Eddy current inspection technique

Details Eddy current inspection technique Eddy current inspection technique

1997-06-30

2001-09-18

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Magnetic

With means to create magnetic field to test material

C324S242000, C324S230000

Reexamination Certificate

active

06291992

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to inspecting an object of electrically conductive material. In the specification and in the claims the phrase “inspecting an object” is used to refer to operations such as measuring the thickness of the object, checking the object for the presence of sub-surface flaws and measuring the thickness of a layer of non-conductive material around the object. The object can be, for example, a wall of a container or the wall of a pipe.
The effective thickness of an object, for example, a wall of a steel container, a pipe or a vessel can be locally affected by external or internal corrosion. In case the object is provided with a layer of insulation material, corrosion detection by visual inspection generally implies temporary removal of the insulation material, which is time consuming and expensive.
U.S. Pat. No. 4,843,320 issued Jun. 27, 1989 to B. R. Spies discloses a device for measuring a thickness of an object of electrically conductive material, such as a wall, comprising a pulsed-signal transmitter for generating an electromagnetic field in the wall and a receiver for measuring the decay of the eddy current generated by the electromagnetic field and for producing a signal representing the decay.
To determine the thickness of the wall, the decay of the received signal over a period of time is compared with the decay of a reference signal indicative of a known wall thickness.
The transmitter includes an emitter coil and the receiver includes a receiver coil. The coils are wound around a core, and, during normal operation, the central longitudinal axis of the core is perpendicular to a surface of the object.
With the known device, corrosion spots of a size comparable to that of the emitter coil can be detected, however, it was found that spots which are relatively small compared to the coil size cannot be detected. Such small corrosion spots, however, can locally reduce the wall thickness to a significant extent. Furthermore, the size of such a small spot can rapidly grow, for example when liquid water is present between the pipe and an insulation layer surrounding the pipe.
It is an advantage of the invention to provide a device which provides an enhanced resolution so as to allow a more detailed inspection of the object.
It is another advantage of the invention to provide an improved method of interpreting measured data to determine wall thickness.
SUMMARY OF THE INVENTION
To this end the device for inspecting an object of electrically conductive material according to the present invention comprises a non-static-signal transmitter for generating an electromagnetic field in the object, and a receiver for measuring the variations of the eddy current generated by the non-static electromagnetic field and for producing a signal representing the decay of the eddy current, wherein the non-static-signal transmitter comprises at least two laterally spaced-apart emitters for emitting an electromagnetic field, which emitters are, during normal operation, so driven that the resulting electromagnetic field in the central region, between the emitters, is intensified.
By intensifying the electromagnetic field in the central region, an eddy current of increased density can be generated in a small selected wall portion, and in this way the eddy current is focused in this selected wall portion. The eddy current receiver means can be arranged so as to measure the decay of the eddy current of increased density without measuring the decay of eddy currents of lower density, i.e. outside the selected wall portion. In this way a better resolution is achieved.
The central region is located between the emitters, and suitably it is located between the centers of the emitters.
In accordance with another aspect of the invention there is provided a method of measuring a thickness of an object of electrically conductive material, which method according to the invention comprises inducing a pulsed eddy current in the object, determining the decay of the eddy current and producing a signal representing the decay, and determining the thickness of the object from the signal, wherein the step of determining the thickness of the object from the signal comprises selecting a first magnitude and a second, smaller magnitude of the amplitude of the signal, measuring the length of the time interval in which the signal decays from the first magnitude to the second magnitude, and determining the thickness of the object from the length of the time interval.


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“A Pulsed Eddy Current Technique for Measureing Clad thickness”, by D. L. Waidelich et al., ANL-5614, pp, 1-49*
“The Response to Pulsed Eddy Currents of a Metal Plate”, by D. L. Waidelich, University of Missouri-Columbia, pp. 23-34.

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