Darrell E. Schlicker

MWM eddy current sensors for monitoring of crack initiation and growth during fatigue tests and in service

Abstract A new surface-mountable (conformable foil) eddy-current sensor called the Meandering Winding Magnetometer-Array (MWM™-Array) has the capability to monitor crack initiation and growth in fatigue test coupons. Fatigue tests with the MWM-Array mounted on the surface inside a 6.4-mm hole in an Al 2024-T3 tension–tension fatigue specimen, demonstrated the capability to detect cracks with l

Residual and Applied Stress Estimation From Directional Magnetic Permeability Measurements With MWM Sensors

Conformable eddy current sensors with grid methods can provide an effective means for applied stress measurements and for characterization of residual stresses in steel components. This paper describes conformable MWM® (meandering winding magnetometer) sensors and high-resolution scanning MWM-Arrays for directional magnetic permeability measurements over a wide range of frequencies with grid methods. MWMs and MWM-Arrays are used for fatigue monitoring, crack detection, characterization of process-affected layers, including coatings, detection of surface-connected and hidden degradation, and damage characterization, e.g., detection and imaging of grinding burns. Tests have shown that these sensors can also provide the capability for residual/applied stress measurements in ferromagnetic materials, e.g., in steels. Specific examples described here illustrate this capability to estimate stresses in steels and other ferromagnetic materials from MWM measured permeability. Results of directional MWM permeability measurements, including permeability mapping, are presented here for (a) a 4340 steel specimen subjected to a series of tensile load tests with an incrementally increasing maximum load, (b) a broken 4340 tensile test specimen, (c) a shot-peened 300M steel specimen subjected to bending load tests, and (d) a shot-peened and subsequently ground high-strength steel component. Quick and inexpensive stress measurements at a point or, in a scanning mode, over a selected area are now becoming possible without permanently mounting sensors, albeit MWM sensors can be readily mounted, if needed, as well.

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.

Moisture dynamic measurements of transformer board using a three-wavelength dielectrometry sensor

The spatial distribution of dielectric permittivity and conductivity of an insulating medium affects the quasistatic distribution of the electric field. Multiple wavelength interdigital sensors using the imposed frequency-wavenumber (/spl omega/-k) dielectrometry technique can measure complex permittivity distributions in dielectrics. Measurements of oil-free and oil-impregnated pressboard transformer insulation using a three-wavelength sensor with changes in ambient moisture concentration are presented. With a calibrated mapping relating conductivity and permittivity to moisture concentration in oil-impregnated pressboard, the spatial distribution of moisture can be measured.

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.

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.

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.

TBC Characterization Using Magnetic and Electric Field Sensors

Recent advances in magnetic and electric field sensors have enabled accurate measurements of bond coat and top coat material properties and thicknesses. This paper reviews current methods of using electric and magnetic field sensors for coating assessment in separate measurements and in a hybrid magnetic-electric sensor construct. Magnetic field (inductive) sensors can be used to characterize gas turbine components by providing critical information about the thickness and condition of the metallic bond coat and thickness of the top coat. Electric field (capacitive) sensors can be used to characterize ceramic top coats by providing information about the thickness, condition, and surface roughness of the top coat. Using multivariate inverse methods and precomputed model-derived databases, multiple unknown coating properties are estimated simultaneously and independently. The capability to independently estimate four unknowns with one sensor and as many as six unknowns with both sensors together enables practical application of this technology for manufacturing quality and in-service condition assessment. Together, these two sensors can provide an effective method of nondestructively characterizing both metallic and ceramic coatings on turbine components.

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.