Abstract: Reference standards or articles having prescribed levels of damage are fabricated by monitoring an electrical property of the article material, mechanically loading the article, and removing the load when a change in electrical properties indicates a prescribed level of damage. The electrical property is measured with an electromagnetic sensor, such as a flexible eddy current sensor, attached to a material surface, which may be between layers of the article material. The damage may be in the form of a fatigue crack or a change in the mechanical stress underneath the sensor. The shape of the article material may be adjusted to concentrate the stress so that the damage initiates under the sensor. Examples adjustments to the article shape include the use of dogbone geometries with thin center sections, reinforcement ribs on the edges of the article, and radius cut-outs in the vicinity of the thin section.

Abstract: One of the issues with planar eddy-current sensors is the placement of the current return for the primary winding. Often the ends of the primary winding are spatially distant from one another, which creates an extraneous and large inductive loop that can influence the measurements. A sensor geometry featuring a primary winding that reduces the effect of this inductive loop is presented. The primary winding may include a plurality of parallel extended winding segments. The segments further include adjacent individual drive coils. Current flows through individual drive coils in an alternating fashion. Current flows through adjacent drive coil portions in a common direction, thereby imposing a spatially periodic magnetic field with at least two periods.

Abstract: Magnetic field sensor probes are disclosed which comprise primary or drive windings having a plurality of current carrying segments. The relative magnitude and direction of current in each segment are adjusted so that the resulting interrogating magnetic field follows a desired spatial distribution. By changing the current in each segment, more than one spatial distribution for the magnetic field can be imposed within the same sensor footprint. Example envelopes for the current distributions approximate a sinusoid in Cartesian coordinates or a first-order Bessel function in polar coordinates. One or more sensing elements are used to determine the response of a test material to the magnetic field. These sense elements can be configured into linear or circumferential arrays.

Abstract: Magnetic or electric field sensors are mounted against a material surface and used for stress, strain, and load monitoring of rotating components such as vehicle drive trains. The stationary sensors are mounted at multiple locations around the component and used assess the stress on the component at multiple rotational positions. The sensor response is typically converted into a material property, such as magnetic permeability or electrical conductivity, which accounts for any coating thickness that may be present between the sensor and mounting surface. The sensors are not in direct contact with the rotating component and are typically mounted on an annular material or ring that encircles the rotating component. Measurements of the annular material properties, such as the stress, are related to the stress on the rotating component and discrete features on the component.

Abstract: Damage and usage conditions in the vicinity of fasteners in joined structures are nondestructively evaluated using the fasteners themselves. Sensors or sensor conductors are embedded in the fasteners or integrated within the fastener construct, either in the clearance gap between the fastener and the structure material or as an insert inside the shaft or pin of the fastener. The response of the material to an interrogating magnetic or electric field is then measured with drive and sense electrodes both incorporated into the fastener or with either drive or sense electrodes external to the fastener on the material surface. In another configuration, an electric current is applied to one or more fasteners and the electric potential is measured at locations typically between the driven electrodes applying the current. The potential is measured circumferentially around the fastener at locations on the material surface or across pairs of fasteners throughout or along the joint.

Abstract: Methods are described for the use of conformable eddy-current sensors and sensor arrays for characterizing residual stresses and applied loads in materials. In addition, for magnetizable materials such as steels, these methods can be used to determine carbide content and to inspect for grinding burn damage. The sensor arrays can be mounted inside or scanned across the inner surface of test articles and hollow fasteners to monitor stress distributions. A technique for placing eddy-current coils around magnetizable fasteners for load distribution monitoring is also disclosed.

Abstract: Combined wound and micro-fabricated winding constructs are described for the inspection of materials and the detection and characterization of hidden features or flaws. These constructs can be configured as sensors or sensor arrays that are surface mounted or scanned over conducting and/or magnetizable test materials. The well-defined geometry obtained micro-fabricated windings and from carefully wound coils with known winding positions permits the use of model based inversions of sensed responses into material properties. In a preferred embodiment, the primary winding is a wound coil and the sense elements are etched or printed. The drive or sense windings can also be mounted under fasteners to improve sensitivity to hidden flaws. Ferrites and other means may be used to guide the magnetic flux and enhance the magnetic field in the test material.

Abstract: An apparatus for the nondestructive measurement of materials that includes at least two layers of electrical conductors. Within each layer, a meandering primary winding is used to create a magnetic field for interrogating a test material while sense elements or conducting loops within each meander provide a directional measurement of the test material condition. In successive layers extended portions of the meanders are rotated so that the sense elements provide material condition in different orientations without requiring movement of the test circuit or apparatus. In a bidirectional implementation the angle is 90° while in a quadridirectional implementation the relative angles are ?45, 0, 45, and 90°. Multidirectional permeability measurements are used to assess the stress or torque on a component. These measurements are combined in a manner that removes temperature effects and hysteresis on the property measurements.

Abstract: Precomputed databases of sensor responses are used to convert measurement data into properties of interest, for example, conductivity or lift-off. These databases of responses may be generated prior to data acquisition and provide sensor response values that span appropriate ranges for the property values. After data acquisition, estimates for the property values are obtained from measurement values by minimizing the difference between the database of responses and the measurement values. The minimization can take the form of a least-squares minimization. Alternatively, iterative techniques can be used to minimize the difference between the database and measurement values.

Abstract: Methods are described for assessing material condition. These methods include the use of multiple source fields for interrogating and loading of a multicomponent test material. Source fields include electric, magnetic, thermal, and acoustic fields. The loading field preferentially changes the material properties of a component of the test material, which allows the properties of the component materials to be separated. Methods are also described for monitoring changes in material state using separate drive and sense electrodes with some of the electrodes positioned on a hidden or even embedded material surface. Statistical characterization of the material condition is performed with sensor arrays that provide multiple responses for the material condition during loading. The responses can be combined into a statistical population that permits tracking with respect to loading history.

Abstract: A set of curved components, such as the dovetail region of engine blades, are inspected by mounting each component into a circular carousel in a vertical orientation and rotating the carousel to move each component toward and away from an inspection site. The inspection site clamps a flexible eddy current sensor array to the curved material surface, scans the array over the surface, records the sensor position. A rigid element having a surface geometry similar to the surface shape of the component can be attached to the component to facilitate scanning of the sensor array over a component edge. The response of each sense element in the array may be converted into an effective material property and sense element proximity to the component material surface to verify the quality of the inspection scan and the presence of a defect such as a crack.

Abstract: Combined wound and micro-fabricated winding constructs are described for the inspection of materials and the detection and characterization of hidden features or flaws. These constructs can be configured as sensors or sensor arrays that are surface mounted or scanned over conducting and/or magnetizable test materials. The well-defined geometry obtained micro-fabricated windings and from carefully wound coils with known winding positions permits the use of model based inversions of sensed responses into material properties. In a preferred embodiment, the primary winding is a wound coil and the sense elements are etched or printed. The drive or sense windings can also be mounted under fasteners to improve sensitivity to hidden flaws. Ferrites and other means may be used to guide the magnetic flux and enhance the magnetic field in the test material.

Abstract: For the inspection of materials and the detection and characterization of hidden objects, features, or flaws sensors and sensor arrays are used to image form two-dimensional images suitable for characterizing the hidden features. Magnetic field or eddy current based inductive and giant magnetoresistive sensors may be used on magnetizable and conducting materials, while capacitive sensors can be used for dielectric materials. Enhanced drive windings and electrode structures permit nulling or cancellation of local fields in the vicinity of the sense elements to increase sensor sensitivity. The addition of calibration windings, which are not energized during measurements, allows absolute impedance and material property measurements with nulled sensors. Sensors, sensor arrays, and support fixtures are described which permit relative motion between the drive and sense elements. This facilitates the volumetric reconstruction of hidden features and objects.

Abstract: An apparatus that measures electrical impedance. The apparatus includes a signal generator controlled by a master microcontroller, a plurality of data acquisition channels, each channel containing a microcontroller, a host computer that processes and stores measured values, and a communication line between the host computer and the master microcontroller.

Abstract: Sensor condition verification may be performed on electromagnetic sensors and sensor arrays mounted to a material surface. The sensors typically have a periodic winding or electrode structure that creates a periodic sensing field when driven by an electrical signal. The sensors can be thin and flexible so that they conform to the surface of the test material. Monitoring the conductivity changes of a test material, with changes in temperature, may provide a mechanism for testing the integrity of the sensor. Changes in the conductivity, due to changes in temperature, without significant lift-off changes may verify the calibration of the sensor and that the sensor elements themselves are intact.

Abstract: Local features such as cracks in materials are nondestructively characterized by measuring a response with an electromagnetic sensor and converting this response into a selected property using a database. The database is generated prior to data acquisition by using a model to generate a baseline response or field distribution for the sensor and combining these results with another model, which may be simpler than the first model or provide a local representation of the field perturbations around a feature, which is evaluated multiple times over a range of values of the selected property. In addition, the presence of a feature may be detected by converting the sensor response into a reference parameter, such as a lift-off factor that reflects the sensor position relative to a material edge, and using this parameter to determine a reference response that can be compared to the measured response.

Abstract: The condition of insulating and semiconducting dielectric materials is assessed by a sensor array that uses electric fields to interrogate the test material. The sensor has a linear array of parallel drive conductors interconnected to form a single drive electrode and sense conductors placed on each side of and parallel to a drive conductor. Subsets of the sense conductors are interconnected to form at least two sense elements sensitive to different material regions. The sense conductors may be at different distances to the drive conductors, enabling measurement sensitivity to different depths into the test material. The material condition is assessed directly from the sense element responses or after conversion to an effective material property, such as an electrical conductivity or dielectric permittivity.

Abstract: Reference standards or articles having prescribed levels of damage are fabricated by monitoring an electrical property of the article material, mechanically loading the article, and removing the load when a change in electrical properties indicates a prescribed level of damage. The electrical property is measured with an electromagnetic sensor, such as a flexible eddy current sensor, attached to a material surface, which may be between layers of the article material. The damage may be in the form of a fatigue crack or a change in the mechanical stress underneath the sensor. The shape of the article material may be adjusted to concentrate the stress so that the damage initiates under the sensor. Examples adjustments to the article shape include the use of dogbone geometries with thin center sections, reinforcement ribs on the edges of the article, and radius cut-outs in the vicinity of the thin section.

Abstract: Observability of damage precursor, damage and usage states, or event occurrence may be enhanced by modifying component materials to include self-monitoring materials or by processing test material to alter the surface properties. The properties of the self monitoring materials, such as magnetic permeability or electrical conductivity, are monitored with electromagnetic sensors and provide greater property variations with component condition than the original component material. Processing includes shot peening or laser welding.

Abstract: Described are methods for pressurizing elastic support structures or balloons in sensor probes used for the inspection of components having areas of limited access. When inflated, the balloons press flexible sensors against the surface of the material under test. When deflated, the balloons permit easier insertion of the probes into the component and reduce the mechanical stresses on the sensors, thereby extending the sensor lifetime. By sequentially partially inserting the sensor into a limited access area from either side of the limited access area and scanning in opposite directions, the entire surface of the test material can be inspected.