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This paper presents results of an eddy current model validation study. Precise measurements were made using an impedance analyzer to investigate changes in impedance due to Electrical Discharge Machining (EDM) notches in aluminum plates. Each plate contained one EDM notch at an angle of 0, 10, 20, or 30 degrees from the normal of the plate surface. Measurements were made with the eddy current probe both scanning parallel and perpendicular to the notch length. The experimental response from the vertical and oblique notches will be reported and compared to results from different numerical simulation codes. [ABSTRACT FROM AUTHOR]

A finite element method (FEM) model was created to calculate the change in impedance of a coil due to the presence of a notch in a plate. The rectangular notches were created via electrical discharge machining (EDM) in a thick aluminum plate and were positioned at normal and oblique angles (10, 20, and 30 degrees) with respect to the vertical axis of the coil. The FEM method was chosen for this model due to its ability to solve problems in complicated geometries with the use of irregular mesh elements to discretize the solution domain. The change in impedance was calculated from the field variables in the simulation for each probe position along the parallel axis of the plate. The error between the model and the experimental data was approximately 5% for the majority of cases. The validated model was used to investigate more complex problems. [ABSTRACT FROM AUTHOR]

In this investigation, a T-shaped aluminum alloy isogrid stiffener element used in aerospace applications was inspected with ultrasonic phased array methods. The isogrid stiffener element had various crack configurations emanating from bolt holes. Computational simulation methods were used to mimic the experiments in order to help understand experimental results. The results of this study indicate that it is at least partly feasible to interrogate this type of geometry with the given flaw configurations using phased array ultrasonics. The simulation methods were critical in helping explain the experimental results and, with some limitation, can be used to predict inspection results. [ABSTRACT FROM AUTHOR]

Comprehensive and accurate inspections of welded components have become of increasing importance as NASA develops new hardware such as Ares rocket segments for future exploration missions. Simulation and modeling will play an increasing role in the future for nondestructive evaluation in order to better understand the physics of the inspection process, to prove or disprove the feasibility for an inspection method or inspection scenario, for inspection optimization, for better understanding of experimental results, and for assessment of probability of detection. This study presents simulation and experimental results for an ultrasonic phased array inspection of a critical welded structure important for NASA future exploration vehicles. [ABSTRACT FROM AUTHOR]

This paper presents a model-assisted probability of detection (MAPOD) study for inspection of a two-layer airframe structure. Eddy current measurements for varying crack length around fastener holes in a two-layer aluminum structure are studied using both experimental and model-generated data. New statistical algorithms are used to calculate the probability of detection. Good agreement was achieved between empirical and model-assisted approaches. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

US Air Force aircraft are managed by Damage Tolerance Assessment (DTA) and Nondestructive Evaluation (NDE) plays a critical role in insuring their flight safety. This paper provides an overview of current NDE capabilities for aircraft structures. The potential migration to Condition-Based Maintenance (CBM) will require NDE to evolve from detection of defects to characterization of their location and size. This represents multiple significant challenges as aircraft have many material and geometry factors that complicate this analysis. This paper provides a strategy to resolve these factors. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Foam shedding from the shuttle external tank remains a critical problem regarding Shuttle orbiter safety. Flaws present in the foam can result in initiation sites for foam loss, and NASA is continuing to look at improving existing NDE methods for foam inspection as well as developing new methods. Terahertz NDE, greatly enhanced over the last several years with respect to its use for external tank foam inspection, has been a focus for continued improvement through signal and image processing improvements. In this study, results from various signal processing approaches to improve terahertz image flaw resolution for external tank foam are described. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper introduces spatial decomposition algorithms to the volume integral method, which will enable the integral equation method to efficiently solve electromagnetic scattering problems where the scatterer extends across several layers or multiple scatterers exist in multiple layers. Model comparisons with the finite element methods are used validate the approach. Studies of typical aging aircraft eddy current inspection problems are conducted using the new formulation. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A series of numerical studies using validated models are presented investigating the influence of probe design and geometric flaw parameters on the eddy current measurement response for the inspection of cracks around fastener holes in multi-layer structures. Multiple quantitative features of the eddy current data were evaluated. The goal of this study is to demonstrate the influence of these design parameters on the ability to resolve the flaw characteristics in the measurement data. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Previous work on the detection of cracks around fastener holes in multi-layer structures using eddy current NDE has met with some success; however, challenges remain in detecting smaller corner cracks while minimizing false call rates. To address this issue, a 3D model was developed for this problem and successfully validated with experimental data. Using this model, a series of numerical studies were performed to identify potential features for defect classification. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents a study of crack detection methods for holes in structures with limited accessibility containing fasteners with variable interface conditions. For far crack detection, a method was first designed using models, and refined using a signal processing parameter optimization scheme and an iterative neural network training process. For near crack detection, a method incorporating the spatial variation of the principal components was developed. Using these methods, improvements in crack detection performance were achieved. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper presents a study of ultrasonic NDE models for a cylindrical hole containing an elastic layer and a fluid-filled hole. For the elastic layer case, both dispersion relations and analytical solutions were solved to assess the sensitivity of leaky Rayleigh waves to annulus properties. A BEM model was formulated for the scattering of shear waves by a fluid-filled hole with a notch. Through simulated studies, a viable inspection technique was derived and validated experimentally. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper investigates the possibility of crack detection for holes containing elastic inserts. A BEM model was formulated for the scattering response of an incident shear wave transducer signal by a cylindrical hole with a radial notch, an elastic insert, and a stiffness interface between the hole and the insert. Through simulated and experimental studies, certain interface conditions were identified for which significant interface waves can be generated and subsequently used for inspection purposes. [ABSTRACT FROM AUTHOR]

In earlier work, an automated protocol was considered to detect bottom cracks in weep holes. During experimentation, this protocol was found to produce some false calls and miss certain small cracks. In this paper, a technique is presented which examines the variation in the A-scan signals as the transducer is incrementally moved along the wing surface. The advantage of this B-scan classification technique is its ability to detect superimposed signals independently from the shape of the pulse. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Weep holes are incorporated in the risers of wing structures in C-141 aircraft to allow fuel to be properly distributed during flight. Two types of fatigue cracks have been observed emanating from weep holes, top cracks that emanate upward and bottom cracks that propagate toward the wing surface. A new approach using creeping Rayleigh waves has been shown to be successful in detecting top cracks in weep holes. The focus of this paper is the development of a robust automated protocol for the detection of both top and bottom cracks in weep holes. A numerical technique was developed using the boundary element method (BEM) to calculate simulated waveforms for reflection and backscattering of ultrasound from weep holes with top or bottom cracks. The simulations were validated by comparison with experimental results. From parametric studies of this model, a robust protocol incorporating both the pulse echo and pitch catch creeping Rayleigh wave signals from top-cracks was defined. Protocols incorporating neural networks were developed for both top and bottom crack classification. A complete automated procedure for crack detection was successfully trained with both simulated and experimental data and was verified with experimental data for the expected variation in weep hole size and crack length. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]