Electricity: measuring and testing – Magnetic – With compensation for test variable
Patent
1991-07-05
1993-02-16
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
With compensation for test variable
324232, 324242, 324262, G01N 2790
Patent
active
051874350
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates to a rotary head for scanning the surface of elongated metal test materials and, more particularly to such a rotary head in a non-destructive eddy current defect test apparatus.
2. Description of Related Art
For many years, rotary heads have been known for use in non-destructive testing of materials, and are widely used. They serve there for testing, in particular, of bare curved surface materials of steel and non-ferrous substances to detect defects located at any point below the surface and extending up to the surface. During long periods of application, such rotary heads have gained a high technological performance providing excellent results. On bare surfaces, e.g., cracks having a depth of only 30 micrometers can be reliably detected, and this accomplished at rather high running speeds.
On the other hand, there are present limits, which would be desirable to overcome, since this would lead to further possible applications. For instance, sensitive testing of "non-bare" ferromagnetic steel is not possible with known eddy current rotary heads because of the high interference level encountered. Another example relates to cracks in ferromagnetic materials, wherein further processing, as rolling, drawing or peeling, will result in a surface being closed again. In this case, the low penetration depth of eddy currents for the high frequencies required to achieve desired sensitivity accounts for the fact that such "smeared" cracks are impossible to detect in most cases. Difficulties have also been encountered in the testing of welded austenitic pipes, wherein ferrous impurities are found to generate a high interference level in the area of the weld seam, and, therefore, prevent an efficient testing of such pipes.
SUMMARY OF THE PRESENT INVENTION
According to the invention, the test material is magnetized resulting in a considerable reduction of the magnetic permeability of the test material, and, if possible, magnetized to reach the saturation range. There is a risk, however, in doing this that the ferrite cores of eddy current transducers will also become saturated, and as a result that the sensitivity of the latter will be undesirably affected. This is compensated for in the invention by locating a ring of magnetically conductive material between the magnetization coil and the eddy current transducer. This ring homogenizes the magnetic field in the test range, and, therefore, the radial component of the magnetic field above the test material surface, will stay small.
By reducing the magnetic permeability of the test material, a comprehensive suppression of all interference sources caused by magnetic inhomogeneities of the material surface, such as scale, heat zones, weld seams or cold consolidation, for example, is achieved. It is now possible, for the first time, to also test "non-bare" ferromagnetic material, using rotating eddy current probes. For "smeared cracks" (i.e. with closed surfaces) the strong reduction of the magnetic permeability, which may be 50 times or more, causes a corresponding increase in the penetration depth of the eddy currents, making defects detectable, which would have normally remained hidden. Also, now for the first time welded austenitic pipes are now fully accessible to testing with eddy current rotary heads, since by the reduction of the magnetic permeability, the influence of ferrous impurities can be made ineffective.
A first embodiment of the invention provides interchangeable protective sleeves supported by the housing and having a borehole adapted to the diameter of the test material, which sleeves serve as conductors of the magnetic flux. According to another embodiment, a protective sleeve extends into the borehole of a hollow shaft allowing the magnetic flux to be conducted in an optimum manner via the rotating hollow shaft into the test material. A still further embodiment requires that between the protective sleeve and the hollow shaft, there is provided a narrow air gap at least at one posit
REFERENCES:
patent: 3299349 (1967-01-01), Tompkins et al.
patent: 4596953 (1986-06-01), Nagasaka et al.
patent: 4673879 (1987-06-01), Harris et al.
patent: 5023550 (1991-06-01), Yamazaki et al.
Dr. Friedrich Forster Pruferatebau GmbH & Co. KG
Netter George J.
Strecker Gerard R.
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