Apparatus and method for eddy current detection of subsurface discontinuities in conductive bodies
First Claim
1. Eddy current inspection apparatus for detecting the presence of subsurface regions within a body wherein the region to be detected is of a different electrical conductivity than portions of said body that surround said region to be detected, said eddy current inspection apparatus comprising:
- drive coil means positionable on the surface of said body to be inspected, said drive coil means being configured for inducing a magnetic field that is directed inwardly into an inspected region of said body in response to an applied drive signal;
electrical signal supply means interconnected with said drive coil for supplying said drive signal, said electrical signal supplying means including means for supplying said drive signal as a current pulse having a relatively long rise time and a substantially shorter fall time; and
magnetic flux detection means positionable on said surface of said body being inspected at a location proximate to said drive coil means, said magnetic flux detection means for supplying a signal representative of the eddy currents produced in said body by that portion of said drive current exhibiting said short fall time.
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Abstract
Nondestructive evaluation of conductive materials is disclosed wherein a magnetic field having a relatively slow rise time and a relatively fast fall time induces eddy currents in the conductive material being tested and subsurface flaws such as internal cracks or other discontinuities are detected by monitoring the magnetic field produced by the eddy currents. The magnetic field is generated by sequentially discharging two capacitor banks through a coil with the currents supplied by the capacitor banks being of opposite polarity and the first capacitive discharge exhibiting a time constant that is substantially longer than the second capacitive discharge. The initial, slowly rising portion of the magnetic field establishes magnetic flux that extends to substantial depths within the conductive material and the fast fall time of the magnetic field causes eddy currents of substantial magnitude to circulate within the region of the conductive material being examined. Flaws within the material cause perturbations in the eddy currents and induce current variations in a pickup coil during a time interval occurring shortly after the magnetic field begins to collapse. Detection equipment, connected for receiving the signal supplied by the pickup coil, monitors the amplitude of the pickup coil signal and, in some situations, its relationship in time relative to the rapid collapse of the magnetic field to determine the presence and location of a flaw.
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Citations
15 Claims
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1. Eddy current inspection apparatus for detecting the presence of subsurface regions within a body wherein the region to be detected is of a different electrical conductivity than portions of said body that surround said region to be detected, said eddy current inspection apparatus comprising:
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drive coil means positionable on the surface of said body to be inspected, said drive coil means being configured for inducing a magnetic field that is directed inwardly into an inspected region of said body in response to an applied drive signal; electrical signal supply means interconnected with said drive coil for supplying said drive signal, said electrical signal supplying means including means for supplying said drive signal as a current pulse having a relatively long rise time and a substantially shorter fall time; and magnetic flux detection means positionable on said surface of said body being inspected at a location proximate to said drive coil means, said magnetic flux detection means for supplying a signal representative of the eddy currents produced in said body by that portion of said drive current exhibiting said short fall time. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A method for detecting subsurface flaws within a conductive body comprising the steps of:
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inducing a slowly rising magnetic field into a region of said conductive body that is to be examined for said subsurface flaws with the flux lines of said magnetic field extending inwardly into said conductive body; rapidly collapsing said induced magnetic field to generate substantial eddy currents which circulate through said region of said conductive body being examined; monitoring the magnetic field variations caused by said eddy currents to supply a detection signal representative of the magnetic field variations caused by said eddy currents; and comparing said detection signal with a reference signal which results when said induced magnetic field is introduced into a flaw-free conductive body having substantially the same geometry as said region of said conductive body being examined. - View Dependent Claims (10, 11, 12, 13, 14, 15)
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Specification