Ebook: Electromagnetic Nondestructive Evaluation (XV)

I deem it an honor and a privilege to be asked to write a foreword for the volume containing the proceedings of the 16^th International Workshop on Electromagnetic Nondestructive Evaluation (ENDE'2011) held at the Indian Institute of Technology, Madras (IITM) in Chennai, India.

The workshop lived up to its reputation as one of the premier meetings focused on electromagnetic nondestructive evaluation methods. The technical sessions were uniformly of high quality and the discussions following each presentation were stimulating. The hospitality and the care lavished on the participants by the organizers were extraordinary. Starting with the evening reception, through the inaugural ceremony, numerous technical sessions and on down to the closing ceremony, the workshop ran smoothly, and an air of professionalism pervaded the three days. In short, the workshop was a spectacular success.

The location and setting also contributed to the uniqueness of the meeting. The workshop was held on the lush and verdant campus of IITM, located in the cultural capital of South India, Chennai in the state of Tamilnadu. It is not surprising that, given the setting, culture was never far away. Tamil, the language spoken in Tamilnadu, is one of the oldest continuously spoken languages in the world with rich literature dating back to 200 BCE. Tirukkural, a collection of 1330 couplets written sometime between the 2^nd and 6^th centuries AD by the celebrated poet Thiruvalluvar, continues to enrich the mind, body and soul of its citizens. The city is the home of the South Indian classical music called Carnatic music, and organizes what is arguably the largest number of music concerts in the world during the months of December and January each year. These concerts attract thousands of music lovers each day. Tamilnadu is also home to one of the oldest forms of classical dance, known as the Bharatanatyam. The origins of this can be traced back to a treatise, Natya Shastra, that dates back to 200 BCE. Those attending the workshop were treated to a tasteful sampling of this art form prior to the banquet. The state has a strong culinary tradition, which draws on the imagination of its people and its bountiful land and oceans. Perhaps one of the most impressive contributions of its people is to architecture and bronze sculptures. Temples, over a thousand years old, with their graceful gopurams (towers) are to be found throughout the land. Bronze sculptures, representing the finest achievement in art and known the world over, adorn many a museum in the west and east. For those blessed with a sense of curiosity and inquisitive nature, the city and its environs are a real treasure house.

The sad note during the workshop, however, was the earthquake off the coast of Japan and the subsequent tsunami. The loss of thousands of lives and the damage inflicted on the land and its people cast a pall over the proceedings of ENDE 2011, with the inability of our Japanese friends at the workshop to connect with their families and loved ones only adding to the distress. The workshop participants were quick to convey their sympathies to the Japanese people and express solidarity with those in distress.

A workshop is considered to have been a success when the technical program is solid and the participants take back something new. A workshop can become spectacularly successful when the solid technical program is leavened with other enriching activities. ENDE'2011 will long be remembered as one such spectacular workshop.

Satish Udpa

A semi-analytical approach is proposed for the rapid calculation of electromagnetic fields induced in a planar stratified media by inductive sensors with complex shapes. Theoretical aspects of the method and new shapes modelled are presented, then comparisons of results obtained in classical cases with reference data are discussed. Finally, an application of this development to the simulation of eddy current testing with a semi-analytical model is detailed.

This contribution presents a fast simulation technique of eddy current testing (ECT) signals due to narrow cracks embedded in a planar stratified medium. Starting from the Boundary Element Method (BEM) approach for ideal and narrow cracks, a collaborative work by the authors has led to the development of a more general semi-analytical model treating several parallel and/or orthogonal narrow cracks embedded in a planar multilayered structure, without any constraint on the numbers of layers or flaws. Simulation results obtained with this method have been compared with experimental data, results from literature and Finite Element Method (FEM). The theoretical approach and results obtained are presented and some perspectives for further extension are discussed.

During some ultrasonic non destructive evaluation inspections implying high temperatures conditions, Electromagnetic Acoustic Transducers (EMATs) are very useful because, contrary to conventional piezoelectric transducer, they do not require any coupling fluid and they have the capability to generate a great number of mode propagation by interchanging the geometry of the emitting electromagnetic apparatus. However, the poor transduction between ultrasonic energy and electromagnetic energy must be improved in order to increase the performances of these transducers. This paper aims to develop some new simulation tools in order to be able to make some parametric studies by using semi-analytical models with reasonable time computations sufficiently flexible for dealing with a great number of geometric configurations. In order to achieve this goal, time harmonic eddy current modules already implemented into the CIVA software are firstly extended to time domain and then they are connected to ultrasonic semi-analytical models to provide at low cost of computation the response of the EMAT transducer. This paper presents the different steps of the developments which have been followed to obtain the new functionalities into the CIVA platform.

In this work we discuss a numerical model of the forward problem in NDE. The model is based on a volume integral formulation. To treat 3D geometries and complex constitutive relationships, the fully populated matrix arising after discretization has been efficiently compressed, using also parallel algorithms. The method utilizes an effective low-rank approximation of the submatrices representing “far” interactions, obtained through a modified Gram-Schmidt - QR factorization method. Some numerical examples show the advantages of the proposed approach

Motivated by quantitative evaluation of aging properties of electrical cables, a computational method for identifying physical parameters of dielectric materials is considered. First, a nondestructive testing using micro-waveguide is described by electric and magnetic fields defined on the problem. Secondly, an inversion technique based on Markov Chain Monte Carlo method (MCMC) is employed for solving the electromagnetic interrogation treated here. Finally, a numerical scheme of the direct problem is developed using the finite-difference time-domain method (FDTD). Applying Metropolis-Hasting algorithm to the inversion presented here, results on computational experiments are demonstrated in order to show the feasibility and applicability of the proposed method.

One of the goals of the steel-based industrial applications is the detection of the mechanical stress state of steel structures. The mechanical tensile stress modifies the magnetic anisotropy in ferromagnetic materials. This fact can be used to detect changes in the stress state of the material using magnetic inspection methods. The Magnetic Barkhausen Noise is one of the most sensitive magnetic non-destructive methods to the presence of applied stress. The present work presents a method for the real-time detection of dynamic changes of the stress field of samples under variable applied tension. The method involves the detection of biaxial applied stress distributions.

The Magnetic Barkhausen Noise (MBN) technique is considered as a potential Non-Destructive Evaluation (NDE) method for assessing material properties in case-hardened gears. For gear manufacturing industries, the evaluation of material properties such as tempering induced changes in hardness level and the associated microstuctural changes, variations in case-depth of hardened layer, grinding process induced alteration in microstructure, hardness and residual stress distributions are of primary importance in order to ensure the quality of gears. During the service life of gears, the evaluation of the contact fatigue damage and the assessment of remaining quality life of gears are important in order to prevent catastrophic failures. This paper presents an overview of different research and developmental studies carried out at Design Unit, Newcastle University on the MBN technique suitable for different applications in the evaluation of quality of gears.

Flexible risers are used to link subsea pipelines to oil and gas production floating installations. These risers are multilayered pipes formed by an inner flexible metal carcass surrounded by polymer layers and spiral wound steel ligaments, also known as armor wires. In this riser, the armor wires sometimes break during installation or due to mechanical fatigue. Some methods have been proposed to monitor the armor integrity. However, until now there is not a practical method that allows the automatic nondestructive detection of individual armor wires rupture. In this work, a very effective method for the automatic detection of armor wires rupture using Magnetic Barkhausen Noise and clustering methods is presented.

Experiments have been carried out to understand the feasibility of using MBN method to estimate the residual stresses and the effect of cold working of steel. In the first set austenitic stainless steel pipe welds of 324mm and 168 mm outer diameters with 25 mm and 14 mm thickness respectively have been used for estimation of residual stress. Pipes are welded using the hot wire GTAW with narrow gap technique. MBN analysis for these pipe welds shows that MBN peak height is sensitive to both microstructure and residual stress. But peak position is insensitive to residual stress however, it varies with micro-structural variation. These pipe welds have the same microstructure and varying residual stress. Second set of experiments are carried out on carbon steel subjected to 10 and 20% cold working and on low alloy steel subjected 8, 10 and 15% cold working. Results shows that rise in MBN parameters like RMS, amplitude and energy is faster till 10% of cold working thereafter it saturates.

NDE techniques that use some form of electromagnetic excitation are termed as electromagnetic NDE techniques and these include eddy current, magnetic flux leakage, Barkhausen emission, micromagnetic, potential drop, microwave etc. techniques. The concurrent advances in sensors, signal processing, instrumentation, modeling etc. have enhanced the capabilities of the traditional electromagnetic NDE techniques enabling detection and characterization of incipient defects, changes in microstructures and accumulated plastic deformation or damage prior to crack formation. This paper discusses some recent advances in electromagnetic NDE techniques for detection of defects, corrosion damage and microstructure changesin both ferrous and non-ferrous materials with the help of example case studies and applications.

This paper is concerned with evaluation of subsurface shape of fatigue crack (FC) by eddy current testing (ECT). Since FC often remains tightly closed, it is difficult to identify deep-lying profiles by visual inspection. Our study is aimed at developing a practical method for detecting and characterizing the direction of crack growth from surface image obtained by scanning transmitter-receiver (TR) probe. A various type of fatigue cracks are made from compact tension (CT) specimen of aluminum alloy samples. The validity and feasibility of the proposed method are demonstrated by collecting data in our laboratory experiments.

This paper proposes the description of a simple method for determination of metallic fillers concentration in composite materials laminated, plastic-metallic fillers type. The bulk resistivity and dielectric permittivity measurements of the studied composites have been performed, these measures being less influenced by the fillers concentration. The amplitude measurement of e.m.f induced in the reception coil of the focused transducer with orthogonal coils and their averaging for the scanning of a relative large zone of the composite has proven to be an efficient method. The average amplitude linearly decreases with the increasing of fillers concentration, and has different values for Cu and AISI 316 fillers.

The residual magnetic field from strain induced α'-martensite phase in cold worked in AISI type 304 stainless steel specimens has been non-destructively measured using Giant Magneto Resistance (GMR) sensor. Correlation has been established between GMR sensor output and volume fraction of martensite accurately determined using X-ray diffraction analysis. Metallographic examination has been carried out to understand the microstructural changes with amount of cold work. Using GMR sensor it is possible to detect 2.8% martensite phase in 10% cold worked specimens. As the GMR sensor saturates after 1.4 mT, identification of severely deformed region i.e. equivalent of more than 40% cold work or 14% martensite phase in components made of AISI type 304 stainless steels is a possibility.

Tensile test experiments were in-situ monitored using Pulsed Eddy Current (PEC) NDE technique for metallic materials. Materials such as Manganese Stainless steel (Mn-SS), SS304, Copper and Aluminum were employed in this study. Experiments were carried out to characterize the changes in PEC response due to the effect of loading pattern either continuous or interrupted loading or due to the effect of loading rate. Plastic deformation induced in the material increases the PEC signal response for all materials studied. Mn-SS material provided the best PEC response due to its property of phase transformation from austenitic (paramagnetic) to martensitic (ferromagnetic) phase as the plastic deformation increases. The effect of loading rate does not appear to influence the PEC response of materials, when the data was analyzed as a percentage of fracture strain. The effect of prior cold work could be identified using the PEC technique by characterizing the slope of PEC signal response in the elastic region when the material was subjected to an interrupted loading/unloading pattern. Offline PEC Measurements were taken along the length of the failed and the plastically deformed specimens. It was observed that from the PEC measurements, the impending failure location could be ascertained. These results suggest that PEC technique could be used as a NDE technique for material characterization and failure location identification.

Fe-Cr alloys with different Cr content were subjected to various heat treated conditions to produce σ phase. Micro-Vickers hardness and magnetic hysteresis loop (MHL) measurements were carried out at various heat treated conditions of the alloys to observe the change in mechanical and magnetic properties respectively. A drastic increase in hardness and coercivity whereas sudden drop in remanence and maximum induction were observed at large volume fraction of σ phase in higher Cr content alloy (48% Cr). At small volume fraction of σ phase no change in hardness was observed. However with the formation of small volume fraction of σ phase and its growth the magnetic properties change systematically. The study revealed that MHL technique would be a good Non-Destructive Evaluation (NDE) tool for the evaluation of σ phase in high temperature structural components made of Fe-Cr alloys.

The behaviors of magnetoacoustic emission (MAE) on cold-rolled and thermally aged Fe-Cu model alloy were investigated. The rms voltage of MAE increases with increasing reduction ratio before thermal aging due to the increase in dislocation density. As to thermal aging, the significant changes were not found on the behaviors for the undeformed specimen. This indicates the morphological change of copper precipitation has no strong effect to the interaction between domain walls and their pinning sites. On the other hand, the rms decreases when aging time elapses for the cold-rolled specimen at first, and then increases in aging over 10³ minutes. These behaviors can be explained on the basis of the microstructure changes including dislocations and copper precipitation.

Early detection of damage in cyclically loaded austenitic steel was performed with electromagnetic acoustic transducer measurements in total strain controlled low cycle fatigue tests at ambient and elevated temperature. The application of physically based measurement data, e.g. time of flight from electromagnetically activated ultrasonic signals in austenitic fatigue specimens and total strain, enables measurements of a new hysteresis relationship. This hysteresis gives, in analogy to the stress-strain hysteresis, information about the actual state of fatigue of the austenitic steel and shows significant changes in shape before specimen failure.

Duplex stainless steel (DSS) has a wide application in industrial structures that needs strong corrosion resistance. The properties that make this class of steel very attractive for the oil refinery industry are: high strength, toughness, ductility, stress corrosion cracking resistance, even in H₂S containing environments, allied to low cost when compared to conventional stainless steel. DSS are two-phase materials which microstructure consists of grains of ferrite (δ) and austenite (γ). Nonetheless, a drawback is pointed out: exposition to temperatures in the range 300°C - 1000°C may lead to precipitation of a deleterious phase, named sigma (σ). When this occurs, the material becomes brittle and its corrosion resistance to H₂S is severely impaired. Eddy currents (EC) testing can be successfully employed to detect and quantify σ phase. The study of magnetic behavior of DSS allows a better understanding of the technological problem of detecting and quantifying the σ phase. The main focus is to understand the difference in EC signals for the different samples with σ phase, because it may be related to either the precipitation of the paramagnetic σ phase or to the decrease of the ferromagnetic δ phase. The present work is a first attempt to understand the origin of the changes observed in EC technique. We considered the hysteresis behavior as the key to understand the differences in magnetic permeability, initially at room temperature. EC measurements were made with a customized probe. It was observed that the changes in the signal of EC are predominantly from deleterious phase

In non-destructive evaluation using magnetic flux leakage, formulation of a function which associates magnetic dipole and magnetic flux density in terms of geometrical coordinates is modified in order to capture the average magnetic flux value along the sensor length for low lift-off area. The effectiveness is verified through numerical simulation scheme. Inverse analysis of two adjacent semi-elliptical surface cracks is conducted. Cracks are detected separately by using the modified function, and the restored crack shape becomes more proper.