Ebook: Electromagnetic Nondestructive Evaluation (XX)

This volume contains selected papers from the International Workshop on Electromagnetic Nondestructive Evaluation (ENDE) held at Instituto Superior Técnico, Lisbon, Portugal, September 25–28, 2016. It was the 21st edition of this Workshop and as the previous editions it was focused on the theoretical and applications research of Electromagnetic Nondestructive Evaluation methods and provided a forum for exchanging ideas and discussing recent developments.

ENDE2016 was organized under the auspices of the ENDE International Steering Committee, by Instituto de Telecomunicações and Instituto Superior Técnico.

There were a total of 97 participants, including 29 students (9 from Portugal), 11 representatives from industry/manufacturing and 3 accompanying persons). The participants were from 17 countries: Brazil, Canada, China, France, Germany, Greece, Hungary, India, Italy, Japan, Poland, Portugal, Romania, Slovakia, South Korea, United States of America and United Kingdom. The technical program was organized in 8 oral sessions: “Advanced eNDE sensors”, “Material characterization” (1) and (2), “New developments in eNDE” (1) and (2), “Analytical and numerical modelling of eNDE, Inverse problems and signal processing”, “Monitoring and diagnoses of mechanical structures” and “Innovative industrial applications of eNDE”. A special panel session on “Promoting Nondestructive Evaluation Studies In Engineering Schools” was organized. There were 70 contributions plus 1 invited and 1 keynote speaker. 41 presentations were presented in the 8 oral sessions and 29 presentations in the poster session.

The technical program encompassed one invited and one keynote speaker. The invited speaker was Professor Luís Oliveira e Silva, Professor of Physics and President of the Scientific Council of Instituto Superior Técnico, one of the few scientists in all the fields of science with two Advanced Grants from the European Research Council. Professor Oliveira e Silva gave a talk entitled “In silico extreme electromagnetic fields”. The keynote speaker was Professor Mário Figueiredo, of the Electrical and Computer Engineering Department of Instituto Superior Técnico. Professor Mário Figueiredo received the 2016 EURASIP Technical Achievement Award and since 2014 has been considered a Web of Science Highly Cited Researcher. The talk by Professor Figueiredo was about the recent advances in computational methods for imaging inverse problems, with a special emphasis on image restoration and reconstruction.

Two Awards were given, including “Best Paper Award” and “Best Poster Award”.

The Workshop Proceedings, including short versions of all contributions, have been published in a pen-drive and distributed to all participants. In this volume, 37 reviewed, revised, accepted papers are published under the title “Electromagnetic Non-Destructive Evaluation (XX)”, in the IOS Press book series “Studies in Applied Electromagnetics and Mechanics”.

To conclude, the editors want to thank all the people who have tirelessly contributed to the Workshop and to the reviewers of the extended papers.

Lisbon, April, 2017

Helena G. Ramos

Artur L. Ribeiro

Editors

We investigated magnetic properties of representative soft magnetic materials, then clarified that non-oriented iron-silicon steel maintains high permeability and shows the most stable temperature dependence of the properties in the temperature range from room temperature to 600°C. The fundamental element for the development of a magnetic field sensor using non-oriented Fe-Si steel was composed and the changes in inductance of the element, are proportional to the permeability of the material, were evaluated by applying a magnetic field to the element. The inductance decreases with increasing the magnetic field almost linearly, indicating the possibility of a magnetic field sensor operating at high temperature up to 600°C.

Conventional electromagnetic testing methods based on inducing alternating current (AC) are confined to detect facial defects because of the skin effect. Aiming at nonferrous metal detection, this paper proposes an injected direct current field measurement (IDCFM) based on a tunnel magneto resistance (TMR) sensor. By injecting a direct current (DC) to nonferrous metal, a constant magnetic field will be excited around the surface of nonferrous metal. As cracks exist inside or near the surface of the nonferrous metal, the current distribution will be disturbed, which causes the change of external magnetic field. Therefore tiny cracks on the surface and inside defects can be detected and located by using a TMR sensor with high sensitivity. A three-dimensional finite-element method was employed to model the magnetic field produced by the DC current. The difference in magnetic field under different magnitudes of direct current was discussed. Simulation results showed a linearity between the external magnetic field intensity and the injected DC intensity. Contrast experiments with coils, Hall elements, GMR sensors and TMR sensors were also carried out. IDCFM signals with different sensors were compared under the same current intensity. The performance of four different sensors under different injected direct current was also compared through experiments. Experiments indicated that TMR sensors have many advantages over other sensors. Applying TMR sensors in IDCFM method, the precision could be 10 times greater than that of GMR sensors, and hundreds times greater than that of Hall elements. Also, the current intensity in IDCFM could be reduced from 100 A to less than 1 A utilizing TMR sensors, which allows IDCFM method to operate continuously for a long time without damaging the workpiece.

This paper presents a study of an innovative front-end for UHF Radio Frequency Identification (RFID) and Partial Discharge (PD) detection application. The front-end system mainly consists of a dual-band antenna, a diplexer, and a PD detection unit. As the antenna design is the major objective of this work, a novel cavity-backed monopole antenna is thus designed, fabricated and tested. Experimental results show that the antenna can cover 649 MHz to 702 MHz and 876 MHz to 1050 MHz, which could be applied for future RFID sensor systems of condition monitoring and fault diagnostics in electrical utilities.

In this paper, a magnetic inductive head probe is used to detect micro-cracks in bearings. For the width of the crack in bearings is less than 15μm and the leakage filed is so weak, a flexible probe system is designed to keep the magnetic inductive head probe fit with the bearing surface closely. Micro cracks in a magnetic particle testing block are detected with the magnetic inductive head probe. The limit detecting crack size of magnetic particle inspection (MPI) is calculated by Boltzmann distribution function. Magnetic inductive head has a higher sensitivity than MPT in bearing parts inspection system.

The inspection of ferromagnetic materials for surface defects is usually made using the Magnetic Flux Leakage (MFL) technique. By magnetically saturating the specimen, some magnetic flux lines will escape the material in the presence of a defect, which can be used to detect and characterize the defect. However, the saturation of the ferromagnetic specimen when motion is involved can be troublesome, which can be a disadvantage in cases such as railroad inspection. This work proposes a new technique to inspect the surface of ferromagnetic materials based on eddy currents. The technique creates two consecutive pulses of currents (up to 1500 A) in a coil that generates a high magnetic field (peak around 3.5 T) in the vicinity of the sample. Simulations were made to show the method is able to detect defects in a ferromagnetic sample. Signal processing was applied to the obtained data in order to reduce the MFL component and give emphasis to the eddy currents perturbations.

For the biosensors based on electromagnetic waves, the material deposited as biosensitive element essentially determines the functioning characteristics. Ag/ZnO/ITO/glass and Ag/ZnO/SiO2/Si mesostructures were analyzed in order to use ZnO as bioactives surface. This paper presents the results obtained at the testing of metallic strip gratings, deposited on different substrates, using the electromagnetic sensor with metamaterials (MM) lens in order to prove its capability to gather the evanescent waves appeared on the edge of strips when the slits are filled with immobilized enzymes.

In this study, to evaluate the structural change due to creep degradation in P91 steels nondestructively by means of the micromagnetic multiparameter microstructure and stress analysis (3MA) device, information of dislocation density change is extracted from micromagnetic properties. Micromagnetic properties of the creep and thermal aging samples are measured by 3MA device. The relationship between measured micromagnetic properties and the low-angle grain boundary length of the samples quantified as the alternative value of dislocation density by Electron Back Scatter Diffraction (EBSD) equipment are investigated. Change of dislocation density due to creep degradation can be evaluated by some micromagnetic properties which are independent on precipitation change.

Terahertz time domain spectroscopy (THz TDS) has a wide variety of applications regarding characterization and inspection of dielectric materials such as plastics, ceramics and wood. Market prices for high-performance equipment have decreased below a hundred thousand Euros already and will soon reach levels where a wide range of industrial applications can be envisaged. Besides the detection of flaws and foreign bodies in many types of plastics and measurement of coating thickness, THz TDS can detect diffusion of water into plastics due to the high absorption coefficient of water in the THz range. This contribution regards imaging of water diffusion into a sheet of polyamide 6.6 with scanning THz TDS equipment in transmission mode. The weight increase of the water-exposed part of the sample due to water absorption was about 3% after about 300 hours of immersion, and the exposed area showed a significant decrease of the transmitted THz amplitude. Future work aims at reconstructing the moisture gradient into depth by means of inverse profiling algorithms.

The deformation-induced phase transformation hardening phenomenon is normally observed in austenitic stainless steels of the AISI 300 series. The amount of hardening is limited by the structural stability of the material. In fact, during the cold forming process, low nickel austenitic stainless steels can undergo deformation-induced phase transformation of fcc gamma-austenite to bcc alpha'-martensite (DIM). On the other hand, by heat treatments, the austenitic structure can be recovered. The reversion of martensite to austenite is studied on a set of austenitic stainless steel specimens (AISI 316). These had been submitted to 63% deformation at a temperature of (−70°C), followed by isochronic, isothermal heat treatments at temperatures between 200 and 900°C. The reversion of martensite in the specimens is studied by optical and electron microscopy, saturation magnetization and electromagnetic non-destructive (ND) methods (conductivity measurements, Feritscope, and permeability evaluation by eddy currents (EC)). A new calibration curve for the assessment of α' phase with Feritscope is presented. The results obtained with the different techniques are compared and discussed.

The theoretical base of the low frequency impedance method and possibilities of its use in the diagnostics of paramagnetic and ferromagnetic alloys (including welds) used in transport and power engineering has been explained in the article. The main attention has been paid to the measurement of stresses in the material using low-cost measurement path, e.g. handheld automated LCR meter. During laboratory tests with five signal frequencies designated relationships describing the impact of stress on the parameters of impedance. Example results are illustrated. For quantitative analysis of observed changes in impedance used precise mathematical formulas that describe the measured resistance and inductance core with its geometry and identifiable material parameters (permeability, conductivity and permittivity). Based on the obtained results it demonstrated the possibility of using low-frequency impedance spectroscopy (multi-frequency eddy currents) and low-cost measuring equipment to monitor the level of stresses. The method can be used in NDE and SHM applications.

This article deals with non-destructive evaluation of austenitic stainless steels, which are widely used as materials in technical praxis and as the biomaterials in medical praxis. Intrinsic magnetic field is investigated using the fluxgate sensor, after the applied heat treatment. The various AISI 316L austenitic steel specimens are studied under the same conditions. Several values of the heat treatment are applied, respectively. Intrinsic magnetic field is sensed and displayed. The obtained results are presented and discussed in the paper.

The degree of structural damage and phase transformations in metallic materials can be monitored by measuring the internal damping of materials, which corresponds to the absorption of mechanical energy in the internal structure of the material when it is under mechanical stress. Damping is a complex variable linked to a number of factors, which are related to modifications in the material structure, the temperature and external magnetic field, as well as the amplitude of the applied mechanical force, its frequency, displacement, deformation and the number of loading cycles. The paper proposes a compact instrument for standard laboratory measurements determining the value of the internal damping, utilizing a monolithic impedance converter. This enables the measurement of a complex electrical impedance of the ultrasonic piezo – transducer within a defined frequency range around the resonance of a mechanical specimen, automatic tracking of an amplitude drift and phase of the resonance curve.

This work deals with investigation of material property changes during metal working – concretely during milling. For eddy current detection, we are using the advanced single chip impedance converter. The device consists of AD5933 impedance converter, voltage controlled current source and MCU. The changes in impedance of a sensing probe are detected by using of four electrodes (couple of voltage, couple of current electrodes). The MBN measurements are performed by us designed device. The MBN device offers MBN measurements for wide range of excitation frequencies. A magnetization coil on a ferrite U – core is connected to an amplifier which powers this part. The MBN signal is sensed by coil placed in U – core gap and it is attached to the sample. The main aim of the work is to obtain information about the scanned surface properties using eddy current and MBN measurements.

A new mode convertor, which has four coaxial cable connectors and four semi-rigid cables situated circumferentially, was fabricated to propagate TE mode microwaves in a circular pipe in order to detect axial cracks appearing in the inner surface of the pipe. In this study, the reflections were measured using a network analyzer by propagating microwaves as a pulse through the mode convertor into a brass pipe that contains the slit on the inner surface. The reflections were evaluated with different orientations, lengths, and positions of the slit. The experimental results showed a clear difference between the signals with and without the slit.

Inspection of multi-layered riveted structures and detection of subsurface cracks under fastener head is critical to ensure safe operation of an aircraft. As defects are embedded deep in the structure, ultra-low excitation frequency is used to achieve larger penetration depth. Further, for rapid inspection of large areas, high scan velocity is required. Eddy current testing with ultra-low frequency exciting and high scan velocity poses additional challenges. This paper studies the effect of scan velocity on ultra-low frequency eddy current signals. Signal processing algorithms are developed and presented for compensating velocity effects.

Pulsed eddy current (PEC) technique is used for detection of surface and near surface flaws in electrically conducting materials. To extend the capability of the PEC technique to detect deep sub-surface flaws in thick non-magnetic materials, it is essential to enhance the sensitivity of the PEC instrument and PEC probe. In this paper, a high-sensitive PEC instrument that driver higher excitation currents is designed and developed. Further, finite element model is used to compare the detection performance of three different PEC probe configurations and to select the optimal probe configuration for detection of deep sub-surface flaws. Using the developed PEC instrument and optimized probe, it is possible to detect flaws located at 6.0 mm below the surface in an 8.0 mm thick stainless steel (SS) plate.

This study attempts to apply low frequency eddy current testing to measure the thickness of double-layered plates which consist of two 304 austenitic stainless steel plates with a thickness of about 30 mm and an air gap with a thickness of 80 mm. The results of numerical simulations show that a change in the magnetic flux density due to the thickness change of the plates appears at the center of the coil when the diameter of the exciting coil is as large as 300 mm and the exciting frequency is in the low tens of Hertz. Experimental verifications were conducted using a magneto-impedance sensor whose sensitivity was 1 mV/nT and sensing range was from 0.2 nT to 2000 nT. We used a large diameter pancake exciting coil and a compensating coil to make up for the short of the range of the magnetic sensor during the measurements. The signals of the sensor situated at the center of the exciting coil changed with the thickness of the double-layered plates, which agrees with the result of the simulations.

In this paper, a hybrid nondestructive testing (NDT) method combined with the pulsed eddy current testing (PECT) method and the electromagnetic acoustic transducer (EMAT) method has been proposed and validated through numerical simulations and experiments. First, a numerical code is developed for the simulation of hybrid EMAT/PECT signals. Second, based on the numerical simulation, the influences of the eddy current induced by the excitation coil and the eddy current due to the velocity effect of the ultrasonic wave are compared and analyzed. Third, several signal separation and extraction methods are proposed on the basis of the spectral analysis and filtering strategies. Finally, hybrid EMAT/PECT experiments are conducted. Based on the numerical and experimental results, the proposed hybrid PECT/EMAT NDT method is demonstrated both high detectability and high efficiency.

This study evaluates the applicability of 3D printing technology for creating artificial flaws that imitate real cracks for the development of electromagnetic nondestructive testing methods. A plate containing an artificial flaw with branching and electrical contact is fabricated using a powderbed-based laser metal additive manufacturing machine. Eddy current testing using an absolute pancake probe is conducted to gather the signals due to the flaw. Subsequent finite element simulations on the basis of the results of destructive testing suggests that one should design a flaw whose signals do not change so significantly even though the profile of the flaw changes almost 0.1 mm.

Lorentz force eddy current testing is a recently developed new technique in the framework of motion induced eddy current testing (MECT). In contrast to existing MECT systems, a new portable system is investigated, where the magnet is moving and the electrically conductive specimen is at rest. In this study new solution approaches for possible patterns of motion are elaborated. Finally, the translational and rotational motion of an axial magnetized permanent magnet above a specimen with a defect are discussed.

Centrifugal casting tubes have uneven surfaces and cannot be inspected by Ultrasonic Testing (UT). Therefore, not all products are checked. Electromagnetic acoustic tests (EMATs) were conducted on centrifugal casting tubes, and their effectiveness was examined. However, signals derived by EMAT is very poor and weak. Acoustic attenuation of centrifugal casting tubes is very high due to their large grains. We developed a C-C impedance-matching circuit to produce electromagnetic acoustic waves, applied it to aluminum specimens, and examined the C-C matching effectiveness.