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Magnetic tangential field-strength-inspection, a further ndt-tool for 3MA

: Dobmann, G.; Pitsch, H.

Höller, P. ; Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V. -DGZfP-, Berlin:
Nondestructive Characterization of Materials III
Berlin/West: Springer, 1989
ISBN: 3-540-51856-8
International Symposium on Nondestructive Characterization of Materials <3, 1988, Saarbrücken>
Fraunhofer IZFP ()
Fourieranalyse; Klirsfaktor; Koerzitivfeldstärke; magnetisches Feld; Material- und Mikrogefügecharakterisierung; Zerstörungsfreie Prüfung

Micromagnetic nd-techniques like Barkhausen-noise-, incremental permeability - and dynamic magnetostriction-inspection are well known with their capability to characterize microstructures and to measure stress in ferromagnetic materials. Different measuring quantities can be derive during a dynamic magnetization performed by an electromagnet as a yoke-tranducer and controlled by a Hall-element-measurement of the magnetic field strength parallel to the surface under inspection i.e. the tangential field component. Though the driving voltage to excite the field coil of the yoke is sinusoidal the current in the coil and therefore the magnetic field reacts with higher harmonics i.e. a distortion factor can be derived. The contribution describes the technique to analyze these higher harmonics in the tangential field. A complex Fast Fourie Transform is used to calculate the amplitudes resp. the phases of the higher harmonics, at which only the 3., 5. and 7. are needed. Technical rules can be derived to estimate directly the field strength in theses signals. Together with the distortion factor two indepent inspection quantities are achieved. The contribution represents results where the technique was applied to separate different microstructures of different steels and to measure the case-hardening depth at the surfaces of a martensitic steel.