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

Preparation and magnetoresistance of Ag 2+x Se thin films deposited via Pulsed Laser Deposition

The preparation of Ag 2+x Se thin films with thicknesses between 4 nm and 3000 nm by pulsed laser deposition on single crystalline NaCl and MgO substrates is reported. The films are perfectly dense and show a good lateral uniformity with a small number of defects. The microstructure of the films corresponds to a nanoparquet, being composed of two different phases of silver selenide. One phase is identified as the Naumannite low temperature phase of silver selenide, the structure of the other phase has not been reported in detail before and probably represents a metastable phase. Silver-rich films contain silver precipitates with typical sizes on the nanoscale. Their presence and their size appears to be responsible for the large and linear magnetoresistance effect of silver-rich silver selenide

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

Nanoindentation of Bridgman YBCO samples

In this study, the mechanical properties of YBa2Cu3O7−x, obtained by the Bridgman technique, were examined using a Berkovich tip indenter on the basal plane (0 0 1). Intrinsic hardness was measured by nanoindentation tests and corrected using the Nix and Gao model for this material. Furthermore, Vickers hardness tests were performed, in order to determine the possible size effect on these measurements. The results showed an underestimation of the hardness value when the tests were performed with large loads. Moreover, the elastic modulus of the Bridgman samples was 128 ± 5 GPa. Different residual imprints were visualised by atomic force microscopy and a focused ion beam, in order to observe superficial and internal fracturing. Mechanical properties presented a considerable reduction at the interface. This effect could be attributed to internal stress generated during the texturing process. In order to corroborate this hypothesis, an observation using transmission electron microscopy was performed

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

Optical excitation thermography with VCSEL-array source

Abstract A novel VCSEL array combines the high temporal flexibility of a laser source with the full field radiation characteristics of an array and yields a new optical excitation source for active thermography that can merge the two regimes of flash and lock-in thermography. Among others, we investigate the possibility of multiplex photothermics for two-dimensional material characterization, e. g. thickness mapping. Several first test results will be presented that demonstrate possibilities and capabilities of this new optical excitation source and compare it to conventional sources

Identyfikacja zanieczyszczeń w połączeniach klejonych kompozytów CFRP poprzez pomiary lokalnej nieliniowości drgań

A new approach based on measurements of a local nonlinear response of the laminate is suggested and applied to characterizing contaminations of adhesive bonding in carbon fibre reinforced polymer (CFRP). It is shown that a contaminated boundary layer of the adhesive contributes to an overall nonlinear response of the laminate that enables to recognise the difference in bonding quality caused by various types and levels of contaminations.W artykule, w celu scharakteryzowania zanieczyszczeń połączeń klejonych polimerów wzmocnionych włóknem węglowym (CFRP), zaproponowano i następnie zastosowano nowe podejście oparte na pomiarach lokalnej nieliniowej odpowiedzi laminatu. Pokazano, że zanieczyszczona warstwa graniczna kleju przyczynia się do ogólnej nieliniowej odpowiedzi laminatu, która umożliwia rozpoznanie różnicy w jakości wiązania spowodowanej różnymi typami i poziomami zanieczyszczeń

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