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Die Zerstörungsfreie Prüfung (ZfP) ist ein wichtiges Werkzeug zur Qualitätssicherung sowie zur Überwachung sicherheitsrelevanter Bauteile. In der industriellen ZfP ist das Interesse an innovativen, kostengünstigen und sicherheitssteigernden ZfP-Methoden sehr groß. Die klassische Streuflussmethode ist die Magnetpulverprüfung, die sehr sensitiv auf Mikrorisse ist. Eine zuverlässige, automatische Prüfung ist hier aber nur bedingt und mit großem Aufwand zu erreichen. Die Lösung liegt im Einsatz von Magnetfeldsensoren, die zudem eine Bewertung der Defektgeometrie aufgrund der gemessenen Rissstreufelder ermöglicht. Insbesondere GMR-Sensoren (giant magneto resistance) eignen sich hierfür aufgrund ihrer kleinen Sensorelemente, welche eine hohe Ortsauflösung ermöglichen, und der sehr guten Feldempfindlichkeit. Jedoch sind kommerzielle GMR-Sensoren nicht an die Bedürfnisse der ZfP angepasst. Daher wurden während dieser Arbeit GMR-Sensoren dahingehend optimiert, dass sie für eine automatisierte Prüfung infrage kommen. Neben dem Design und der Charakterisierung der angepassten Sensoren wurden Messungen zur Detektionswahrscheinlichkeit durchgeführt. Um die Praxistauglichkeit zu untermauern, erfolgte ein quantitativer Vergleich mit alternativen ZfP- Oberflächenmethoden, der Wirbelstrom-, Magnetpulver- und Thermografieprüfung. Zusätzlich konnte der erfolgreiche Einsatz der GMR-Streuflussprüfung in einer industriellen, automatisierten Prüfeinrichtung unter Beweis gestellt werden.
High-precision magnetic field sensors are of increasing interest in non destructive testing (NDT). In particular GMR-sensors (giant magneto resistance) are qualified because of their high sensitivity, high signal-to-noise ratio and high spatial resolution. We performed magnetic flux leakage measurements of artificial cracks with a GMR-gradiometer and a 3-axes GMR-magnetometer. Cracks of a depth of 44 μm still could be detected with a sufficient high signal-to-noise ratio. A semi-analytic magnetic dipole model was used for swiftly predicting magnetic stray fields. The reliable reconstruction based on measurements of artificial rectangular-shaped defects is demonstrated.
GMR sensors are increasingly used for magnetic surface inspection due to their high sensitivity and high spatial resolution. In case of simple planar or cylindrical shaped components, the GMR-based inspection procedure can be automated easily. We present GMR measurements of real fatigue cracks. In addition, we present a probe design using a local magnetization unit and commercially available GMR sensors. The design was carried out by means of finite-element method (FEM) simulations. Using the local probe we measured bearings containing artificial reference cracks of different depths and orientations. Cracks with a depth of 40 µm could be resolved with a signal-to-noise ratio better than 6. A further reduction of the measuring time can be obtained using a sensor array. For this purpose we present a study of the optimized size of the sensing GMR-layers for a NDE-adapted sensor array. The geometric sensor parameters were investigated through simulations of the magnetic flux leakage of surface cracks using an analytic model.
Evaluation of high spatial resolution imaging of magnetic stray fields for early damage detection
(2016)
Metal magnetic memory (MMM) technique with associated ISO 24497-1:3 is gaining considerable interest in the magnetic NDT community. In contrast to traditional Magnetic Flux Leakage (MFL) testing, the inspection objects are not intentionally magnetized by an external magnetic field. Due to the physical coupling between mechanical stress and magnetization of ferromagnetic materials, it is assumed that the distribution of the residual MFL correspond to the internal stress of the specimen, or in the most general sense, to a degradation of the material.
Usually, MMM measurements are performed by relatively bulky magnetic inspection sensors. The evaluation of local magnetic field distribution is limited thereby. Highprecision GMR (Giant Magneto Resistance) measurements in the micrometer regime can provide a higher degree of information due to better spatial resolution.
We present a concise summary of studies on the correlation of magnetic structure and microstructure of steels. In particular, we compare residual stress measurements in S235JRC steel welds by means of neutron diffraction with high resolution magnetic field mappings. Results indicate a qualitative correlation between residual stresses and local stray field variation. In addition, stray field measurements of plastically deformed specimens for quasi-static and cyclic loading cases are discussed. The present study concludes that GMR sensors can detect inhomogeneous plastic deformations of S235JR steel in a very early stage, without specific signal processing according to the ISO 24497-1:3.
Development of adapted GMR-probes for automated detection of hidden defects in thin steel sheets
(2016)
Thin steel sheets with a thickness of 0.3 mm and less are the base materials of many everyday life products (cans, batteries, etc.). Potential inhomogeneities such as non-metallic inclusions inside the steel can lead to a rupture of the sheets when it is formed into a product such as a beverage can. Therefore, there is a need to develop automated NDT techniques to detect hidden defects and inclusions in thin sheets during production. For this purpose Tata Steel Europe and BAM, the Federal Institute for Materials Research and Testing (Germany), collaborate in order to develop an automated NDT-system. Defect detection systems have to be robust against external influences, especially when used in an industrial environment. In addition, such a facility has to achieve a high sensitivity and a high spatial resolution in terms of detecting small inclusions in the μm-regime. In a first step, we carried out a feasibility study to determine which testing method is promising for detecting hidden defects and inclusions inside ferrous thin steel sheets. Therefore, two methods were investigated in more detail – magnetic flux leakage testing (MFL) using giant magneto resistance sensor arrays (GMR) as receivers [1,2] and eddy current testing (ET). The capabilities of both methods were tested with 0.2 mm-thick steel samples containing small defects with depths ranging from 5 μm up to 60 μm. Only in case of GMRMFL-testing, we were able to detect parts of the hidden defects with a depth of 10 μm trustworthily with a SNR better than 10 dB. Here, the lift off between sensor and surface was 250 μm. On this basis, we investigated different testing scenarios including velocity tests and different lift offs. In this contribution we present the results of the feasibility study leading to first prototypes of GMR-probes which are now installed as part of a demonstrator inside a production line.
We present a prototype for automated magnetic stray field testing of ferromagnetic roller bearings. For this purpose NDE-adapted GMR sensor arrays (giant magneto resistance) are used for the detection of surface breaking cracks. The sensors are miniaturized down to the lower µm-regime to achieve adequate spatial resolution. In doing so, sensor arrays with up to 48 elements are used to inspect the bearing surface within a few seconds only. In contrast to magnetic particle inspection (MPI), where the global magnetization requires a further inspection step and succeeding demagnetization, the presented prototype only locally magnetize the surface area in the vicinity of the GMR Sensors. For the local magnetization, the applied sub-surface magnetic field was simulated and proofed for detecting flaws with a depth of a few 10 µm. By multiplexing the sensor array with an adapted read out electronics we quasi simultaneously detect the normal field component of about 100µm above the surface. The detection of artificial notches with a depth of 40 µm and more could be resolved with a SNR better than 20 dB. The presented testing facility is fast and provides a step towards automated testing of safety relevant steel components.
At BAM, the Federal Institute for Materials Research and Testing, a group of scientists develops NDT-applications based on GMR sensor technology. In particular, the knowledge gained in the field of automated testing systems based on the magnetic flux leakage (MFL) were combined with GMR sensors to achieve high resolution testing of ferromagnetic materials.
In cooperation with Europe's second largest steel producer, Tata Steel Europe, BAM is working on solutions for the detection of small inhomogeneities in thin steel plates. The objective is to incorporate an automated testing facility in a production line.
Before setting up an automated testing system, a feasibility study was carried out in order to verify the ability of GMR-MFL-testing and eddy current testing (ET) for the detection of hidden defects in thin steel plates. For this purpose, Tata Steel Europe fabricated in a first step test samples of 0.2 mm thick steel plates in which defects of different depth (5 – 60 µm) were introduced. Only in case of GMR-MFL-testing, we were able to detect parts of the hidden defects trustworthily with a SNR better than 10 dB. The lift off between sensor and surface was 250 ìm. On this basis, we investigated different testing scenarios including velocity tests and different lift offs. The achieved results for this special testing problem were presented leading to a practical example for GMR-based testing and paving the way to an automated testing system in a production line.
Seit einigen Jahren beschäftigt sich eine Gruppe der BAM mit der Entwicklung angepasster GMR-Sensorik (giant magneto resistance, Riesenmagnetwiderstand) für spezielle ZfP-Probleme. Hierbei wurden reichhaltige Erfahrungen auf dem Gebiet der automatisierten Streuflussprüfung mit GMR-Sensoren gesammelt (DACH-Tagung 2012, Graz).
Die an der BAM gewonnenen Erkenntnisse veranlassten Europas zweitgrößten Stahlhersteller, Tata Steel Europe, mit der BAM eine Lösung für ein spezielles Prüfproblem zu finden. Ziel ist es, dünne Bleche im Fertigungsprozess auf kleinste Inhomogenitäten automatisiert zu prüfen.
In einem ersten Schritt wurden durch die Tata Steel Europe 0,2 Millimeter starke Testbleche hergestellt, in die Fehler von 5-60 Mikrometern Tiefe eingebracht wurden. Anschließend wurden von der BAM vergleichende Prüfungen mit Wirbelstrom und GMR-Streufluss durchgeführt. Nur mit letzterer Methode gelang es, einen Teil der verdeckten Testfehler sicher zu detektieren. In einem zweiten Schritt wurden verschiedene Untersuchungen zur Praxistauglichkeit der Sensorik (Prüfgeschwindigkeit; Sensorabstand etc.) durchgeführt. Die Ergebnisse der verschiedenen Untersuchungen und ein möglicher Weg zum praktischen Einsatz der Sensorik werden vorgestellt.
We present a simulation study which pursues the objective to find probe geometries for a MR-based eddy current (EC) probe (MR magneto resistance, e.g., GMR giant magneto resistance, TMR tunnel magneto resistance). MR sensor technology exhibits two significant advantages compared with conventional coil systems. First, MR sensors are relatively frequency-independent within common EC-frequency ranges which enable us to operate them in hidden defects testing problems. Secondly, MR technology is well suited for miniaturization helping us to design small elements in the order of below 100 µm. In this paper simulation and experimental results obtained with the probes for low frequency application, i.e. for hidden defects detection are discussed.
Our simulations are based on two different approaches for a better validation, a commercial finite element method software (Opera, Vectorfields) and the semi-analytical software CIVA. We investigated both coil arrangement in order to excite sufficient high eddy currents inside the test samples and position of MR-elements at the array chip. In doing so the MR sensors were positioned that they are not exposed to excitation fields. In addition, different coil geometries, in particular coil length, e.g. Ι = 20 mm, were analyzed in order to generate a consistent eddy current distribution beneath an array of up to 32 MR-elements. To prove obtained probe principles we built GMR-EC-probes. The first test measurements are in good agreement with the simulations performed by BAM and CEA. On basis of our findings the IMAGIC consortium developed new MR-EC-probes using integrated ASIC technology.