We have developed a low frequency electromagnetic technique using sensitive room temperature magnetoresistive (MR) sensors for a variety of nondestructive evaluation (NDE) applications. These applications include the NDE of medical implants and aircraft structures, the detection of cracks and corrosion in metals, the detection of ferromagnetic foreign objects in the eye and the brain, and the noninvasive determination of iron content in the liver. Our technique consists of applying a low frequency ac magnetic field to the sample and detecting the sample response. The low excitation frequency enables us to probe deep into metal structures; the sensitivity of the MR sensor allows us to detect weak responses from the sample without applying too large an excitation field, particularly in the case of human tissue. The MR sensors are small and relatively inexpensive compared to other sensitive magnetic field sensors such as fluxgates and superconducting quantum interference devices or SQUIDs; hence the resulting NDE instrument will be compact and cost-efficient, enabling its commercialization for practical applications. In this paper, we focus primarily on NDE of orthopedic implants.

This paper explores the feasibility of detecting damage within structures such as air foils by application of eddy current based techniques and reduced order modeling. To identify the geometry of a damage, an optimization algorithm is employed which requires solving the forward problem numerous times. Therefore, the forward algorithm must be solved with extremely fast and accurate solution methods. In constructing these forward methods, we employ reduced order Proper Orthogonal Decomposition (POD) techniques. The POD technique is a method which creates an 'optimal' ordered basis in the sense that information captured in the first few basis elements is maximized. One then uses a fixed number (based on a quantitative formula for percentage energy captured) of the first few basis elements, called the reduced POD basis, in the forward algorithm. Since one uses only a small number of basis elements, one is able to create a fast forward algorithm that accurately represents the relevant information. In this paper, for illustrative purposes and proof-of-concept, we consider rectangular 'cracks' parameterized by a vector parameter q representing the length, thickness, depth, center, etc. of the damage. We attempt to recapture the parameters of a damage assuming we have access to the magnetic flux density B. Our analysis uses simulated data perturbed with normally distributed noise to represent corrupted experimental data. When recapturing the length and thickness of a damage using the component of the magnetic flux density orthogonal to the eddy current flow in the sample, the methods are shown to be efficient and robust even with data containing 10% relative noise.

The remote-field eddy-current (RFEC) technique has been shown to be highly sensitive to cracks and corrosion that are embedded deep in multi-layer aircraft structures. This paper shows the effectiveness of the approach in detecting cracks in double-layered specimens with fasteners, as well as corrosion specimens. The crack specimens, made by Lockheed George Company in 1980, had two layers held together with ten fasteners. The total thickness is 0.356' for Group A and 0.446' of Group B, respectively. Fatigue cracks were made on different layers and at different depths. The corrosion specimens are of 0.063' thick with 0.006' or 0.002' corrosion wall thinning. Another one or two much thicker plates of aluminum are placed on top of one corrosion specimen during a test. All tests were conducted using a newly developed RFEC system that includes a probe specified for inspecting thick plates. The effect of different parameters, such as excitation frequency, excitation to pick-up coil separation distance, and probe to fastener distance, were studied to determine the optimal test parameters. The system will be demonstrated along with the presentation.

A new eddy-current system based on low-noise magnetoresistive sensors has been developed to detect cracks and corrosion in thick, multi-layer metal structures. The new instrument has detected narrow slots as short as 6.3 mm, in the lowest layer of a stack of three aluminum plates totaling 25 mm in thickness. These flaws were detected through 19 mm of overlying aluminum, in spite of the presence of steel fasteners and the proximity of a plate edge that attenuated the eddy currents impinging on the flaw. This performance was achieved by combining the low-frequency sensitivity of magnetoresistive sensors with a special probe design that minimized background errors due to liftoff, plate-edge effects and the steel fasteners. This new system is potentially useful for inspecting thick, layered structures in aircraft, as well as ferrous metal structures such as pipelines and storage tanks, where the shortness of the skin depth has previously limited the usefulness of eddy-current inspection.

This paper describes experiments conducted with the lock-in thermographic procedure. A carbon fiber reinforced composite specimen with defects of various sizes and depths below the test surface was analyzed. The detectivity of AGEMA 900 lock- in system was investigated. The experimental results show that the detectivity of lock-in thermography depends on inspection frequency, intensity of heat source, resolution of lock-in system, distance between the IR camera and the object. It was found that inspection frequency has significant effect on the phase difference produced by a certain defect. At blind frequency, the defect produces no difference or very small difference. There are 2 optimum frequencies at which the defect produces maximum positive and negative phase differences respectively. At a depth that both high frequency and low frequency thermal waves can reach, lock-in thermography is more sensitive at high frequency. Lock-in phase sensitive thermography was found to be more sensitive than conventional reflection and transmission thermography. Sight line angle, which is the angle between the surface plane of the object and the sight line of the camera, has no significant effect on detectivity of lock-in thermography.

As structural integrity models for aging aircraft begin to include the effects of corrosion and corrosion-fatigue, nondestructive evaluation (NDE) techniques will be called upon to provide metrics of corrosion for input to these models. It is unlikely that any one NDE technique can provide all the required metrics to characterize the condition of complex airframe structures. This paper discusses how data fusion can be used to integrate the results of multiple NDE techniques into a form suitable for input to structural models. Examples of the inspection of service-retired lap joints with NDE techniques including pulsed eddy current, conventional eddy current, Edge of Light, and D Sight are given. Significant metrics for structural models of the joint are discussed, and the performance of the individual NDE techniques on the metrics of corrosion and fatigue is evaluated. The results are used to generate a set of requirements for data fusion system to successfully transform the NDE data to a form a suitable for input to the structural models.

Currently, the NDE procedure for Alouette helicopter rotor blades consists of a visual inspection followed by a manual acoustic inspection procedure by trained personnel using purpose manufactured tapping hammers. The former inspection is aimed at detecting surface cracks and corrosion whereas the latter is intended to inspect the rotor blade for possible areas of delamination between the alloy skin and the leading edge spar or blade root reinforcing strip. This paper investigates the feasibility of using either the authors locally developed Portable Digital Shearography or Electronic Speckle Pattern Interferometry in conjunction with Mechanical Impedance Analysis in order to determine the possible presence and extent of defects more accurately and reliably. Both optical inspection techniques are discussed, their theory and apparatus presented and the inspection procedure described. The principle of Mechanical Impedance Analysis is also outlined and the inspection method described. The successful results of the study as well as defects detected are presented and discussed. Outlining the potential of using this alternative NDE method as an on-site, in-situ inspection procedure concludes the paper.

The issue of detecting corrosion between riveted plates of aluminum continues to be of interest, especially for the application of detecting hidden corrosion in aging aircraft. In this presentation, a preliminary investigation of the use of localized ultrasonic resonance and acousto-ultrasonic methods has been completed to determine the suitability of these techniques for the detection of hidden corrosion. The initial results have demonstrated the ability to identify components of the received resonance and acousto-ultrasonic signals that can be used to detect the presence of corrosion. A computer algorithm has been implemented to simplify the classification process, enabling the user to detect and categorize the hidden corrosion in the riveted structure. In addition, systematic changes in the received signals from the hidden corrosion can be used to monitor the evolution of the corrosion and, thus, determine its severity in the riveted structure.

Optical device based on the thermoplastic recording of information of is intended for the continuous registration of data from radar indicator. This device can be treated as an optical processor due to capacity to produce the summarizing on the photothermoplastic storage medium optical image caused the visual subtraction of the real trajectory of a moving object. The screen with graticule can include trajectory image of normal landing. It allows operator to determine turning out situation, to make corrections in the pilot actions, to make a documentation of the flight data without any equipment. Designed devices have been checked in two versions of using by the oscillograph with discrete manual moving of the light spot and at recording from screen of the CRT indicator of the radar systems (Stavropol airport) worked at the landing at the dispatcher regimes.

Many of today's methods of inspecting structures are very time consuming, labor intensive and in many cases (due to limited access), impractical. In addition, long shutdown times are required to perform the inspections, thus creating tremendous expenses associated with manpower, materials and lost production. With continuing advances in signal processing and communications a significant interest has been shown in developing new diagnostic technologies for monitoring the integrity of structures with known defects, or for detecting new defects, in real time with minimum human involvement. The continued use of aging structures, especially in regard to the airworthiness of aging aircraft, is a major area of concern. Recent developments in both active and passive Acoustic Emission monitoring as an advanced tool for 'Structural Health Management Systems (SHMS),' are illustrated by using two recently developed acoustic emission systems; the Acoustic Emission-Health and Usage Monitoring System (AE-HUMS) helicopter drivetrain health monitoring system, and the Acoustic Emission Flight Instrument System (AEFIS) composite health monitoring system. The data collected with these types of systems is processed with advanced data screening and classification techniques, which are employed to take full advantage of parametric and waveform-based acoustic emission.

Repairs of secondary structure can be accomplished by restoring structural integrity at the damaged area and increasing the structure's damping in the repair region. Increased damping leads to a reduction in resonant response and a repair that will survive for the life of the aircraft. In order to design a repair with effective damping properties, the in-service structural strains and temperatures must be known. A rugged, small and lightweight data acquisition unit called the Damage Dosimeter has been developed to accomplish this task with minimal impact to the aircraft system. Running autonomously off of battery power, the Damage Dosimeter measures three channels of strain at sample rates as high as 15 kilo-samples per second and a single channel of temperature. It merges the functionality of both analog signal conditioning and a digital single board computer on one 3.5 by 5 inch card. The Damage Dosimeter allows an engineer to easily instrument an in-service aircraft to assess the structural response characteristics necessary to properly select damping materials. This information in conjunction with analysis and design procedures can be used to design a repair with optimum effectiveness. This paper will present the motivation behind the development of the Damage Dosimeter along with an overview of its functional capabilities and design. In-service flight data and analysis results will be discussed for two applications. The paper will also describe how the Damage Dosimeter is used to enable the Durability Patch design process.

An early prototype of the Vertical Stabilizer Assembly (VSA) of the Shuttle Orbiter was modal tested at healthy and damaged states to study vibrational nondestructive damage evaluation in aerospace structures. Frequency Response and Coherence functions were collected with a Laser Vibrometer at 84 points when the healthy and damaged VSA was shaken with a continuous random force from 0 to 300 Hz. The measurements were used to extract the resonant frequencies and modal shapes for the healthy and damaged states. After pairing of the mode shapes between the healthy and damaged states through the Modal Assurance Criterion and an analysis of the differences in the frequency response functions (FRFs), the strain energy densities of the elements due to modal deformations were determined through a finite element model. The energy densities were normalized so that the total energy of the structure be the same for the mode pairs of the undamaged and damaged sates. The differences in the modal strain energy densities between the healthy and damaged structures provide features that allow for the localization of damage. This is achieved by implementing fusion techniques that combine competing and complementing information obtained from the energy density differences of several mode pairs. This paper evaluates the detectability and performance of two fusion methods to localize damage in the VSA. These are averaging and Bayesian fusion. The performance is based on detectability and the number of false calls.

Most fiber grating sensor technology that has been developed to support strain sensing involves the measurement of axial strain. Fiber grating sensors are however capable of monitoring transverse as well as axial strain. This paper reviews a series of applications of this technology that are of particular interest to aerospace applications.

An experimental method is presented for quantifying impact damage in composite materials using specimens of glass fiber reinforced polyurethane and epoxy. Thermoelastic stress analysis (TSA) was used to quantify the stress concentration associated with impact-damage. Following impact and TSA imaging, the samples were fatigued to failure over a range of stress amplitudes. The stress concentration factors acquired from TSA were used to determine a modified stress for each sample that collapsed the impact, then fatigue data onto a master curve on stress-life axes. This approach provides a quantitative measure of impact damage and a rational estimate of the expected residual fatigue lifetime of impact damaged composites.

The eddy current method is used to trace crack propagation under a composite patch repair of a cracked metallic structure, after mechanical testing in fatigue. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thickness are checked for several patch thicknesses. Notched specimens 6 mm thick were fabricated using 2024-T3 Aluminum. Boron Epoxy patches bonded with film adhesive were applied to the one side of the metallic specimens. Initial notches were 10 mm long, while the thickness of the reinforcement was varying from 2 layers (0.25 mm) to 7 layers (0.875 mm) in order to represent actual structural composite patch repairs. Crack propagation from the tip of the notches was achieved by fatigue loads. The estimation of required loads to cause fatigue crack propagation was done by means of three-dimensional finite elements analysis. The eddy current method was then applied to trace the crack tip under the patch after their mechanical testing. Accuracy of the eddy-current method was verified by measuring the crack lengths on both sides of the specimen and comparing the results. The eddy-current method was found to be fully capable of tracing the crack propagation under the composite patch, requiring only proper calibration for the generator. Small differences in the crack lengths between the patched and the unpatched side of the specimen were explained by their non-symmetric configuration, which induced different stress intensity factors at the patched and the unpatched sides, as finite elements analysis has clearly shown.

In recent years, significant emphasis in the aerospace industry has been placed upon the development of new composite manufacturing methods. One approach that has shown a great deal of promise in the aerospace industry is the 'Co-curing' of composites. A multi-scan ultrasonic testing (UT) approach for the evaluation of composite co-cured structure has been developed and is being implemented at Boeing. The general approach will be described, as will the specific techniques, each of which is essential to full inspection coverage for these unique structures.

Lamb waves have been used for internal defect detection in multilayered composite plates. Different Lamb modes generate various stress levels in different layers. As a result, all Lamb modes are not equally sensitive to internal defects located in different layers. A number of studies have been carried out to identify which Lamb mode is most effective for detecting defects in a specific layer. However, one shortcoming of the Lamb wave inspection technique is that in a symmetrically layered composite plate stress and displacement magnitudes and energy distribution profiles for all Lamb modes are symmetric about the central plane of the plate. As a result, the ability of a Lamb mode to detect defects in a specific layer of the plate is identical to its ability to detect defects in the corresponding layer of mirror symmetry. Hence, from the Lamb wave generated image one cannot distinguish between the defects in two layers of mirror symmetry. In this paper it is investigated how by fine-tuning the frequency and the striking angle of the incident beam in the neighborhood of a Lamb mode one can separately detect internal defects in layers of mirror symmetry in the upper and lower halves of a plate.

This paper focuses on the experimental study of the effects of exciting frequencies, grain (aggregate) sizes, and damage upon the ultrasonic P-wave velocity when performing the ultrasonic nondestructive testing (NDT) for concrete specimens. Two batches of concrete and mortar specimens were prepared in the laboratory for the investigation of the effects from the stated factors upon the P-wave velocity. Damage here mostly refers to microcracks and microvoids in concrete. Five different aggregate sizes, 0' (mortar), 3/8', 1/2', 3/4', and 1', were selected to demonstrate the grain (aggregate) size effect. Exciting frequencies of the ultrasonic wave were set to range from 100 kHz to 1,000 kHz, with increment of 50 kHz, to demonstrate the frequency effect. Styrofoam particles were mixed into the comparison concrete and mortar specimens to simulate the distributed microvoids (damage). Different volume fractions of styrofoam particles were mixed into the mortar specimens in order to study the effect of different porosities (damage) upon the P-wave velocity. The experimental observations show that, for mortar and concrete specimens with aggregate sizes from 0 to 1 inch, the P-wave velocity would not be affected significantly within the tested frequency range (100 - 1000 kHz). The normalized P-wave velocity exhibits almost identical pattern upon the exciting frequencies for all specimens.

The Road Deflection Tester (RDT) is a testing vehicle for highways capable of assessing a road deflection profile at traffic speeds by using two arrays of laser range finders. The deflection profile in turn is evaluated to determine layer properties in the road structure. The present study looks at some geometric relationships that can be used for this purpose. It was found that evaluation techniques used for stationary devices are less suitable. Other methods may however, be of great value.

The application of Nondestructive Evaluation, particularly ultrasonic evaluation, in the automotive components industry has been accelerated in the last several years. The quality and integrity of materials and structures employed for automotive components are routinely examined by radiographic, eddy current and ultrasonic methods. Some discontinuities can be detected with one or all of these methods. Certain defects, in the form of delamination, can only be detected by the ultrasonic method. Both manual testing and highly automated system have been used for ultrasonic testing of automotive components. The automated system is particularly attractive when high reliability is required for automotive components used in critical safety systems applications. This paper gives a practical overview of the application of ultrasonic evaluation in automotive component development and manufacturing. Areas covered include ultrasonic evaluation of materials, automated inspection of cold rolled components, and methodology for setting up acceptance and reject standards. This manuscript is focused on the ultrasonic application to safety related automotive components.

The high pressure turbine blades of jet engines show internal channels designed for air cooling. These recesses define the internal walls (partitions) and external walls of the blade. The external wall thickness is a critical parameter which has to be systematically checked in order to ensure the blade strength. The thickness evaluation is usually lead by ultrasonic technique or by X-ray tomography. Nevertheless, both techniques present some drawbacks related to measurement speed and automation capability. These drawbacks are bypassed by the eddy current (EC) technique, well known for its robustness and reliability. However, the wall thickness evaluation is made difficult because of the complexity of the blade geometry. In particular, some disturbances appear in the thickness evaluation because of the partitions, which exclude the use of classical EC probes such as cup-core probe. In this paper, we show the main advantages of probes creating an uniformly oriented magnetic field in order to reduce the partition disturbances. Furthermore, we propose a measurement process allowing to separate the wall thickness parameter from the EC signals. Finally, we present some experimental results validating the proposed technique.

A difficult problem in eddy current inspection of airplane parts is the detection of cracks near sharp edges. In particular, cracks initiate close to the edge in the case of aircraft engine turbine disks. Since the edge is a geometric discontinuity, it produces a strong eddy current response that can distort the crack signal, resulting in a degradation of the capability to reliably detect the crack. A new probe that uses giant magnetoresistive (GMR) sensors as detecting elements provides an elegant solution for this problem. The GMR sensor is placed in the center of a pancake-type excitation coil. Being a uni-directional device, the sensor detects only the field along its sensitive axis. In the measuring configuration, the sensing axis is oriented coplanar with the specimen surface, so that the sensor is insensitive to the excitation field. When the probe is scanned above a crack initiating perpendicular to an edge, orienting the sensing axis parallel to the edge, the output signal is produced only by the crack, while the usually high signal due to the edge is eliminated. A discussion of experimental observations obtained by scanning surface and subsurface edge cracks are included in the paper.

Interest continues to increase in applying MEMS technology for advancing both NDE and condition based maintenance (CBM) applications. The advantages of drastic size and weight reduction of MEMS enable consideration of developing low-cost, high-performance, ultra-portable, MEMS-based diagnostic systems for field inspections. For system health monitoring, it is feasible to envision permanently attached, distributed networks of micro-sensors on structural surfaces, e.g, acoustic emission sensors, accelerometers, strain gages, and pressure sensors. In addition, embedded MEMS based sensors could be used for continuous health monitoring of structures and systems. Under sponsorship of the Naval Air Warfare Center, NTIAC has performed an extensive review of the current status of MEMS technology and an assessment of how this technology can be used in applications of NDE and CBM. This presentation will provide a summary of the results of this technology assessment and describe recent MEMS developments in ultrasonics, eddy current, structural health monitoring and the medical field with potential for application to NDE and CBM.