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.
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.
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.
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.
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.
Despite ongoing research and development efforts, landmine and unexploded ordnance infestation continues to be a problem. Remediation efforts typically utilize inexpensive handheld metal detectors that rely on the principle of electromagnetic induction but have a limited depth of sensitivity and are unable to discriminate the shape, size, depth, and material type of the detected object. Conventional metal detectors can often detect all relevant metal objects. However, when instrument thresholds are set to a high level of sensitivity, unacceptably high false alarm rates result. To address these issues, a new design for inductive array detectors is under development. This design is founded on an advanced Nondestructive Evaluation (NDE) sensor called the MWM that was originally developed at MIT and has been successfully commercialized for manufacturing and NDE applications. The MWM-Array offers several advantages: (1) high resolution imaging with deep penetration, (2) varied applied field direction across sensor footprint, (3) potential for wide bandwidth continuous wave of pulsed mode operation, (4) simultaneous sampling and parallel processing of data form sensing element arrays for rapid image building, and (5) when measurements of the sensor element responses are combined with model-based measurement grids, quantitative estimates of size and depth for known object shapes can assist in the classification and discrimination of detected objects, as well as the elimination of false detections.
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 Laplace's equation in air or poorly conducting soils. Electroquasistatic (EQS) devices satisfy Laplace's 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.
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