To test a material with the eddy current method, a coil is moved into proximity with a test piece, the coil inducing eddy currents in the test piece by producing magnetic alternating fields. Such induced eddy currents produce a secondary field in the coil or in a respective sensor coil. The signals appearing during a scanning cycle at the output of the coil or sensor coil are converted into a sequence of digital values corresponding to complex numbers.
During the testing of materials with the eddy current method, very often disturbing effects will appear, generating higher and/or different signal levels than those generated by the defects present in the material being tested. Such disturbance effects are caused, e.g., by the lifting of the coil from the test piece, by ferritic influx in the material, etc. In order to detect the flaws, these interfering signals must be separated from those signals caused by the defects in the material For this separation one can use the amplitudes and the phase positions of the complex values. Many times, however, it is not possible to definitely assign the flaw signals and disturbance signals to certain amplitude and phase ranges.
In a multifrequency testing apparatus, one uses linear algorithms to distinguish the flaw signals from the disturbance signals (i.e., these signals attributable to field variations produced by phenomena other than flaws in the material). Thereby stationary independent data measured at different measuring frequencies are combined by matrix operations in order to generate zero points in the directions of the disturbance producing the disturbance field to be suppressed. Flaw signals are indicated only when the phase of the disturbance signals falls within a predetermined range. As the amplitude of the disturbance signal increases, so do the phase angles relative to the direction of the disturbance. If the size of the defect influences not only the amplitude but also the phase position, this will result in losses unfavorably affecting the determination of the defect size. These losses will appear independent therefrom whether or not a disturbance signal is coinciding with a flaw signal.