The aging of commercial and military aircraft fleets in the US has led to increased emphasis on detecting accumulated damage such as corrosion and wide-spread fatigue damage in multi-layer airframe structures. Eddy currents are the method of choice for this task, since they can penetrate multiple layers whether or not the layers are mechanically bonded. We have developed a new pulsed eddy-current instrument that is fast, rugged, portable, and relatively inexpensive to build. It has the ability to acquire wide- bandwidth information that can quantitatively characterize the thickness of multi-layered airframe structures. The instrument can also detect cracks growing from fastener holes in the second or third layers of a lap-joint structure. The probe is mounted in a rugged scanner that easily attaches to an aircraft. Data from the instrument are presented to the user in two formats: an instantaneous A- scan of the time-domain pulsed eddy-current signal and a C- scan image of the scanned area. The C-scan image can be time-gated to provide images representing signals form various depths and to discriminate against signals from fasteners or other interfering features. We describe the instrument in detail and provide examples of its use to detect both corrosion and second-layer cracks.

Eddy current methods are used extensively in the nondestructive testing of aircraft structures and parts. One of the challenging problems in the inspection of aircraft structures is the detection of flaws in multi-layer geometries in the vicinity of edges. Very often the large contribution to the signal from the edges mask the information related to the defects. The design of new eddy current probes for reducing edge effects is therefore of significant interest. This paper investigates the design of a new eddy current probe for eliminating the edge effect. A simple method to focus the field is devised using a combination of two coils. The total field distribution is controlled by choosing the coil geometry and coil currents appropriately. Both the magnitude and phase of the currents in the tow coils are varied in order to obtain the desired flux pattern. The efficiency of these probe designs is demonstrated using finite element models. The probe parameters are optimized using finite element predictions. A probe built on the basis of this study is used to demonstrate experimentally the feasibility of the approach.

The aging of the US aircraft fleet poses serious economic and safety challenges. Fatigue cracks in the 7079-T6 aluminum fuselage skin of aging transports have presented zn opportunity to test a prototype repair. GLARE, a fiber metal laminate, has been applied to repair fuselage cracks in the fuselage skin of a US transport aircraft. This affordable prototype solution to extend the life of aging aircraft requires an inspection method to track crack growth and monitor the effectiveness of the patch on repaired fuselage skin. The fiber metal laminate patch is opaque and the fuselage skin at the damage location generally can only be accessed from the outside surface requiring the use of a non-destructive means to monitor crack length. Advances in eddy current inspection technology have provided a means to detect and track crack growth beneath patches on fuselage skins. This paper describes the development of low-frequency eddy current techniques to monitor cracks under bonded composite repair patches applied to stiffened fuselage structures. The development involved the use of a rugged portable eddy current inspection unit. The results show crack growth can be monitored to ensure the continued structural integrity of repaired flawed structures; however, the influence of substructure present a challenge to the inspector in detecting crack growth.

Eddy current inspection has long been recognized as a very sensitive and highly reliable means of detecting surface cracks in aerospace components, especially gas turbine engines. A number of factors such as equipment cost, false rejections, and throughput impacts have limited its application in the industry. All of these factors ultimately relate to the cost associated with the inspection. In the Air Force products produced to the ENSIP or PPSIP criteria, fracture critical components with relatively small flaw sizes are typically inspected as part of the production process and at their predetermined overhaul interval. The use of a hard inspection interval that requires the engine be disassembled and inspected at predetermined points in its life is fundamental for Air Force engines. In both the production and overhaul environment, dedicated facilities that utilize fully automated systems have become the norm. Fully automated systems utilize as many as seven axes and typically cost well over 1 million dollars per system require highly trained individual s to program them for new inspections. Typically, these systems are housed in their own facility which also adversely affects cost effectiveness. Once in operation, these systems can achieve crack sensitivities as small as 0.005 inches deep. To use these systems, the parts must be fully disassembled and cleaned prior to inspection. In the case of very small flaw size requirements, the parts are frequently polished to achieve an acceptable surface condition. These systems have been used very effectively in the maintenance of modern fighter engines.

Shearography and thermography have shown promising results on orbiter structure and external tank (ET) and solid rocket booster (SRB) thermal protection systems (TPS). The orbiter uses a variety of composite structure, the two most prevalent materials being aluminum and graphite-epoxy honeycomb. Both techniques have detected delaminations as small at 0.25 inches diameter in the orbiter payload bay doors graphite-epoxy honeycomb structure. Other applications include the robotic manipulator system (RMS) and the rudder speed brake structure. The ET uses spray-on foam insulation (SOFI) as the TPS and the SRB forward section uses marshall sprayable ablative as the TPS. Debonding SOFI damage to the orbiter 'belly' tile and exposes the ET to thermal loading. Voids in SOFI test panels as small as 0.375 inch were detected in 1.75 inch thick foam using a pressure reduction of not more than 10 inches of water or 0.4 pounds per square inch. Preliminary results of the X33 metallic TPS are presented. Ultrasonic testing approved for orbiter bond integrity testing, is time consuming and problematic. No current non-destructive inspection technique is approved for inspection of ET/SRB TPS or the orbiter RMS honeycomb at Kennedy Space Center. Only visual inspections are routinely performed on orbiter structure. The various successes of these two techniques make them good candidates for the aforementioned applications.

The concurrent inspection of calibrated test panels manufactured with artificially created, although realistically behaving flaws is essential to providing confidence in the thermographic inspection process of advanced composite structure. For honeycomb type composite structures of principle interest is identifying delamination and disbond type defects along the bondline between the core and faceplate, as well as within the faceplate itself. To ensure that these types of flaws will be caught during the inspection cycle of a structural component the test panels must have similar behaving artificial defects. A common practice for the manufacture of artificial flaws in test panels is the use of embedded Teflon tape, or other release agents, for force an unbond condition within the laminate. These procedures though, yield results that are questionable, since one is not sure whether or not the inspection process is identifying the unbond or the inserted materials. Several fabrication methods are compared and contrasted in this paper, for controlling the degree of disbond to simulate defects resulting from mishandling or manufacturing errors without the need for inserting foreign materials in the laminate. These results are also compared to those obtained by inspecting a composite inter-tank test structure which used Teflon tape as the means to simulate critically sized defects.

The surface contamination analysis technology (SCAT) team was originated as a working roup of NASA civil service, Space Shuttle contractor, and university groups. Participating members of the SCAT Team have included personnel from NASA Marshall Space Flight Center's Materials and Processes Laboratory and Langley Research Center's Instrument Development Group; contractors-Thiokol Corporation's Inspection Technology Group, AC Engineering support contractor, Aerojet, SAIC, and Lockheed MArtin/Oak Ridge Y-12 support contractor and Shuttle External Tank prime contractor; and the University of Alabama in Huntsville's Center for Robotics and Automation. The goal of the SCAT team as originally defined was to develop and integrate a multi-purpose inspection head for robotic application to in-process inspection of contamination sensitive surfaces. One area of interest was replacement of ozone depleting solvents currently used for surface cleanliness verification. The team approach brought together the appropriate personnel to determine what surface inspection techniques were applicable to multi-program surface cleanliness inspection. Major substrates of interest were chosen to simulate space shuttle critical bonding surface or surfaces sensitive to contamination such as fuel system component surfaces. Inspection techniques evaluated include optically stimulated electron emission or photoelectron emission; Fourier transform infrared spectroscopy; near infrared fiber optic spectroscopy; and, ultraviolet fluorescence. Current plans are to demonstrate an integrated system in MSFC's Productivity Enhancement Complex within five years from initiation of this effort in 1992. Instrumentation specifications and designs developed under this effort include a portable diffuse reflectance FTIR system built by Surface Optics Corporation and a third generation optically stimulated electron emission system built by LaRC. This paper will discuss the evaluation of the various techniques on a number of substrate materials contaminated with hydrocarbons, silicones, and fluorocarbons. Discussion will also include standards development for instrument calibration and testing.

As part of the advanced Launch System technology development effort begun in 1989, the Air Force initiated a program to automate, to the extent possible, the processing of NDE data from the inspection of slid rocket motors during fabrication. The computerized system, called the Automated NDE Data Evaluation System or ANDES, was developed under contract to Martin Marietta, now Lockheed Martin. The ANDES system is generic in structure and is highly tailorable. The system can be configured to process digital or digitized data from any source, to process data from a single or from multiple acquisition systems, and to function as a single stand-alone system or in a multiple workstation distributed network. The system can maintain multiple configurations from which the user can select. In large measure, a configuration is defined through the system's user interface and is stored in the system's data base to be recalled by the user at any time. Three operational systems are currently in use. These systems ar located at Hill AFB in Ogden, Utah, Kelly AFB in San Antonio, TX, and the Phillips Laboratory at Edwards AFB in California. Each of these systems is configured to process x-ray computed tomography, CT, images. The Hill AFB installation supports the aging surveillance effort on Minuteman third stage rocket motors. The Kelly AFB system supports the acceptance inspection of airframe and engine components and torpedo housing components. The installation at Edwards AFB provides technical support to the other two locations.

Nondestructive evaluation instruments and sensors are becoming smaller with enhanced computer controlled capability and increasingly use commercially available hardware and software. Further, robotic instruments are being developed to serve as mobility platforms allowing automation of the inspection process. This combination of miniaturized sensing and robotics technology enables hybrid miniature technology solutions for identified aircraft inspection needs. Integration of inspection and robotics technologies is benefited by the use of a standard computing platform. JPL investigated the application of telerobotic technology to inspection of aircraft structures using capabilities that were developed for use in space exploration. A miniature crawler that can travel on the surface of aircraft using suction cups for adherence was developed and is called multifunction automated crawling systems (MACS). MACS is an operational tool that can perform rapid large area inspection of aircraft, which has a relatively large platform to carry miniature inspection instruments payload. The capability of MACS and the trend towards autonomous inspection crawlers will be reviewed and discussed in this paper.

We present a design for a compact residual stress measurement sensor based on laser annealing and laser speckle interferometry. The instrument integrates laser diodes, holographic optical elements, a compact CO₂ laser, and advanced data reduction techniques to provide quantitative measurements. In this paper we present a review of residual stress measurement using laser annealing, details of our design and preliminary measurement data.

An overview is presented of several techniques under development at NASA Langley Research Center for detection and quantification of flaws in aircraft structures. The techniques have been developed as part of the NASA Airframe Structural Integrity Program. The techniques focus on the detection of cracks, corrosion and disbonds in thin laminated structures. The techniques range from thermal procedures which give a rapid indication of the regions of concern to eddy current instrumentation for detecting small cracks and multilayer corrosion. Results are presented on specimens with both manufactured defects for calibration of the techniques and on specimens removed from aircraft.

The magneto-optic imaging system is being used for the inspection of surface and sub-surface defects in aircraft aluminum structures. The technique is based on the combination of eddy current excitation and magneto-optic sensing and imaging. A current sheet is excited with AC excitation which induces eddy currents in the sample. These eddy currents generate normal magnetic fields when they encounter a defect. The magneto-optic sensor has its easy axis of magnetization int he direction of the normal magnetic fields, and memory effect. Thus by imaging the reflected polarized light through the sensor, one can image defects. The sensor has an area of 60 cm² which makes the instrument very attractive for rapid inspection of aircraft structures. The magnetic fields from subsurface corrosion are weak and so the defect indications get hidden by the magnetic domains and other noise. The challenge is to eliminate the magnetic domain structure from the image and to enhance the image created by the defect. Thus image processing algorithms need to be evaluated to overcome this problem. A number of schemes such as large area averaging, morphological operators, correlation filter have been implemented. Results obtained from applying these methods to some corrosion samples is discussed in the next few sections.

Hydrogen embrittlement of metal alloys used in aircraft components can cause serious deterioration of the mechanical properties of those components. Especially vulnerable are titanium jet engine fan blades which are exposed to hydrogen at high temperatures and pressures. For this reason non- destructive detection of hydrogen down to concentrations of a few hundred ppm will be a significant addition to maintenance inspection capabilities. The McClellan Nuclear Radiation Center is investigating the use of neutron tomography to obtain quantitative hydrogen concentration information. This paper reports results of the characterization of this system. Image resolutions of a few hundred microns and noise signals of .5 percent have been found. The signal for hydrogen at a few hundred ppm has been found to be above the noise. The measured attenuation coefficients for titanium and hydrogen show beam hardening behavior consistent with the neutron beam energy spectrum. Reconstruction of titanium aircraft engine fan blades show artifacts which may mask hydrogen concentrations as low as 100 ppm; however, procedures for removing those artifacts are presented.

As part of the advanced launch system technology development effort begun in 1989, the Air Force initiated a program to automate to the extent possible the processing NDE data from the inspection of solid rocket motors during fabrication. The computerized system, called the automated NDE data evaluation system (ANDES), was developed under contract to Martin Marietta. The generic ANDES system has been tailored to support inspection tasks at two Air Logistic Centers in the Air Force. These centers are the Ogden Air Logistic Center at Hill AFB, UT and the San Antonio Air Logistic Center at Kelly AFB, TX. The ANDES systems can be configured to process digital or digitize NDE data from any source. The system will analyze the data for anomalies, classify detected anomalies, and make a recommendation ont he serviceability of the component containing the anomalies base on established criteria. The ANDES systems at Hill AFB is configured to process x-ray computed tomography (CT) images in support of the surveillance activities on the Minuteman III third stage solid rocket motors. The system functions as a supplement to the visual image analysis effort and as an automated report generation and electronic report storage device. The ANDES system installed at Kelly AFB also processes CT images. The components supported are the oil scavenging assembly from the F-100 aircraft engine,t he yoke pivot block casting data images for void content and cracks and a braised joint on the oil scavenge tube for leak paths and total wetted area. The paper will briefly discuss the ANDES development history, the system hardware and software architectures employed at both Hill AFB and Kelly AFB, and a brief description of the performance of the operational systems.

The problems of guided waves propagating across a lap joint as well as reflected by the free edge of a semi-infinite plate with defects are investigated both theoretically and experimentally. The theoretical analysis is accomplished by using a hybrid method called the global local finite element method. A bounded region enclosing the lap joint, or the free edge, is described and analyzed by the finite element method, and the Lamb wave modal expansion is used to represent the wave field outside this region. The experiments are performed by using the fracture wave detector. The theoretical and experimental results are compared and show good agreement.

Superconductive quantum interference devices, patterned in copper oxide superconductor, offer new technology for eddy current evaluation of airframes, using pulsed currents. Their high sensitivity at low frequencies, with minute pickup loops, enables arrays that can give scanned images of millimeter fatigue cracks and corrosion hidden in underlayers of airframes, without disassembly. High temperature superconductors can bring the arrays into common use. They enable packaging them in a hand-held, refrigerated instrument without cryogenic encumbrances. The instrument would be a hand-held, electromagnetic microscope for eddy current inspection of airframes.

The problem of early detection of sub surface lap joint corrosion in the fuselage section of aircraft is critical to the continued air worthiness of aging aircraft. The large area of lap joint to be inspected requires a high speed system of measurement. D SIGHT is an enhanced visual technique that allows rapid inspection and early detection of lap joint corrosion. Previously stored D SIGHT images, compared with current images can readily determine a change in a specific area of an aircraft, while constantly updating maintenance records. Nondestructive inspection of aircraft and other large complex structures using modern inspection devices leads to the generation of vast quantities of data, generally in an 'image' format. The types of inspection devices used in nondestructive inspection activities include but are by no means limited to ultrasound, eddy current, holography, D SIGHT and MOI. This paper describes a graphical user interface that has been implemented for data acquisition, storage, and retrieval of corrosion D SIGHT image data from aircraft lap splices. Features of this interface are described, as well as the benefits that can be realized from good data organization, presentation, and storage for monitoring and maintaining valuable capital assets.

The Federal Aviation Administration Technical Center has sponsored a visual inspection reliability program at its airworthiness assurance nondestructive inspection validation center (AANC). We report on the results of the benchmark phase of that program in which 12 inspectors were observed in two days of inspections on a Boeing 737 aircraft. All of the inspectors were currently employed with major airlines and all had experience inspecting the Boeing 737 aircraft. Each inspector spent 2 days at the AANC facility where they inspected to the same ten job cards. Each inspector was videotaped and all nonroutine repair actions were recorded for each inspector. Background information on each of the inspectors, including vision test results, was also gathered. The inspection results were correlated with the background variables. Aviation experience and a test time reflecting visual acuity were significantly correlated with performance factors. An analysis of the video tapes was performed to separate decision errors from search errors. Probability of detection curves were fit to the results of inspecting for cracks from beneath rivet heads in a task using prepared samples with known cracks.

In a project funded by the Federal Aviation Administration's (FAA) Aging Aircraft Program, a Portable Holographic Inspection System (PHITS) has been further developed. The technique involves taking a double exposure white light reflection hologram of aircraft structures. Each exposure is taken at a slightly different load state, and the resulting interferogram shows the deformations that occur between the two load states. Results showed that the rivets in a simple lap joint, designed to simulate the longitudinal lap splice on a Boeing 737, behaved in two distinct and easily recognizable modes. The first mode occurred at low loads and was an indication that friction forces between the two sheets of the lap joint dominated the load transfer mechanism. Indications were that the second mode related to higher loads for which the friction forces played a much lesser role. The load at which the changeover begins to occur has been called the critical load. Preliminary experiments showed that structures with a high value of critical load had a fatigue life of order ten times that of a normally fastened splice. Critical load can be readily determined in the field using the PHITS system. Research designed to establish the relationship between fatigue life and critical load is continuing. An understanding of that relationship could lead to a technique capable of fatigue life determination in typical aircraft structures.

A new enhanced optical technique for NDI has been developed at IAR/NRC. This technique converts changes in surface slope into light intensity changes in an image created with a computer based line scanner. Light passing through a slit (edge of light) is reflected from the inspected surface at a shallow angle and captured by the scanner detector. Typically a plastic zone exists in the vicinity of a crack. This zone leaves a deformed surface trail along a crack. It is this surface deformation which is enhanced by the edge of light method. In this study, a prototype edge of light scanner was used to inspect low cycle fatigue cracks in engine disk bolt holes. The results showed surface breaking cracks which were not visible using optical microscopy. The probability of detection of the inspection was calculated by opening the cracks. The prototype edge of light scanner performed significantly better than liquid penetrant and magnetic particle techniques for this application. The ability of this technique to enhance very small scale surface deformation holds promise for may NDI applications and will be the subject of future studies and evaluations.

The FAA Center for Aviation Systems Reliability has as its objectives: to develop quantitative nondestructive evaluation (NDE) methods for aircraft structures and materials, including prototype instrumentation, software, techniques and procedures; and to develop and maintain comprehensive education and training programs specific to the inspection of aviation structures. The program, which includes contributions from Iowa State University, Northwestern University, Wayne State University, Tuskegee University, AlliedSignal Propulsion Engines, General Electric Aircraft Engines and Pratt and Whitney, has been in existence since 1990. Efforts under way include: development of inspection for adhesively bonded structures; detection of corrosion; development of advanced NDE concepts that form the basis for an inspection simulator; improvements of titanium inspection as part of the Engine Titanium Consortium; development of education and training program. An overview of the efforts underway will be provided with focus on those technologies closest to technology transfer.

With over seven million departures per year, air transportation has become not a luxury, but a standard mode of transportation for the United States. A critical aspect of modern air transport is the jet engine, a complex engineered component that has enabled the rapid travel to which we have all become accustomed. One of the enabling technologies for safe air travel is nondestructive evaluation, or NDE, which includes various inspection techniques used to assess the health or integrity of a structure, component, or material. The Engine Titanium Consortium (ETC) was established in 1993 to respond to recommendations made by the Federal Aviation Administration (FAA) Titanium Rotating Components Review Team (TRCRT) for improvements in inspection of engine titanium. Several recent accomplishments of the ETC are detailed in this paper. The objective of the Engine Titanium Consortium is to provide the FAAand the manufacturers with reliable and costeffective new methods and/or improvements in mature methods for detecting cracks, inclusions, and imperfections in titanium. The consortium consists of a team of researchers from academia and industry-namely, Iowa State University, Allied Signal Propulsion Engines, General Electric Aircraft Engines, and Pratt & Whitney Engines-who work together to develop program priorities, organize a program plan, conduct the research, and implement the solutions. The true advantage of the consortium approach is that it brings together the research talents of academia and the engineering talents of industry to tackle a technology-base problem. In bringing industrial competitors together, the consortium ensures that the research results, which have safety implications and result from FAA funds, are shared and become part of the public domain.

A substantial and coordinated program in inspection system research was initiated at the FAA Technical Center in 1990 as part of the National Aging Aircraft Research Program. the primary objectives of the inspection systems initiative are to develop improved inspection techniques to address specific aging airframe and engine inspection problems and to evaluate and validate existing and emerging inspection systems. Advanced conventional technologies, emerging technologies, or combinations of technologies are investigated for their ability to accurately and reliably detect cracks, disbonds, corrosion, and other damage. This paper will present an overview of FAA inspection system research initiatives, but will focus primarily on the technical issues which have defined and prioritized those initiatives.

The questions of amplitude fiber sensors utilization and optical reflectometry development for built-in systems used for nondestructive evaluation of solidity and integrity of basis aircraft constructions and units made from composite materials are discussed.

Aging of current commercial and military aircraft has become a major concern as many older aircraft are reaching their original design life. Service failures due to inaccurate characterization of aging responses might result in costly repair, premature component replacement, and loss of human lives. The properties of aluminum alloys, titanium alloys, and nickel-based superalloys used in aircraft structures and engines might degrade with service conditions associated with the operation of the aircraft. Important aspects of environmental conditions encountered in service cannot be accurately simulated. Thus, it will be a great advantage that the in-situ mechanical properties can be obtained nondestructively. A novel portable/in-situ stress-strain microprobe (SSM) system was developed to use an automated ball indentation technique to measure, yield strength, true- stress versus true-plastic-strain curve, strength coefficient, strain-hardening-exponent, and to estimate fracture toughness. Example test results on metallic structural components and samples are given in this paper and a video demonstration will be presented at the conference. Furthermore, potential applications of the SSM technology to assess the integrity of aging aircraft are briefly discussed.

An acoustic emission tester for aircraft Halon bottles has been developed. The necessary load is applied by heating the bottles. Acoustic emission is monitored during the heating by six sensors held in position by a special fixture. This fixture was designed to fit spheres with diameters between 5 and 16 inches. A prototype has been undergoing testing in two commercial Halon bottle repair and test facilities. Results to date indicate that about 97 percent of the bottles tested show no indications of any flaws. The other three percent have had indications of flaws in non-critical areas of the bottles. All bottles tested to date have passed the hydrostatic test required by the DOT.

Through the use of an 'integrated product team' approach and new inspection techniques incorporating the latest in imaging capabilities and automation, the costs of some man- power intensive tasks can now be drastically reduced. Also, through the use of advanced eddy current techniques, the detectable size of cracks under flush-head fasteners can be reduced while maintaining a reliable inspection. Early in this decade, the FAA Technical Center and NASA LaRC formulated an aging aircraft research plan. The unique aspect about the research is that it is driven by the aircraft manufacturers and airlines in order to center only on those areas in which help is needed and to keep it focused. Once developed, the manufacturer works with the FAA Validation Center at Sandia National Labs., the airline, and the researcher to transfer technology to the field. This article describes the evaluation and results obtained using eddy current technology to determine the minimum detectable crack size under installed flush-head fasteners. Secondly, it describes the integrated efforts of engineers at McDonnell Douglas Aerospace and Northwest Airlines in the successful application of MAUS eddy current C-scanning of the DC-10 circumferential and axial crown splices. The eddy current C-scanning greatly reduced the man-hour effort required for the existing radiographic inspection. Thirdly, it describes the use of a novel ultrasonic technique coupled to a scanner and graphics for the detection and quantification of corrosion thinning and stress corrosion cracking of the DC-9 lower wing tee cap. This successful effort resulted from a rather large integrated task team. It also results in a vast man-hour savings over the existing internal visual inspection.

The Swedish National Road Administration has been using a laser based system for assessing pavement surface characteristics for over a decade. Typically, important distress data like roughness and rutting are being sampled at speeds up to 25 meters per second. Other parameters include textures and geometric data, such as cross slope and curvature. The Royal Swedish Fortifications Administration has just recently looked into these techniques for airfield pavements. The objectives are similar but not exactly the same as for highway pavements. A promising aspect is using data for building terrain models of the surface so that overlay design procedures can take place in a computerized environment. The objectives are similar but not exactly the same as for highway pavements. A promising aspect is using data for building terrain models of the surface so that overlay design procedures can take place in a computerized environment. Two different ways of treating the data for this purpose was tried. One model uses several parallel continuous longitudinal profiles. The other consists of snap-shot transverse profiles at five meter intervals. The former model yields good assessment of the volumetric needs of either filling or milling operations. The latter is better for identifying features on the surface. The present paper describes some of experiences obtained with the two methods. However, in both cases the most advantageous aspect of using the non-contact devices is the short time needed for collecting the data.

Seismic Pavement Analyzer and a portable version of it have been extensively used for quality control and monitoring the structural condition of pavements. With them, a pavement can be tested at closely spaced points and at a fraction of the cost and time of coring. The main tests used are the impact echo for determining the thickness of the slab, ultrasonic body wave and ultrasonic surface wave for determining the moduli of top layer, impulse response for determining the condition of subgrade, and SASW method for determining the modulus profile of pavement. Based on extensive field testing on numerous types of base and subgrade,the techniques in general, and the two devices in particular are quite suitable for many quality and pavement evaluation projects.

The state-of-the-art for nondestructive testing (NDT) of highway and airport pavements was evaluated in the late 1980's as part of the Strategic Highway Rehabilitation Program (SHRP). This program included many research projects in pavement construction, testing, maintenance, and rehabilitation. The limitations, as well was the capabilities of the various NDT methods then extant were examined by joint teams of federal, state, academic, and private sector engineers and researchers. The SHRP program, and the allied long-term pavement performance program clearly demonstrated that certain NDT methods performed well on asphalt pavement but performed unsatisfactorily on concrete pavements, and vice-versa. Despite this, some methods are still being used inappropriately, while other promising techniques are virtually being ignored. This paper examines some of the reasons for this, and summarizes the research and current state-of-the-art for NDT of concrete airport and highway pavements. Promising avenues for further development of test equipment, applications, data analysis, and interpretation are also discussed.

This paper describes the nondestructive evaluation devices, visual distress survey and coring used to investigate jointed concrete pavement performance in northern Mississippi. 3D finite-element models were developed to simulate in-service conditions and to characterize in-situ material properties. Reasonable good agreement is found between in-situ moduli backcalculated from the dynamic analysis of falling weight deflectometer (FWD) deflections measured on selected pavements and laboratory moduli. Effects of load pulse shape, cracking, and discontinuities on the surface deflection response of pavements subjected to FWD load wee also investigated. It is shown that 3D analysis of temperature distribution and resulting thermal stresses play a significant role int he performance of concrete pavements. The study results demonstrated the extensive usefulness of the finite-element dynamic analysis and limitations of the static multilayered analysis and other pavement analysis programs which do not allow for crack modeling and dynamic analysis.

The Center for Quality Engineering and Failure Prevention (CQEFP) at Northwestern University is actively involved in the development of new stress-wave based non-destructive evaluation techniques for airport pavements. This paper summarizes recent accomplishments and outlines current research directions. The development of a new stress-wave source is detailed first. The stress-wave generating technique allows for a high degree of control of the input stress wave while at the same time enabling the generation of significant wave amplitudes. Experimental results on concrete specimens demonstrate the controllability and penetrating ability of the developed stress wave generation technique. Its performance is compared to that of an impact source. Experimental results from an existing stress-wave based NDE technique, the impact-echo method, are presented and limitations of that approach are demonstrated. Finally, directions of future airport pavement NDE research at CQEFP, which focus upon application of the developed stress wave generation technique to pavement NDE problems, are outlined.

One of the major thrusts established under the FAA's National Aging Aircraft Research Program is to foster new technologies associated with civil aircraft maintenance. Recent DOD and other government developments in the use of bonded composite doublers on metal structures has supported the need for research and validation of such doubler applications on US certificated airplanes. Composite doubler technology is rapidly maturing and shows promise of cost savings on aging aircraft. While there have been numerous studies and military aircraft installations of composite doublers, the technology has not been certified for use on commercial aircraft. Before the use of composite doublers can be accepted by the civil aviation industry, it is imperative that methods be developed which can quickly and reliably assess the integrity of the doubler. In this study, a specific composite application was chosen on an L-1011 aircraft in order to focus the tasks on application and operation issues. Primary among inspection requirements for these doublers is the identification of disbonds, between the composite laminate and aluminum parent material, and delaminations in the composite laminate. Surveillance of cracks or corrosion in the inspection (NDI) method can inspect for every flaw type, therefore it is important to be aware of available NDI techniques and to properly address their capabilities and limitations. This paper reports on a series of NDI tests which have been conducted on laboratory test structures and on a fuselage section cut from a retired L-1011 aircraft. Specific challenges, unique to bonded composite doubler applications, will be highlighted. In order to quickly integrate this technology into existing aircraft maintenance depots, the use of conventional NDI, ultrasonics, x-ray, and eddy current, is stressed. The application of these NDI technique to composite doublers and the results from test specimens, which were loaded to provide a changing flaw profile, are presented in this paper. The development of appropriate inspection calibration standards will also be discussed.

As the application of advanced composite materials and adhesively bonded components becomes increasingly numerous in aircraft structures, so is the number of aircraft containing such structures that can be classified in the aging aircraft category. The effect of environmental and in- service aging of such structures is not well known or understood, neither have NDE techniques been able to satisfactorily qualify and quantify the loss of structural integrity due to the aging process. This paper will present the latest developments in the practical use of a field portable holographic interferometric testing system. The system results, known as holographic interferograms, provide a better understanding of how a structure is behaving under realistic loads in the presence of defects, damage and material property aging. The system has been applied to a variety of defects in composite and adhesive bondlines, as well as artificial environmental aging of these materials. The holographic interferograms produced form these investigations will be briefly reviewed and their impact on structural integrity of the component discussed.

The percentage of composite materials used on new aircraft continues to rise, creating a greater need for NDI methods that can be used on-aircraft and are effective in identifying degradation and damage in composite structures. A particular type of degradation (delamination due to impact), can produce significant strength or stiffness reduction without being visible to the naked eye. Methods such as pulse echo ultrasonics, shearography, and thermography are being applied to this problem, but are costly to implement. The traditional coin tap or tap hammer method, is low cost, but is inherently subjective, and operator dependent. Boeing has developed a low cost instrumented tap hammer that provides a quantitative measure of the hammer/composite impulse time that can be correlated to delaminations in the structure. The instrumented tap hammer supplements the tonal discrimination of the operator with a numeric readout that can readily be related to local part quality. The effect of background noise and operator differences on the inspection results can be eliminated. An increased sensitivity is also shown over the audible tap test method. This paper describes the theoretical basis for the impulse measurement, the tap hammer design, test methods, and test results which validate the device.

Boeing Defense and Space Group in Kent, WA is investigating forced harmonic shearography nondestructive testing technique for detection of disbonds in bonded structures of both existing nd future generation aircraft. This paper describes the forced harmonic shearography inspection technique by using real time electronic shearography and sine swep vibration system. Results from NDI of aluminum bonded and composite bonded structures will be shown.

Microwave and millimeter wave nondestructive methods using open-ended sensors have shown great potential for detecting minute thickness variations in laminate structures, in particular those backed by a conducting plate. Similarly, slight variations in the composition of dielectric materials may also be detected using a set of optimal parameters which include the standoff distance and the frequency of operation. In this paper the potential of using an open- ended rectangular waveguide for detecting the presence of rust under common paint and composite laminate coatings will be demonstrated. An experimental scanned image of a steel specimen with an area of induced corrosion will also be shown.

Systems that associate positions with eddy current probe readings present inspection data more directly and usefully than the more common impedance plane display. We have economically constructed such an integration of an eddy current testing device with a position feedback device which gives 0.005 inch achievable resolution. The position and probe readings are fed to a PC and plotted in real-time as a projection into a user-defined rectangle in 3-space. All spatial coordinates and all inspection data are retained for model building and record keeping. Unsampled eddy-current readings are interpolated using a spatial average, consistent with the observed averaging effect of the probe. An empirically-determined convolution kernel permits an estimate of the surface profile on the opposite side of the sample by deconvolution. This can then be combined with the position to provide a model of the sampled area.

A next generation magneto-optic imaging system, the MOI 303, has recently been introduced with the ability to generate real-time, complete, 2D eddy current images of cracks and corrosion in aircraft. The new imaging system described features advanced, digital remote control operation and on- screen display of setup parameters for ease of use. This instrument gives the inspector the capability to more rapidly scan large surfaces areas. The magneto-optic/eddy current imaging technology has already been formally approved for inspection of surface cracking on an aircraft fuselage. The improved magneto-optic imager is now poised to aid rapid inspection for corrosion and subsurface cracking. Previous magneto-optic imaging systems required the inspector to scan the surface twice for complete inspection coverage: a second scan was necessary with the imager rotated about 90 degrees from the orientation of the first pass. However, by providing eddy current excitation simultaneously from two orthogonal directions, complete, filled-in magneto-optic images are now generated regardless of the orientation of the imager. THese images are considerably easier to interpret and evaluate. In addition, there is a synergism obtained in applying eddy current excitation simultaneously in multiple directions: better penetration is obtained and the resulting images have better signal to noise levels compared to those produced with eddy current excitation applied only in one direction. Examples of these improved images are presented.

We describe an IR thermal wave imaging technique for making corrosion thinning determinations on aging aircraft skins. The technique uses pulsed surface heating and fast, synchronous IR imaging of subsurface structure, such as skin corrosion and disbonded doublers or tear straps. Sensitivity to corrosion thinning of less than two percent is demonstrated. Practical implementation of a simplified numerical measurement algorithm is presented, and the results are compared with profilometry and ultrasonic measurements of calibration standards. Examples are presented of thermal wave imaging of fuselage skin corrosion of a B737 testbed aircraft in a hangar environment at the FAA's Aging Aircraft NDI Validation Center.

Application of ultrasonic imaging to the inspection of multilayered airplane structures can provide comprehensive information on possible defects such as corrosion and cracks. A quantitative assessment of defects in internal layers of airplane structures using this technique can be accomplished form an external surface of the airplane skin. No disassembly is required for component evaluation. Therefore this ultrasonic technique can be extremely useful for cost-effective maintenance of airplanes. Three different ultrasonic techniques developed for inspection of the DC-9 and DC-10 airplanes are discussed in this paper. One of the techniques was used for evaluation of corrosion and stress corrosion cracks in a DC-9 tee cap, other techniques have been developed for fatigue crack characterization in a DC-9 rear spar and a DC-10 spar-cap/strap connection, respectively. All three techniques were developed to be applied with the same type of data acquisition and imaging system. However for each kind of flaw and structural configuration substantial modifications of the scanning and the data acquisition procedures have been made. {Peculiarities of each ultrasonic technique will be discussed. The ultrasonic technique for the inspection of the DC-9 tee cap has been approved by the FAA as an alternate means of compliance to meet the requirements of the DC-9 Service Bulletin.

The early detection of corrosion in aging structures is probably one of the most important challenges of the aeronautic maintenance services. Laser-ultrasonics offers interesting characteristics to become an industrial technique able to solve this problem. Neverless, to become quantitative, this non-destructive method requires a precise description of the laser-ultrasonic generation. This paper presents a new and original model which takes into account the layered structure which is generally encountered in aeronautic materials subjected to impacts, fatigue and corrosion. This model solved the Christoffel equations in an axisymmetrical configuration over an infinite plate of finite thickness presenting a cylindrical orthotropy. The sample is a flat plate made of two layers of different materials and the laser impinges the sample normally to the surface. The method of resolution used allows fast calculation and observation of the displacements over a long time period. This is very useful in NDT, especially in the case of thick samples. Validations were conducted by comparing the results calculated by this model to the ones obtained with a previous model and with experimental measurement using a Nd:YAG pump laser and an interferometric detection.

A need exists in airline maintenance for a method of producing high quality, easily interpreted images that provides details on the soundness of aircraft fuselage structures. The 'dripless bubbler' technique combines the use of broadband focused-beam ultrasonic immersion transducers with a method of maintaining a contained, bubble-free water pool. The use of the dripless bubbler, when combined with a portable robotic scanner and data acquisition system, produces ultrasonic C-scan and B-scan images for evaluating the extent of corrosion within lap and butt joints and ont he interior of fuselage skins. Also easily identifiable are disbonds in the skin-to-structure adhesive joints and delamination type defects in composite structures. The high image quality is not affected when scanning over surface irregularities; the dripless bubbler can be scanned over button-head rivets and lap splices with minimal change in transducer orientation and no loss of couplant.

We describe a library of image enhancement and understanding algorithms developed to enhance and recognize surface defects from remote live imagery of an aircraft surface. Also described are the supporting mobile robot platform that generates the remote stereoscopic imagery and the inspection console containing a graphical user interface, through which the inspector accesses the live imagery for remote inspection. We will discuss initial results of the remote imaging process and the image processing library, and speculate on their future application in aircraft inspection.

We propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor thickness changes of plate-like structures. A pair of pin transducers are used to excite and detect the A₀ mode Lamb wave in a test plate or a pipe wall and the wave velocity is obtained by time of flight measurement. Any change in plate thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of the A₀ mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of thickness measurements of a test plate with 1 percent accuracy, and erosion/corrosion monitoring in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.

Second layer cracking of the C-14 lower inner wing spanwise slice has been identified by the C-141 system program office as the life-limiting feature of the C-141 aircraft. The current inspection method is bolt hole eddy current (BHEC) that detects second layer fatigue cracks that are 0.070 inch or longer. ALthough the BHEC has the detection capability, this method requires the removal of 3,700 fasteners in the lower wing splice joints. In addition, the BHEC inspection effort requires 9,000 man-hours per aircraft and is performed every five years under the programmed depot maintenance. Ultrasonic inspection of two-layer structures can be accomplished if sealant is located between the faying surfaces to conduct the ultrasonic signal from one layer to the next. Such an ultrasonic inspection has been performed and validated under a FAA-sponsored, technology-transfer task on DC-9 wing-box tee caps. On C-141 aircraft structures, recent proof-of-concept studies have shown that an ultrasonic inspection process has the potential to reliably detect 0.050 inch or smaller second layer cracks in simulated spanwise joint test specimens. Automation of the inspection process improves the reliability of the inspection and accomplishes the aircraft inspection in 200 man-hours per aircraft. Implementation of such an ultrasonic method results in a 45:1 reduction in the manpower as compared to the BHEC inspection and returns the aircraft to service much sooner. This paper describes a methodology for developing a prototype inspection process. Involved in that process is the design, fabrication, and testing of a prototype scanner for multi-channel, real-time data collection and display of A-scan, B-scan, C-scan ultrasonic data formats. Functional testing of the inspection process is described on fatigue crack test articles in the laboratory. Typical ultrasonic image results and a description of a probability-of-detection study will be presented.

Recent microwave methods have shown to be capable of detecting and sizing surface slots and cracks in metals. These methods have incorporated the use of an open-ended rectangular waveguide probe for such measurements. A new microwave method utilizing an open-ended coaxial line sensor has been under investigation for some time now. Coaxial line sensors have certain features that make them quite attractive for surface crack detection. These features include their high level of sensitivity to the presence of very narrow cracks as will as the fact that their geometry may include complicated bends allowing access to hard to reach places. This paper presents and comments on some preliminary experimental results of using this sensor for hairline surface crack detection.