

Zou F, Cegla F, 2017, High Accuracy Ultrasonic Corrosion Rate Monitoring, Corrosion, Vol: 74, Pages: 2663-2679, ISSN: 0010-9312

Ultrasonic testing with permanently installed transducers is widely used for online corrosion monitoring in the field. In this paper, a carefully optimized ultrasonic corrosion monitoring technique for carrying out measurements in the laboratory is presented. It is shown that for thickness measurements of a 10 mm steel component, a repeatability of ~40 nm can be maintained over the period of a day. The technique has been applied to monitoring the wall losses of a steel sample during forced and unforced corrosion experiments. All ultrasonic wall loss measurements reported have been validated by optical surface profile scans and, where possible, by analytical predictions based on Faraday’s law. Further analysis of the results shows that wall loss rates in the order of 0.1 – 0.2 mm/year can be detected within 1 – 2 hours. This state-of-the-art laboratory technique is highly accurate and responsive, and possesses the potential for becoming a powerful alternative corrosion assessment tool that is convenient to use.

Journal article

Zou F, Cegla, 2017, High accuracy ultrasonic monitoring of electrochemical processes, Electrochemistry Communications, Vol: 82, Pages: 134-138, ISSN: 1388-2481

Ultrasonic testing (UT) can be used for non-intrusive corrosion monitoring. In this paper, we firstly show that UT is not only capable of monitoring wall-thickness losses, but can also be exploited for tracking electrodeposition processes. All ultrasonic measurements reported are in agreement with analytical predictions and optical surface profile measurements. Since UT is highly sensitive to the coupling conditions and the relative acoustic properties of substrates and deposited materials, it can become an effective tool for studying the interface phenomena in which dissolution and deposition compete. Examples of these include passivation layer formation and scale deposition which are corrosion-inhibiting electrochemical processes.

Journal article

Howard R, Cegla F, 2017, Detectability of corrosion damage with circumferential guided waves in reflection and transmission, NDT & E International, Vol: 91, Pages: 108-119, ISSN: 0963-8695

There is an increasing interest in high frequency short range guided waves to screen or monitor for corrosion. This contrasts with long range guided waves (LRGWs) which screen pipes for large patches of corrosion and have been successfully used in corrosion management for the past twenty years. The fundamental setup described in this paper uses circumferential guided waves, which are excited at a single location on a pipe and travel around the pipe wall and are detected at the same location. The study uses a finite element model assisted method to evaluate the detection capability of two short range circumferential guided wave setups which use both the reflected and transmitted signals. The setups themselves consist of either an axial array of transducers, for monitoring, or a single transducer which axially scans a pipe. Both setups have an array or scan pitch between either adjacent transducers or measurements. The detection capability of the fundamental Lamb wave modes (A0 and S0) in both reflection and transmission have been compared, as well as a hybrid shear horizontal wave setup, which uses the SH0 mode in reflection and the SH1 mode in transmission. A sensitivity analysis was conducted using two separate methods to determine the probability of detection (POD) for either the reflection or transmission signals. Both methods determine a POD for a specific defect, noise level, and array or scan pitch. Probability images are produced which map the POD for a range of defect sizes. For the parameters investigated in this study, it was found that in transmission large diameter defects have a higher detectability, whereas deep, narrow diameter defects are more detectable in reflection. A generalised overview of the sensitivity of short range guided waves is presented by combining both the reflection and transmission PODs. The data fused sensitivity of the S0 and SH hybrid modes are given as 0.6% and 0.75% cross sectional area (CSA) respectively, allowing for the comp

Journal article

Isla J, Cegla FB, 2017, EMAT phased array: a feasibility study of surface crack detection, Ultrasonics, Vol: 78, Pages: 1-9, ISSN: 0041-624X

Electromagnetic-acoustic transducers (EMATs) consist of a magnet and a coil. They are advantageous in some non-destructive evaluation (NDE) applications because no direct contact with the specimen is needed to send and receive ultrasonic waves. However, EMATs commonly require excitation peak powers greater than 1 kW and therefore the driving electronics and the EMAT coils have to be bulky. This has hindered the development of EMAT phased arrays with characteristics similar to those of conventional piezoelectric phased arrays. Phased arrays are widely used in NDE because they offer superior defect characterization in comparison to single-element transducers. In this paper, we report a series of novel techniques and design elements that make it possible to construct an EMAT phased array that performs similarly to conventional piezoelectric arrays used in NDE. One of the key enabling features is the use of coded excitation to reduce the excitation peak power to less than 4.8 W (24 Vpp and 200 mA) so that racetrack coils with dimensions can be employed. Moreover, these racetrack coils are laid out along their shortest dimension so that 1/3 of their area is overlapped. This helps to reduce the crosstalk between the coils, i.e., the array elements, to less than −15 dB. We show that an 8-element EMAT phased array operating at a central frequency of 1 MHz can be used to detect defects which have a width and a depth of 0.2 and 0.8 mm respectively and are located on the surface opposite to the array.

Journal article

Isla J, Cegla F, 2017, EMAT Phased Array Probe for Detecting Surface Cracks, Joint IEEE International Symposium on Applications of Ferroelectrics (ISAF) International Workshop on Acoustic Transduction Materials and Devices (IWATMD) / Piezoresponse Force Microscopy Workshop (PFM), Publisher: IEEE, Pages: 41-44

Conference paper

Isla J, Cegla FB, 2017, Coded excitation for pulse-echo systems, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol: 64, Pages: 736-748, ISSN: 0885-3010

Pulse compression has been used for decades in radar, sonar, medical, and industrial ultrasound. It consists in transmitting a modulated or coded excitation, which is then cross-correlated with the received signal such that received echoes are time compressed, thereby increasing their intensity and hence the system resolution and signal-to-noise ratio (SNR). A central problem in pulse-echo systems is that while longer coded excitations yield higher SNRs, the length of the coded excitation or sequence is limited by the distance between the closest reflector and the transmitter/receiver. In this paper, a new approach to coded excitation is presented whereby receive intervals or pauses are introduced within the excitation itself; reception takes place in these intervals. As a result, the code length is no longer limited by the distance to the closest reflector and a higher SNR increase can be realized. Moreover, the excitation can be coded in such a way that continuous transmission becomes possible, which reduces the overall duration of the system response to changes in the medium. The optimal distribution of the receive intervals within the excitation is discussed, and an example of its application in industrial ultrasound is presented. The example consists of an electromagnetic-acoustic transducer driven with 4.5 V, where a clear signal can be obtained in quasi-real-time (e.g., ~9-Hz refresh rate), while commercially available systems require 1200 V for a similar performance.

Journal article

Benstock D, Cegla FB, 2017, Extreme value analysis (EVA) of inspection data and its uncertainties, NDT & E International, Vol: 87, Pages: 68-77, ISSN: 0963-8695

Extreme value analysis (EVA) is a statistical tool to estimate the likelihood of the occurrence of extreme values based on a few basic assumptions and observed/measured data. While output of this type of analysis cannot ever rival a full inspection, it can be a useful tool for partial coverage inspection (PCI), where access, cost or other limitations result in an incomplete dataset. In PCI, EVA can be used to estimate the largest defect that can be expected. Commonly the return level method is used to do this. However, the uncertainties associated with the return level are less commonly reported on. This paper presents an overview of how the return level and its 95% confidence intervals can be determined and how they vary based on different analysis parameters, such as the block size and extrapolation ratio. The analysis is then tested on simulated wall thickness data that has Gaussian and Exponential distributions. A curve that presents the confidence interval width as a percentage of the actual return level and as a function of the extrapolation ratio is presented. This is valid for the particular scale parameter (σ ) that was associated with the simulated data. And for this data it was concluded that, in general, extrapolations to an area the size of 500–1000 times the inspected area result in acceptable return level uncertainties (<20% at 95% confidence). When extrapolating to areas that are larger than 1000 times the inspected area the width of the confidence intervals can become larger than 30–50% of the actual return level. This was deemed unacceptable: for the example of wall thickness mapping that is used throughout this paper, these uncertainties can represent critical defects of nearly through wall extent. The curve that links the confidence interval width to the return value as a function of extrapolation ratio is valid only for a particular scale parameter value of the EVA model. However, it is imagineable that a few of such relati

Journal article

Cegla FB, Herdovics B, 2016, Structural Health Monitoring (SHM) using torsional guided wave EMATs, Structural Health Monitoring: an international journal, Vol: 17, Pages: 24-38, ISSN: 1741-3168

Torsional guided wave inspection is widely used for pipeline inspection. Most commonly piezoelectric andmagnetostrictive transducers are used to generate torsional guided waves. These types of transducers require bondingor mechanical contact to the pipe which can result in changes over time which are undesirable for Structural HealthMonitoring. This paper presents a non-contact Lorentz force based Electromagnetic Acoustic Transducer for torsionalguided wave monitoring of pipelines. First, the excitation mechanism of the transducer is simulated by analyzing theeddy current and the static magnetic field using the finite element method. An EMAT transformer model is presentedwhich describes the eddy current generation transfer function and the ultrasound excitation. Independently simulatededdy current and magnetic fields are used to calculate the Lorentz force that an EMAT array induces on the surface ofa 3 inch schedule 40 pipe and an explicit finite element solver is then used to simulate the elastic wave propagationin the pipe. Then, the reception mechanism and the expected received signal levels are discussed. The constructionof an experimental transducer is described and measurement results from the transducer setup are presented. Themeasured and modeled performance agree well. Finally, a monitoring example is presented where an artificial defectwith 3% reflection coefficient is introduced and successfully detected with the designed sensor.

Journal article

Howard R, Cegla F, 2016, On the probability of detecting wall thinning defects with dispersive circumferential guided waves, NDT & E International, Vol: 86, Pages: 73-82, ISSN: 0963-8695

Two ultrasonic techniques are well established for pipe inspection and monitoring: highly sensitive ultrasonic spot thickness measurements, which can be presented in C-scan form, or low frequency guided waves that rapidly screen large areas for big defects. Recently there has been a growing interest in pipe inspection and monitoring systems exploring the middle ground between these two techniques by using higher frequency guided waves over shorter distances. In this paper the use of an axial transducer array (more suitable for monitoring applications) or a single axially scanned transducer (more suitable for inspections) that sends and receives dispersive circumferential guided waves around a pipe has been studied. The presence of a defect is detected as a result of a change in the wave travel time around the pipe circumference as a result of the thickness reduction. Both measurement modalities have a pitch between adjacent transducers/measurements. By approximating the pipe to a plate, a finite element model assisted method to evaluate the detection capabilities (probability of detection-POD) of this short range guided wave technique as a function of scan or transducer pitch is presented. The performance of three guided wave modes (A0, S0, and SH1) are compared in a 10mm thick plate. The results help to optimize the pitch and defect sensitivity of the setup. For the parameters investigated in this study, it was found that the S0 mode, at 2MHz.mm, is the most suitable for detecting wide shallow defects. For the same detection capabilities a much wider pitch can be used for S0 mode transducers as compared to SH1 mode transducers. Whereas the SH1 mode, at 3MHz.mm, is better suited to detecting narrow and deep defects using a relatively small pitch. The S0 mode is much less sensitive to these defects. The A0 mode was excluded from the POD analysis because it had a much higher variability in average thickness measurements, at comparable SNRs, compared to the other two m

Journal article

Cegla FB, Gajdacsi A, 2016, The effect of corrosion induced surface morphology changes on ultrasonically monitored corrosion rates, Smart Materials and Structures, Vol: 25, ISSN: 1361-665X

Corrosion rates obtained by very frequent (daily) measurements with permanently installed ultrasonic sensors have been shown to be highly inaccurate when changes in surface morphology lead to ultrasonic signal distortion. In this paper the accuracy of ultrasonically estimated corrosion rates (mean wall thickness loss) by means of standard signal processing methods (peak to peak—P2P, first arrival—FA, cross correlation—XC) was investigated and a novel thickness extraction algorithm (adaptive cross-correlation—AXC) is presented. All of the algorithms were tested on simulated ultrasonic data that was obtained by modelling the surface geometry evolution coupled with a fast ultrasonic signal simulator based on the distributed point source method. The performance of each algorithm could then be determined by comparing the actual known mean thickness losses of the simulated surfaces to the values that each algorithm returned. The results showed that AXC is the best of the investigated processing algorithms. For spatially random thickness loss 90% of AXC estimated thickness trends were within −10 to +25% of the actual mean loss rate (e.g. 0.75–1.1 mm year−1 would be measured for a 1 mm year−1 actual mean loss rate). The other algorithms (P2P, FA, XC) exhibited error distributions that were 5–10 times larger. All algorithms performed worse in scenarios where wall loss was not distributed randomly in space (spatially correlated thickness loss occured) and where the overall rms of the surface was either growing or declining. However, on these surfaces AXC also outperformed the other algorithms and showed almost an order of magnitude improvement compared to them.

Journal article

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