Electricity: measuring and testing – Magnetic – Magnetic sensor within material
Patent
1997-09-17
1999-06-22
Strecker, Gerard
Electricity: measuring and testing
Magnetic
Magnetic sensor within material
324232, 324240, G01N 2790
Patent
active
059145951
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The invention relates to an eddy current sensor using separate receiver and transmitter windings for carrying out the nondestructive testing of electrically conductive parts.
The invention also relates to a tool for the nondestructive testing of tubes, said tool having at least one eddy current sensor.
The sensor according to the invention can be used for the nondestructive testing of a part having random shape and dimensions, provided that the nature of the material or materials constituting said part permit the induction therein of eddy currents. A preferred, but in no way limitative application, relates to the testing of steam generator tubes equipped nuclear power stations.
2. Prior Art
In view of their ease of implementation, eddy current sensors have been subject to numerous developments in the nondestructive testing field.
The principle of such sensors is based on creating a primary magnetic field in a winding supplied with alternating current. When the winding is placed in the vicinity of an electrically conductive material, said primary magnetic field induces eddy currents in the material. These eddy currents produce a secondary magnetic field opposing the primary magnetic field. The thus formed secondary magnetic field has the effect of modifying the impedance of the winding in proportions dependent on the value of the air gap between the winding and the part and the different factors linked with the shape and internal structure of the part. Nondestructive testing by means of eddy currents is essentially based on the fact that the presence of faults in the material modifies the impedance of the winding.
The simpler sensors have a single winding used both as the transmitter and as the receiver.
The most widely used sensors are employed in differential measurements. These sensors generally use two windings connected in series, both being used as transmitters and receivers. Due to the fact that the windings face two adjacent regions of the part, any impedance difference between the two windings reveals the presence of a fault in the material and also the extent thereof.
Eddy currents, whereof each winding acts both as a transmitter and a receiver, perform local measurements making it possible to establish the cartography of faults present within a part to be tested. However, such sensors only detect faults present over a limited depth from the surface of the part close to the measuring windings. Thus, in the case of the testing of tubes of steam generators, certain point sensors only detect faults on the outer wall when they have a depth exceeding 40% of the thickness of said wall.
EP-A-370 691 proposes testing a tube by means of an apparatus incorporating a single eddy current sensor fitted coaxially between two end parts. This sensor has two transmitter windings placed around a common axis to be located in accordance with the axis of the tube, as well as a plurality of receiver windings located in the annular gap separating the transmitter windings, so that their axes are oriented radially with respect to the axis of the transmitter windings. The latter are excited in opposition, so that the primary magnetic fields are summated in the gap containing the receiver windings. The detection cases place in turn on successive receiver windings, so as to perform a circumferential scan during the displacement of the apparatus in the tube.
Due to the fact that the primary magnetic fields are summated at the location of the receiver windings, the apparatus described in the aforementioned document requires an electronic compensation treatment in order to eliminate from the signals emitted by the receiver windings the fraction resulting from the summated primary magnetic fields.
The apparatus described in EP-A-370 691 is insensitive to cracks oriented in accordance with the circumference of the tube. Finally, this apparatus can only be used for the testing of a tube. It consequently does not permit the testing of parts having different shapes, such as plates.
DESCRIPTION OF TH
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Piriou Marc
Slazak Jacky
Intercontrole
Strecker Gerard
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