A.C. field measurement system for detecting and sizing defects i

Details A.C. field measurement system for detecting and sizing defects i A.C. field measurement system for detecting and sizing defects i

1993-07-06

1996-11-12

Wieder, Kenneth A.

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

For fault location

324238, 324242, G01N 2790

Patent

active

055743765

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an a.c. field measurement system for detecting and sizing defects in a conductor.
The techniques currently in use for the detection and sizing of surface-breaking cracks on ferritic steel structures require access to the bright metal. It is therefore necessary to carry out extensive pre-cleaning of the area to be inspected. This is particularly onerous underwater where marine growth and calcareous deposits have to be removed as well as oxide layers prior to inspection. Any protective paint layer present also has to be removed and then reapplied after inspection. Magnetic particle inspection (MPI) is then commonly used for crack detection and crack length measurement, possibly followed by a.c. potential drop (ACPD) inspection if crack depth information is required but more often material is removed by grinding until the MPI indication disappears. The a.c. field measurement (ACFM) technique was developed to extend the crack sizing capability of ACPD to a non-contacting form capable of inspecting through the non-conducting coatings. ACFM therefore differs from ACPD only by its measurement of above surface magnetic fields instead of surface potential differences. As long as the surface current density induced into the specimen is locally of uniform strength and direction, theoretical modelling of the expected magnetic fields above a defect is possible which allows crack sizes to be estimated from experimental measurements. However, a problem associated with ACFM is that signals indicative of defects in a test piece are conditioned by the speed at which a probe is scanned relative to the test piece and it is often difficult to distinguish between cracks in a test piece and spurious signals resulting from probe lift off.
According to the present invention there is provided an a.c. field measurement system for detecting and sizing defects in a conductor comprising a probe and a processor including at least one display, the probe including a yoke for inducing a uniform a.c. field in a first direction, sensor means comprising at least two coils a first coil of which having a longitudinal axis extending in the said first direction and a second coil of which having a longitudinal axis extending in a second direction orthogonal to the first direction and means for effecting energisation of the field the arrangement being such that signals produced by the coils indicative of defects in a test piece, when the probe is located adjacent the test piece, are processed by the processor and plots of signal magnitude derived from at least two of the coils are recorded on the display as a function of time or against each other.
The present invention is designed to implement a practical application of the ACFM technique by a combination of novel probe and graphical display formats. The probe is designed to achieve the two requirements of ACFM by the production of a locally uniform input current, perpendicular to the expected crack edge, and by the simultaneous measurement of at least two mutually perpendicular components of magnetic field at the same point in space. The technique can also be implemented with an array of double coil measurements, to give instantaneous display of the surface magnetic field distribution, or with controlled probe motion to give improved spatial resolution. The remote graphical display combines the conventional time-base plots of the individual magnetic field component strengths with a novel display format in which one component is used as the abscissa and another as the ordinate. This format, which has been dubbed a "butterfly plot" because of the characteristic shape produced by a crack, has several practical advantages for crack detection. Firstly, the removal of the time-base gives a display independent of the speed at which the probe is scanned and which can be retraced for confirmation. Secondly, the simultaneous monitoring of two components helps to distinguish between cracks and spurious indications due to probe lift-off. The combination of point measurements of magn

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