Eddy current wide probe

Details Eddy current wide probe Eddy current wide probe

1999-01-14

2001-04-10

Oda, Christine (Department: 2862)

Electricity: measuring and testing

Magnetic

With means to create magnetic field to test material

C324S238000, C324S240000

Reexamination Certificate

active

06215300

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to eddy current probes for detecting flaws in conductive materials and, more particularly, to a wide eddy current probe suited for rapid inspection of axial dovetail or shaped holes.
BACKGROUND OF THE INVENTION
Eddy currents provide a measurable indicator of flaws in the surface and sub-surface of conductive materials. They are generally confined to the surface and near surface regions of the material. The eddy currents are affected by changes in the resistivity of the conductive material. Flaws in the material, such as microscopic hairline cracks or pits, affect the localized resistivity of the material. Flaws in a material cause localized variations in the eddy currents in the material. Accordingly, a conductive material can be inspected for flaws by inducing eddy currents in the material.
Eddy current probes detect material flaws by sensing variations in eddy currents. These probes have coils with high-frequency current that project a fluctuating magnetic field into the conductive material being measured. This imposed magnetic field induces eddy currents in the material. The strength of the eddy currents depends on the local resistivity of the material, which resistivity is affected by the presence of material flaws and cracks. The eddy currents create a magnetic field that varies in intensity with the strength and, hence, the presence of material flaws.
The magnetic field created by the eddy currents extends above the material surface up to the probe. The magnetic field from the eddy current induces its own voltage in the probe coil. The eddy magnetic field opposes the coil field. These coupled magnetic fields measurably influence the net current and inductance of the probe coils, and variations in the coil currents vary in response to material flaws and are measured to detect these flaws.
Generally, the current probe is moved axially along the length of the surface to be scanned. As the probe completely traverses each scan line across the surface, the probe is circumferentially indexed to the next scan line around a reference frame. The probe is then drawn in reverse along the next scan line. This scanning and indexing sequence is repeated until the probe completely scans the entire surface. The probe must cover the entire surface to ensure that all material flaws are detected. To do this, the probe travels along straight scan lines parallel to the axis of the surface. If the probe wanders off a scan line, portions of the material surface will be missed, and flaws in the material may escape detection. Moreover, it is difficult to accurately specify the location of flaws when the probe drifts off the intended scan line.
Probe sensitivity to small flaw detection is limited by the size of the probe sense coil. The conventional small probes require long scanning times for scanning and indexing the probe over the entire surface of the part.
DISCLOSURE OF THE INVENTION
If probe sensitivity is not critical for a particular application, it is thus desirable to provide a wider configuration for the eddy current probe to reduce scanning time. According to the present invention, one axis of the electrical coil in the eddy current probe is elongated to span a much larger distance than is true of the conventional coils. Desirably, a single pass of each of the two coils according to the present invention can inspect a path of up to one-half inch width, while to obtain the same coverage, the conventional coil requires up to 50 passes.
According to one aspect of the invention, there is provided an eddy current probe including a housing block, a pair of elongated ferrite cores disposed end-to-end in the housing block along respective longitudinal axes, a pair of receive coils disposed in the housing block respectively surrounding the elongated ferrite cores, a transmit coil wound around the receive coils in the housing block, and a pair of ferrite shields disposed in the housing block. Ferrite shields sandwich the transmit coil, the pair of receive coils and the pair of elongated ferrite cores. The active length of each of the pair of receive coils has been demonstrated as about 0.4″. In other embodiments, the housing block is shaped corresponding to a contour of a turbine dovetail. The housing block may be further split axially to allow for part-to-part tolerances. In this context, one of the pair of receive coils is mounted in each half of the housing block.


REFERENCES:
patent: 4719422 (1988-01-01), DeWalle et al.
patent: 6114849 (2000-09-01), Price et al.

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