The Federal Aviation Administration established the Airworthiness Assurance Center of Excellence (AACE) in September 1997, through a cooperative agreement grant with Iowa State University (ISU) and The Ohio State University (OSU). A technical support contract with the Center is now in place as well. Initially the Center has five areas of concentration supporting advances in airworthiness assurance. These are 1. Maintenance, inspection, and repair, 2. Propulsion and fuel systems safety, 3. Crashworthiness, 4. Advanced materials, and 5. Landing gear systems performance and safety. AACE has nine core members who provide guidance to the Program Management Office at ISU/OSU through a Board of Directors. The core members are: Arizona State University, Iowa State University, Northwestern University, The Ohio State University, University of Dayton, University of Maryland, University of California - Los Angeles, Wichita State University, and Sandia National Laboratories. The organization also includes numerous academic affiliates, industry partners, government laboratories and other organizations. The Center now has over thirty technical projects supporting technical advances in airworthiness assurance. All these projects have industry guidance and support. This paper discusses the current technical program of the center and the highlights of the five-year plan for technical work. Also included is a description of the factors that make the Center an innovative partnership to promote aircraft safety.

Boeing investigated corrosion effects on crack growth rates under a USAF contract with Tinker AFB in an engineering assignment performed by the Boeing Wichita. Sixty-eight crack growth specimens were notched, corroded, and fatigue test to determine crack growth rates. After salt spray exposure and crack growth testing the specimens were nondestructively inspected (NDI) by five different NDI methods to determine the amount of material lost. The NDI methods were chosen to represent different methods that could detect the presence of corrosion and digitize the data for analysis later to determine percent material lost. The NDI methods evaluated were computed tomography, pulse-echo ultrasonic c-scan, eddy current c-scan, thermography and digital radiography. Microscopy cross sectioning was also performed to visually document the corrosion damage. This data was used to compare the effectiveness of the NDI methods for detecting and quantifying corrosion damage and was used in the reanalysis of pre-corroded crack growth data. The results of NDI portion of this study are presented.

Magneto-optic imaging based upon Faraday rotation of polarized light has been successfully applied to the problem of non-destructive testing of cracks, stress fractures, corrosion, and other surface and subsurface defects in both ferromagnetic nd nonmagnetic metal structures. Some of these applications have been successfully applied to aircraft fuselage and wing structural examination, as well as to the inspection of tanks and other low-accessibility containers. There are significant needs and opportunities for improving upon the accuracy, sensitivity, portability, and automation of such non-destructive evaluation, particularly for aircraft which are by virtue of age, design, or condition subject to dangerous metal fatigue developments in between scheduled examination. There has been a need for improvement in the basic magneto-optic sensing technology as well as in the image processing of data gathered from the sensor, and in the refinement of crack and corrosion recognition algorithms and methods that can enhance automated and assisted recognition. The current research and development program in non-destructive testing applications at MODIS Corporation has developed several innovations within these areas that enable wider application of magneto-optic imaging. These include new Fe-Ga based thin-film technology resulting in (R, Bi)₃(Fe, Ga)₅O₁₂ wafers that are demonstratably more sensitive to low-strength magnetic fields. These films contain (Y, Lu, Bi)₃ (Fe, Ga)₅O₁₂ composition, grown on a transparent single- crystalline substrate of Gd₃ Ga₅ O₁₂ composition. Other more sensitive films and substrates have been developed as well. These films have uniaxial anisotropy due to crystallographic orientation, although with orientation films can be customized for more spatial resolution and sensitivity due to the almost uniformly planar anisotropy. The MODE sensor technology is incorporated into a modular scanning apparatus that enables the operation of several modes of inspection using replaceable video or digital still camera devices as well as variable optics for magnification. Instead of relying upon tradition eddy current technology for introducing measurable magnetic fields in the sample object being examined, the MODIS apparatus operates with a high-current, micro-burst application to the test surface. The sensitivity of the MODE Fe-Ga wafers has been demonstrated in laboratory experiments to operate with magnetic fields that are weaker than those produced by long-duration high-current eddy currents such as are presently being used in NDT applications. The coupling of higher magneto-optic sensitivity plus a reduction in the eddy current generation and heat dissipation opens a path to a number of variations and extensions of magneto-optic NDT. Algorithms and software developed by MODIS and partners for processing and analysis of the scanner output images reside on a Windows 95/NT computer and are compatible with body- wearable PC systems to enable completely hands-free, mobile inspection and data collection. The recognition algorithm are based upon standard digital image processing and neural network pattern recognition that has been successfully applied in other applications.

This paper will provide an overview of an approach that will autonomously detect and monitor crack growth in aircraft structure. Present day computer technologies make it possible to put a small computer and sensor system on an aircraft to monitor structural health at locations that are difficult and costly to inspect through more traditional techniques. The health of the monitored locations can be determined by the end user through normal operations, thereby providing a significant reduction in scheduled maintenance requirements. All types of aircraft suffer from structural cracking. Even though the location of the crack can often be predicted, the inspections required are costly, often requiring significant structural disassembly. Sometimes there is an unacceptable level of uncertainty in NDI results, leading to more frequent inspections and even greater costs due to prematurely replaced structure. Broadband Acoustic Emission (BAE) systems have demonstrated the capability to detect crack growth in structure. Such a system can autonomously monitor aircraft structure during operation and provide information necessary to detect crack growth, determine the source location and monitor crack growth without disassembly. This paper will present an overview of the BAE approach to crack monitoring. Results from proof-of-concept testing will be included with comparisons to BAE waveform analysis results. Conclusions, system requirements and a preliminary design of the electronics and software necessary for an aircraft crack monitoring flight, system will also be presented.

The use of adhesive joints in aerospace structures is becoming increasingly important. From this, arises the problem of assessing joint integrity quickly, non- intrusively, accurately, and inexpensively. Current methods of assessing joint integrity, such as ultrasonics and x- rays, are time intensive and difficult to interpret. Blue Road Research's solution to monitoring adhesive joint integrity quickly and accurately is to embed non-intrusive, multidimensional optical fiber grating strain sensors into or adjacent to the joints. Aluminum double lap adhesive joints were instrumented with the multi-axis grating strain sensors into or adjacent to the joints. Aluminum double lap adhesive joints were instrumented with the multi-axis senors and subjected to tension and fatigue test. Each specimen contained one sensor located either near the bond, embedded at the edge of the bond, or embedded towards the inner bond area. The joints with senors embedded into the adhesive showed minimal strength degradation. Basically, the multiaxis fiber grating strain sensors were found to provide information about transverse strain, axial strain, and transverse strain gradients that can provide important information throughout the adhesive joint. By changing the orientation of the sensor, shear strain and its effects can be clearly measured.

The structural defects on aircraft are not directly monitored and indicated like hydraulic failures. The identification of structural damage is important for safety flights, but inspecting airplanes at regular intervals is very costly. A structural damage identification method is presented which examines the elastic and dynamic behavior of the structure by means of deflections stemming from a discrete load case and from normal mode shapes of the structure respectively as well as the information of the change of natural frequencies. The main objective of the study is to provide an economical and reliable damage detection method for aeronautical structures. An updated finite element model of the structure must be available. The reduced stiffness due to damage will be localized using non- linear mathematical optimization codes. Displacements for selected load cases and normal modes are taken as constraints. Minimum sizing changes with respect to the initial structure is used as objective function. Numerical examples with different structures show that the proposed method can accurately detect the variations in stiffness in certain cases.

Microradiographic and ultrasonic characterization of fatigue crack emanating from pitting corrosion is discussed. Crack initiation and growth from the artificial pit of different depths in Al-2024-T3 alloy is studied experimentally. The effect of crack closure on the radiographical and ultrasonic detection of fatigue crack is also illustrated. The experimental results were analyzed using fracture mechanics models including those for small cracks. The model shows very good agreement with experiment in describing the initial and growth of a crack emanating from a pit and in predicting the dependence of the reduction of fatigue life on pit size. Using this analysis, a relation between the depth of the corrosion pit and the fatigue life is established, and thus the prediction of fatigue life of the sample with corrosion pit can be predicted based on the radiographic and ultrasonic measurements of pit and crack sizes.

Nondestructive testing of aircraft components through ultrasonic testing is well established as one of the industry's benchmark techniques. Its capability to penetrate both thin and thick material provides arguably the best information to inspectors on subsurface faults. However, there are tow basic drawbacks to it use: its difficulty to employ and its slow speed. Real-time C-scan solves both of these issues while maintain high quality subsurface information. Cracking, corrosion, voids, delaminations and impact damage can be observed in 1/30 second. The basis for this technology is a novel 2D imaging array that creates immediate, high-resolution images of subsurface faults. The latest developments of the technique include commercial introduction of a through-transmission scanning product which can inspect large structures, as well as significant progress in the development of a hand held device which produces instantaneous high quality imagery of defects in reflection over an area as the user simply holds a probe up to the target. This work is funded in part by the Navy SBIR 'Fasttrack' program.

In order to inspect sets of cooling holes being properly drilled on an aircraft engine blade, a robotic controlled IR imaging system was designed. The system physically having a robot held the drilled blade at different viewing positions in front of an IR camera where a sequence of images were obtained for analysis and evaluation where a hot-air cool- air heating and cooling cycle was being administered at the base of the blade. An initial teach model was needed to enable the robot to remember the different viewing position of the blade in front of the camera so as to establish a position and orientation reference and sequence of cooling hole inspection. Two data processing algorithms had been achieved in this paper: 1) A position adjustment during the regular operation after the tach-mode so as to made sure that he cooling holes to be inspected matched the reference established by the teaching phase; 2) An image processing method to extract four meaningful features and to drive a pattern recognition system for the determination of cooling hole operating. Blade classification was based on the number of bad cooling holes as being good if all but one hole is determined to be good, unless the bad hole is positioned in one of four critical locations, in which case the blade was classified as bad. A hole is classified as being bad if the hole is blocked in some manner, or if the hole's diameter does not fall within a specified range. Two types of data which were considered to be significant in characterizing the hole were the temperature intensity of the hole as it was heated and cooled, and the change in the intensity as a function of time during the same period. Image data features for the holes were extracted from the images and applied to several classifiers to determine an optimal classification method. Different data sets and/or combinations of features for both training and testing sets were formed and tested. Over 90 percent performances were achieved under different evaluation methods.

The state-of-the-art of Thermal Wave Imaging as an NDE technique for Aging Aircraft is summarized. The technique is described, and examples are given of application to the thermal wave inspection of aircraft for such subsurface defects as corrosion, disbonded doublers, fluid intrusion, and delaminations of composite structures.

For workpiece inspection applications such as internal edge measurement verifications, 3D data processing for detecting the internal and external surface of the object is required. Rather than detecting edge points on each slice of CT image and stacking them up, we detect the 3D surface points directly from a sequence of CT images. Using this strategy, surfaces whose normals are vertical to the slices can be detected. We present a set of 3D edge detection methods, facet-model-based, morphology-based, and wavelet-based for evaluation. The facet-model-based surface detection method uses facet model to estimate the local 3D directional derivatives and location of the zeros of the second 3D directional derivatives along the direction of gradient. Subvoxel accuracy can be achieved using this method. The morphology-based method performs a 3D dilation-erosion residue operation first. Then the zero crossings of the residue result images are detected, and the surface points are extracted. The wavelet-based method performs a 3D wavelet transform on the CT images, and the local maxima of the gradient are detected and marked as surface points. The performance of these surface detectors are compared. Finally, as an illustration, we apply the facet-model-based method on a set of engine blade CT images with resolution of 5 mil by 5 mil and the measurements of thickness of wall are taken. A linear regression model is used to correct the systematic error. The results are very close to the direct optical measurements of cut-up sections of the blade with a maximum error of 3 mil.

The Swedish National Road Administration has been using a laser range finder system for assessing longitudinal and transversa profiles of pavements for a number of years. The Civil Administration has expressed interest in trying automated methods for surveying airfield runways. Regardless of sampling method, the data re used as input for computer aided design of new pavement surfaces. Typically, the output will consist of tables and maps of where to pave and mill the old pavement surface. Adjusting an old surface to perfectly smooth standards may require continuously changing the new pavement layer thickness. Traditionally, this was often carried out with mechanical devices, usually in direct touch with a wire. Some manufacturers now offer non-contact devices for pavers such as ultra sound sensors as an alternative to the mechanical sensors thus eliminating the need for setting up sires. The method was tested in late 1997 after the output data were converted for a design to be read directly by a paver control unit. A larger test was done in the summer of 1998. The results were promising and the Royal Swedish Fortifications Administration decided to try the method on a taxiway in 1999. The present paper describes some of the experiences obtained with this method and computer controlled equipment in general.

A nondestructive evaluation technique for monitoring damage due to crack growth in concrete beams is presented in this paper. The technique is based on monitoring the resonant frequencies of vibration. An introduction to the concepts of vibrational modes in beams is detailed first followed by a description of the experimental procedure for resonance frequency measurement. The result of a finite element (FE) analysis that is performed to identify the different modes of vibration of the beam specimen, are then presented. The resonance frequencies determined by the FE analysis are shown to match closely with the experimental values. A notch is introduced in the specimen and the effect of notch length on the resonant frequencies is studied by varying the notch depth. Experimental results are compared with the result of FE simulation of the beam with different notch lengths. The influence of a real crack on the frequencies of the vibrational modes is also studied by loading a specimen in a three-point bending configuration and propagating a crack in a controlled manner using a closed-loop testing machine. Analysis of the obtained data is performed to evaluate the response of the different vibrational modes of the concrete beam specimen to varying crack and notch lengths. Frequencies of the vibrational modes decrease consistently with increasing crack and notch lengths. There is a larger decrease associated with increasing notch length. Resonance frequencies are shown to be sensitive to crack and notch growth in concrete beams and can be used to effectively monitor the decrease in structural stiffness due to crack progress progression.

The high-speed rolling deflectometer is one of the result of almost twenty year of research in pavement condition using laser technique. The latest research vehicle is the laser Road Deflection Tester, built in the mid-nineties using experiences from a prototype truck from the early nineties. Apart from the laser range finders used for finding used for finding the deflection, the truck is also equipped with optical speedometers for both longitudinal and transversal speed, accelerometers and force transducers on the rear wheel axle and a gyro for assessing the deviation. Presently, only the laser range finders are being used as the rest of the sensors has not been calibrated in a satisfying way. During the spring and summer of 1998 a first test program was carried out, and about twenty different roads were studied as a first step towards a more thorough investigation on a road network level. The results from this first major test with the high-speed rolling deflectometer are very promising and, even though many questions remains to be answered, the method has most certainly a strong potential. A general view of some different ways to evaluate the data, as well as more thorough evaluation of some specific roads, will be presented in this paper.

In this paper, we present a holographic technique which is used for the monitoring of thin shell structures. The technique provides a quantitative measurement of the stress concentration in the early stage of load and permits one to predict the magnitude of critical loads and location of faults in the construction. Failure of the structure is shown to be correlated to the point in the structure where the stress concentration in highest.

Ultrasonic inspection methods are often used to detect defects in ferrous and non-ferrous metal casting processes. The finite sensor recovery time following emission of the ultrasonic pulse leads to a 'shadow zone', from the surface to a depth of about 1 cm, that cannot be probed by ultrasonics. Conventional eddy current methods, operating at frequencies of several kHz or higher, can only detect surface-breaking flaws. Giant Magneto-REsistive and Anisotropic Magneto-Resistive sensors offer their full performance independently of frequency from 100kHz down to DC. This allows eddy current-based technique to prove ferrous materials deeper than heretofore, and thus to provide coverage through the shadow zone. The work presented here demonstrate the performance and sensitivity that can be achieved. The separation of eddy current and ferromagnetic signals allows the structure to be probed and the material properties to be imaged. We have detected artificial defects in ferrous steel samples to a depth of 1 cm as well as surface flaws smaller than those found by conventional eddy current methods. We also show results of probing a cast and rolled steel sample with real defects, where a band of presumed subsurface ferromagnetic inclusions masks the signal from tight surface cracks.

The degradation of civil infrastructure has placed a focus on effective nondestructive evaluation techniques to correctly assess the condition of existing concrete structures. Conventional high frequency ultrasonic response are severely affected by scattering and material attenuation, resulting in weak and confusing signal returns. Therefore, low frequency ultrasonic transducers, which avoid this problem of wave attenuation, are commonly used for concrete with limited capabilities. The focus of this research is to ascertain some benefits and limitations of a low frequency ultrasonic phased array transducer. In this paper, we investigate a novel low-frequency ultrasonic phased array and the results of experimental feasibility test for practical condition assessment of concrete structures are reported.

We propose that heterogeneous systems fail by exhibiting a critical behavior, characterized by the presence of log- periodic patterns. This prediction has been tested extensively during our continuing collaboration with the French Aerospace company Aerospatial on gas pressure tanks embarked on the European Ariane rockets. Our theory was applied to about 50 pressure-tanks and the results indicate that a precision of a few percent in the determination of the stress at rupture. This non-destructive evaluation technique is now used routinely in the qualifying procedure of the industrial fabrication process. We provide an introduction to the underlying theory, the physical mechanisms, as well as to generalization to other systems.

The fusion of multiple modal strain energy differences is proposed for the detection of damage at single and multiple locations. The approaches assume the existence of several modal shapes of the structure in the undamaged and damaged states. Modal curvatures obtained through iterative high- order spline fits of the shapes permit the determination of the modal strain energy content of the structure in both states. Locations with increases in the modal strain energy between the undamaged and damaged structure are indicative of possible damage. The damage indications resulting from multiple modes are fused according to three rules: (1) an Average Standard Norm, (2) a union of probability mass functions and (3) a weighted intersection of the probabilities that the strain energy are greater than zero. the third rule requires knowledge of the statistics of the modal shape measurements. While not strictly adhering to the classical data fusion methodologies such as, Dempster-Shafer or Bayesian, the combination of information at the probabilistic level yields result that are consistent with those expected in a formal data fusion formulation. The fusion algorithms for the location of damage are tested using five modal shapes obtained from aluminum beams in undamaged and damaged states. One and two locations at different damage magnitudes are considered. The results indicate that the last two approaches are superior to the standard norm method.

An aluminum stiffened-plate panel resembling aircraft- fuselage construction was tested in the laboratory with a laser doppler velocimeter. The purpose of the test was to extract out-of-plane mode shape data before and after the infliction of damage to evaluate a global NDE damage localization technique. The NDE damage localization technique is based on modal strain energy differences between the undamaged and damaged states. The modal strain energies were computed from bending and twisting curvatures obtained using an iterative bi-variate curve-fit procedure on estimated curvatures obtained from finite differences of the mode shapes. Strain energy differences between pairs of matching modes of the undamaged and damaged structure locate the inflicted damage by indicating increases in the modal strain energy. The damage indications provided by several modes are normalized using a standard norm and fused using an average approach to create damage maps.

Most of the current practices of manual visual inspection of aircraft wiring bundles can be replaced or assisted by a portable test system consisting of a miniature B/W or color TV camera and controllable uniform illumination mounted in a flashlight-sized, hand-held unit weighing less than one pound. The location and configuration of the battery power supply and image viewing and storage means are at the discretion of the inspector. A typical viewed area is nominally 1 inch by 1.3 inches, with a depth of field up to 1.5 inches. Tradeoffs among pixel dimensions, geometrical optics, and lens diffraction which arise in the design of such a unit are discussed. Data are presented showing actuator measured depths of field and image resolutions vs. variable camera lens aperture under real conditions. The image size in these tests provided an overall 12X enlargement of the target, as viewed by the inspector in real time. On insulated wires as small as 1mm OD, printed markings are easily seen and read, as are common chafes, radial surface cracks, and other surface anomalies of the insulation. The present mechanical design permits the small unit not only to inspect areas in any direction to which the hand and wrist can point, but also allows insertion of the unit in areas not easily accessible to the hand or eye.

Pulse Phase Thermography (PPT) has been reported as a novel powerful technique of the thermal NDE. It employs application of the Discrete Fourier Transform (DFT) to thermal images obtained following flash heating of the front surface of a specimen. The computed phasegrams are excellent for defect visualization in a wide range of materials. This is in part due to their low sensitivity to uneven heating. This work is an attempt to analyze advantages and limitations of PPT. Results of application of the DFT to simulated temperature decays are presented. The temperature evolution on a surface has been simulated based on an analytical solution of the 1D heat diffusion problem. A more sophisticated study has been done for different sizes of defects using numerical solution of the 3D mathematical model. Capabilities of PPT for in-depth scanning and for monitoring of the material loss are discussed. The recommendations for the practical application of the PPT are presented. Experimental results obtained following these recommendations are reported.

A real-time speckle-based shearography system will be presented. It is a tow camera system that records four phase stepped images simultaneously, two per camera. This is achieved by the two Savart elements in the optical setup, that are used to create the proper phase steps between the two sheared wavefronts and to split the two mutually orthogonal components into two fully separated beams, thus creating two images, side by side on a single CCD camera. From those recorded images the phase can be computed using the four bucket algorithm. By measuring the phase before and after loading the sample under inspection, and subtracting the two phase distribution, information on the homogeneity of the sample is obtained. Loading is performed by heating the sample by use of a halogen lamp. Defects in the sample show up as irregularities in the phase difference distribution. The system has been used on parts of helicopters and airplanes. All within a laboratory environment. The use of the system in more hostile environments has been investigated and proves to be much better than conventional, non real-time systems. Finally, a concept for an even more compact setup will be presented. In this setup only a single camera will be used. By employing the method of temporal phase unwrapping the phase front, and therewith the surface shape, can be monitored as a function of time.

The leaky Lamb wave (LLW) technique is approaching a maturity level that is making it an attractive quantitative NDE tool for composites and bonded joints. Since it was first observed in 1982, the phenomenon has been studied extensively, particularly in composite materials. The wave is induced by oblique insonification using a pitch-catch arrangement and the plate wave modes are detected by identifying minima in the reflected spectra to obtain the dispersion data. The wave behavior in multi-orientation laminates has ben well documented and corroborated experimentally with high accuracy. The sensitivity of the wave to the elastic constants of the material and to the boundary conditions led to the capability to measure the elastic properties of bonded joints. Recently, the authors significantly enhanced the LLW method's capability by increasing the speed of the data acquisition, the number of modes that can be identified and the accuracy of the data inversion. In spite of the theoretical and experimental progress, methods that employ oblique insonification of composites are still not being applied as standard industrial NDE methods. The authors investigated the issues that are hampering the transition of the LLW to industrial applications and identified 4 key issues. The current capability of the method and the nature of these issues are described in this paper.

Hole expansion is a cold working method used for fatigue life improvement of aircraft structures containing fastener holes. The insertion of an oversized mandrel through the hole induces inhomogeneous plastic deformation in the surrounding regions. The resulting unloading residual stresses play a major role in determining the fatigue life improvement of the structure. It is therefore the purpose of this investigation to conduct comprehensive 3D elasto- plastic finite element analysis to evaluate the development and growth of the plastic zone and unloading residual stress resulting from the cold expansion of two adjacent holes. In this study, we focus our attention to the sequential expansions of the two holes. The contact between the mandrel and the hole was modelled using special contact elements, which employ the combined penalty-Lagrange multipliers formulation using ANSYS. Some of the results contained in this paper are based on an article published in the International Journal of Mechanical Sciences, which is currently in press.

Cold working processes are well established for improving the fatigue life of critical load bearing components. The resulting unloading residual stresses are beneficial but there is now a need to nondestructively quantify and verify their levels in engineering components. It is therefore the purpose of this investigation to evaluate the capability of the leaky Lamb wave technique to characterize residual stresses. The component is ted as a transversely isotropic layered solid with wave velocities varying in each layer. Modeling indicates that the technique can characterize depth varying residual stresses. Experimental work indicates that the technique has potential as a quality assurance tool.

A consorted effort is ongoing to utilize spectral analysis of Lamb Waves to rapidly characterize and to detect damage in plates. To optimize test set-up, to understand the limitations of the methodology, and to verify the experimental results, an effort is under way to simulate conditions normally encountered in actual cases. The impact of propagation of Lamb waves in the presence or absence of damage has been simulated using a finite element algorithm, 2D Fourier transform can be used to identify individual modes, and to measure the amplitude and propagation velocity of each mode in a thin plate. Mode conversion due to the presence of a crack can be readily identified with this method. The damage was modeled as notches with different widths and depths. Three different materials were modeled for this purpose: Aluminum 6061-T6, Steel A36, and Graphite- Epoxy; the latter being analyzed in directions parallel nd perpendicular to the fibers. The focus of the work was on low-frequency range where the fundamental symmetric and anti-symmetric modes are dominant. The ratio of notch depth to plate thickness and the absolute notch depth were considered as possible controlling parameters for the sensitivity in the damage detection. The effect of the width of the flaw on the transmissivity of the Lamb waves was also considered and found to be minimal.

Lamb waves are guided ultrasonic waves capable of propagating relatively long distances in plate-like structures such as airframe skins. Their propagation depends on frequency-thickness and material properties, and because structural flaws present changes in effective thickness and/or material properties Lamb waves can be employed to assess the integrity of these structures. For aging aircraft structures a full integrity evaluation can be a time- consuming operation, but with Lamb wave techniques this evaluation can be performed with waves propagating along one dimension of the inspection area as the probing transducer pair is moved in the perpendicular dimension. Such an approach yields information about the presence of flaws within the scanned area. Then, in order to quantitatively characterize the flaws, Lamb wave measurements can be made for a number of projections and an image of the flawed region can be reconstructed tomographically. In this paper, contact scanning Lamb wave tomography for metallic aircraft structures with flaws is discussed as a practical technique for quantitative nondestructive evaluation.

A tomographic imaging system using ultrasonic Lamb waves for the nondestructive inspection of aircraft components such as wings and fuselage is being developed. The computer-based system provides large-area inspection capability by electronically scanning an array of transducers that can be easily attached to flat and curved surface without moving parts. Images of the inspected area are produced in near real time employing a tomographic reconstruction method adapted from seismological applications. Changes in material properties caused by structural flaws such as disbonds, corrosion, and fatigue cracks can be effectively detected and characterized utilizing this fast NDE technique.

Ultrasonic elastic waves have traditionally been examined using single point measurement techniques. In this paper, we present a technique capable of recording out-of-plane ultrasonic displacement data over wide areas. The method employs 2D surface vibration data collected via electronic speckle pattern interferometry used in combination with laser modulation and optical phase stepping. Using this rapid, wide-area measurement method, images of ultrasonic Lamb waves on ideal and flawed plate structures are presented. For a single ultrasonic excitation frequency, multiple data records are collected as an externally excited traveling plate wave moves through the sample. When an optical phase shift is introduced between the data records, the data can be processed to yield quantitative displacement fields. The resulting processed images demonstrate evidence of Lamb wave reflection, transmission and scattering. Following the full field data collection, a novel inverse scattering algorithm was applied to reconstruct images of the scattering sources responsible for the measured displacement field data. The results of these investigations will be presented and discussed in the context of detecting hidden flaws in aging aircraft.

Conventional methods of ultrasonic non-destructive evaluation (NDE) use liquids to couple sound waves into the test samples. This either requires immersion of the parts to be examined or the use of complex and bulky water squirting systems that must be scanned over the structure. Air-coupled ultrasonic systems eliminate these requirements if the losses at air-solid interfaces are tolerable. Micromachined capacitive ultrasonic transducers (cMUTs) have been shown to have more than 100 dB dynamic range when used in the bistatic transmission mode. In this paper, we present results of a pitch-catch transmission system using cMUTs that achieves a 103 dB dynamic range. Each transducer consists of 10,000 silicon nitride membranes of 100 micrometers diameter connected in parallel. This geometry result in transducers with a resonant frequency around 2.3 MHz. These transducers can be used in transmission experiments at normal incident to the sample or to excite and detect guided waves in aluminum and composite plates. In this paper we present ultrasonic defect detection results from both through transmission and guided Lamb wave experiments in aluminum and composite plates, such as those used in aircraft.

Methods for enhanced detection and characterization of corrosion in aircraft structures are presented. A unique Reverse Geometry X-ray system was utilized for real time radiographic imaging of structures. This system has several possible advantages as compared to conventional radiography. First, the x-ray detector can be miniaturized and easily positioned inside a complex structure enabling images of each surface of the structure to be obtained separately. Second, multiple detectors enable the simultaneous acquisition of data from several different perspectives without moving the structure or the measurement system. This provides a means for locating the positions of flaws and enhances separation of features at the surface from features inside the structure. A method is presented for calibration of the eight detectors for reduction of radiographic data to thickness measurement. Results are presented for measurements on a varying thickness sample. Laminographic reconstruction of data from the multiple detector system is presented. Laminographic imaging methods are shown to be capable of separating surface features from corrosion in the lap joint and locating corrosion in multilayer structures. Removal of surface features to enhance definition of corrosion boundaries is discussed.

State of the art of holographic research performed within different space flight programs of several countries is discussed. Some of previous methods are not well compatible with unpromising conditions of weightlessness. Quite novel solutions are proposed for real time in situ holographic data acquisition and display on the board of a spacecraft. They are extremely simple, cheap and require minimal holographic hardware. Based on these solutions several novel supersmall holographic devices were devised. One of them is presented. It is portable holographic interferometer with no lenses and no alignment problems enabling monochrome or color data output. No optically skilled personnel are required for operating this device. Experimental data properly illustrating novel vast possibilities for on the board space research are presented.

Corrosion is a relatively slow material degradation process to which metallic structures of aircraft are subjected during service and it can appear in many forms. Generally, corrosion protection for preventing or inhibiting the formation and growth of corrosion damage on aircraft structures is well-established technology. Unfortunately, despite preventive measures, corrosion does occur and its probability of formation significantly increases as structures age. Corrosion detection and characterization at the initiation stages, while hidden under paint or in concealed areas, still poses a challenge to inspection science and technology. Corrosion damage is costly and it carries the risk of loss of life as well as hardware in case of catastrophic failure. The authors are investigating the application of obliquely backscattered ultrasonic signals (OBUS) as a means of detecting and characterizing corrosion under paint in metallic panels. OBUS were measured using oblique insonification and were used to produce C-scan images of corrosion damage located on both top and bottom faces of test panels through the paint. A combination of OBUS data and a sensor-array real-time imaging (SARTI) system is being developed for field applications. SARTI uses CCD to display ultrasonic data nd the integrated system has the potential to reduce the need for paint stripping prior to inspection. The main features of the combined OBUS and SARTI are described in this paper.

The application of the acoustic emission (AE) technique has been severely limited during the past several decades due to the difficulties of source location on general shell structures. Two major obstacles are the lack of search algorithms on shell structures, where wave paths are guided by structure shapes, and the lack of the ability to process AE data for meaningful source location. This paper discusses a systematical approach for precise source location on shell structures. The approach not only provides the satisfactory answers to these two problems but also addresses the issue how a precise source location techniques can be used on a daily and routine basis. The approach has been applied to two important studies: analysis of AE data from a rocket motor case and AE monitoring of a railroad tank car. The success in both applications is remarkable. In the case of the railroad tank car where 12 sensors were used, the typical error margin is only 50-150 mm with the best accuracy of 10 mm for a set of data. In viewing of the consistently of the high performance of this approach as demonstrated in both cases as well as the fact that these studies were carried out under the normal industrial monitoring conditions, including structure size, sensor array arrangement and data gathering procedure, we believe that the approach is reliable, technically and economically feasible and ready for general industrial applications.

The remote field eddy current (RFEC) technique is based on the RFEC phenomenon which is characterized by differences in the energy flow patterns in the near and remote field regions. The energy released by the probe excitation coil traverses the pipe al twice before reaching the pickup coil. The RFEC technique, currently used in metallic tube inspection, is therefore characterized by its equal sensitivity to a flaw irrespective to its location in the tube wall. It can be used for detecting defects located several skin-depths away from the excitation source, since its inspection capacity is limited by the skin-depth of the specimen, but by the signal-to-noise ratio for a particular measurement condition.

A surface detection method using 3D directional for extracting surface points of a solid object from its cross- sectional slices' CT images is presented. The formulas for calculating 3D directional derivatives are derived using facet model. The subvoxel accuracy is obtained by locating the zeros of the 3D second directional derivative along the estimated gradient. Experiments for applying this method to CT images of real industrial parts are performed and some results are shown.