Several emerging technologies affording new opportunities for the nondestructive evaluation of aging aircraft structures. Among these technologies are ultrasonic attenuation detection of fatigue and corrosion damage; non-contact ultrasound including laser-laser, laser-EMAT, laser-air, and air-air; laser based acoustic emission source identification; full-field double-pulse holographic imaging; non-linear assessment of adhesive bond quality; x-ray diffraction topographic imaging of the quality of nickel based alloy single crystal turbine blades.

Paint systems containing color-change or fluorescing compounds have been found to be sensitive to underlying corrosion processes by reacting to the pH increase associated with the cathodic reaction that accompanies corrosion. The sensitivity of these coating systems for detection of cathodic reactions associated with corrosion was determined by applying constant cathodic current and measuring the charge at which color change or fluorescence is detected. Visual observation of coated samples with the unaided eye can detect changes resulting from a charge corresponding to a hemispherical pit with depth on the order of 3–15 microns. Electrochemical Impedance Spectroscopy was also performed to test the influence of the indicating compound addition on the coating corrosion protectiveness.

Nominally smooth surfaces subjected to repeated contact may develop characteristic fretted textures. An important example concerns fretted surfaces that may develop on disk dovetail joints that secure fan blades in turbine engines. These surfaces may form sites for crack nucleation and subsequent catastrophic disk and engine failure. Thus a practical and reliable method for the nondestructive evaluation of such fretted surfaces is critically needed. In this paper straightforward optical techniques are considered for identifying and quantifying fretted (and other surface damaged) regions. For example, digital images of candidate surfaces may be processed to separate fretted from unfretted regions and to evaluate the relative degree of fretting. Ideally, optical characterization should be rapid, convenient, inexpensive, reliable, and insensitive to details of lighting and image capture.

Method of electromagnetic acoustic resonance (EMAR) was applied to the noncontact measurement of the shear wave attenuation during the creep test of 2.25Cr-1Mo steels. Two electromagnetic acoustic transducers were manufactured for this purpose, which generate the polarized shear waves through the magnetostriction effect. The attenuation coefficient increased with the creep deformation. The evolution range of the attenuation was beyond 0.1 μs−1 before the failure, which was much larger than the attenuation variation among the samples.

The surface damage introduced by general corrosion attack of surfaces is considered deleterious to long term structural integrity. As a result, the quantification of this damage represents an interest in the NDE community. In this document, the chemical kinetics of general attack are used to model the morphology of the surface as a function of time of exposure. Timeof- flight ultrasonic data for corroded surfaces are presented which appear to agree with the model predictions. The experimental results are critically reviewed with respect to the practical limitations of the ultrasonic experiments. The effect of exposure on surface morphology and surface stress concentration are shown and related to the classic ultrasonic measurement techniques. The relationships are used to suggest potentially important ultrasonic measurement techniques and to underline the inherent limitations of many classical ultrasonic measurements.

We present examples of the application of thermal wave imaging to the detection of structural defects in aging aircraft. Examples include the imaging of bonded and disbonded internal doublers, together with quantitative corrosion thinning measurements made from thermal wave images of aircraft fuselages and wing skins. We also show example images of impact damage, disbonds and delaminations in composite aircraft materials.

This paper discusses the role electrical anisotropy plays in the structural integrity assessment of polycrystalline titanium alloys from the standpoint of fatigue crack detection and the related issue of microstructural noise. In eddy current inspection of noncubic crystallographic classes of polycrystalline metals the electric anisotropy of individual grains produces an inherent microstructural variation or noise that is very similar to the well-known acoustic noise produced by the elastic anisotropy of both cubic and noncubic materials in ultrasonic characterization. The presented results demonstrate that although the electrical grain noise is detrimental in eddy current nondestructive testing for small flaws, it can be also exploited for characterization of the microstructure in noncubic polycrystalline materials such as titanium alloys in the same way acoustic grain noise is used for ultrasonic characterization of the microstructure in different materials.

Selected electronic, digital x-ray imaging systems and techniques were evaluated as possible replacements for the use of x-ray film at Air Force field aircraft maintenance facilities. Incentives for the introduction of digital systems include the elimination of recurring costs of film, processing, and waste disposal, and the advantages of electronic data analysis, digital signal processing, and the ease of storage, retrieval, and transfer of digital data. Initial experimental evaluations included photostimulable storage phosphors, scintillator-camera systems, flat panel imagers and the reverse geometry method. The technical evaluations were conducted with a test object consisting of typical honeycomb structure containing various faults, line pair gages, and image quality indicators on ¼–inch aluminum. Several electronic systems were capable of meeting most of the technical requirements for x-ray inspection, with system prices attractive for field locations with annual film costs in the range of $15,000 or more. All the electronic methods eliminate processing chemicals and offer advantages in certain applications. However, the storage phosphor systems were particularly well suited for field implementation because of their similarity to film, but without most of the recurring costs and the environmental problems associated with the use of film. These evaluations were conducted under a Small Business Innovation Research (SBIR) Phase I program. More extensive system evaluations are planned as the program continues into an SBIR Phase II project.

Dielectric techniques have the potential of allowing observation of changes in the dipole mobility within an adhesive joint structure as a result of its exposure to a warm humid environment. Water molecules absorbed by an adhesive resin will exhibit a series of distinct relaxation features, which are characteristic of the environment in which the molecules are located. Hydration of the surface oxide of an aluminium-epoxy joint will produce a distinctive dielectric relaxation at approximately 1 MHz which is quantitatively related to the amount of hydroxide formed. Data on aged adhesive joints indicates that the dielectric technique has potential for the study of the changes occurring within the joints and the technique may be also used for studies of carbon fibre - epoxy - carbon fibre structures.

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fiber composites by DC electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continuous carbon fiber carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fiber fracture as well. For the short carbon fiber polymer-matrix and cement-matrix composites, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibers and the resulting increase in the chance that adjacent fibers touched one another.

By measuring the electrical resistance of a continuous unidirectional carbon fiber epoxy-matrix composite along the fiber direction during loading in this direction, fiber breakage was progressively monitored in real time. Fiber breakage occurred in spurts involving 1000 fibers or more. It started at about half of the failure strain during static tensile loading and at about half of the fatigue life during tensiontension fatigue testing. Immediately before static failure, 35% of the fibers were broken. Immediately before fatigue failure, 18% of the fibers were broken. The fiber breakage was accompanied by decrease in modulus.

A strategy for selecting and developing nondestructive evaluation (NDE) methodologies for fiber reinforced polymer composite structural components to be used in civil infrastructure applications is described. Conventional and innovative evaluation approaches are discussed.

Actual experience with NDE of hybrid carbon and glass fiber reinforced vinyl ester pultrudedbeams that are installed in a bridge carrying automotive traffic is described.

An interdigitated gate electrode field-effect transistor (IGEFET) was designed, fabricated and used to monitor the cure of a common epoxy. The IGEFET sensor consists of an interdigitated gate electrode structure which is coupled to the gate contact of a conventional metal-oxide-semiconductor field-effect transistor (MOSFET). The epoxy was deposited on the interdigitated gate electrode, and the IGEFET's electrical performance was observed as the epoxy cured. The cross-linking chemical reaction during epoxy cure caused electrical impedance changes that were quantified when the IGEFET was operated with a periodic voltage pulse signal. Charge transferred through the chemically-active epoxy is manifested as a temporally-dependent potential applied to the MOSFET's gate contact. By operating the MOSFET as a linear amplifier, a potential corresponding to the temporally-dependent gate voltage was directly measured at the amplifier's output. The Fourier transform of the IGEFET's time-domain response at specific time increments was computed. The resulting epoxy cure spectra were compared to a reproducible baseline spectrum, and an ensemble of difference spectra were computed to reveal the epoxy's chemical state at specific instances of time. The difference spectra features yield valuable information concerning the state of the epoxy's cure.

The service of many military aircraft is being extended far beyond original design lives. When the C-130 is eventually retired, for example, it will have been in service for 79 years, well beyond its planned life expectancy of 40 years. Similarly, the line of KC-135's is presently expected to remain operational for 86 years, and the venerable B-52 an astounding 94 years! Not only are inventories of parts in short supply, but it may be necessary to acquire parts no one ever expected to replace. The first step in any reprocurement activity is the creation of a data package that can be submitted to suppliers for bid. If no computer-aided design (CAD) model exists, which is likely with older parts, the demands of modern electronic commerce dictate that one be created. If the original alloy is no longer obtainable, which is also likely with older parts, the demands of functional equivalence dictate that a suitable substitute be found and certified. Computed tomography (CT) offers an ideal way to obtain dimensional and material data critical to reverse engineering efforts. Industrial CT systems have progressed to the point where they can nondestructively measure part dimensions to an accuracy competitive with coordinate measuring machines and a speed competitive with laser scanners. Of the existing methods for obtaining part coordinates, only CT can dimension interior surfaces. Moreover, only CT has the ability to densitometrically quantify internal defects — a key consideration for computer-aided engineering activities. This paper describes the use of CT to help create data packages for resupply efforts and examples are presented.

It is possible to monitor the initiation and progress of various mechanical or environmentally induced perturbations in concrete elements by way of fully integrated optical fiber sensors. Geometric adaptability and ease by which optical fibers can be embedded within concrete elements has led to the development of a number of innovative applications for concrete elements. This article is intended for a brief introduction into the theories, principles, and applications of fiber optic sensors as they pertain to applications in concrete.. However, due to the fact that the transduction mechanism in optical fibers is invariant of the materials employed, the principles introduced here also correspond to other structural materials. The only application related differences among various materials pertain to sensitivity and choice of optical fiber sensor types.

Structurally integrated optical fiber sensors form the basis for smart structure technology. Over the past decade a variety of sensor configurations have been developed for measurement of strains and deformations in structures. Strains and deformations alter the refractive index and the geometry of the optical fiber material. These changes perturb the intensity, phase, and polarization of the light-wave propagating along the probing fiber. The optical perturbations are detected for the determination of strain. The research presented here describes the development of a new optical fiber sensor system for measurement of structural strains based on white light interferometry. An optical switch provides for multiplexing of strain signals from various locations in the structure. Redundant Bragg grating type fiber optic sensors as well as strain gauges were employed for comparison and verification of strain signals as measured by the new system. The system provides capability for distributed sensing of strains in large structures.

This paper presents the results of creep tests on three notched dam models made with three type of concrete. In order to characterise the time-dependent behaviour of the process zone, a series of tensile tests was performed for each kind of concrete. On the basis of the law obtained in this way, crack propagation was simulated numerically through the Cohesive Crack model.

Studying the behavior of steel and concrete as a composite is of fundamental importance to the understanding of the cracking of reinforced concrete structures. In this article, a technique leading toward the development of a constitutive model for the interaction of steel and concrete is described. Experiments are based on pull-out specimens, where the shear stress and the two displacement components at the interface are measured. Phase Measurement Interferometry is used for accurate surface displacement measurement, and crack growth detection. The normal stress is then deduced using the measured crack length and crack opening displacements, along with a fracture mechanics based numerical simulation.

The properties of concrete can be altered in aggressive media such as pure water. A leaching test was performed for Reactive Powder Concretes (RPC) in order to evaluate their durability when used as storage of nuclear wastes. Emphasis is placed on the changes of their microstructure, on the tritium diffusion coefficient and on the porosity after leaching. Using mainly non destructive techniques such as SEM, BET and diffusion cell, we found that the leaching affects the RPC microstructure, particularly the anhydrous cement grains. The tritium diffusion coefficient, even lower than that of granite, is however ten times higher after a three month-leaching.

Rigid airport pavement structures suffer damage from multi-axial high magnitude cyclic stresses resulting from passing heavy aircraft. It is of interest to model the response of plain portland cement concrete to such loading conditions; thus, the sensitive detection and characterization of such damage during the loading process is important. Low strain vibrational resonance frequency measurement offers direct information concerning the global, apparent elastic moduli of the material and preliminary results have shown such measurements are sensitive to the presence of damage in concrete.

The work reported here includes the theoretical foundation and experimental results of a non-destructive technique, based on vibrational resonance measurement. The tests are applied to monitor damage imparted to end-mounted hollow concrete cylinders subjected to monotonic and cyclic torsional (bi-axial) loads. An introduction to the concepts of vibration testing and details of mechanical and vibrational test procedures employed are given first. The most significant vibrational modes, of all of the possible modes setup within the specimen, are identified. The frequency value of these significant modes in the concrete specimen are experimentally obtained throughout a controlled cyclic testing procedure to failure. The behavior of these modes is then monitored duringa controlled cyclic testing procedure to failure. Distinctions between the frequency values of the various excited resonance modes are noted. Moreover, effects of the two damage types (monotonic and cyclic) on the frequency values of the modes are studied. Finally, conclusions concerning the applicability of the vibrational resonance techniques for monitoring imparted damage in these concrete specimens are drawn.