Neil J. Goldfine

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

Optimal compliance design for grinding robot tool holders

A simple and effective solution to the robot grinding problem is introduced, which significantly reduces vibrations during grinding without additional actuators or active control. The objective is to determine the optimal compliance design for grinding robot tool holders. It is found that the degree of correlation between the dynamic behavior of the wheel in the directions normal and tangent to the desired workpiece surface has a direct effect on the grinding performance. This fact is utilized to determine the optimal tool holder compliance design through analysis, simulation and experimentation. The resulting design conclusions have been incorporated in an end-effector which was successfully tested for the grinding of weld seams.

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.

Dielectrometers and magnetometers suitable for in-situ inspection of ceramic and metallic coated components

Quasistatic spatial mode (QSM) sensing is a new form of nondestructive evaluation developed to address the increasing need for quantitative materials characterization. Two types of QSM sensors and measurement methods are described: QSM magnetometry and QSM dielectrometry. These measurement methods were invented at the MIT Laboratory for Electromagnetic and Electronic Systems and are being developed at JENTEK Sensors, Inc. for specific applications, such as coating porosity characterization. The sensors discussed here are the meandering winding magnetometer and the inter-digital electrode dielectrometer. These sensors are thin and flexible, permitting inspection of complex and difficult-to-access surfaces. Using continuum electromagnetic models of the sensor field interactions with multiple layered media, repeatable and quantitative measurement of physical and geometric properties are obtained. The specific application addressed here is the characterization of coating and component condition for turbine blades. This includes measurement of thickness and porosity for both ceramic and metallic coatings. Future research will focus on age degradation monitoring as well.

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.

Introduction to the Meandering Winding Magnetometer (MWM) and the grid measurement approach

A new sensor called the Meandering Winding Magnetometer (MWM) and associated grid measurement algorithms is described. The MWM can be used to determine property profiles for ferrous and nonferrous components, to provide repeatable and reproducible measurements on curved surfaces or inspection of difficult to access locations, and to detect and characterize cracks. This paper describes the application of the MWM to detection and characterization of early stage fatigue damage in aluminum and stainless steel. Other potential applications include coating characterization, case depth measurement, crack detection, and embedded sensing. The MWM is a thin and conformable sensor that incorporates both eddy current type sensing and magnetic induction sensing methods to measure conducting and magnetic properties of nonferrous and ferrous metals. The grid measurement approach is a model-based technique used to measure two properties independently, at a single frequency. This grid method also provides a convenient framework for MWM system calibration and processing of multiple frequency data. For example, this permits measurement over a wide frequency range using a single MWM sensor geometry. This paper provides a general introduction to the MWM technology and specific capability demonstrations on ferrous and nonferrous alloys.

Validation of Multi-Frequency Eddy Current MWM Sensors and MWM-Arrays for Coating Production Quality and Refurbishment Assessment

Coatings for oxidation, corrosion, and thermal protection provide the required materials performance for gas turbine blades and vanes in state-of-the-art industrial gas turbines. These turbines must withstand severe operating conditions for well over ten thousand hours. Variations in the coating thickness, and increased porosity, can influence the lifetime of such coatings significantly. For components that have been removed from service, effective assessment of the aged coating and substrate condition is critical for refurbish/replace/continue-to-run decisions. A suitable device for coating thickness measurement and detection of unacceptable porosity is needed for ensuring the quality of such coatings. In this paper, we present new results on coating thickness measurements for metallic MCrAlY overlay coatings on gas turbine parts. These measurements were performed with a Meandering Winding Magnetometer (MWM® ) eddy-current sensor using grid methods. This technique allows proper coating measurements even after a diffusion heat treatment for a better coating adhesive strength. The MWM technology enables measurement of the coating thickness, the absolute electrical conductivity (which may in turn be related to porosity or other properties of interest), and lift-off, which is related to surface roughness. Single-channel MWM sensors and multi-channel imaging MWM-Arrays permit capture of features of interest for a population of components. New capabilities for inspecting gas turbine components are, thus, provided. Inspection applications include metallic and non-metallic coating thickness measurements, porosity measurements, and detection of cracks on complex surfaces. Results of coating assessment for a production line of gas turbine vanes by means of a multifrequency MWM technique are presented for various combinations of coatings and base metals. A description of improved multiple frequency quantitative inversion methods is provided for simultaneous and independent measurement of multiple unknowns such as metallic bond coat thickness, metallic bond coat porosity, and top coat thickness. Ongoing research focuses on characterization of aged components using MWM sensors and imaging MWM-Arrays as well as on development of enhanced algorithms for four and five unknown coating / substrate properties. In a recent study of hot corrosion, uncoated nickel alloy specimens were characterized using an MWM sensor with grid methods. Preliminary results indicated that, within the limitations of the three-unknown single-layer model used, the method could readily identify specimens with no apparent corrosion damage, specimens with moderate corrosion damage, and specimens with severe corrosion damage.© 2003 ASME

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.