Andrew P. Washabaugh

Conformable Eddy-Current Sensors and Arrays for Fleetwide Gas Turbine Component Quality Assessment

The conformable Meandering Winding Magnetometer (MWM®) eddy current sensors and MWM-arrays provide new inspection capabilities for gas turbine components. The sensors provide measurements of coating thickness and absolute electrical conductivity, which can capture features of interest for a population of components, e.g., for tracking fleetwide trends in quality and aging, failure evaluations, and correlating failure origins to features of specific fleet population segments. Inspection applications include metallic and nonmetallic coating thickness and porosity measurement, detection of cracks on complex surfaces, imaging and detection of small flaws, thermal degradation monitoring, and cold work quality assessment. For example, the U.S. Air Force uses the MWM for cold work quality control on all of the C-130 propeller blades that go through the Warner Robins ALC. For P-3 and C-130 propeller blades, trend analysis is being performed fleetwide. This paper describes MWM technology advances for absolute property measurements and specific capability demonstrations. Multifrequency quantitative inversion methods used for coating characterization are also used for characterization of process-affected zones, such as shot peen quality or titanium alpha case characterization.

Surface-mounted periodic field eddy current sensors for structural health monitoring

Surface mountable eddy current sensors are a revolutionary new concept in nondestructive inspection. These eddy current sensors can be mounted, like a strain gage, at critical locations for detection of crack initiation and monitoring of crack growth. This can be accomplished on a fatigue test article, as well as on in-service aircraft or other structures (patents pending). The mountable periodic field eddy current sensors, described in this paper, can be used as a replacement for standard eddy-current sensors without introducing new requirements. This is not the case with other proposed health monitoring sensors. For critical structures, substantially reduced inspection costs and life extension is possible with permanently mounted eddy current sensors. This is particularly true for difficult-to-access locations that require surface preparation (e.g., sealant or insulation removal) and disassembly when conventional eddy current testing is performed. By enabling eddy current testing in areas currently not accessible to conventional inspection, such as locations deep in an aircraft structure, damage tolerance can be achieved with low cost inspections. Embedded versions might even be mounted between layers, such as in a lapjoint. Surface mountable eddy current sensors are suitable for on-line monitoring and in-service inspections. This paper provides an introduction to surface mountable eddy current sensors, presents specific results from fatigue coupon tests and describes upcoming full-scale aircraft fatigue tests. Also, ongoing efforts to implement this technology on commercial and military aircraft are described. This research has been funded in part by the U.S. Navy, U.S. Air Force, JENTEK Sensors, Inc., and Lockheed Martin Aeronautics Company. The goal of this paper is to provide a basic understanding of surface mounted eddy current sensor capabilities and potential, and to promote their broader use in fatigue testing, aircraft health monitoring as well as for health monitoring of non-aerospace structures.

Dielectric measurements of semi-insulating liquids and solids

Sinusoidal steady state and transient decay measurement techniques were used to monitor the dielectric properties of electrical insulation. Impedance measurements of an air-gap capacitor, immersed in Shell Diala A transformer oil and driven with a 1 V peak sinusoidal signal (5 mHz-10 kHz), showed that the oil conductivity increased with temperature (15-70/spl deg/C) and aging in air but was independent of the oil moisture content (2-30 ppm). Interfacial double layer effects were also observed. Surface charge effects were studied in a cylindrical electrode apparatus with EHV-Weidmann Hi-Val pressboard on the inner cylinder and transformer oil filling the gap. The pressboard dielectric properties were obtained from time transient decay measurements of the open-circuit voltage after disconnecting a DC voltage source from across the cylinders. These measurements indicate that the conduction mechanism through the pressboard follows a drift dominated unipolar conduction law rather than ohmic conduction. Further frequency sweep dielectrometry measurements using a three-wavelength interdigital electrode sensor confirm the dispersive properties of oil-impregnated pressboard. This sensor was also used to measure the time and space distributions of moisture diffusion into oil-impregnated pressboard.

Shaped-field eddy-current sensors and arrays

Shaped-field eddy current Meandering Winding Magnetometer (MWM) sensors and MWM-Arrays, designed to fit physical models provide new inspection capabilities for materials characterization, quality control, and damage detection in aerospace structures. Accurate modeling is enabled by designing primary winding distributions that create either a spatially periodic magnetic field or a single period shaped- field. Accurate modeling of the sensor response permits absolute property measurements with minimal calibration, e.g., calibration in air without a reference standard. This paper will provide an overview of shaped-field eddy- current sensors and their use in several aerospace applications. In one group of applications, the sensors are permanently mounted on test components and can be mounted on actual structures for on-line fatigue damage monitoring. This supports the damage tolerance and retirement for cause methods for life extension and safe operation of numerous commercial and military aircraft, particularly in locations where the high cost of inspection is associated with disassembly and surface preparation. This capability can also be used to make damage standards having known flaws, including representative crack clusters. In a second group of applications, scanning of inductive element arrays permit high-resolution wide-area imaging of the properties revealing quality, damage state, or spatial variations of properties of conductive and magnetic materials. Model-based inversion methods convert each sensing element response into property measurements and permit independent property and lift-off measurements with each element. Furthermore, new MWM sensors incorporating giant magnetoresistive sensors allow low frequency measurements, even down to dc. This permits inspection for hidden cracks or hidden corrosion in thick multilayer structures.

Applications of spatially periodic field eddy current sensors for surface layer characterization in metallic alloys

Spatially periodic field eddy current sensors such as the Meandering Winding Magnetometer (MWM) with Grid Methods provide a powerful capability to nondestructively characterize surface layers introduced during fabrication as well as those modified by service exposure. This is critical for process quality control and component condition assessment. Conformable MWM sensors provide absolute property measurements (conductivity, permeability) and dimensional data (coating thickness, proximity) over flat and curved surfaces with minimal calibration requirements. Using a three-layer (substrate, coating, air) model and an inversion algorithm, a new measurement module has been developed to estimate the thickness of a process-affected zone and to provide a measure of property variations in this zone in real time. This paper presents results for two specific process quality control applications: (1) alpha case thickness measurements in a titanium alloy and (2) characterization of shot peening for aluminum alloys.

Imposed ω-k Magnetometer and Dielectrometer Applications

Dispersive properties (i.e., frequency dependent properties) of multiple layered media cannot be measured with conventional multiple-frequency techniques, because such techniques cannot uniquely distinguish between the impedance transitions caused by property variations with frequency and those caused by spatial property variations. In addition, at a single frequency, conventional eddy current sensor techniques can measure only the conductivity-thickness product for a thin metal layer or coating.

Health monitoring using MWM-array and IDED-array sensor networks

This paper describes the use of MWM eddy current array sensor networks and IDED dielectrometer array sensor networks as well as hybrid MWM-IDED sensor networks for monitoring of absolute electrical properties for the purposes of detecting and monitoring damage, usage and precursor states within an Adaptive Damage Tolerance (ADT) framework. We present specific results from MWM-Array fatigue monitoring demonstrations, temperature measurement and dynamic stress monitoring, along with IDED methods for age degradation monitoring. We also describe the use of such sensor networks as part of an ADT framework, as well as for generation of real damage standards (e.g., real cracks without starter notches), and for prognostics model validation.

Remote temperature and stress monitoring using low-frequency inductive sensing

The use of giant magnetoresistive (GMR) sensing elements in inductive sensors permits low frequency operation for materials characterization and defect detection in aerospace and engineering materials. This offers a substantially increased depth of sensitivity over conventional eddy-current sensing coils and also allows new measurement capabilities, such as the non-contact remote monitoring of temperature and stress variations through material layers. This paper provides an overview of the Meandering Winding Magnetometer (MWM) drive winding constructs that incorporate GMR based sensing elements. The sensors are designed so that the magnetic field distribution created by the primary winding and the resulting response of sensing elements can be accurately modeled. Representative applications to be described include (1) detection and imaging of 3% material loss in a 6.4-mm (0.25-in.) thick aluminum plate, (2) monitoring of temperature variations of an aluminum plate located behind another 6.4-mm thick aluminum plate with an air gap between the plates, and (3) independent measurements of stress (through magnetic permeability measurements) in a steel plate located behind an aluminum plate with an air gap between the plates.

New quasi-static magnetic and electric field imaging arrays and algorithms for object detection, identification, and discrimination

Unlike radar-based imaging technologies that use electromagnetic waves, quasistatic imaging technologies operate at lower frequencies where electric and magnetic fields are decoupled. Magnetoquasistatic (MQS) devices, such as metal detectors, that impose magnetic fields satisfy the diffusion equation in conducting media and Laplaces equation in air or poorly conducting soils. Electroquasistatic (EQS) devices satisfy Laplaces equation. In Laplacian or diffusion decay, the amplitude of the magnetic and electric fields decay exponentially with distance from the drive windings or electrode. For quasistatic sensors, objects are detected and imaged through perturbations to the applied magnetic or electric fields that change the mutual transimpedances or transadmittances at the sensor terminals, rather than through time delays of reflected electromagnetic waves as in GPR.

High-resolution inductive sensor arrays for UXO detection, identification, and clutter suppression

The efficiency of unexploded ordnance (UXO) remediation is currently limited by the inadequate discrimination capability of present detection technologies, such as single sensing coil inductive sensors. While these methods often detect all relevant metal objects, they generally cannot discriminate between harmful objects and harmless clutter. False indications continue to far outnumber verified detections. To help address the need for a fieldable detection and clutter suppression capability, high resolution inductive arrays are being developed for UXO imaging. This development effort leverages existing MWM-Array sensor and instrumentation technology used in nondestructive testing to create quantitative images of geometric and material property variations. This program is being funded by SERDP and JENTEK Sensors. This paper reviews the MWM-Array technology and its extension to UXO detection and discrimination. The technology uses unique designs for electromagnetic induction sensor arrays incorporating a single drive with multiple sense elements. The drive creates a shaped magnetic field pattern that concentrates the field energy into longer wavelength spatial modes for deeper and “focused” penetration into the ground. Arrays of small inductive coils, placed throughout the shaped field, sense the response from conducting or magnetic UXO and clutter. Images obtained from scans over buried objects provide a basis for spatial filtering and signal processing. Multiple sensor arrays placed at different positions within the drive provide different “views” of buried objects and clutter. Model-based grid measurement methods are also reviewed as a real-time method for multiple property measurements, and real-time data analysis/image generation.