Method and apparatus for non-destructive testing of electrically conductive materials
First Claim
1. A method of non-destructively testing electrically conductive articles for the presence of flaws using eddy currents, the method comprising the steps of:
- applying a repetitive square wave excitation signal to a coil placed upon an article to be tested, said coil providing a circular footprint of magnetic field;
moving a magnetic field detector along a circular scan path on the surface of the article within the footprint of the magnetic field, said movement synchronized with said excitation signal, said detector being responsive to low frequency variations of magnetic field;
examining the field signal sensed by said detector for variations in echo component of the field signal consequent upon the change in position of the detector on the scan path;
gating the detected field signal at set time intervals synchronised to the excitation signal to define at least one series of time bands, each time band of a series falling within a respective half-wave portion of the excitation signal, such that each series of time bands registers echoes of the excitation signal from a depth zone within the article under test for which the aggregate signal transit times to and from the depth zone fall within the gated interval; and
measuring the signal level for each time band of each series and comparing the measured signal levels within a series to identify any change in level from a predominant background level,wherein a change in measured signal level is indicative of an echo from a discontinuity in the material of the article under test in the depth zone corresponding to the gated interval, the azimuth position of said discontinuity being indicated by the temporal position of the change in level within the respective series of time bands.
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Accused Products
Abstract
Eddy current based method and apparatus uses a repetitive square wave signal applied to an excitation coil and operates in a pulse echo mode by gating the detected field signal at synchronized intervals to generate one or more sequences of like gated slices obtained from respective detector positions on a circular scan path. Suitable time gating to exclude surface and near-surface reflections overcomes surface clutter and flaws are detected by examination of the sequence of slices for characteristic changes in level. Both method and apparatus have special application for crack investigations at rivet fasteners or similar, e.g. in airframes, and have ability to cope with ferrous fasteners in non-ferrous material. Excitation coil and magnetic detector are combined in a probe and the latter is driven around a circular scan path. A centering display is derived by examination of the near-surface reflection signal to determine the position and level of the peak slice level on the scan path sequence of slices.
25 Citations
12 Claims
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1. A method of non-destructively testing electrically conductive articles for the presence of flaws using eddy currents, the method comprising the steps of:
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applying a repetitive square wave excitation signal to a coil placed upon an article to be tested, said coil providing a circular footprint of magnetic field; moving a magnetic field detector along a circular scan path on the surface of the article within the footprint of the magnetic field, said movement synchronized with said excitation signal, said detector being responsive to low frequency variations of magnetic field; examining the field signal sensed by said detector for variations in echo component of the field signal consequent upon the change in position of the detector on the scan path; gating the detected field signal at set time intervals synchronised to the excitation signal to define at least one series of time bands, each time band of a series falling within a respective half-wave portion of the excitation signal, such that each series of time bands registers echoes of the excitation signal from a depth zone within the article under test for which the aggregate signal transit times to and from the depth zone fall within the gated interval; and measuring the signal level for each time band of each series and comparing the measured signal levels within a series to identify any change in level from a predominant background level, wherein a change in measured signal level is indicative of an echo from a discontinuity in the material of the article under test in the depth zone corresponding to the gated interval, the azimuth position of said discontinuity being indicated by the temporal position of the change in level within the respective series of time bands. - View Dependent Claims (2, 3, 4, 12)
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5. Apparatus for non-destructively testing electrically conductive articles for the presence of flaws using eddy currents, the apparatus comprising:
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a probe unit locatable on the article under test, said probe unit comprising; an excitation coil; a magnetic field detector for detecting incoming field signals, said detector being responsive to low frequency variations of magnetic field; and rotational drive means for driving the detector along a circular scan path overlying and adjacent to the article under test; an excitation signal generator for energising the excitation coil with a repetitive square wave excitation signal in synchronism with the motion of the detector along said scan path; a field signal processor for processing the output of detector, said processor having at least one channel, said at least one channel comprising; a switching device for gating the incoming field signal from the detector at intervals in synchronism with the excitation signal so as to pass a series of time bands of the field signal, there being one time band for each half-wave portion of the excitation signal; and an integration circuit for integrating the field signal level within each passed time band of the series; measurement means for measuring at each of said at least one channel, the integrated signal level for each time band in a respective series; and means for comparing the integrated signal levels of a series and for indicating variations from a predominant background level. - View Dependent Claims (6, 7, 8, 9, 10, 11)
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Specification