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Organisationseinheit der BAM
For non-destructive testing (NDT) appropriate reference blocks are required in order to verify and calibrate a testing procedure. At BAM a special electric discharge machining (EDM) system has been developed which is able to manufacture artificial defects having a width down to 30 μm.
Especially in the case of austenitic materials conventional EDM leads to a transformation of austenite to martensite. The martensite transformation causes a higher sensitivity of electromagnetic NDT methods (e. g. eddy current testing) at the artificial defects compared to natural defects of same size. The EDM system developed at BAM uses very low energy to avoid this material transformation. A side effect of the low-energy EDM is a lower surface roughness compared to conventional EDM. The artificial defects manufactured at BAM are measured optically and delivered with a certificate. A comparison of artificial defects shows the influence of material transformation on NDT and how differently the quality of the artificial defects can be.
The use of composite pressure vessels instead of conventional vessels made of steel or aluminum grew strongly over the last decade. The reason for this trend is the tremendous weight saving in case of composite vessels. However, the long-time behavior is not fully understood for filling and discharging cycles and creep strength and their influence on the CFRP coating (carbon fiber reinforced plastics) and the internal liner (steel, aluminum, or plastics). The CFRP ensures the pressure resistance while the inner liner is used as container for liquid or gas. To overcome the missing knowledge of ageing BAM started an internal project to investigate degradation of these material systems. Therefore, applicable testing methods like eddy current testing are needed. Normally, high-frequency eddy current testing (HF-ET, f > 10 MHz) is deployed for CFRP due to its low conductivity of the fiber, which is in the order of 0.01 MS/s, and the capacitive coupling between the fibers. Nevertheless, in some cases conventional ET can be applied. We show a concise summary of studies on the application of conventional ET of composite pressure vessels.
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.
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).
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.
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.
The recognition of surface damage on rails is of critical importance in order to guarantee a high level of safety for normal train operation. Without early recognition systems, defect damage (e.g. cracks) in the rail, will increase in size and can eventually lead to fracture of the rail. Special grinding trains are in general use for rail maintenance and are employed to machine down worn rails and prevent surface damage in an early stadium. The company SPENO INTERNATIONAL SA along with the BAM, DB AG and other partners have integrated an eddy current instrument into a grinding train. The equipment has been developed for detection of Head Check type defects and provides information on defect position and depth. Initial experiments were presented at the DACH-conference 2004 in Salzburg. This system has been continually developed and as will be shown is ready for practical application. It is named HC Grinding Scanner. The actual measuring system and results will be presented.
Non-destructive testing for surface crack detection and head check depth quantification at the gauge corner of railway tracks can be achieved using eddy current methods. With the extension of the tested zone to the running surface, rail defect signal types other than head checks can be measured. Due to their mostly irregular shape, a quantitation based on a calibration against regular test cracks of varying depth may not be linear. Estimates of the expected influence of more complex crack patterns may be obtained by a finite element simulation of sufficiently simple limiting cases, like two displaced or intersecting cracks or a simply branched or flexed crack. As a first step, a 3D finite element model of the HC10 eddy current probe distributed by Prüftechnik Linke und Rühe (PLR), Germany was built and verified against measured results from an (easily fabricated) reference block with isolated long cracks.