An Airport located inland central Sweden is susceptible to frost heave. The runway is uneven especially at the end of each winter. The Swedish Road and Transportation Institute Laser Profiler was brought to the site in the spring of 1997 in order to study this seasonal effect. Several longitudinal profiles were sampled along the entire length of the runway. The test was then repeated in the fall when the runway had settled. The profiles were then investigated to see if certain criteria were fulfilled, like the International Civil Aviation Organization straightedge guideline. Several different wavelength intervals of unevenness were also examined. It was found that the frost heave affected certain wavelength bands more than others. It was also possible to determine exactly where the most troublesome spots were located and if they would adversely interfere with an expansion of the runway. Data from the profile could also serve as help in preparing guidelines for safety rules related to roughness.

Maintenance of airport infrastructure presents many unique challenges. Airport engineering and maintenance personnel must maintain around the clock service to millions of people each year while maintaining millions of cubic meters of concrete distributed throughout the facilities. This infrastructure includes runways, taxiways, roadways, walkways, bridges, building walls and roofs. Presently only a limited number of accurate and economical techniques exist to test this myriad of concrete structures for integrity and safety as well as insure that they meet original design specifications. Remote sensing, non-destructive testing techniques, such as IR thermography, ground penetrating radar, magnetometer and pachometer, measure physical properties affected by the various materials and conditions found within, and under, concrete infrastructure. These techniques have established reputations for accurate investigations of concrete anomalies.

The galvanic action between steel fasteners and aluminum wing skins of aircraft often leads to hidden exfoliation corrosion around the countersink surface of the fastener heads. To detect and evaluate the severity of such corrosion defects, the Dripless Bubbler ultrasonic scanner was applied. This technique uses a focused beam of high frequency ultrasound in a closed-cycle, water-coupled scan of wing skin test panels containing corroded and uncorroded fasteners. With full waveform acquisition, not only the lateral extent but also the depth profile of the corrosions around the fastener heads were mapped out, subject to shadowing of defects at different depth. The technique is capable of providing quantitative assessment of the severity of the corrosion. In tests conducted to evaluate different techniques, the Dripless Bubbler has shown high probability of detection and low false call rate. The presence of paint on the surface did not degrade the performance of the technique. In addition, the Dripless Bubbler was also used on wing skin panels containing repair 'blend-out' regions that had 0.020' to 0.100' of metal removed from the surface by grinding. Corrosions around fasteners in the blend-out regions were also detected.

Life extensions on numerous military and commercial aircraft have heightened the need for quantitative, nondestructive detection of deeply buried damage in aircraft structures. Traditional coil-based eddy-current sensors are severely limited in their ability to detect small buried defects, defects under fasteners and deeply buried cracks and corrosion. TPL has developed eddy-current sensors based on the use of Giant Magnetoresistance (GMR) sensor elements. GMR offers high sensitivity, very wide bandwidth and low noise from DC to over 1 GHz. Coupled with the ability to fabricate GMR sensors with micron-level dimensions, these new eddy-current sensors offer an ideal technology for inspections requiring high spatial resolution and low- frequency, deeply-penetrating fields. This paper discusses magnetoresistance and results obtained using a prototype GMR sensor for both contacting and non-contacting, C-Scan measurements on samples containing crack and corrosion damage.

Many studies have shown inspector variability to be a crucial parameter in nondestructive evaluation (NDE) reliability. Therefore it is desirable to automate the decision making process in NDE as much as possible. The automation of inspection data handling and interpretation will also enable use of data fusion algorithms currently being researched at IAR for increasing inspection reliability by combination of different NDE modes. Enhanced visual inspection techniques such as D Sight have the capability to rapidly inspect lap splice joints using D Sight and other optical methods. IARs NDI analysis software has been sued to perform analysis and feature extraction on D Sight inspections. Different metrics suitable for automated interpretation have been developed and tested on inspections of actual service-retired aircraft specimens using D Sight with solid film highlighter.

Contact between galvanically dissimilar metals, such as cadmium plated steel fasteners and aluminum wing skins are known to be a source of corrosion. There is a design requirement to fill the void between the contacting surfaces of steel fasteners with a wet sealant. However, if the contacting surface is damaged or a void exists between the fastener head and the aluminum skin, moisture can collect and intergranular corrosion may occur along aluminum grain boundaries, which run parallel to the surface of the wing skin. If intergranular corrosion is allowed to propagate, delamination of the thin layers of aluminum, known as exfoliation corrosion will occur. When this intergranular corrosion reaches an exfoliated state, extensive rework is involved in removing the corrosion. This paper discusses the results of a USAF E-3A Engineering Service Task 89-E3B3-16 to develop a nondestructive inspection procedure to detect intergranular corrosion in an incipient state before it reaches exfoliation. Eddy current and ultrasonic inspection techniques were evaluated. A novel ultrasonic pulse echo technique was developed which utilizes a focus transducer with a hand held fixture. Inspections were performed on test parts which were removed from the upper wing skin of a retired 707 which had varying degrees of intergranular and exfoliation corrosion. Inspection results are compared to the results from the mechanical rework of the wing skin and dissection of a wing skin fastener hole.

This paper is concerned with the quantitative nondestructive evaluation of corrosion around and cracks emanating from rivet holes in typical aircraft structural components. Current techniques for detecting defects around rivet holes are time consuming and qualitative. A recently developed ultrasonic technique using plate guided Lamb waves as probes has been found to be successful in laboratory specimens and appear to have the potential to improve the speed and robustness of the inspection. The general features and future prospects of the method is described in the paper.

We present an analytical solution of the transmitted and reflected wave fields due to the interaction of Lamb waves with localized, small-scale defects on late surfaces. The surface defect is represented by a distribution of surface sources, and the overall solution is obtained as a superposition of the incident wave field plus the 'small' wave field associated with the surface sources. The reflection and transmission coefficients of various Lamb modes are also determined. It is shown that the scattered field consists of a superposition of various possible modes at the frequency of the incident mode. For the fundamental modes, it is found that the reflection coefficients are periodic functions of the defect width with the period and magnitude being dependent on the surface defect profile. The transmission coefficient for the converted mode is also found to be a periodic function of the defect width while that corresponding to the incident mode does not deviate significantly from unity. A parametric study has also shown that to this approximation the numerical values of the reflection and transmission coefficients are not sensitive to the maximum slope of the defect profile.

A method base don an inverse solution of the 1D heat conduction equation is applied to the general problem of estimating the local thickness of a structure form measurements of its thermal response to a flash excitation. Results are presented which demonstrate the efficacy of the method when applied to specimens with simulated corrosion damage.

A technique for detecting and locating structural damages is presented in this paper. It is based on the analysis of experimentally evaluated frequency response functions (FRFs) and consists of a comparison of the FRFs of the healthy structure which are assumed as reference and the FRFs collected at different times. A damage detection index interprets the differences between the FRFs. The results obtained by this technique when tested on a partial frame of a commercial aircraft were very interesting. It is possible to detect and locate all damages which were simulated/induced and also gave an indication of the extent of the damage. Moreover, the technique has the basic features required of a new NDE technique such as being non- model related and having the possibility of performing real- time monitoring.

Previously, the applicability of laser-based ultrasonic techniques for the automated inspection of polymer-matrix airframe structures, having either flat or contoured geometries, has been demonstrated without restriction to large radii of curvature. More recently our LBU inspection capability has been extended to include higher spatial resolution for the inspection of complexly contoured metallic structures. In this paper, progress regarding the application of LBU to the inspection of complex geometry polymer-matrix composite structures and metallic rocket engine components is summarized.

Non-destructive damage detection is an important issue in almost all structural areas ranging from aerospace/aeronautical structures, civil infrastructures, and structural materials. The use of vibrational-based nondestructive evaluation techniques to locate structural damage has been attempted to evaluate the integrity of civil infrastructures, composite laminates, continuum structures, and especially aircraft and large space structures. In an attempt to develop a structural health monitoring system for rocket engines, hundreds of technical papers in the vibrational assessment area have been reviewed. This paper provides a comprehensive overview of various vibrational- based nondestructive evaluation techniques, including a brief introduction of the theoretical background of different methods, an analysis of their advantages and drawbacks, and a foresight of the applications of different methods towards different type of structures. To date most research into vibrational-based structural damage detection has been performed by a handful of researchers at a wide variety of sites with little or no coordination in research efforts. Many of these methods have been tested using mass- spring test models or simple planar truss models. Few of standard test problems truly embrace the essence of real- world structures and as such poor judges of the performance of a few method. There clearly is a gap between theoretical research and practical application. This paper would be considerably helpful for future research, and especially beneficial for the development of a structural monitoring system in choosing an applicable and realistic method as a basis.

The principles of thermal wave imaging for NDE are described. The technique utilizes high-power photographic flash lamps for pulse-heating of the surface of the composite. The cooling of the surface is monitored by means of an IR video camera. Disbonds, delaminations, and inclusions are seen in the resultant thermal wave images, with deeper features appearing systematically at later times. Examples of the application of thermal wave imaging to nondestructive evaluation of boron fiber composite reinforcement patches, and graphite-fiber aerospace composite materials are described.

Several data processing procedures for the pulse thermal inspection require preliminary determination of an unflawed region. Typically, an initial analysis of the thermal images is performed by an operator to determine the locations of unflawed and the defective areas. In the present work an algorithm is developed for automatically determining a reference point corresponding to an unflawed region. Results are obtained for defects which are arbitrarily located in the inspection region. A comparison is presented of the distributions of derived values with right and wrong localization of the reference point. Different algorithms of automatic determination of the reference point are compared.

A nondestructive evaluation method is desired for ensuring the 'as manufactured' and 'post service' quality of graphite/epoxy instrumentation rack shells. The damage tolerance and geometry of the racks dictate that the evaluation method be capable of identifying defects, as small as 0.25 inch² in area, over large acreage regions, tight compound radii and thickness transition zones. The primary defects of interest include voids, inclusions, delaminations and porosity. The potential for an IR thermographic inspection to replace ultrasonic testing for qualifying the racks as 'defect free' is under investigation. The inspection process is validated by evaluating defect standard panels built to the same specifications as the racks, except for the insertion of artificially fabricated defects. The artificial defects are designed to closely match those which are most prevalent in the actual instrumentation racks. A target defect area of 0.0625 inch² was chosen for the defect standard panels to ensure the ability to find al defects of the critical size.

Not always during in-field measurements it is possible to reproduce laboratory conditions. This is especially true in thermography-based techniques which are most likely to be affected by unwanted external heat fluxes. Typical examples of parasitic heat sources may be spatial heater unevenness, reflections, heater power variations in time, real external heat flux sources, specimen curvature variation, misplacement of heater effects, etc. These heat sources contribute to reduce the overall SNR. As a consequence, a technique perfectly working in laboratory, when ported to field environment may not yield the expected results. Moreover the repeatability of the measurement is not ensured due to the randomness of these sources. D2D is a transient thermography technique based on moving a linear heater over a specimen's surface and recording the thermal evolution of the hated surface by means of a thermal imager.

Shearography has been shown to be a powerful tool for on- site non-destructive evaluation applications, especially in aircraft component inspections. However, current commercially available portable shearography systems are qualitative, low signal to noise ratio, and low flaw detection sensitivity. In this paper, we will introduce a real-time, high-resolution, portable, phase-stepping shearography system developed at the Applied Research Lab of The Pennsylvania State University. The system can fit into a wheeled cart and run at a speed of 1-2 inspections/sec for a 7 inch X 5 inch field of view. It provides full quantitative analysis ability and substantially improves flaw detection sensitivity. The applicability and portability of the system to on-site field NDE applications have been tested by a successful field demonstration conducted at a naval air station.

The phase-stepping technique is becoming a common enhancement to digital shearography and holography systems used in NDE. Capturing a series of phase-stepped images, rather than just a pair, allows the calculation of surface deformation at every pixel; data from a equispaced, spatially dense set rather than a sparse collection of nonuniformly spaced points. Phase maps are calculated modulo 2(pi) . If surface deformations are of sufficiently high amplitude, the phase will appear 'wrapped'. The next logical step is to 'unwrap' the phase and make it continuous again. For flaw detection purposes, however, this difficult step should be questioned; it is sometimes not only unnecessary, but deleterious to the flaw detection process. Further image processing steps must sometimes be applied to the unwrapped phase in order to expose flaws once readily visible in the wrapped phase map. These steps, only applicable to an unwrapped phase map, can also serve to uncover some flaws not previously visible in the wrapped phase map. Finally, effective data visualization plays an important role in conveying information role in conveying information embedded in the wrapped phase map. Finally, effective data visualization plays an important role in conveying information embedded in the wrapped or unwrapped phase maps. This paper introduces these issues and gives several examples, wrapped and unwrapped, with varying excitation type, material, flaw type, and image processing.

Holographic interferometry has been successfully employed to characterize the materials and behavior of diverse types of structures under stress. Specialized variations of this technology have also been applied to define dynamic and vibration related structural behavior. Such applications of holographic technique offer some of the most effective methods of modal and dynamic analysis available. Real-time dynamic testing of the modal and mechanical behavior of aerodynamic control structures for advanced missiles systems has always required advanced instrumentation for data collection in either actual flight test or wind-tunnel simulations. Advanced optical holography techniques are alternate methods which define actual behavioral data on the ground in a noninvasive environment. These methods offer significant insight in both the development and subsequent operational test and modeling of advanced composite control structures and their integration with total vehicle system dynamics. Structures and materials can be analyzed with very low amplitude excitation and the resultant data can be used to adjust the accuracy of mathematically derived structural models.

Helicopter rotor blades are highly sophisticated products composed of a variety of materials and components and therefore, 100 percent quality control has to be assured. Therefore, the French helicopter producer, Eurocopter S.A., Paris, has installed a system for automatic and 100 percent inspection of rotor blades on structural defects. In this system, laser shearography is used as an inspection technique. The rotor blades are mounted in a vacuum chamber and loaded with a relative pressure difference. At this load, bonding and structural defects show up as tiny deformations of the surface and are recorded with two shearography cameras, positioned on both sides of the rotor blade. After each measurement, they are automatically moved to the next inspection position. In this way, the entire rotor blade is automatically inspected in several measuring steps. As this helicopter blade inspection system is the first automatic production control system based on laser shearography in Europe, this application is an important step in bringing shearography techniques into production control. In this paper, the complete inspection system is presented.

Considerable attention has been focused on the life extension of ageing military and commercial aircraft by the government and major aircraft fabricators. A vital, but often neglected segment of the aircraft industry is the are of inspecting ageing fleets of corporate and privately-owned aircraft. Many of these aircraft are inspected and maintained by the various FAA-approved repair stations located around the country. Nondestructive inspection (NDI) methods, equipment, and trained inspectors are a key aspect of maintaining these aircraft; however, there are currently several issues that need to be addressed by the private sector NDI community. Personnel training and certification to an accepted standard is critically needed in this industry since experience and capability in NDI can vary considerably between FAA stations and inspectors. Also, the updating of NDI methods are standards is needed. A review of these issues and suggestions for improvement are presented.

Advantages and limitations of different approaches to determine probability of detection are provided and a practical and economical approach based on using actual components with real flaws is described. Important factors that affect POD measurements are mentioned and POD results generated on engine components with service-induced fatigue cracks using different inspection techniques, both manual and automated, are presented. It is found that the majority of service induced cracks, in the engine parts investigated, are small corner cracks that are difficult to simulate and detect while laboratory-grown cracks tend to form as through cracks that are easier to detect. The most widely used liquid penetrant and magnetic particle inspections (LPI and MPI) are not very effective in detecting service cracks in the bolt holes of engine components as indicated by the POD- crack size relationships and the crack sizes detectable at 90 percent POD with 95 percent confidence level. Eddy current techniques, in particular automated eddy current, are more suitable and should be used in place of, or in addition to the traditional LPI and MPI for such applications.

SIMS has developed a simple means for detecting and monitoring both absolute and relative clamp load, or bolt tension, in fastener systems. More than twenty-five percent of automotive failures are known to be due to undetected loss of fastener clamp load. While the equivalent aerospace maintenance statistics are not known, the average automobile has 3,500 fasteners while a Boeing 747 has closer to one million. It is therefore anticipated that the new SensaBolt clamp load tracking system could find wide applications in the aerospace arena. We describe a visually-evident and retrofitted clamp load monitoring design which is based on the differential joint substrate compression at, and immediately adjacent to, the fastener location. This intrinsically-accurate indicator does not necessarily require alteration in either the bolt or nut geometries, thereby facilitating product introduction and retrofit in aging aircraft applications. In addition, SensaBolt's sole reliance on substrate compression renders it more accurate then torque wrench or turn-of-nut techniques. Readout may be accomplished by any of three principal methods: for those applications with ease of access to the sensor, loss of tension can be determined by direct visual inspection. Application of a standard wrench can then be made to restore the fastener's proper tightness, per the SensaBolt indicators. In those instances where line-of-sight is unimpeded and more formal inspection is desired, the SensaBolt may be interrogated by a laser scanner bar code reader. Finally, SensaBolt may be addressed by the SIMS fiber optic harness for those instances where full-time remote interrogation is desired.

The service life of military aircraft is being extended beyond original design intents. When the C-130 is eventually retired, it will have been in service for 79 years, well beyond its planned life expectancy of 40 years. Similarly, the KC-135s are presently expected to remain operational for 86 years, and the B-52 for 94 years. Not only are inventories of parts in short supply, but it is necessary to acquire parts no one expected to replace. The first step in any resupply activity is the creation of a data package. If nor computer-aided design (CAD) model exists, the demands of modern electronic commerce dictate than one be created. Creating a CAD model of an existing part is referred to as 'reverse engineering.' Computed tomography (CT) offers an ideal way to obtain metrology data critical to reveres engineering activities. Industrial CT systems have progressed to the point where they can nondestructively measure part dimensions at an accuracy competitive with coordinate measuring machines and a speed competitive with laser scanners. However, of the existing methods, only CT can nondestructively dimension interior surfaces, and only CT has the ability to densitometrically quantify the internal state of materials. The use of CT to help create CAD models for resupply efforts will be described and examples presented. Additionally, examples will be presented how the CT-created CAD models were then sued to fabricate replacement parts for aging systems.

Development trends of on-board fiber optic system for safety parameters monitoring of aerospace transport facilities are considered. Concept of joint field of integrated fiber optic sensors is incorporated and possibilities of on-board systems creation with improved metrological and information features, based on field advantages, are discussed. Questions of optical reflectometry applications for information removal from sensors of field are considered and constructive particularities of last are determined. Application of sensor based on two twisted together fibers with locked ends as amplitude, polarimetric, and reflectometric measuring transducer are discussed.

Nonlinear acoustic technique has been recently introduced as a new tool for nondestructive inspection and evaluation of fatigued, defective, and fractured materials. Various defects such as cracks, debonding, fatigue, etc. lead to anomalous high level of nonlinearity as compared with flawless structures. One of the acoustic manifestations of such nonlinearity is the modulation of ultrasound by low frequency vibration. Two methods employing the nonlinear interaction of ultrasound and vibration were developed, namely vibro-modulation (VM) and impact-modulation (IM) methods. VM method employs forced harmonic vibration of a structure tested, while IM method uses impact excitation of structure natural modes of vibration. The feasibility tests were carried out for different objects and demonstrated high sensitivity of the methods for detection of cracks in steel pipes and pins, bonding quality in titanium and thermoplastic plates used for airspace applications, cracks in combustion engine, adhesion flaws in bonded composite structures, and cracks and corrosion in reinforced concrete. The model of the crack allowing to describe the modulation of sound by vibration is discussed. The developed nonlinear technique demonstrated certain advantages as compared with the conventional linear acoustic technique, specifically discrimination capabilities, sensitivity, and applicability to highly inhomogeneous structures.

A superconductive probe presently can detect a crack at a rivet hole that is two to three times smaller than the smallest crack detectable by a conventional probe. As the technology matures and noise resolution approaches a limit set by SQUIDS, approximately 1 fH, it will enable detecting submillimeter cracks down to approximately 15 mm.

A novel NDE system with an ultrasound sensitive multi- element array has been demonstrated and is currently is product development. The tool is capable of imaging internal defects and provide real-time, depth sensitive C-scan information. The system uses a patented sensor array of ultrasound sensitive elements that generates images in real time. This low cost, portable tool requires no mechanical scanning to acquire C-scan images and can be used for inspection of subsurface corrosion and other flaws. Since the system requires no mechanical scanning, it is be implemented as a handheld probe, not requiring targets to be submerged in a fluid at all. The probe can be used for spot inspection by an operator, or integrated into manufacturing processes for immediate production control over large areas. The application of the system for composites will enable characterization both while they ar forming, as well as for filed testing of in-service materials. The system operates at thirty frame a second allowing rapid movement over larger areas. This work is supported in part by the Navy SBIR program.

The fuselage of an aircraft is made by rivetting thin aluminum skins to the sub structure which is then exposed to the environmental elements. Moisture can penetrate between these aluminum surfaces causing inter laminar corrosion. This research describes an optical procedure to inspect large areas of quasi flat surfaces in order to identify areas of suspected inter laminar corrosion followed by a focussed ultrasonic test in the suspected area to confirm the inter laminar corrosion. After a contour scan of the offending are to determine the resulting contour amplitude an FEA model of the area was used to estimate the stress in the aluminum skin.

A new system for making dimensional measurements through a rigid borescope is described. Unlike commercially available systems, the random error in the measurement is proportional to the range to the object, rather than to the square of the range. The system requires only the addition of a video cursor generator, a computer, software, and a precision translator to any standard, substantially side-looking borescope. the system is implemented so that the user can effectively interpolate between camera pixels along the axis to which the measurement is most sensitive to pointing errors. Under ideal conditions, the relative positions of object points can be located in three dimensions to within 0.1 percent of the range, and the precision to which the 3D distance between points can be determined is several times better than that. Measurements of distances under field realistic conditions are demonstrated to have a precision of 0.4 percent of the range. In all cases, the systematic error in the measurement is demonstrated to be consistent with the level of precision.