Ebook: Electromagnetic Nondestructive Evaluation (XIII)

The 14th International Workshop on Electromagnetic Nondestructive Evaluation (ENDE) was held at the Crowne Plaza Hotel in Dayton, Ohio, USA from 21 July through 23 July 2009. The last time the workshop was held in the USA, was in East LansingMichigan in 2004. The Ende activities in the Dayton area reflect the local interest in aerospace sustainment.

Eighty participants officially registered from the United States, Japan, Korea, France, Italy, Romania, Greece, Poland, India, and Germany. There were 59 oral and poster presentations made at the workshop. 39 papers were submitted for publication, and 37 peer reviewed papers have been accepted and appear in these proceedings.

The effort put forth by many individuals was essential to the success of the workshop. I would like to express my sincere gratitude for the help I received from the co-editors of these proceedings, Dr. Mark Blodgett (AFRL), Dr. Buzz Wincheski (NASA), and Dr. Nicola Bowler (ISU). I also wish to thank the session organizers and chairs: Mr. Juan Calzada, Dr. Buzz Wincheski, Dr. Nicola Bowler, Dr. Michael Havrilla, Dr. Mark Blodgett, Dr. Marcus Johnson, Dr. Fumio Kojima, Dr. Toshiyuki Takagi, Dr. John Aldrin, Dr. Eric Lindgren, Mr. Gary Steffes, Mr. Charles Buynak, Dr. Lalita Udpa, and Dr. Antonello Tamburrino.

Others who deserve acknowledgement include: Ms. Cathy Griffith and Ms. Nancy Johnson from General Dynamics Information Technology for their fantastic organizing efforts, and the NDE division and branch chiefs, Dr. Chuck Ward and Ms. Monica Poelking for their guidance. Special thanks to Dr. Neil Goldfine and Dr. Yanko Sheiretov for the excellent keynote presentation and Dr. Ronald Kerans for the dinner talk on the “Innovations in the Advent of Powered Flight and the Wright Approach to Program Management”. I am indebted to the students at the AFRL NDE branch Mr. Ryan Mooers, Ms. Brooke Johnson, and Mr. Matt Cherry for their technical help during the conference. Last but not least, I wish to thank the attendees, for their presentations and manuscripts, and the reviewers, for their time and energy which was necessary to produce a high quality volume of work.

Mr. Jeremy Knopp

We have computed the impedance variations of coils due to cracks in bore holes using a boundary element method. The problem is formulated using a scalar analysis of the electromagnetic field into transverse electric and transverse magnetic modes. A formal solution of the scalar equations gives the potentials in integral form with scalar kernels. The kernels take account of cross-coupling between the modes that is a consequence of ensuring the continuity of the tangential electric and magnetic fields at the surface of the hole. A dyadic kernel, formed using the scalar Green's kernels, incorporates the field continuity conditions at the surface of the hole. As a results, the flaw field is simply expressed as an integral over the flaw region. Following an earlier calculation for planar structures, the effect of a crack is represented by a current dipole layer. An electric integral equation determines the dipole density at the crack but instead of solving the equation directly, a numerical approximation is found from a system of linear simultaneous equations derived using a boundary elements approximation. The probe impedance variation can be computed for infinitesimally thin cracks and for slots with a small opening. Numerical predictions of the coil impedance due to the crack have been compared with results from finite element calculations showing close agreement.

Eddy current coil signals due to a crack at the edge of a right angled conductive wedge have been calculated using a boundary element scheme based on an electric field integral equation with a quarter space kernel. We consider the case in which the edge is perpendicular to plane of a crack which intersects both horizontal and vertical surfaces of the conductive wedge. The properties of the kernel mean that numerical solutions based on boundary or volume elements, can be found by using discrete approximation of the field in the domain of the flaw. Here we describe in general terms the development of a quarter space kernel for finding the quasi-static field due to a corner crack. Coil impedance variations due to the flaw have been calculated from the field and compared with experimental measurements to validate the predictions.

In this paper, we accurately and carefully characterize a ferrite-core probe that is widely used for aircraft inspections. The characterization starts with the development of a model that can be executed using the proprietary volume-integral code, VIC-3D©, and then the model is fitted to measured multifrequency impedance data taken with the probe in freespace and over samples of a titanium alloy and aluminum. Excellent results are achieved and will be discussed.

The numerical modeling of the eddy current inspection of Steam Generator (SG) tubes near Tube Support Plates (TSP) with Quatrefoil-shaped openings is examined. The modeling approach, developed in the CIVA platform, is based on a hybridization of the Volume Integral Method (VIM) with the Finite Elements Method (FEM). In this way, the complexity of the tube – TSP is resolved using the FEM whereas the flaw interaction can be efficiently calculated by means of the VIM. Material deposit in the TSP openings, responsible for clogging up effects, can be also taken into account with this approach and will be considered in a next stage.

The Interdigitated Electrode Dielectrometer (IDED™) is suitable for measurements on insulating or slightly conducting dielectric materials. The JENTEK team is now developing solutions for several practical applications:

• Quality control and degradation monitoring in ceramic matrix (CMC) and glass fiber composites.

• Cure monitoring of polymers, epoxy, sealants, etc.

• Measurement of porosity and thermal conductivity in ceramic thermal barrier coatings.

• Moisture measurement in transformer oil and pressboard.

• Asphalt porosity measurement

• Thin film characterization.

A microwave technique for nondestructive evaluation of radome sandwich structures is presented. The method employs a rectangular patch sensor for detecting defects of the radome core, such as water ingression, in the form of a change in resonant frequency of the sensor due to permittivity variations in the test-piece. Using well-established models that predict the resonant frequency of a rectangular patch antenna in the presence of a dielectric superstrate, the effects of different patch sensor parameters on sensor performance are analyzed. Based on this analysis, design of a half-wave patch sensor is optimized. A prototype half-wave patch sensor is fabricated and tested. Experimental results show that the sensitivity of the sensor to permittivity changes in the core of a laminar structure is better than Δε = 0.046.

A simple method is presented to extract the dielectric constant and conductivity of a low-loss conductor-backed material layer using measured natural resonance frequencies. Because the material is assumed to have a small value of conductivity, a number of approximations can be invoked that simplify the computation of the theoretical natural frequencies and thus make the technique more robust. Experimental results are used to validate the approximations and demonstrate the usefulness of the technique.

A concentric coplanar capacitive sensor is designed for the quantitative characterization of material properties for dielectrics. The sensor output signal, transcapacitance C_T, is related to the thickness and dielectric constant of the material under test. Electrostatic Green's functions due to point charges over different dielectric structures are derived through the Hankel transform in cylindrical coordinates, given the cylindrical symmetry of the proposed sensor. Numerical implementations based on the Green's functions are presented. Benchmark experiment results are provided and excellent agreement with numerical results is observed.

Micro residual stresses (MRS) of the 2nd and 3rd order play an important role in the fracture mechanical analysis of thermally-cycled materials and thus in lifetime analysis of such components. In multi-phase materials there can exist two kinds of MRS: Thermally-induced MRS of the 2nd order and coherent MRS of 3rd order. The first appear when individual material phases exhibit different thermal expansion coefficients and the second occur when the lattice parameters of the second phase particles which are embedded coherently in the matrix and the lattice parameter of the matrix are different. The main emphasis of the presented research work is the development of a micro-magnetic non-destructive technique for quantitative characterization and separation of MRS of 2nd and 3rd order in iron-based materials.

Modifications on surfaces in mechanical engineering or electronic industry are used to enhance performance or lifetime of technical systems. Surfaces and surface coatings are specifically processed to adjust required properties depending on the component's task. There are several processes, e.g. coating of conductive or non-conductive layers, or special mechanical treatments. In many applications it is essential to control the surface layer properties, because the thickness and quality of these layers define future performance of the entire product and may change during fabrication or operation. This paper presents a new Eddy Current device, which is able to measure at very high frequencies up to 100 MHz. By using high frequencies, we are able to characterize thin conducting layers, near to the surface with minor influences of the substrate. Due to the skin effect depending penetration depth, the use of frequency sweeps in the range of 3 decades allows to obtain information on electrical conductivity from different depths below the surface. The information about the conductivity can be used to distinguish parameters of mono and multi layer structures, such as the material itself, thickness, hardness, stress, states of cold work, and microstructure properties. The paper presents results from samples with different conductive coating on conductive substrates and the analysis of different mechanically treated surfaces like shot peened surfaces

The interaction of electromagnetic radiation with small metallic particles has been studied in greater detail during the last decade. It is well known that the noble metallic nanoparticles, like gold and silver exhibit remarkable optical properties, viz, strong colors exhibited by these nanoparticles. These particles acquire a characteristic color due to plasmon resonance. Plasmon resonance occurs due to coherent oscillation of the conduction band electrons induced by the incident electromagnetic field at optical frequencies. Optical techniques are conventionally used to detect the surface plasmon resonance modes in metallic nanoparticles with nanometer resolution. In these techniques, the electric field around the nanoparticle is usually sensed and imaged. Moreover, these techniques are used at optical frequencies. The imaging of magnetic field around the nanoparticles at high frequencies is very complicated. The imaging of magnetic field around nanoparticles at low frequency electromagnetic radiation has not been reported so far in the literature, to the knowledge of the authors. In this paper, we report a new methodology to image magnetic moments of metallic nanoparticles in low frequency electromagnetic fields. To accomplish this, a traditional atomic force microscope (AFM) is externally modified to detect near-field magnetic fields of the nanoparticles. Samples of silver and platinum nanoparticles are kept in the electromagnetic field excited at frequencies in the range of 30-100 kHz. The magnetic field around the nanoparticles is then detected by a magnetic AFM tip-cantilever positioned at a distance of few nanometers. The output of the cantilever is used to separate topography and magnetic field image using external electronic instrumentation. Magnetic field images are obtained at different frequencies and the effect of size, shape and frequency on the magnetic field distribution is studied. The results of the magnetic field distribution are analyzed in view of the possibility of using the methodology for sensing application.

Recent research results indicated that eddy current conductivity measurements might be exploited for nondestructive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. This paper presents new results that indicate that in some popular nickel-base superalloys the relationship between the electric conductivity profile and the sought residual stress profile is more tenuous than previously thought. It is shown that in IN718 the relationship is very sensitive to the state of precipitation hardening and, if left uncorrected, could render the eddy current technique unsuitable for residual stress profiling in components of 36 HRC or harder, i.e., in most critical engine applications. The presented experimental results show that the observed dramatic change in the eddy current response of hardened IN718 to surface treatment is caused by very fine nanometer-scale features of the microstructure, such as γ' and γ" precipitates, rather than micrometer-scale features, such as changing grain size or carbide precipitates.

The aim of this research is to make a high performance EMAT, highly sensitive, with low electric power consumption and wireless data communication. For this purpose, we introduced the following new devices and techniques into the EMAT: 1) A high efficiency power supply circuit to generate a strong oscillating magnetic field in the excitation coil, and 2) Effective use of high quality noise reduction to increase S/N ratio of the received signal. This EMAT system detects fatigue cracks in thick austenitic stainless steel 316L specimens by angle beam inspection as well as detecting EDM slits by the normal beam method.

The objective of this paper is to find sizing methodologies of pipe wall thinning using Electro Magnetic Acoustic Transducer (EMAT). First, SUS304 test specimens fabricated by an artificial corrosion on the back surface are provided and measurements are made by scanning the transducer on the front surface. Secondly, in order to clarify the propagation of an ultra-sound wave inside sample materials, simulation experiments are implemented using our developed numerical code. A sizing methodology for detecting and characterizing corrosion geometries is proposed based on time of flight diffraction method (TOFD). We report the method proposed is effectively examined through various kinds of test specimens.

Prior work has demonstrated the potential to characterize surface and sub-surface pitting corrosion in both aircraft structures and tubing through the application of model-based inversion schemes to eddy current inspection data. However, there has been limited progress to transition such technologies to application due in part to questions about the robustness of the inverse method schemes to the expected variability with in-field NDE measurements. In this paper, a sensitivity analysis is performed for this case study problem to quantify the impact of potential sources for variation on the performance of NDE procedures incorporating inverse methods. Certain data processing steps, including careful feature extraction, background clutter removal and compensation for variation in the scan step size through the tubing, were found to be critical to achieve good estimates of the pit depth and diameter. Variance studied in model probe dimensions did not adversely affect inversion performance.

A particular application of the Volume Integral Method (VIM) dedicated to the efficient simulation of eddy current testing configurations involving several objects with very different dimensions is presented in this paper. Other problems addressed by this development are the modeling of flaws with complex shapes and the efficient computation of interactions between remote objects. Simulation results obtained with this direct model have been processed by the statistical module of the CIVA platform in order to carry out Probability of Detection (POD) calculations in the case of the ECT of a rivet neighbored by a flaw. Two parametric models used for the POD estimation are presented and simulation results are discussed.

A significant portion of aging aircraft nondestructive inspection (NDI) applications exist due to the frequent occurrence of maintenance induced damage. The maintenance of Boeing 737 fuselage lap joint structures, for instance, includes the removal of sealant which can result in scribing of the fuselage skin panels. Scribing is a precursor to crack generation and, therefore, it is imperative that this damage is detected and repaired. Cracks emanating from scribes may grow under the lap joint requiring the inspection of crack defects in a second or third layer of a multi-layered structure.

Wyle is developing a nondestructive inspection (NDI) system based on a Giant Magneto-Resistance (GMR) eddy current approach [1]. Unlike conventional eddy current coils, GMR sensors enable crack detection of high sensitivity deep into the test structure and are, unlike conventional ultrasonic testing, not limited to first layer inspection.

The multi-layer lap joint structure inspection has proven to be a difficult problem; previously fielded ultrasonic phased-array approaches for inspection were considered overly burdensome in their complexity of setup and use. Furthermore, they require the expensive removal of the aircraft paint and decals prior to inspection to ensure proper transducer coupling. The Wyle GMR system probes the multi-layer aluminum airframe by inducing electrical eddy currents via an alternating magnetic field transparent to paint and therefore requires no significant preparation of the aircraft. Wyle has worked with Boeing to package this technology into a handheld instrument that relies on limited expertise by the user to easily perform this time intensive inspection in a simplified go/no-go output.

Scribes and cracks will alter the flow of the eddy currents and therefore the magnetic field they produce which are detected by the GMR sensor. However, eddy current, as well as any NDE method, are susceptible to non-critical sources of indication responses due to the structural geometry. Recent advancements toward understanding these sources of noise in a lap joint, recognizing them and separating their responses from the target response are presented.

This paper assesses the impact of various eddy current probe parameters in terms of the probe's crack using Wyle's fully automated Eddy Current Inspection Station (ECIS). Probes were manipulated by the ECIS to simulate a certain coil orientation, rotation, or tilt then scanned and indexed across 15 × 8 × 3 mil EDM notch for calibration. Following calibration a set of fatigue cracks ranging from 3 to 23 mils in depth were inspected in both transverse and longitudinal (in-line) orientations with respect to the scan direction with indexing consistent with calibration. Layers of tape were also added to the probe shoe to simulate liftoff, and a potentiometer was added in series with the drive coil to vary the field produced by the drive coil. Drive frequency was varied to assess the system response as a function of drive frequency, and probes of various degrees of imbalance were used to assess the impact of probe balance. For all tests the gain calibration sensitivities were recorded and used to normalize crack responses to effectively obtain un-calibrated crack responses. Calibrated and un-calibrated results were then compared and the experimental results were analyzed and reported. Some work was done to evaluate possible techniques to compensate for various critical probe parameters, but that work will be the topic of a future paper.