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Magnetic Flux Leakage Testing

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Situation

Increasing quality demands regarding manufacturing can often only be met by using non-destructive testing methods. In this context, crack testing plays a significant role. The most frequently used method is the manual or (half-) automated flux leakage testing with magnetic powder and optical evaluation.

Regarding optical methods, subjective influences cannot be ruled out and much manpower is required. A complete automation of the process was not possible in all cases.

Realisation

Within magnetised materials, magnetic field lines are deflected by possibly existing material discontinuities like cracks. Discontinuities that are at a right angle (i.e. perpendicular) to the direction of the magnetic field have the biggest influence. In the case of near-surface flaws this leads to a local increase of the tangential magnetic field strength on the surface of the component that is to be tested (see figure 1).

Figure 1: Course of the magnetic field along a crack in a magnetised component

GMR Sensors

The tangential magnetic field parts are detected by Giant Magneto-Resistance sensors. The Giant Magneto-Resistance effect is based on the dramatic decrease in electric resistance of the sensor materials when the sensor is in an outer magnetic field.

Figure 2: GMR sensor array

Automated Flux Leakage Testing With GMR Sensors

Due to the development of new, highly sensitive magnetic field sensors, we are now able to measure magnetic leakage fields directly. In this process, separate sensor elements or sensor groups (see figure 2) are moved along the surface of the magnetised component by a manipulator system. In this way, the physical effects described above can be utilised for crack detection even without magnetic powder testing. See the following example:

Figure 3: Result of flux leakage testing of an induction-hardened part ("C-scan")

Advantages of Sensor-Based Flux Leakage Testing

• Technique is based on the same physical effects as magnetic particle testing
• No testing medium needed
• Soiling and geometry have no influence on the result
• Largely independent of material inhomogenities
• Fully automated testing process
• Comprehensive, automated documentation of the testing results

Applications

• Automated surface flaw testing of steel bars
• Automated surface flaw testing of steering racks
• Automated surface flaw testing of slewing rings

Contact

Paul Kröning
Phone: +49 (0) 6 81/9 76 71 53
Fax: +49 (0) 6 81/9 76 71 58