This paper describes an experimental study of inducing disbonds in adhesively bonded aluminum lap splices by flexural fatigue and the use of ultrasonic methods for detecting such disbonds.Disbonds were imaged using focused beams of high frequency (15MHz) and low frequency (1 MHz) ultrasound. Detectability of disbonds above and below scrim cloth in the adhesive bond was investigated. Ultrasonic NDE results were compared with optical edge-on observations of disbond propagation during mechanical testing and also to other NDE methods such as thermal wave imaging and electronic speckle pattern interferometry. Methods for exploiting focused ultrasonic beam for inspection are discussed.

We are engaged in the testing of a 22 ship fleet of 25 - 30 year old aging aircraft using an advanced acoustic nondestructive technique. The purpose of our testing is to provide real-time detection of structural defects anywhere within the entire pressure hull of the aircraft as they are pressurized to 12 psig, in order to prevent catastrophic failure and potential loss of the aircraft. Periodic proof pressurization is a requirement of the regulatory authority for the aircraft, and is a condition for continued airworthiness certification.

High contrast sensitivity (0.2%) and high throughput for aircraft inspection is obtained in a large area, real-time/near real-time system. X rays originate in the source, which is modelled upon the cathode ray tube. Magnetic deflection coils cause electrons to sweep in a raster pattern across the high z target of a broad anode plate. The resulting microfocus x rays scan across a specimen placed in close proximity -- typically 1 inch or less -- to the large source. A computer synchronizes the beam sweep, the detector readout rate, and the monitor sweep. Digital output from the detector unit allows the system to display first generation images, without recourse to an image intensifier or vidicon.

Computed Tomography (CT) using penetrating radiation (x- or gamma-rays) can be used in a number of aircraft applications. This technique results in 3D volumetric attenuation data that is related to density and effective atomic number. CT is a transmission scanning method that must allow complete access to both sides of the object under inspection; the radiation source and detection systems must surround the object. This normally precludes the inspection of some large or planar (large aspect ratio) parts of the aircraft. However, we are pursuing recent limited-data techniques using object model information to obtain useful data from the partial information acquired. As illustrative examples, we describe how CT was instrumental in the analysis of particular aircraft components. These include fuselage panels, single crystal turbine blades, and aluminum-lithium composites.

Ultrasonic nondestructive evaluation is a valuable technique for finding defects in aircraft structures. It can detect unbonds, corrosion damage, and cracks in various aircraft components. Ultrasonic nondestructive evaluation techniques interrogate materials with high frequency acoustic energy. A piezoelectric transducer generates acoustic energy and converts returned acoustic energy into electrical signals which can be processed to identify the reflector. The acoustic energy propagates through the component and is reflected by abrupt changes in modulus and/or density that can be caused by a defect. Ultrasonic nondestructive evaluation typically provides a two dimensional image of internal defects. These images are either a planar view (C-scan) or a cross-sectional view (B-scan) of the component. The planar view is generated by raster scanning an ultrasonic transducer over the area of interest and capturing the peak amplitude of internal reflections. Depth information is generally ignored.

We apply dual-band infrared (DBIR) imaging as a dynamic thermal tomography tool for wide area inspection of a Boeing 737 aircraft (owned by the FAA/AANC at the Sandia hangar in Albuquerque, NM) and several Boeing KC-135 aircraft panels (used for the round robin experiment conducted at Tinker AFB, OK). Our analyses are discussed in this report. After flash-heating the aircraft skin, we record synchronized DBIR images every 40 ms, from onset to 8 seconds after the heat flash. We analyze selective DBIR image ratios which enhance surface temperature contrast and remove surface-emissivity clutter (from dirt, dents, tape, markings, ink, sealants, uneven paint, paint stripper, exposed metal and roughness variations). The Boeing 737 and KC-135 aircraft fuselage panels have varying percent thickness losses from corrosion. We established the correlation of percent thickness loss with surface temperature rise (above ambient) for a partially corroded F-18 wing box structure (with a 2.9 mm uncorroded thickness) and several aluminum plates (with 1.0, 1.1, 2.3, and 3.9 mm thicknesses) which had 6 to 60% thickness losses at milled flat-bottom hole sites.

The thermographic NDT using a contact cooling technique of an IR transmissive optical solid was newly developed for the defect inspection of the laminated composites. The resolution of the defect inspection using the proposed contact cooling technique was examined both in the computational simulation and the experimental study. The thermal transfer technique for the samples with the less emissivity was developed together with the contact cooling technique using the IR transmissive solid. The usefulness of the thermal transfer technique was demonstrated in the thermographical inspection of the delaminated defects in laminated FRP and the debonded defects in copper-polyimide laminates with less emissive bare surfaces.

Pulse-Echo Thermal Wave IR techniques have been presented at recent SPIE Thermosense Conferences. A brief review is presented along with applications to the detection of adhesion disbonds and corrosion defects in bonded aluminum lap joints. Results for both test panel specimens and aircraft panels are presented. The feasibility of utilizing the technique for rapid, contactless, wide area field inspection of aircraft is discussed. Examples from a recent field test on a B737 aircraft are presented.

A new thermographic technique, Forced-Diffusion Thermography (FDT), is described. Forced-Diffusion Thermography uses a continuously projected dynamic heat pattern to set up appropriate combinations of lateral and through-depth heat flow in structures. A synchronous imaging IR camera is used to measure the structures thermal response to a patterned heat flux in order to find flaws such as cracks and delaminations. A numerical model of the technique is used to describe the thermal responses to cracks and delaminations. Experimental data is presented for the case of a fatigue crack in a 2 mm thick aluminum skin.

This paper describes the use of time-resolved infrared radiometry (TRIR) to identify characteristic temperature-time signatures resulting from different subsurface thermal structures in aging aircraft. Central to the TRIR technique is the analysis of the temperature- time signatures at various locations as a step heating pulse is applied to the structure. Of particular interest is determining whether a signature can be identified which discriminates the presence of corrosion product from the simple thinning of the aircraft skin as might occur as a result of a previous repair. A technique is proposed which implements both area and localized heating sources. The area heating source provides one-dimensional heating of the specimen and allows suspect areas to be rapidly detected. A localized heating source is then used to further characterize the suspect regions.

This paper provides an overview of a program established at Sandia National Laboratories by the Federal Aviation Administration (FAA) to validate nondestructive inspection (NDI) processes for application to aging aircraft. The paper describes an NDI Validation Center, an evolving library of specimens with typical defects, a validation process, ongoing related field experimentation, and current and future work activities.

Evaluating the reliability and capabilities of nondestructive evaluation (NDE) methods has normally been done empirically with many costly round-robin test block exercises. These results are necessarily problem specific and limited in scope. Here, an alternative approach is described where model-based simulations are used to predict the probability of detection versus flaw size curves for components which can then be validated experimentally with a few samples. It is demonstrated how knowledge of these curves can be used to evaluate NDE inspection performance, to determine the influence of NDE tests on component reliability, and to design components with built-in inspectability criteria.

Under the FAA Aging Aircraft Research Program we are developing robots to deploy conventional and, later, new-concept NDI sensors for commercial aircraft skin inspection. Our prototype robot, the Automated NonDestructive Inspector (ANDI), holds to the aircraft skin with vacuum assisted suction cups, scans an eddy current sensor, and translates across the aircraft skin via linear actuators. Color CCD video cameras are used to align the robot with a series of rivets we wish to inspect using NDI inspection sensors. In a previous paper we provided a background scenario and described two different solutions to the alignment problem: a model-based system built around edge detection and a trainable neural network system. In this paper, we revisit the background and previous research and detail the first steps taken towards a method that will combine the neural and the model based systems: a neural edge detector.

Thermoelastic stress analysis is a relatively new experimental method which shows considerable promise for quantitative NDT of structures such as aging aircrafts. Whereas most previous thermoelastic applications have been qualitative in nature, this paper emphasizes the ability to develop the technique into a practical, non-contacting, quantitative NDT method for analyzing actual engineering structures made of either isotropic or orthotropic materials and while in their operating environment.

A program of research was undertaken to develop a method to find stress intensity factors of fatigue cracks propagating in metal structures using the method of photoelastic coatings. The method is described in this paper along with test results to demonstrate how it can be used in the analysis of fatigue crack growth.

Superconducting quantum interference device (SQUID) magnetometers offer promise as multi- mode instruments capable of obtaining high resolution images of extremely low frequency injected currents or eddy currents, and they can be configured to image the magnetic susceptibility of titanium, aluminum, and nonmetallic composites. While high resolution SQUID magnetometers will generally be noisier than conventional SQUIDs, the small coils and reduced coil-to-source spacing more than compensate to provide low-noise, high- resolution images. To explore SQUID NDE, we have developed research facilities that include the high-resolution MicroSQUID magnetometer, a magnetic shield, a scanning stage, and a computer-based control and data acquisition system. Using this instrumentation, we have imaged magnetic fields produced by varied sources. In support of the experimental studies, we have developed analytical and numerical models for the simulation of flaws with several geometries inside thick and thin current-carrying plates and thin-walled tubes, and have demonstrated that two-dimensional magnetic images can be deconvolved into images of current or magnetization by filtering techniques, finite element models, lead field analyses, and maximum entropy methods.

Subsurface flaws in electrically conducting components are not easy to detect by conventional eddy current techniques because the skin depth (delta) decreases at high frequencies, while the signal decreases at low frequencies. However, SQUID magnetometers are capable of measuring dc and low frequency magnetic fields, and have been used for imaging current distributions. We have now extended SQUID NDE by utilizing a technique to induce an extended eddy current parallel to the surface in a conducting plate. The magnetic field due to the eddy current perturbation caused by a subsurface flaw has been calculated and compared with experimental results, which gives information about the depth of the flaw. We also present data recorded from aluminum samples that simulate lap joints in aircraft wings.

The magnetic signature associated with electrochemical corrosion in standard aircraft lap joints has been investigated with a superconducting quantum interference device (SQUID) magnetometer. Preliminary results on both spatial and temporal scans of single aluminum electrodes and lap joint samples exposed to dilute NaOH are presented for the first time. We have observed definite corrosion signals from a wide variety of aluminum samples, in all physical orientations relative to the instrument, down to an electrolyte concentration of 0.01 M. We have also detected natural filiform corrosion taking place in a dry 40 year old joint not exposed to additional corrosives. These preliminary results indicate that SQUID based magnetometers can be applied to detect corrosion in aging aircraft.

A new magneto-optic/eddy current imaging technology promises to revolutionize the nondestructive inspection of aging aircraft and also to provide new ways to inspect unlikely materials such as non-metallic composites. As the name implies, the technology is a hybrid of magneto-optic imaging techniques and novel eddy current excitation methods. The result, a device we call a magneto-optic/eddy current imager, or MOI, allows one to inspect such things as aircraft lap joints for both cracks and hidden corrosion in approximately one tenth the time it would take with conventional eddy current inspection equipment. The device produces realistic, real-time images of both cracks and corrosion and has proven to be a reliable in-service tool. The MOI has been approved for the inspection of Boeing, Douglas, and Lockheed commercial aircraft. It is also currently being used by the U.S. Air Force, NASA, and many other organizations.

Electronic shearography has emerged as a promising new technology for non-contacting, full- field optical nondestructive evaluation. In particular, the `common-path' interferometric configuration that is typically used with shearography offers greater immunity to environmental disturbances as compared to traditional electronic speckle pattern interferometry (ESPI) or holographic interferometry. This advantage gives electronic shearography a potential edge for testing applications in industrial environments. This paper presents results of recent experiments towards evaluating the capability of electronic shearography for the inspection of aluminum airskin structures. The defect classifications targeted in this study included disbonds in adhesively bonded skins as well as fatigue cracks emanating from rivet holes. Selection of an effective loading strategy to isolate the defect regions in the fringe patterns was a primary consideration in this study, as with any type of interferometric inspection. Vacuum stressing proved effective locating disbonds while mechanical point loading with vacuum grippers was used to identify fatigue cracks. In each case, the resolution of the shearographic inspection technique was intrinsically dependent on the size of the field of view.

Electronic speckle pattern interferometry (ESPI) utilizing a phase-modulation of the object beam and a continuous reference-updating technique is proposed to provide noise reduction in optical NDE methods. Unlike conventional ESPI techniques, this method uses phase modulation between successively subtracted additive speckle interference images. The ability of this technique to work in a turbulent environment is demonstrated, and application to detection of structural defects in adhesively bonded structures, a problem of interest to the NDE community, is shown.

The inspection of large area aircraft structures for damage is a time consuming process, based on the large surface areas involved and the small size of damaged areas. Damage can come from impacts for composites or corrosion in metal structures. D SIGHT, a non-contacting optical technique, allows large areas to be inspected quickly and with a high degree of sensitivity. Several examples of aircraft inspection are shown.

The inflight structural failure of an Aloha Airlines 737-200 in April, 1988 brought international attention to the ageing aircraft issue and caused operators to improve inspection and maintenance procedures for their fleets. The use of nondestructive visual inspection equipment such as borescopes, fiberscopes and videoimagescopes allow maintenance personnel to inspect internal aircraft structure for corrosion and fatigue without costly and time consuming disassembly.

The decision was made in 1982 to introduce neutron radiography to complement other nondestructive inspection methods available for inspection of aircraft parts (Stationary Neutron Radiography System) and intact aircraft (Maneuverable Neutron Radiography System). Both the systems have now produced their initial results: aircraft show significant trends in the type of defects found (moisture is the most frequent finding); trends in the location of defect occurrence in specific parts (e.g., along leading edges of critical control surfaces); and trends between groups of aircraft (depending on whether they operate primarily in dry or damp environments).