Routes for GMR-Sensor Design in Non-Destructive Testing

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

Development, analysis, and application of GMR sensor arrays for non-destructive evaluation of ferromagnetic components

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.Non-destructive testing is important for both quality control and maintenance of safety-related components. Modern industry steadily undergoes competition and cost pressure. Therefore, new innovative testing methods are key to increase safety and cost-effectiveness. The conventional magnetic flux leakage testing method (MFL) using magnetic particle inspection (MP) is a manual procedure which is very sensitive in terms of the detection of micrometer-scaled cracks. An automated reliable testing however calls for adapted magnetic field sensors. Additionally the quantification of stray fields allows an evaluation of defect geometry. GMR sensors (giant magneto resistance) are particularly well-suited for this purpose. Their low costs, excellent field sensitivity, and capacity to be miniaturized lead to high resolution test results. However, drawbacks exist for commercial GMR sensors which include nonadaption for NDT applications. To overcome this drawback one objective of this thesis was to optimize the geometry of the sensing elements for a GMR sensor array. After characterization, the new sensor arrays were used for validation and investigation of a probability of detection. In addition, by comparing GMR-MFL with other testing methods related to surface breaking defects (eddy current testing, MP, thermography), it was possible to classify in a first step GMR MFL testing in NDT. Finally, an automated GMR was tested successfully established for industrial purposes

Development, analysis, and application of GMR sensor arrays for non-destructive evaluation of ferromagnetic components

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.Non-destructive testing is important for both quality control and maintenance of safety-related components. Modern industry steadily undergoes competition and cost pressure. Therefore, new innovative testing methods are key to increase safety and cost-effectiveness. The conventional magnetic flux leakage testing method (MFL) using magnetic particle inspection (MP) is a manual procedure which is very sensitive in terms of the detection of micrometer-scaled cracks. An automated reliable testing however calls for adapted magnetic field sensors. Additionally the quantification of stray fields allows an evaluation of defect geometry. GMR sensors (giant magneto resistance) are particularly well-suited for this purpose. Their low costs, excellent field sensitivity, and capacity to be miniaturized lead to high resolution test results. However, drawbacks exist for commercial GMR sensors which include nonadaption for NDT applications. To overcome this drawback one objective of this thesis was to optimize the geometry of the sensing elements for a GMR sensor array. After characterization, the new sensor arrays were used for validation and investigation of a probability of detection. In addition, by comparing GMR-MFL with other testing methods related to surface breaking defects (eddy current testing, MP, thermography), it was possible to classify in a first step GMR MFL testing in NDT. Finally, an automated GMR was tested successfully established for industrial purposes

Routes for GMR-Sensor Design in Non-Destructive Testing

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

Routes for GMR-Sensor Design in Non-Destructive Testing

sensor

Evaluation of High Spatial Resolution Imaging of Magnetic Stray Fields for Early Damage Detection

Metal magnetic memory (MMM) technique with associated ISO 24497-1:3 [1] 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 [1,2]. Due to the physical coupling between mechanical stress and magnetization of ferromagnetic materials [3], it is assumed that the distribution of the residual MFL correspond to the internal stress of the specimen [2,4], or in the most general sense, to a degradation of the material [1,2]. Usually, MMM measurements are performed by relatively bulky magnetic inspection sensors [2]. The evaluation of local magnetic field distribution is limited thereby. High precision GMR (Giant Magneto Resistance) measurements in the micrometer regime can provide a higher degree of information due to better spatial resolution [5]. 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.</p

Eddy Current Probes Based on Magnetoresistive Array Sensors as Receivers

International audienceEddy Current (EC) Technique is a powerful method for detection of surface notches and of buried flaws during inspection of metallic parts. This technique is used for inspection at different industrial domains like aeronautics and nuclear one. Classical winding coils are the most commonly used EC sensors. Nevertheless, when the size of flaws decreases or the defect is rather buried deep inside the material, traditional winding coil probes turn out to reach their limits. For this reason, other technologies are investigated to improve this technique. Magnetoresistive sensors present the advantages of flat frequency response and dimensions at the micron size. These sensors are hence very attractive for the detection of buried defects that require low frequencies because of skin depth effect. Also, they are suitable for small surface defects due to high spatial resolution because of their manufacturing down to one hundred µm without losing their field sensitivity. An optimization of such probes based on magnetoresistive sensors (GMR - giant magnetoresistive and MTJ – magnetic tunnel juntions) specially designed to be integrated into an eddy current probe has been experimentally studied. Measurements using MR array probes consisting of 32 GMR- and MTJ-elements, an ASIC, subsequent readout components, and emitters for EC generating inside the material under test are shown. These probes have been developed in the IMAGIC-project1 for detection and imaging of surface breaking defects and buried flaws. The performances of developed probes have been investigated for several mock-ups

High-Resolution Nondestructive Test Probes Based on Magnetoresistive Sensors

International audienc