Wissenschaftliche Artikel der BAM
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It is widely accepted that the magnetic state of a ferromagnetic material may be irreversibly altered by mechanical loading due to magnetoelastic effects. A novel standardized nondestructive testing (NDT) technique uses weak magnetic stray fields, which are assumed to arise from inhomogeneous deformation, for structural health monitoring (i.e., for detection and assessment of damage). However, the mechanical and microstructural complexity of damage has hitherto only been insufficiently considered. The aim of this study is to discuss the phenomenon of inhomogeneous “self-magnetization” of a polycrystalline ferromagnetic material under inhomogeneous deformation experimentally and with stronger material-mechanical focus. To this end, notched specimens were elastically and plastically deformed. Surface magnetic states were measured by a three-axis giant magnetoresistant (GMR) sensor and were compared with strain field (digital image correlation) and optical topography measurements. It is demonstrated that the stray fields do not solely form due to magnetoelastic effects. Instead, inhomogeneous plastic deformation causes topography, which is one of the main origins for the magnetic stray field formation. Additionally, if not considered, topography may falsify the magnetic signals due to variable lift-off values. The correlation of magnetic vector components with mechanical tensors, particularly for multiaxial stress/strain states and inhomogeneous elastic-plastic deformations remains an issue.
Sicherheitsrelevante und zyklisch hoch belastete Bauteile erfordern zur Vermeidung von kostenintensiven Ausfällen besonders stabile Prozessparameter. Bereits sehr kleine Randzonenfehler können unter zyklischer Bauteilbelastung zu Risswachstum und letztendlich zum Bauteilversagen führen. Die frühzeitige Erkennung von Randzonenfehler in Hochleistungsbauteilen wie z.B. Zahnräder, Ritzelwellen und Kurbelwellen erfordert daher eine leistungsfähige zerstörungsfreie Oberflächenrissprüfung, die es ermöglicht in den hochbeanspruchten Funktionsflächen auch Härterisse, Schleifrisse oder Zundereinschlüsse zu detektieren.
Hierzu sind in den letzten Jahren einige neue, innovative Oberflächenprüfverfahren wie die laserangeregte Thermografie und die Streuflussprüfung mit hochauflösenden GMR-Sensoren oder magnetooptischen Verfahren entwickelt worden. Zusätzlich zur hohen Empfindlichkeit zeichnen sich diese innovativen Verfahren durch einen schnellen und teils auch berührungslosen Einsatz aus. Da die noch relativ neu-en Verfahren naturgemäß noch nicht normativ verankert sind, wurden auch bereits erste Validierungen durchgeführt. Um die Leistungsfähigkeit der Verfahren eingehend zu untersuchen, erfolgten Testreihen an verschiedenen Testkörpern in Bezug auf Ortsauflösung, Empfindlichkeit, Automatisierung und Bewertung der Messsig-nale.
Neben den neuen Verfahren und ihren ersten Schritten hin zur Validierung kamen als Referenz auch die „klassischen“ Verfahren der Magnetpulver- und Wirbelstromprüfung zum Einsatz, deren Leistungsfähigkeit durch angepasste Sondenentwicklung auch für sehr kleine Oberflächendefekte nochmals unter Beweis gestellt wurde. Zusätzlich wurde an einigen Testkörpern eine hochauflösende CT durchgeführt. Die Ergebnisse dieses Vergleiches werden vorgestellt und Möglichkeiten sowie Grenzen der einzelnen Verfahren herausgearbeitet.
Magneto-resistive (MR) sensor arrays are suited for high resolution eddy current testing (ET) of aerospace components due to two significant advantages compared to conventional coil systems. First, to obtain high spatial resolution they can be manufactured down to the µm-regime without losing their outstanding field sensitivity. Secondly, MR technology has a relatively frequency-independent sensitivity in the range of common ET-frequencies thus providing a benefit for low frequency applications.
This paper presents measurements using MR array probes consisting of 32 TMR-elements (tunnel magneto resistance), an ASIC, and subsequent readout components. A source for generating the eddy currents inside the material under test is also implemented onboard of the PCB. These probes were developed in the IMAGIC-project* for detection and imaging of surface breaking defects.
The performance of the new sensor system has been investigated for several mock-ups, Aluminum and Titanium plate specimens having small adjacent boreholes with diameter of 0.44 mm and micro notches in the µm-range, respectively. To compare our results we used conventional eddy current probes. The MR sensor elements have a length of around 60 µm leading to a nearly 'point like' measurement. Neighbouring boreholes (depth 0.25 mm) with a separation of 0.6 mm between their centres could be resolved with a good SNR, and more important, the boreholes could be confidently distinguished using the TMR-probes. In case of conventional probes a reliable separation was not possible. In this paper we present the MR-ET-probes of the IMAGIC consortium and a comparison with conventional techniques.
*The IMAGIC-project ('Integrated Magnetic imagery based on spIntronics Components', 2011 – 2014, project reference: 288381) was funded by the European Commission, Seventh Framework Programme. Further partners involved in the consortium beside BAM and CEA were INESC-ID and INESC-MN (Portugal), Sensitec GmbH (Germany), Tecnatom S.A. (Spain), and Airbus Group (France).
Measurement of spontaneous magnetic stray field signals has been reported to be a promising tool for capturing macro-scale information of deformation states, defects and stress concentration zones in a material structure. This paper offers a new method for self-magnetic leakage field detection using a magneto-optical (MO) hand-held microscope. Its sensor has a dynamic field range between ±0.05 and ±2 kA/m and a lateral optical resolution of approx. 10 µm. We examined flat tensile test specimens of metastable austenitic steel AISI 304. Static tensile tests were repeatedly interrupted at various predetermined states of strain and the magnetic information was measured by the MO system. Comparative measurements using a high-precision magnetic field GMR-sensor, verify the outstanding capability of the MO microscope regarding spatial resolution of magnetic fields.