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Eingeladener Vortrag
- nein (27)
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.
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. In order to
reduce the inspection time we present a GMR-based NDT-system consisting of a yoke
and a coil as a local magnetization unit. This way the global magnetization step and, if
necessary, the corresponding demagnetization cycle can be avoided reducing the
number of working steps. Using a local probe we measured plates, bearings, and rails,
each of which containing real fatigue cracks and reference artificial cracks of different
depths and orientations. Cracks with a depth of 40 ìm could be resolved with a signalto-
noise ratio of about 20. A reduction of the measuring time can be obtained using a
sensor array. We present an optimized sensor array for nondestructive testing
application, where gradiometric arranged GMR layers were fabricated on a board with
up to 48 GMR sensors. Each sensor detects the vertical field gradient. In our example
the baseline was chosen to be 250 ìm which efficiently suppress external background
fields without losing sensitivity for the detection of surface breaking defects.
Fast defect parameter estimation based on magnetic flux leakage measurements with GMR sensors
(2011)
We present a fast inverse scheme that is capable of simultaneously estimating the parameters depth, opening and length for rectangular 3D geometries of surface-breaking defects. The parameter estimation is realized by an iterative least-squares minimization using the trust-region reflective algorithm. A semi-analytic magnetic dipole model that allows the sensor characteristics to be incorporated is used for predicting the stray magnetic fields. Giant magneto-resistance (GMR) measurements were carried out on a test specimen that includes a series of artificial defects. For the estimation of the defect depths relative errors between 0.6% and 15.9% have been obtained. Due to its very low computational costs, the inverse scheme can suitably be employed in automated production environments.
In dieser Arbeit wird ein kameragestütztes Verfahren zur Flying-
Spot-Thermografie vorgestellt. Die Anregung wird über einen Diodenlaser mit einem optischen Scanner realisiert. Bei der Auswertung kommt ein effizienter und robuster Algorithmus zum Einsatz, der direkt auf die mit der Kamera aufgenommene Thermografiesequenz angewendet werden kann, ohne auf eine
Synchronisation zwischen Kamera und Lasersystem angewiesen zu sein. Daher eignet sich diese Herangehensweise besonders zur Nachrüstung bestehender Thermografiesysteme.
Die prinzipielle Eignung dieser Methode wird anhand einer Messung an einem Schienenstück mit Rollkontaktermüdungsrissen gezeigt. Als Referenzverfahren werden die Magnetpulverprüfung und eine auf GMR (Giant Magnetoresistance)- Sensoren basierende Streufeldmessung herangezogen. Mit dem thermografischen Verfahren konnten sämtliche enthaltenen Fehler erkannt werden. Dabei wurden Risse im mm-Abstand getrennt nachgewiesen, was etwa der geometrischen Auflösung der eingesetzten Kamera entspricht.
GMR-Sensor Arrays ermöglichen eine örtlich hoch aufgelöste Messung magnetischer Streufeldamplituden an Stahlbauteilen, bei der abhängig von der Größe der zu prüfenden Oberfläche erhebliche Datenmengen erzeugt werden.
Eine automatische Detektion von Rissen kann anschließend mittels digitaler Bildverarbeitung erreicht werden, welche hauptsächlich auf einer Kantenerkennung basiert. Diese wurde anhand einer Magnetometer-Messung getestet, die an einem zylindersymmetrischem Bauteil mit funkenerosiv eingebrachten Rissen geringer Tiefe durchgeführt wurde. Die erzielte Selektion der Defekte wird für eine angestrebte Rekonstruktion der Defektgeometrie benötigt.
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.
Angepasste Wirbelstromsonden mit GMR Sensoren für die zerstörungsfreie Prüfung von Alluminium
(2013)
NDT is a multidisciplinary research area fusing the disciplines of natural Science and engineering. Particularly in the early development stages of a new testing method, feasibility investigations focus on the fundamental physical interaction between the sensing mechanism and specimen. Research activities during subsequent phases of modular prototype development, technical System Integration and Validation primarily attempt to solve the practical engineering and real-life aspects of a testing method. We show several examples of new methods in different stages of development and highlight the potential for further industrial use. The first section deals with Steel surfaces of lightweight components which are
inspected with pm-sized magneto resistive sensor arrays. This quantitative magnetic stray field approach combines high spatial resolution and very high sensitivity, enabling the user to detect even very small micro-defects. Tuming to CFRP components we also show new developments in airbome ultrasonic testing. Here we use ferroelectret foils to reduce the acoustical mismatch between transducer and air, which enhances the signal-to-noise ratio. A further example illuminates the use of metallic nano layers as a broadband thermo acoustical emitter. Their use provides both high sound pressures and very broadband Signal characteristics. When it comes to CFRP components and other anisotropic and highly Scattering materials, ultrasonic matrix arrays in conjunction with many angles of incidences per transducer position can help to enhance the statistics of a testing problem. In the field of conventional water-coupled UT testing we also show the use of matrix array transducers
providing a tomographic-like 3D-image of impact damage in CFRP samples.
GMR sensors are widely used in many industrial segments such as information technology, automotive, automation and production, and safety applications. Each area requires an adaption of the sensor arrangement in terms of size adaption and alignment with respect to the field source involved. This paper deals with an analysis of geometric sensor parameters and the arrangement of GMR sensors providing a design roadmap for non-destructive testing (NDT) applications. For this purpose we use an analytical model simulating the magnetic flux leakage (MFL) distribution of surface breaking defects and investigate the flux leakage signal as a function of various sensor parameters. Our calculations show both the influence of sensor length and height and that when detecting the magnetic flux leakage of µm sized defects a gradiometer base line of 250 µm leads to a signal strength loss of less than 10% in comparison with a magnetometer response. To validate the simulation results we finally performed measurements with a GMR magnetometer sensor on a test plate with artificial µm-range cracks. The differences between simulation and measurement are below 6%. We report on the routes for a GMR gradiometer design as a basis for the fabrication of NDT-adapted sensor arrays. The results are also helpful for the use of GMR in other application when it comes to measure positions, lengths, angles or electrical currents.
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.
The rising popularity of additive manufacturing processes leads to an increased interest in possibilities and methods for related process monitoring. Such methods ensure improved process quality and increase the understanding of the manufacturing process, which in turn is the basis for stable component quality, e.g., required in the aerospace industry or in the medical sector. For laser powder bed fusion, a handful of process monitoring tools already exist, such as optical tomography, thermography, pyrometry, imaging, or laser power monitoring. Although these tools provide helpful information about the process, more information is required for an accurate in-depth understanding. In this article, advanced approaches in eddy current testing (ET) are combined, such as single wire excitation, magnetoresistive (MR) sensor arrays, and heterodyning to build up a system that can be used for online process monitoring of laser powder bed fusion. In addition to detailed information about the developed ET system and underlying signal processing, the first results of magnetoresistance-basedonline ET during the laser powder fusion process are presented. While producing a step-shaped cuboid, each layer is tested during recoating.
Test results show that not only the contours of the topmost layer are detected but also the contours of previous layers covered by powder. At an excitation frequency of 1 MHz, a penetration depth of approx. 400 μm is obtained. To highlight the possibilities of ET for online process monitoring of laser powder bed fusion, results are compared with postexposure images of the integrated layer control system (LCS).
Background Due to physical coupling between mechanical stress and magnetization in ferromagnetic materials, it is assumed in the literature that the distribution of the magnetic stray field corresponds to the internal (residual) stress of the specimen.
The correlation is, however, not trivial, since the magnetic stray field is also influenced by the microstructure and the geometry of component. The understanding of the correlation between residual stress and magnetic stray field could help to evaluate the integrity of welded components.
Objective This study aims at understanding the possible correlation of subsurface and bulk residual stress with magnetic stray field in a low carbon steel weld. Methods The residual stress was determined by synchrotron X-ray diffraction (SXRD, subsurface region) and by neutron diffraction (ND, bulk region). SXRD possesses a higher spatial resolution than ND. Magnetic stray fields were mapped by utilizing high-spatial-resolution giant magneto resistance (GMR) sensors.
Results The subsurface residual stress overall correlates better with the magnetic stray field distribution than the bulk stress.
This correlation is especially visible in the regions outside the heat affected zone, where the influence of the microstructural features is less pronounced but steep residual stress gradients are present.
Conclusions It was demonstrated that the localized stray field sources without any obvious microstructural variations are associated with steep stress gradients. The good correlation between subsurface residual stress and magnetic signal indicates that the source of the magnetic stray fields is to be found in the range of the penetration depth of the SXRD measurements.
Additive manufacturing (AM) technologies, generally called 3D printing, are widely used because their use provides a high added value in manufacturing complex-shaped components and objects. Defects may occur within the components at different time of manufacturing, and in this regard, non-destructive techniques (NDT) represent a key tool for the quality control of AM components in many industrial fields, such as aerospace, oil and gas, and power industries. In this work, the capability of active thermography and eddy current techniques to detect real imposed defects that are representative of the laser powder bed fusion process has been investigated. A 3D complex shape of defects was revealed by a μCT investigation used as reference results for the other NDT methods. The study was focused on two different types of defects: porosities generated in keyhole mode as well as in lack of fusion mode. Different thermographic and eddy current measurements were carried out on AM samples, providing the capability to detect volumetric irregularly shaped defects using non-destructive methods.
In den letzten Jahren haben additive Fertigungstechnologien an Bedeutung gewonnen. Für komplexe Funktionsbauteile oder die Produktion von Werkstücken in kleinen Stückzahlen kann das Laser-Pulverbettschmelzen eingesetzt werden. Hohe Sicherheitsanforderungen, z. B. in der Luft- und Raumfahrt, erfordern eine umfassende Qualitätskontrolle. Daher werden nach der Fertigung zerstörungsfreie Offline-Prüfverfahren wie die Computertomographie eingesetzt. In jüngster Zeit wurden zur Verbesserung der Rentabilität und Praktikabilität zerstörungsfreie Online-Prüfverfahren wie die optische Tomographie entwickelt. In diesem Beitrag wird die Anwendbarkeit der Wirbelstromprüfung mit GMR Sensoren für die online Prüfung von PBF-LB/M Teilen demonstriert. Die Ergebnisse einer online Wirbelstromprüfung mit GMR Sensoren und einer Ein-Draht-Anregung werden gezeigt. Während des Produktionsprozesses wird für jede Lage eine Wirbelstromprüfung durchgeführt. Trotz hochauflösender Arrays mit 128 Elementen wird durch eine angepasste Hardware die Prüfdauer geringgehalten. So kann die Messung während des Beschichtungsvorgangs durchgeführt werden, ohne den Fertigungsprozess signifikant zu verlangsamen. Eine online Wirbelstromprüfung eines stufenförmigen Testkörpers aus Haynes282 über 184 Lagen zeigt, dass die Kanten nicht nur in der aktuellen Lage detektiert werden können, sondern auch in einer Tiefe von 400 µm, wenn eine Anregungsfrequenz von 1,2 MHz gewählt wird.
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.
Size adapted GMR arrays for the automated inspection of surface breaking cracks in roller bearings
(2014)
Their small size together with a remarkable field sensitivity are the most prominent features of present-day GMR sensors paving the way for various applications in automated non-destructive testing (NDT). This work presents a prototype for fast and automated magnetic testing of roller bearings. A local magnetization unit excites the magnetic field inside the bearing. As a result of a design study and the following wafer fabrication the probe was equipped with NDT-adapted GMR sensor arrays in which 48 elements measures the field response. The detection of artificial and 40 µm deep defects could be resolved with a SNR better than 20 dB. In addition, we report of first results of a POD (Probability of Detection) analysis using GMR sensors to investigate bearings with EDM (electronic discharge machining) notches having depths down to 10 µm. Finally, we estimate successfully the depth of a 57 µm notch from the measured data.
Eddy Current Technique is a powerful method for detection of surface notches and of buried flaws during inspection of metallic parts. Recent EC array probes have demonstrated a fast and efficient control of large surfaces. Nevertheless, when the size of flaws decreases or the defect is rather deep, traditional winding coil probes turn out to be useless. Magnetoresistive sensors present the advantages of flat frequency response and micron size. These sensors are hence very attractive for the detection of buried defects that require low frequencies because of skin depth effect. An optimization of the probe with magnetoresistive sensors as receivers has been made by simulations using CIVA software and finite elements methods with OPERA. EC probes for buried flaw detection have been designed. Experimental results have been compared with simulations.
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.
Zunehmend werden hochpräzise Magnetfeldsensoren in der
Materialprüfung verwendet. Aus dem Zoo der Magnetfeldsensoren eignen sich
GMR-Sensoren (Giant Magneto Resistance) aufgrund ihrer hohen Feld-
Empfindlichkeit, ihrem hohen Signal-Rausch-Verhältnis sowie ihrer kleinen
Sensorflächen (verbunden mit einer hohen Ortsauflösung) hierfür im Besonderen.
Mit GMR-Sensoren wurden Risse untersucht und konnten mit einer Tiefe ≤ 50 µm
noch mit einem guten Signal-Rausch-Verhältnis aufgelöst werden. Die an
Prüfkörpern mit künstlich eingebrachten Defekten gemessenen Streufeldsignale
wurden anhand von analytischen Lösungen verifiziert. Anhand der analytischen
Lösungen wurden auch Defektparameter zu den aufgezeichneten Signalen
rekonstruiert.
Development of adapted GMR-probes for automated detection of hidden defects in thin steel sheets
(2015)
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.
Micro-cracks in a magnetized ferromagnetic material cause stray fields that can be observed using giant magnetoresistive (GMR) sensors. This work investigates the applicability of GMR sensors to the non-destructive evaluation of micro-cracks via the observation of stray fields. For this purpose, our measurement setup is assessed using a fast new sensitivity analysis based on adjoint states, employing the finite-element method. A model for the GMR sensor is developed and verified. We are able to resolve micro-cracks with an opening of 3 µm and a depth of 30 µm. GMR positioning inaccuracies are analyzed.
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.
BAM und TATA Steel untersuchen gemeinsam, ob sich die Prüfung von ferritischen Feinblechen mit einer Stärke von etwa 0,2 mm während der Fertigung durch den Einsatz von GMR-Sensorik verbessern lässt. Hierzu wurde eine Vorstudie an Feinblechen mit eingebrachten Mikronuten durchgeführt. Diese Nuten variieren in ihrer Tiefe zwischen 5 μm und 60 μm und wurden verdeckt mit ZfP-angepassten GMR-Sensorarrays geprüft. Die Ergebnisse der Vorstudie zeigten, dass bei verdeckter Prüfung schon kleinste Defekte mit einer Tiefe von 10 μm - gleichbedeutend mit einer Restwanddicke von 90 % – mit einem SNR > 6 dB detektiert werden konnten, wobei der Abstand der Prüfsonde zur Oberfläche bis zu 500 μm betrug. Zudem wurden in der Vorstudie Untersuchungen zur Prüfgeschwindigkeit und Praxistauglichkeit durchgeführt.
Basierend auf den hier gewonnenen Ergebnissen wird ein erster Demonstrator für eine zuverlässige Prüfung eines Teilbereiches von 50 mm des Feinbleches innerhalb einer Produktionslinie aufgebaut. Zum Einsatz kommen Prüfsonden mit jeweils 24 GMR-Sensorelementen. Hierbei wird das Ziel einer umfassenden Prüfung des Feinblechs über die gesamte Breite von 1200 mm angestrebt. Eine besondere Herausforderung stellt dabei die hohe Anzahl von mehr als 1000 Sensorelementen dar. Das hierfür entwickelte Prüfkonzept umfasst neben einer angepassten Erzeugung des Magnetfeldes die Entwicklung einer geeigneten schnellen und sicheren Elektronik für die neuen Mehrkanalprüfköpfe. Neben der Vorstellung des Prüfkonzeptes und seiner Eigenschaften vergleichen wir die erzielten Prüfergebnisse an verschiedenen Vergleichsfehlern mit denen der Vorstudie.
Hohe Empfindlichkeit und Ortsauflösung sowie eine kurze Prüfzeit
sind Anforderungen an ein leistungsfähiges Prüfsystem basierend auf magnetischen
ZfP-Verfahren für die Oberflächenprüfung. Dieses soll das sichere Auffinden feinster
Risse mit Tiefen von weniger als 50 Mikrometern gewährleisten.
Um eine gute Ortsauflösung zu gewährleisten, müssen in der Regel der Defektgröße
angepasste Sensoren zum Einsatz kommen. Speziell im Fall von
Risslängen kleiner als ein Millimeter sollten die Sonden für die Streuflussprüfung
ebenfalls Wirkbreiten kleiner als ein Millimeter aufweisen. Durch die daraus folgenden
geringen Spurbreiten ergeben sich allerdings lange Prüfzeiten. Der
begrenzten Prüfzeit und präzisionsbedingten mechanischen Grenzen in der Verfahr-
Geschwindigkeit einer Sonde kann mit einem Multikanalansatz begegnet werden.
Eine hohe Anzahl an Kanälen wiederum führt zu einem Platzproblem in der
Prüfsonde, schließlich erfordert die Verstärkung schwacher Signale im eine Reihe
von aktiven Bauelementen.
Der Schlüssel zur Auflösung dieses Konflikts ist der Einsatz moderner Schaltkreise
wie Multiplexer und FPGA. In diesem Beitrag stellen wir am Beispiel eines
48 kanaligen Testaufbaus für die GMR-basierte Streuflussprüfung ein flexibles miniaturisiertes
Elektronikkonzept vor. Dabei gehen wir auf die einzelnen
Komponenten zur Ansteuerung der Sensoren und zur Signalkonditionierung ein.
Wir berichten vom Design, Aufbau und der Charakterisierung des Multikanalmoduls
und seiner Erprobung im realen Prüfeinsatz.
Composite pressure vessels consist of an internal liner (Steel, aluminum, or plastic) which stores the liquid or gas and CFRP (carbon fiber reinforced plastics) which serves as pressure resistance. These vessels comprise some advantages compared with conventional vessels made of steel or aluminum and their application grew strongly over the last decade. The reason for this trend is the tremendous weight saving.
However, the long-time behavior for filling and discharging cycles and the creep strength of composite vessels is not fully understood. Therefore, BAM started an internal project to investigate degradation of these material systems. Normally, high-frequency eddy current testing (HF-ET, f > 10 MHz) is deployed for CFRP due to the low conductivity of the fibers, which is in the order of 0.01 MS/s, and the capacitive coupling between them. Nevertheless, in some cases conventional ET can be applied.
Some first results using conventional ET for detecting leakages in the internal liners are already presented. This contribution gives an update of our investigation using both conventional and high-frequency ET. Therefore, we developed an ET equipment based on a vector network analyzer using frequencies between 300 kHz and 2 GHz. The focus concerns the study of the aging effects.
Magneto resistance sensors like GMR (giant magneto resistance) or TMR (tunnel magneto resistance) are widely used in industrial applications, examples are position measurement and read heads of hard disk drives. However, in case of non-destructive testing (NDT) applications these sensors, although their properties are outstanding like high spatial resolution, high field sensitivity, low cost and low energy consumption, never reached a technical transfer to an application beyond sci-entific scope.
This paper deals with benefits of GMR/TMR sensors in terms of high spatial resolution testing for different NDT applica-tions. The first example demonstrates the preeminent advantages of MR-elements compared with conventional coils used in eddy current testing (ET). The probe comprises one-wire excitation with an array of MR elements. This led to a better spatial resolution in terms of neighboring defects.
The second section concentrates on MFL-testing (magnetic flux leakage) with active field excitation during and before test-ing. The latter illustrated the capability of highly resolved crack detection of a crossed notch. This example is best suited to show the ability of tiny magnetic field sensors for magnetic material characterization of a sample surface. Another example is based on characterization of samples after tensile test. Here, no external field is applied. The magnetization is only changed due to external load and magnetostriction leading to a field signature which GMR sensors can resolve. This gives access to internal changes of the magnetization state of the sample under test.