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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.
Magneto resistive (MR) sensors are suited for a wide range of different applications. Especially due to their small sizes and their frequency-independent behaviour, they are eligible candidates for non-destructive testing methods where high spatial resolution is required. The use of specialized magnetic field probes enables not only a defect evaluation, but also a localized characterization of ferromagnetic materials.
This paper presents some NDT (non-destructive testing) examples concerning magnetic flux leakage applications (MFL) and eddy current testing (ET). For MFL-testing we used an NDT-adapted GMR (Giant MR) sensor array for the detection of small inclusions in thin steel sheets. We also investigated the effects of microstructure on magnetic properties using GMR sensors. The ET-example describes an MR array probe consisting of 32 MR-elements, an ASIC, subsequent readout components and a one-wire excitation to generate eddy currents inside the material (developed in the EU-funded IMAGIC-project, 2011-2014, 7th Framework Programme). Using this MR-probe we achieved results with a better spatial resolved defect detection of neighbouring boreholes compared to “conventional” ET-probes.
Magneto resistive (MR) sensors are suited for a wide range of different applications. Especially due to their small sizes and their frequency-independent behaviour, they are eligible candidates for non-destructive testing methods where high spatial resolution is required. The use of specialized magnetic field probes enables not only a defect evaluation, but also a localized characterization of ferromagnetic materials.
This paper presents some NDT (non-destructive testing) examples concerning magnetic flux leakage applications (MFL) and eddy current testing (ET). For MFL-testing we used an NDT-adapted GMR (Giant MR) sensor array for the detection of small inclusions in thin steel sheets. We also investigated the effects of microstructure on magnetic properties using GMR sensors. The ET-example describes an MR array probe consisting of 32 MR-elements, an ASIC, subsequent readout components and a one-wire excitation to generate eddy currents inside the material (developed in the EU-funded IMAGIC-project , 2011-2014, 7th Framework Programme). Using this MR-probe we achieved results with a better spatial resolved defect detection of neighbouring boreholes compared to “conventional” ET-probes.
Electromagnetic methods are widely used in Non-destructive Testing industries (NDT). In order to meet the requirements, safety-relevant products and structures have to be tested reliably during production or within subsequent maintenance cycles. For this purpose industrial users call for adapted testing methods which allow the detection of relevant defects such as cracks and pore in ferromagnetic or conductive materials. Automated testing systems and the application of sensor arrays are of great interest in order to enhance inspection quality and to decrease testing time and costs. Magneto resistance (MR) sensors like giant magneto resistance (GMR) or tunnel magneto resistance sensor (TMR) have proofed their suitability in many NDT-applications due to their extraordinary properties which combine high field sensitivity, a high spatial resolution and low costs. Due to their small size these sensors can be useful – in addition to the pure detection task – for a more detailed defect characterization and defect reconstruction providing defect size, orientation and geometry. This chapter gives a brief introduction of electromagnetic testing methods, i.e., magnetic particle inspection (MPI) and eddy current testing (ET), followed by two different approaches for which MR sensor arrays were successfully adapted.
Main concept of magnetism and, therefore, of magnetic imaging can be subdivided into different levels, macroscopic, magnetic domain, and atomic. While conventional sensor solutions cover only the macroscopic level, the spatial resolution of GMR (Giant Magneto Resistance) sensors go down to the domain scale. In addition, those low cost sensors are well suited for automotive and industrial applications, particularly high-speed solutions. Main reason is their outstanding performance in terms of high spatial resolution, high accuracy, high bandwidth combined with field sensitivity, energy efficiency and durability.
In contrast to industrial use, down to the present day GMR sensors do not get beyond scientific scope in case of non-destructive testing (NDT) applications. Nevertheless, there are scientific and industrial NDT applications in which adapted GMR sensor can be promising compared to the conventional NDT methods.
This contribution summarizes findings at the BAM over the last decade which demonstrates the preeminent properties of GMR-based testing solutions. This comprises the active and passive testing of different materials with hidden defects and flaws near geometric boundaries like edges where conventional methods meet their limits. Another promising application for adapted GMR sensors is the characterization of magnetic materials, where the sensors provide additional information on microstructure, mechanical stress state, phase transformations and their interaction with magnetic fields. The examples show the need and benefit of NDT adapted GMR sensors.