Aircraft wheel testing with remote eddy current technique using a SQUID magnetometer

An aircraft wheel testing system using a planar HTS SQUID gradiometer with Joule-Thomson machine cooling in conjunction with a differential eddy current (EC) excitation has recently been developed El], From a routine performance test in the wheel testing facility at the Lufthansa Base, Frankfurt/M, airport, we learned that the quadrupolar flaw signatures complicate signal interpretation considerably. In order to overcome these difficulties, the system was equipped with a HTS rf magnetometer SQUID sensor and an absolute EC excitation coil. The coil was mounted with a lateral displacement with respect to the SQUID. The geometry was chosen similar to the remote EC technique: a given point on the rotating wheel first passes underneath the excitation coil and then underneath the sensor. We analyzed the dependence of the response field of an inside crack on excitation coil displacement, EC frequency and lock-in phase angle and found an optimum rotation velocity for deep lying defects. The depth selectivity of the technique is discussed

Extraordinary Magnetoresistance in Hybrid Semiconductor-Metal Systems

We show that extraordinary magnetoresistance (EMR) arises in systems consisting of two components; a semiconducting ring with a metallic inclusion embedded. The im- portant aspect of this discovery is that the system must have a quasi-two-dimensional character. Using the same materials and geometries for the samples as in experiments by Solin et al.[1;2], we show that such systems indeed exhibit a huge magnetoresistance. The magnetoresistance arises due to the switching of electrical current paths passing through the metallic inclusion. Diagrams illustrating the flow of the current density within the samples are utilised in discussion of the mechanism responsible for the magnetoresistance effect. Extensions are then suggested which may be applicable to the silver chalcogenides. Our theory offers an excellent description and explanation of experiments where a huge magnetoresistance has been discovered[2;3].Comment: 12 Pages, 5 Figure

Defect Detection in Thick Aircraft Samples Based on HTS SQUID-Magnetometry and Pattern Recognition

SQUID technology has recently evolved to the point that it can be used for industrial applications in Non-Destructive Evaluation (NDE). We present the implementation of an HTS SQUID magnetometer in an eddy current testing system to measure very thick structures in large aircraft. We measured a 62 mm-thick, bolted aluminum sample from EADS-Airbus, similar to the three-layered outer wing splice that is being proposed for the Airbus A-380. The combination of field sensitivities of a few pT/rootHz and a large dynamic range of about 140 dB/rootHz enabled us to detect defects at a depth of up to 40 mm. However a problem was presented by the fact that deep-lying defects which caused small field variations-were superimposed on field changes, in their turn caused by current distortions in the vicinity of the titanium bolts.. Separation of these two contributions was achieved through parameter optimization based on FEM simulations and signal processing. We report on the possibilities for flaw detection using adapted eddy current excitation

Defect detection and classification using a SQUID based multiple frequency eddy current NDE system

The probability of detection (POD) of hidden fatigue defects in riveted multilayer joints, e.g. aircraft fuselage, can be improved by using sophisticated eddy-current systems which provide more information than conventional NDE equipment. In order to collect this information, sensor arrays or multi-frequency excitation schemes can be used. We have performed simulations and measurements with an eddy current NDE system based on a SQUID magnetometer. To distinguish between signals caused by material defects and those caused by structures in the sample, such as bolts or rivets, a high signal-to-noise ratio is required. Our system provides a large analog dynamic range of more than 140 dB/root Hz in unshielded environment, a digital dynamics of the ADC of more than 25 bit (>150 dB) and multiple frequency excitation. A large number of stacked aluminum samples resembling aircraft fuselage were measured, containing titanium rivets and hidden defects In different depths in order to obtain sufficient statistical information for classification of the defect geometry. We report on flaw reconstruction using adapted feature extraction and neural network techniques

Multiplexed SQUID array for non-destructive evaluation of aircraft structures

SQUID sensors offer a significant advantage for eddy-current (EC) testing of aircraft components for material flaws hidden deeply in the tested structure. However, the requirement to take maps of the magnetic field, usually by meander-shaped scans, leads to unacceptably long measurement times. Due to their inductive coupling to a tank circuit, several rf SQUID sensors may be read out sequentially by selectively coupling to their tank circuits, using only one electronics with a multiplexer [1]. The multiplexed operation of three planar HTS rf SQUID gradiometers with one electronics and one cable is shown, demonstrating the advantage of lower liquid nitrogen boil-off. Independent operation and switching is confirmed using local coil excitation of the individual SQUIDs. We report on the implementation of two multiplexed SQUID sensors in conjunction with an EC excitation and lock-in readout at unshielded laboratory environment. Scanning is performed while continuously switching the operating SQUID, thus obtaining two traces simultaneously. The applicability to EC testing of riveted sections of aircraft fuselage is discussed

Entwicklung eines Systems zur zerstoerungsfreien Materialpruefung mittels Wirbelstrommethode unter Verwendung von Hochtemperatur - Supraleiter - SQUIDs. Untersuchungen zur oertlichen Aufloesung und Diagnosezuverlaessigkeit bei Gegenwart von Stoersignalen Forschungsbericht/Abschlussbericht

Available from TIB Hannover: F98B2197+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman